Teleprotecio CIGRE

PROTECTION USING

TELECOMMUNICATIONS Joint Working Group 34/35.11 December 2000

Cigré JWG 34/35.11

PROTECTION USING TELECOMMUNICATIONS CIGRE JWG 34/35.11

Protection using Telecommunications Cigré Joint Working Group 34/35.11 - Final Report -

Regular members of JWG34/35.11:

Per Odd GJERDE (Convenor)

(Norway)

Hermann SPIESS (Secretary)

(Switzerland)

Alastair ADAMSON

(United Kingdom)

Ken BEHRENDT

(United States)

Michael CLAUS

(Germany)

Alouis W. H. GEERLING

(Netherlands)

José Angel GONZALES VIOSCA

(Spain)

Christopher HUNTLEY

(Canada)

Carlos SAMITIER OTERO

(Spain)

Yoshizumi SERIZAWA

(Japan)

Kent WIKSTROM

(Sweden) Corresponding members:

Ricardo de AZEVEDO DUTRA

(Brazil)

Stephen HUGHES

(Australia)

David C. SMITH

(South Africa) Comments and contributions received from:

Hervé HOUKE

(France)

Trygve JORDAN

(Norway)

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Contents 1

FOREWORD, SCOPE, OBJECTIVE ..............................................................................................................7

2

POWER SYSTEMS AND FAULT CLEARING .............................................................................................9 2.1 ELECTRIC POWER SYSTEMS ..............................................................................................................................9 2.2 ELECTRIC POWER SYSTEM FAULTS AND CLEARING ........................................................................................12 2.2.1 Electric Power System Faults ................................................................................................................12 2.2.2 Fault Clearing .......................................................................................................................................13 2.3 WHY DOES PROTECTION NEED TELECOMMUNICATION? ..................................................................................15 2.4 INTRODUCTION TO POWER SYSTEM PROTECTION ...........................................................................................15 2.4.1 Fault clearing system.............................................................................................................................17 2.5 HOW IS TELECOMMUNICATION USED..............................................................................................................19

3

PROTECTION USING TELECOMMUNICATIONS .................................................................................21 3.1 LINE PROTECTION ...........................................................................................................................................21 3.1.1 Analog Comparison Schemes ................................................................................................................21 3.1.1.1 Current differential protection ........................................................................................................................... 22 3.1.1.2 Phase comparison protection ............................................................................................................................. 28 3.1.1.3 Charge comparison protection ........................................................................................................................... 31

3.1.2

State Comparison Schemes....................................................................................................................33

3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.2.5 3.1.2.6

Intertripping Underreach Distance Protection ................................................................................................... 34 Permissive Underreach Distance Protection ...................................................................................................... 36 Permissive Overreach Distance Protection ........................................................................................................ 37 Accelerated Underreach Distance Protection..................................................................................................... 38 Blocking Overreach Distance Protection ........................................................................................................... 39 Deblocking Overreach Distance Protection ....................................................................................................... 40

3.2 BUSBAR PROTECTION ......................................................................................................................................42 3.2.1 Two-breaker busbar configuration........................................................................................................42 3.2.1.1 Normal fault clearing ......................................................................................................................................... 42 3.2.1.2 Breaker failure ................................................................................................................................................... 43

3.2.2

One- and a half breaker busbar configuration ......................................................................................43

3.2.2.1 Normal fault clearing ......................................................................................................................................... 44 3.2.2.2 Breaker failure ................................................................................................................................................... 45

3.2.3

Two zones / one breaker configuration..................................................................................................46

3.2.3.1 Normal fault clearing ......................................................................................................................................... 46

3.3 OTHER PROTECTION SCHEMES ........................................................................................................................47 3.3.1 Generator protection .............................................................................................................................47 3.3.2 Transformer protection..........................................................................................................................47 3.3.3 Reactor protection .................................................................................................................................48 3.4 SYSTEM PROTECTION ......................................................................................................................................48 3.4.1 Back-up protection ................................................................................................................................49 3.4.2 System-wide protection..........................................................................................................................53 4

TELECOMMUNICATION SYSTEMS FOR PROTECTION ....................................................................55 4.1 TELECOMMUNICATION CIRCUITS ....................................................................................................................56 4.1.1 Private and rented circuits ....................................................................................................................56 4.1.2 Analogue and digital circuits.................................................................................................................56 4.2 TELECOMMUNICATION NETWORKS .................................................................................................................57 4.3 TRANSMISSION MEDIA ....................................................................................................................................58 4.3.1 Pilot wires / Copper wires .....................................................................................................................58 4.3.2 Power Line Carrier (PLC).....................................................................................................................60 4.3.3 Microwave Radio...................................................................................................................................62 4.3.3.1 Multichannel radio............................................................................................................................................. 63 4.3.3.2 Single channel radio .......................................................................................................................................... 64

4.3.4 4.3.5

Optical fibres .........................................................................................................................................65 Satellites ................................................................................................................................................67

4.3.5.1 GEO - Geosynchronous Earth Orbit Satellites................................................................................................... 67

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4.3.5.2 MEO - Medium Earth Orbit Satellites ............................................................................................................... 68 4.3.5.3 LEO - Low Earth Orbit Satellites ...................................................................................................................... 68

4.4 MULTIPLEXING TECHNIQUES AND DIGITAL HIERARCHIES ..............................................................................69 4.4.1 Multiplexing Techniques........................................................................................................................69 4.4.1.1 Frequency Division Multiplexing (FDM).......................................................................................................... 69 4.4.1.2 Time Division Multiplexing (TDM) .................................................................................................................. 70 4.4.1.3 Code Division Multiplexing (CDM).................................................................................................................. 71

4.4.2

Digital Hierarchies................................................................................................................................72

4.4.2.1 PDH - Plesiochronous Digital Hierarchy........................................................................................................... 72 4.4.2.2 SDH - Synchronous Digital Hierarchy .............................................................................................................. 73

4.5 NETWORK TECHNOLOGIES ..............................................................................................................................75 4.5.1 Transport Networks ...............................................................................................................................77 4.5.2 Service Networks ...................................................................................................................................78 4.5.2.1 4.5.2.2 4.5.2.3 4.5.2.4

4.5.3

Circuit Switched Networks (POTS, ISDN) ....................................................................................................... 79 Packet Switched Networks (X.25, Frame Relay)............................................................................................... 80 Cell Switched Networks (ATM) ........................................................................................................................ 80 Datagram Networks (IP)................................................................................................................................... 81

Local Area Networks .............................................................................................................................82

4.5.3.1 Topology............................................................................................................................................................ 83 4.5.3.2 Media Contention and Protocols........................................................................................................................ 84 4.5.3.3 Advanced topologies ......................................................................................................................................... 85

4.6 NETWORK DESIGN AND OPERATION ...............................................................................................................86 4.6.1 Introduction ...........................................................................................................................................86 4.6.2 Technological considerations................................................................................................................88 4.6.2.1 PDH/SDH Networks.......................................................................................................................................... 88 4.6.2.2 ATM Networks.................................................................................................................................................. 89 4.6.2.3 IP Networks ....................................................................................................................................................... 91

5

TELEPROTECTION INTERFACES ............................................................................................................93 5.1 CONTACT INTERFACES ....................................................................................................................................93 5.2 ANALOG INTERFACES ......................................................................................................................................94 5.2.1 Pilot-wires (50/60Hz) ............................................................................................................................94 5.2.2 Voice frequency circuits (2-wire/4-wire) ...............................................................................................94 5.3 DIGITAL DATA INTERFACES.............................................................................................................................94 5.3.1 Electrical interfaces...............................................................................................................................94 5.3.2 Optical fibre interfaces ..........................................................................................................................95 5.3.3 LAN / Ethernet interfaces ......................................................................................................................96

6

PERFORMANCE AND RELIABILITY REQUIREMENTS ......................................................................99 6.1 REQUIREMENTS ON TELECOMMUNICATION SYSTEM ........................................................................................99 6.1.1 Introduction ...........................................................................................................................................99 6.1.1.1 Terminology and General Requirements ......................................................................................................... 100 6.1.1.2 Definitions ....................................................................................................................................................... 103

6.1.2

Requirement from analog comparison protection ...............................................................................108

6.1.2.1 Time synchronization through GPS................................................................................................................. 108 6.1.2.2 Time synchronization through communication network.................................................................................. 108

6.1.3

Requirements from state comparison protection .................................................................................109

6.1.3.1 Propagation Time............................................................................................................................................. 109

6.1.4 Requirements from intertripping .........................................................................................................109 6.1.5 Requirements from system protection..................................................................................................110 6.2 REQUIREMENTS ON TELEPROTECTION ...........................................................................................................111 6.2.1 Requirements on interface compatibility .............................................................................................112 6.2.2 Functional requirements......................................................................................................................112 6.2.2.1 6.2.2.2 6.2.2.3 6.2.2.4 6.2.2.5

Analog comparison protection control and monitoring ................................................................................... 113 State comparison protection control and monitoring ....................................................................................... 113 Erroneous signal detection............................................................................................................................... 114 Loop-back and misconnect detection............................................................................................................... 114 Actions on alarm conditions ............................................................................................................................ 114

6.3 REQUIREMENTS ON PROTECTION ...................................................................................................................115

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6.3.1

Requirements on analog comparison protection .................................................................................115

6.3.1.1 Need for delay compensation .......................................................................................................................... 115

6.3.2

Requirements on state comparison protection.....................................................................................116

6.3.2.1 Interface co-ordination..................................................................................................................................... 116 6.3.2.2 Delay Compensation........................................................................................................................................ 116

6.3.3 Requirements on other protections......................................................................................................116 6.4 CONSIDERATIONS ON INTERFACES AND INSTALLATION PRACTICES ...............................................................116 7

PROTECTION SYSTEM CONFIGURATIONS AND DESIGN ..............................................................119 7.1 PROTECTION SCHEMES AND TELECOMMUNICATION SYSTEMS COMPATIBILITY ............................................119 7.2 DESIGN CHECKLIST .......................................................................................................................................124 7.2.1 Application ..........................................................................................................................................124 7.2.2 Interfaces .............................................................................................................................................124 7.2.3 Contractual..........................................................................................................................................125

8

FUTURE TRENDS AND PROBLEMS TO BE SOLVED..........................................................................128 8.1 TRENDS IN COMMUNICATION ........................................................................................................................128 8.1.1 General Network Development............................................................................................................128 8.1.2 Transport Technologies.......................................................................................................................128 8.1.3 Networking Technologies ....................................................................................................................129 8.1.4 Service Access/Provisioning Technologies..........................................................................................129 8.1.5 Integration of Technologies.................................................................................................................129 8.1.6 New Technologies for QoS provision ..................................................................................................130 8.1.7 Intra- and inter-substation communication .........................................................................................131 8.1.7.1 Intra-substation communication....................................................................................................................... 131 8.1.7.2 Inter-substation communication....................................................................................................................... 132

8.2 TRENDS IN PROTECTION ................................................................................................................................134 8.2.1 Considerations on new protection philosophies ..................................................................................134 8.3 OPEN ISSUES AND PROBLEMS TO BE SOLVED.................................................................................................137 8.3.1 Protection relay interoperability .........................................................................................................137 9

CONCLUSIONS .............................................................................................................................................139

ANNEX A1

TELEPROTECTION SYSTEM CONFIGURATIONS ............................................................141

ANNEX A2

TELECOMMUNICATION SYSTEMS CHARACTERISTICS ..............................................144

ANNEX A3

QUALITY OF SERVICE .............................................................................................................146

A3.1 INTRODUCTION TO QOS............................................................................................................................146 A3.2 QOS DEFINITION IN ATM NETWORKS.......................................................................................................147 A3.2.1 ATM Service Categories..................................................................................................................149 A3.2.2 ATM over SDH/SONET...................................................................................................................151 A3.2.3 Applications Summary.....................................................................................................................152 A3.3 QOS DEFINITION IN IP NETWORKS ............................................................................................................152 A3.4 IP TO ATM SERVICE MAPPING..................................................................................................................155 A3.5 QUALITY OF SERVICE STANDARDS ...........................................................................................................156 ANNEX A4

PROTECTION SYSTEM TIME SYNCHRONIZATION TECHNIQUES.............................157

A4.1 TIME SYNCHRONISATION FOR SIMULTANEOUS SAMPLING.........................................................................157 A4.1.1 Internal timing synchronization ......................................................................................................157 A4.1.2 External timing synchronization .....................................................................................................159 LIST OF FIGURES.................................................................................................................................................161 LIST OF TABLES...................................................................................................................................................163 BIBLIOGRAPHY ...................................................................................................................................................164

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ABBREVIATIONS..................................................................................................................................................166 INDEX ......................................................................................................................................................................169

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1 FOREWORD, SCOPE, OBJECTIVE Deregulation in both the telecommunication and electric power industry, together with new telecommunication network technologies and advances in numerical protection, has resulted in the need to reconsider traditional methods of delivering teleprotection schemes and their associated bearer services. Fibre-optic technology is commonly deployed in new telecommunication networks for inter-station communication, and utility-owned and public telecommunication networks from third parties are available for protection purposes. Trends in substation automation move towards the use of bus- and LAN technologies within substations and interchange of information in numerical form. Numerical protection has been state-of-the-art for protection relaying for some years. In September 1996, Cigre SC34 "Power System Protection and Local Control", and SC35 "Power System Communication and Telecontrol", decided to form the joint working group Cigré JWG 34/35.11, with the following scope of work: -

Assess the state of development of advanced protection using inter-site communications Analyze the relevance and opportunities of newly released telecommunication technologies (referring to the work of WG 35.07) Identify and promulgate opportunities for future advances in the joint discipline of teleprotection Examine the need for, and if necessary compile, a lexicon of terminology to suit the new environment Develop a new report to update the Technical Brochure Ref. No. 13, 1987.

JWG 34/35.11 met for a kick-off meeting in Oslo in September 1997. The working group agreed that a new version of the former Technical Brochure "Protection systems using telecommunication" (Ref. No. 13, 1987) should be produced. The document should create awareness for the requirements, opportunities and risks of protection systems using telecommunications, and guide protection and telecommunication engineers towards a common understanding for the design and operation of reliable teleprotection schemes that meet performance requirements in the most economical way. This Technical Brochure has the following content: Chapter 2 describes power systems from a teleprotection point of view, with focus on power system faults, their reasons and characteristics, and fault clearing requirements. It continues with the definition of fault clearing systems, protection systems, protection schemes, and ends up with explanations why teleprotection is needed, and how protection can use telecommunication to meet fault clearing requirements. Chapter 3 describes protection relaying principles and protection schemes using telecommunications, and deals - from a power system point of view - with various aspects around the need of teleprotection, its benefits and adverse implications if the teleprotection service would fail. Chapter 4 gives an overview of telecommunication systems, with focus on capabilities and limitations related to protection signal transmission. Problems and risks that may arise with different types of telecommunication technologies are addressed, and functional and reliability aspects are dealt with, both under normal conditions and - most important - under power system

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fault conditions. Chapter 5 deals with interfaces. Requirements on interfaces between protection, teleprotection and telecommunication devices are given. Chapter 6 focuses on performance requirements on protection, teleprotection and telecommunication functions. Chapter 7 deals with protection system configuration and design. Compatibility issues between protection schemes and telecom technologies are addressed to provide a guide for protection and telecommunication specialists to design teleprotection systems that will meet fault clearing requirements. Chapter 8 gives an outlook on future trends and addresses some problems to be solved. In Chapter 9 the document is summarized some conclusions are drawn. Annexes A1 to A4 contain some related topics and additional information, which the JWG has found valuable for the better understanding of the subject.

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2 POWER SYSTEMS AND FAULT CLEARING 2.1 ELECTRIC POWER SYSTEMS Electric power systems consist of three principal components: generating stations, transmission systems, and distribution systems. Generating stations convert mechanical or thermal energy to electric energy, typically in the form of 50 or 60 Hz alternating current. Transmission systems transmit electric energy from the generating stations to the distribution system. To transmit electric energy efficiently over long distances, transmission system power lines are typically operated at 200 kV to 800 kV. The operating voltage of generators and distribution systems is typically in the range of 2.4 kV to 25 kV. Electric power systems deliver electric energy to power consuming equipment owned by residential, commercial, industrial, and governmental customers. Consumer products typically operate at several hundred volts. Power transformers are required to step the power system voltage up and down to connect various power system segments having different system operating voltages. Power lines designed to transmit electric energy, called transmission lines, are often networked to improve service capability and reliability. This permits lines to be taken out of service for planned maintenance, or forced out of service by fault clearing, without disrupting the delivery of electric energy from the generating source to the customer. Branches of the network are connected at nodes, called busbars or buses. Power systems are almost always three phase systems, including conductors for 3 phases and ground wires. Throughout this report only single line diagrams are shown. Some simple busbar configurations are shown in Figure 2.1-1 and Figure 2.1-2.

Node in network = busbar Breakers Generator and transformer unit 1

Overhead power line Overhead power line

Generator and transformer unit 2

Underground or submarine cable

Breakers Figure 2.1-1:

Single-line diagram of a typical power station

Nodes at different voltage levels are connected by transformers. These connection points, transformers and other units are made within a limited geographical area, called a station.

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Configuration of a typical power generating station is shown on Figure 2.1-1, and Figure 2.1-2 shows a typical transformer station.

Node in network = busbar Breakers Power lines/cables

Transformer 1

Overhead power line

Transformer 2

Overhead power line

Underground or submarine cable

Load Breakers Figure 2.1-2:

Single line diagram of a typical transformer station

Power flows through all healthy transmission lines in the electric power system network as it moves from generation sources to consuming equipment owned by customers. Electric power system networks operated by more than one electric power utility are often tied together to form a large grid that supports the transmission of power over a very large area, sometimes spanning several countries. Figure 2.1-3 shows the routes of major power lines connected in the Scandinavian power grid.

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TROMSØ ALTA

ISLAND NARVIK

REYKJAVIK

OFOTEN

TORNEHAMN

FINNLAND PETÄJÄSKOSKI

SVARTISEN

PIRTTIKOSKI

LETSI

SCHWEDEN AJAURE

PIKKARALA

GRUNDF.

TUNNSJØDAL LINNVASSELV.

VUOLIJOKI

STORNORRFORS

STORFINNF.

TRONDHEIM

ALAPITKÄ JÄRPSTRÖMMEN

NEA

ALAJÄR VI

HJÄLTA

HUUTOKOSKI

NORWEGEN

PETÄJÄVESI KANGASALA ULVILA

(132)

TOIVILA

OLKILUOT O

HIKIÄ HYVINKÄÄ

BERGEN

LOVIISA

RJUKAN

HELSINKI

OSLO

INKOO

BORGVIK

ENKÖPING

HASLE

V

STOCKHOLM

STAVANGER LISTA

NORRKÖPING

KRISTIANSAND

DC

DC

GÖTEBORG NÄSSJÖ

OSKARSHAMN

RINGHALS

TJELE

HELSINGBORG

KARLSHAMN

[04/ \

MALMÖ KøBENHAVN

KASSØ

/

50 100 150 km

FLENSBURG

HAMBURG

DEUTSCHLAND

Figure 2.1-3:

The Scandinavian Power Grid

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2.2 ELECTRIC POWER SYSTEM FAULTS AND CLEARING 2.2.1 Electric Power System Faults Power system conductors energized to extremely high voltages in three phase systems must be properly insulated from each other and from ground. This insulation is achieved by special insulation materials covering each conductor and/or by air insulation. Air is a very inexpensive insulator, but requires very large spacing. Special non-conducting materials typically insulate the energized conductors in generators, transformers, capacitors, reactors and cables where compact design is essential. Overhead power lines are insulated by air, except at the point they are attached to the supporting poles and towers. Special insulators made of porcelain, glass, or insulating plastic with special surface design and shape achieve the combination of strength and electric insulation to make this attachment. Overhead line design principles are shown in Figure 2.2-1 which also indicates possible arc fault tracks. The three phase conductors

Phase - Ground fault Ground wires

Insulator Phase - Ground fault

Tower

Ground Phase - Phase fault

Figure 2.2-1:

Power line with examples of fault types and fault positions

All power system components are exposed to faults due to insulation breakdowns. The Scandinavian power system shown in Figure 2.1-3, for instance, typically experiences approximately 3000 faults per year. Voltage stresses caused by lightning and switching transients, and contamination due to polluted air are major sources of insulation breakdowns. Mechanical stresses caused by wind, vibration, ice, and snow-loading are major sources of insulator and supporting structure damage that also leads to insulation breakdown. For power lines, most insulation breakdowns are in air between phases and/or phases and ground. Most frequently, insulation breakdowns are along the surface of insulators due to excessive voltage stresses. An example of insulator flashover is shown in Figure 2.2-2

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A flashover in air influences a few rather narrow ‘corridors' where the air loses electrical insulation capabilities. The air does not recover its insulation capability as long as the current is flowing. Therefore it is very important to interrupt the fault current as soon as possible to recover insulation capability. When the current is interrupted, air recovers it’s full insulation capability within a fraction of a second. If the fault current is interrupted rather fast, normally no damage is caused to conductors, insulators or towers. Then the power line can be re-energized within a short time so it can again carry power in the grid. One of the most important design and operational criteria for a power transmission system is that the power system should withstand tripping of at least one power line (unit) without any unnecessary interruption of consumers or power producing units. Wide area weather disturbances, like lightning storms, severe wind storms, and ice storms, expose multiple transmission lines to the risk of faults within the same time period. Consequently, high speed tripping and fast reclosing of tripped transmission lines may be very important to avoid power system collapse due to two or more power lines out of service at the same time. Faults on power apparatus like breakers and units like generators, transformers and cables are most probably breakdowns and damage of special insulation materials. This causes damage that must be repaired before the unit can be re-energized to carry power. This may take a considerable length of time, depending on the availability of spare parts, and trained service personnel. Sometimes units are completely destroyed and must be replaced before normal operation can be achieved.

Figure 2.2-2:

Insulator flashover

Faults on both power lines and other power units can also be caused by misoperation of earth switches and "forgotten" security ground connections. Power system faults caused by weather, animals, high trees, humans, or equipment failure disrupt normal power flow by diverting current through a short-circuited connection and collapsing power system voltage. In addition to equipment damage, power system faults cause transients that adversely affect sources of generation and customer loads. Consequently, faults must be detected and isolated very quickly. Electric power system generators, transformers, busbars, and power lines are therefore monitored by protective relays designed to detect power system faults and operate isolating devices designed to interrupt damaging fault current.

2.2.2 Fault Clearing Power system fault clearing requirements are very important design and operational criteria for power systems. Faults can cause damage that requires expensive repair work or investments for equipment replacement. Faults also cause severe operational disturbances. Generators

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accelerate, motors retard and severe voltage drops can easily result in tripping of complex industrial plants. For the power system itself, severe disturbances can result in collapses and blackout for regions, and, in severe cases, even for several countries. Today’s society does not accept frequent severe disturbances and blackouts because of their heavy reliance on electric power consuming devices for business activities, safety, lighting, heating, cooking, communication and many other conveniences. Therefore, to avoid severe disturbances and blackouts, sound protection practices are used to provide rapid fault clearing. In some cases, formal requirements for fault clearing are provided to assure consistent levels of reliability levels throughout the power system. Formal requirements may be grouped in external requirements and utility requirements as already done in [3]. External requirements may encompass: - Customer power quality and interruption requirements. - Requirements from insurance companies who underwrite equipment failures. - Legal requirements to meet ‘prudent utility practice and industry standards’ in case primary equipment failures result in personal injury or property damage and legal actions are taken against the utility by the parties incurring damage. - International and national safety regulations, imposed by governmental and other agencies. - Requirements imposed by manufacturers of primary equipment in order to validate equipment warranties. - Requirements from occupational safety and hazard prevention. Utility requirements The power system must be designed and operated to avoid instability, loss of synchronism, voltage collapse, undesired load shedding, and unacceptable frequency or voltage. Good protection practices help meet these objectives by detecting and clearing faults rapidly. Rapid fault clearing helps: - Prevent severe power swings or system instability - Minimize disruption of system power transfer capability - Prevent unreliable services - Limit or prevent equipment damage It is very important to clear the fault within specified ‘limits’ to ensure that the healthy remainder of the power system can continue to serve it’s customers with acceptable quality and reliability. Requirements on protection Protection performance requirements are issued to satisfy external and utility requirements. These requirements specify how protective schemes must perform on specific contingencies to fulfill external and utility requirements. They typically provide a balance between the conflicting goals of dependability and security. Dependability goals require maximum sensitivity and fast response time to detect and clear all faults quickly with very low probability of a failure to trip. Security goals require maximum selectivity and slow response time to minimize the probability of an unwanted trip on an unfaulted circuit. Security is an issue during fault conditions as well as during normal, unfaulted conditions. Simply stated, the implementation of these protection requirements should result in dependable operation of only those relays protecting the faulted unit, and secure non-operation of the relays during non-fault conditions and when faults occur on adjacent power system units. This balance is met only through proper protection scheme design, proper relay and equipment selection,

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and proper connection and setting of these relays and equipment to achieve appropriate sensitivity and coordination. When protection schemes detect a fault on the equipment or line they protect, they signal isolating devices, called circuit breakers, to open, isolating the faulty segment of the system, and restoring normal system voltage and current flow in the power system. Protection schemes command circuit breakers to isolate faults with no intentional time delay. When the protection scheme and circuit breakers operate properly, the fault is isolated within the required fault-clearing time. Protection applied on high voltage systems, where fault-clearing times are most critical, typically detect faults and operate in about one to two cycles. Some schemes operate in less than one cycle. Circuit breakers operate in one to three cycles. The combination of high-speed protection schemes and fast circuit breakers can interrupt a fault in about two cycles, although more common fault-clearing times range from three to six cycles.

2.3 WHY DOES PROTECTION NEED TELECOMMUNICATION? Protection systems must meet sensitivity, time response, selectivity and reliability requirements in order to meet fault clearing requirements. Fault clearing systems (see 2.4.1) for generators, busses, transformers or other units within a substation can normally meet these requirements without using telecommunication. Telecommunication may be needed for the protection of these substation units only if a breaker is missing or fails to interrupt fault-currents. Protection schemes for extremely high voltage transmission lines, however, very seldom meet all these requirements without using telecommunications. Some protection schemes, such as stand-alone step-distance schemes, provide very reliable and sensitive protection capable of clearing all power system faults without using telecommunications, but time response and/or selectivity requirements can only be met by using telecommunications. Telecommunications are therefore needed to ensure that time response and selectivity requirements are met for all power system fault conditions! Telecommunications is also essential for some types of protection schemes, like analogue comparison schemes, to operate. If telecommunication fails, backup protection schemes ensure that power system faults will be cleared, but they may not be cleared within specified performance requirements. Then the probability of uncontrollable power swings and partial or complete system blackout increases significantly. Alternative methods for reducing the probability of fault-induced blackouts is to build additional generating stations and transmission lines, or add redundant telecommunications. In virtually all cases, it will be far less expensive to add redundant telecommunications. Telecommunications is therefore vital to the reliability and economy of modern electric power systems.

2.4 INTRODUCTION TO POWER SYSTEM PROTECTION Power system protection schemes are designed to detect and clear faults, in accordance with requirements on protection, as discussed in the previous sub-chapter, to: -

Minimize adverse affects on customer loads Minimize disruption of system power transfer capability Coordinate tripping with protective relays in other protection zones Prevent severe power swings or system instability Limit or prevent equipment damage

Power units and lines are protected in zones to coordinate fault detection and clearing. A

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protection zone is defined as all high voltage power system equipment (and all necessary control, supervision and protection equipment) between two or more circuit breakers. Selective fault clearing (selectivity) is to trip the breakers for the faulty zone, and not trip any additional breakers for non-faulty zones. Five basic zones of protection are shown in Figure 2.4-1. These zones of protection are identified as Generators (1), Transformers (2), Busses (3), Lines (4), and Loads, such as Motors (5). 5

M

5

M

3

2

Station C 3

Station A 1

G

3

3

Station B 2 1

4

4

1

2 4

G

4

G

3

3

Station D 4

Figure 2.4-1:

4

Typical power system and its zones of protection

The boundaries of each zone of protection, as it applies to protective relays, are determined by the location of the current transformers that provide the representation of primary system currents to the protective relays. Other parameters, such as voltage, are also used by some protective relays to perform their protection function, but the current transformer location determines the protection zone boundary. Overlapping zones of protection is an established protection concept represented by Figure 2.4-2. As shown, the current transformers are typically located on opposite sides of the circuit breaker, or on one side and as close as possible to the circuit breaker that is tripped to clear faults in the respective protection zones. Protection zone boundaries for power units such as generators, transformers, busses, and motors are typically within the same substation, permitting one relay to monitor currents at the boundary of its protection zone. Likewise, the same relay can easily be connected to issue trip signals to all circuit breakers at the boundary of its protection zone. The boundary for line protection, however, is typically located at two different stations that may be separated by a considerable distance. This separation makes it impossible for one relay to sense currents at both ends of the line, or control breakers at both ends of the line. It is therefore common practice to install at least one relay and circuit breaker at each end of the

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line. These relays may operate independently, or they may share information to improve their operating speed, or they may require communication between them to operate.1

CT for Zone B Zone B

Zone A CT for Zone A a. CTs on opposite sides of breaker

CT for Zone B Zone B

Zone A CT for Zone A

b. both CTs on same side of breaker Figure 2.4-2:

Overlapping protection zones established by current transformer location

2.4.1 Fault clearing system A Fault Clearing System is defined in this report according to Figure 2.4-3. Fault currents must be interrupted from both (all) sides. The fault clearing system therefore includes : - Protection system - Mechanisms of circuit breakers Fault Clearing System includes one or more protection systems and the circuit breakers required to clear (interrupt) a fault and isolate the faulted portion of the circuit. Protection System includes a complete arrangement of protection equipment and other devices required to achieve a specified function based on one protection principle. A protection system is all embracing and includes protection functions as well as auxiliary power systems, sensors for detecting measured quantities, controls and circuitry for closing/opening circuit breakers, teleprotection and telecommunications for interchange of information between protective functions and all necessary connections between these functions and units. (Example: A phase comparison protection system, or a line current differential protection system.) Sensors include voltage transformers and current transformers that scale primary system voltages and currents down to secondary values compatible with the protective device design. The Teleprotection Function converts the signals and messages from the protection function into signals and messages compatible with the telecommunication system, and vice versa. The teleprotection function may be integrated with the protective device, or the telecommunication equipment, or it may be in a stand-alone device. The Telecommunication System provides a communication link between ends of a protected 1

Protection schemes that share information to improve operating speed are sometimes referred to as “non-unit” protection schemes. Protection schemes that require communication to operate are sometimes referred to as “unit” protection schemes.

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circuit, permitting the exchange of information (analogue data and/or status) or transmission of commands. In Figure 2.4-3, the telecommunication system may be dedicated point-to-point, shared point-to-point, or a network. High Voltage Equipment

Fault Clearing System Protection System Auxiliary power

Protection Functions

Control Circuit Breaker

Mechanism

Sensors Teleprotection Function

Protection Scheme Protection Zone

Telecommunication System

Teleprotection Function Sensors Circuit Breaker Protection Functions

Control

Mechanism

Auxiliary power

Figure 2.4-3:

Fault clearing system

Protection Function(s) may be performed by multiple protective relays working together, or more commonly in modern protection systems, by one or more multi-function protective relays.

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Protection functions in one station interchange information with protection functions in a remote station via teleprotection and telecommunications. This sub-total functionality forms a Protection Scheme.

2.5 HOW IS TELECOMMUNICATION USED Telecommunication is essential for analog comparison protection schemes (see 3.1.1) to share data between relays at each end of the protected line. Telecommunication is needed for direct intertripping schemes to pass tripping commands from the protection and control scheme at one line terminal to the power circuit breaker at the other line terminal. Telecommunication is used with state comparison protection schemes (see 3.1.2) to reduce the overall tripping time for faults on the protected line section. Analog comparison protection schemes typically share data, such as line current magnitudes and phase angles, to differentiate between power system faults within the protected zone or outside the protected zone. Communication between relays at each line terminal is essential to the operation of analog comparison protection schemes. State comparison protection schemes share the logical status of relay elements to determine if the fault is internal or external. These schemes are generally built by adding and interfacing communication to stand-alone relays to improve tripping speed for faults in the end-zone areas not protected by direct tripping relays. Schemes that use communication to improve tripping speed are referred to as communication assisted schemes. Telecommunication is also used for intertripping schemes that must communicate a trip command to a remote substation circuit breaker to isolate a fault within the local station, block and control schemes, and wide area protection schemes. All of these schemes are described in greater detail in Chapter 3. Telecommunication systems used for protection are described in Chapter 4. Protection using telecommunication provides consistent relay tripping times in the order of 2 to 3 cycles for faults over the entire length of a protected transmission line. Stand-alone protection schemes may take upwards of 20 to 30 cycles to trip both line terminals of a faulted line. Protection schemes using telecommunication can thereby reduce the tripping and clearing time for line faults by as much as 18 to 28 cycles compared with stand-alone protection schemes. This reduced tripping time greatly reduces the affect of faults on generators, power transfer, and customer loads, and reduces the damage to faulted and unfaulted equipment. The faster fault clearing speeds are essential to the efficient and economic operation of modern power systems. As described in this document, protective relays are interfaced with telecommunication systems through the teleprotection function. The teleprotection function may be performed by a standalone device, or it may be integrated with the protective relay or with the telecommunication equipment. Interfaces between protection relays, teleprotection, and telecommunication systems are described in Chapter 5. The following chart is excerpted from the IEC 60834-1 standard to help show the relationship between protection, teleprotection, and telecommunication. From the teleprotection point of view, the relatively selective protection schemes shown in Figure 2.5-1 are typically communication-aided state comparison schemes (see 3.1.2), and the absolutely selective protection schemes in Figure 2.5-1 are typically communication dependent analog comparison-schemes (see 3.1.1.).

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Figure 2.5-1:

Fundamental terms on protection and teleprotection (From IEC60834-1)

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3 PROTECTION USING TELECOMMUNICATIONS 3.1 LINE PROTECTION 3.1.1 Analog Comparison Schemes Analog comparison protection is based on the transmission and comparison of electrical parameters such as primary currents (amplitude and/or phase) between the ends of a protected line. Each end sends its registered values to each other and compares them with the remote ones. When an internal fault occurs, the result of the comparison will be a differential value, so that, if it is higher than a threshold, the relay will initiate the trip. These systems are called analogue comparison protection systems because they exchange analogue quantities such as amplitude and/or phase with the other ends. They are sometimes also referred to as "unit protection" or "closed" schemes. The term “unit” refers to the clear interdependence between the ends for operation and to the closed and absolutely selective characteristic of this protection. Obviously, the comparison must be made between magnitudes at the same instant, which implies a transmission and comparison system as fast as possible. A delay must be provided for the local signal to compensate for the transmission time of the remote value. Unlike the time-grade protection such as distance and time overcurrent relays, the trip of the analog comparison protection is instantaneous for every fault on the protected line. It is applicable to any overhead line or cable at all voltage levels and for any type of system neutral arrangement. It is particularly suitable where: -

-

Step distance relays (without acceleration schemes) have limitations, for example: à Very short lines and cables due to their low impedance, which makes it difficult to find an adequate setting to get a instantaneous trip for faults on the main part of the line. à Multi-terminal lines, since the intermediate infeeds modify the impedance seen by the distance relays, which depends not only on the distance to the fault, but also on the infeed from the remote terminals, making impossible an accurate measure of the impedance. No potential transformers and only current transformers are installed at each end of the line.

We can distinguish two types analog comparison protection systems: longitudinal current differential protection and phase comparison protection. The current differential protection compares the power frequency signals proportional to the primary power system currents (amplitude and phase angle), while the phase comparison one is based on comparison of the phase angle (or sign) between currents of each end of the protected line. Since both of them use only current information, in comparison with the distance or other system protections, analog comparison protections have the following advantages: -

Not responsive to system swings and out-of-step conditions Unaffected by inadvertent loss-of-potential (i.e., due to a blown potential fuse)

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-

No mutual coupling problems from parallel lines. This may cause the line-to-ground fault current reverses and flows into a weak source terminal, causing faulty directional discrimination if other protection systems are used Not subject to transient problems associated with coupling capacitor potential devices With segregated current differential there are no problems of phase selection for single pole auto-reclosing at simultaneous faults on different circuits and phases close to one line end, because it operates only for faults between current transformers in each phase. Some relaying problems in EHV transmission lines due to applying series capacitors are also overcome, e.g. voltage reversal, current inversion or phase imbalance.

When phase selection is required for single phase tripping, especially at simultaneous faults on different circuits and phases or in a faulty line when handling heavily loaded EHV lines, the phase-segregated technique is used. The analogue information is transmitted separately for each phase. In cases where the complete information about the polyphase conditions is not essential and single-phase tripping is not needed, the non-segregated technique is used. It reduces the threephase system of currents to a single-phase one by means of a mixing device. The communication link needs therefore to only accommodate the transmission of this single phase information. Some mixing techniques are described in [1]. 3.1.1.1 Current differential protection Operating principles As mentioned above the current differential protection is an absolutely selective protection system for transmission lines, tripping instantaneously for faults in the protected zone defined by the current transformers of each end of the line. It is based in the principle of current comparison. The Figure 3.1-1 shows a basic scheme of the differential protection. In each terminal, an evaluation circuit compares the sum of the local and remote current values, i.e. the differential current, with an operation threshold value Iop. In normal operation conditions or external faults, the current entering at one end is practically the same as one leaving at the other end, so the differential current value is practically zero and the protection will remain stable. For a fault on the protected power line the differential current value will exceed the operation value and the protection will trip. When very large currents flow through the protected zone for a fault external to the zone a differential current appears due to the different ratio error and saturation characteristic of the current transformers, which could exceed the operation level. Such a maloperation of the protection is prevented by the stabilizing. The stabilizing characteristic uses a bias current, which is usually proportional to the sum of the absolute values of the currents at each terminal, i.e. |iA| + |iB|, in order to make the protection less sensitive for higher through currents. This technique is also called percentage restraint.

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A

IA

B

IB

SA

TX

iA

Telecommunication system

TX

SA

iB

DEL

DEL

iB

+

RX

Id

RX

TPF

Id> Iop

TPF

iA

+ Id Id>Iop

SA = Signal adapter (filtering, mixing circuit, A/D conversion, etc.) TX = Transmitter RX = Receiver Iop = Operation threshold according to stabilizing characteristic DEL = Delay compensation TPF = Teleprotection Function

Figure 3.1-1:

Principle of differential protection

Figure 3.1-2 shows an example of percentage restraint characteristic with two slopes: the lower slope ensures good sensitivity to resistive faults under heavy load conditions, while the higher slope is used to improve relay stability against saturation of the current transformers and other distortion effects under heavy through fault conditions. The selection of the minimal operation current Is1 is based upon the magnitude of line capacitance current and switching transients expected on the protected line. The capacitance of the three conductors to earth and, except in single core cable, also between each other, makes that under undisturbed conditions the current at both ends differs in angle and magnitude. Particularly in cables, the capacitive charging current can attain significant values. Nevertheless, usually the necessary rise of the Is1 does not involve an important loss of sensitivity. Idiff Idiff > k2xIbias - (k2-k1)Is2 + Is1

TRIP slope k2

NO TRIP

Idiff > k1xIbias + Is1

slope k1

Is1

Is2

Ibias

Idiff = iA + iB Ibias = |iA| + |iB|

Figure 3.1-2:

Differential protection: Example of percentage restraint characteristic

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The differential principle may be applied to multi-terminal lines. The protection relies on the sum of the inflowing currents, which are added geometrically. For this purpose, the measuring circuits have to be so arranged that at each end of the line, the local current and the currents from each of the others ends of the line are available for comparison. Generally, the most recent designs allow up to three terminals applications. For a multi-terminal system, the master/slave or centralized configuration is also used. In this case, the current values are sent to a specific terminal for evaluation of the differential current. This terminal will henceforth be noted as a master, while the terminal sending information about currents will be denoted as a slave terminal. For a two-terminal system, the master/slave configuration can, of course, also be used, but a master/master or distributed configuration, where the current information is exchanged between both terminals and evaluated at both ends is normally preferred, since this gives a shorter operating time than that in a master/slave configuration. See Figure 3.1-4 and Figure 3.1-5 for more details about centralized and distributed configurations. The saturation of the current transformers for heavy through currents normally requires the selection of a higher slope setting which involves a loss of sensitivity for internal faults. Recent protections include some techniques to detect the saturation, so in only such conditions is the protection desensitized increasing the restraint slope. To avoid the maloperation of the remote protections, the terminal that detects the saturation includes a code in the message transmitted to the other ends, so that all terminals increase the degree of stabilization. Time delay compensation As mentioned, the current values used in the differential protection must be taken at the same instant at all ends of the power line for comparison, so a delay circuit is needed to compensate the transmission time for the remote values. Classical designs incorporate an adjustable delay for aligning the current values. However, when digital communication systems with automatic route switch are used, the time delay can change and the protection must continuously adjust the time alignment. For this purpose, digital devices incorporate different techniques in which the messages of current values sent through the communication channel are tagged with the sampling time. The principles of some synchronization techniques are described in more detail in A4.1. An error in delay compensation results in a differential current that - according to Figure 3.1-2 - increases the risk of unwanted tripping. For more information see 6.1.2.2 and 6.3.1.1. Additional functions Generally, differential protections use intertrip functions, i.e. the sending of trip commands to the remote ends. Intertrip commands are sent through the same communication channels used to transmit the current values (switching the channel frequency to a specific intertrip frequency when analogue links are used, or flagging the corresponding command bits in the out-going data messages in digital links). The intertrip function is activated either when the relay reaches a trip decision, or by closing an external contact connected to an input of the relay. The intertrip function can be used for: - Breaker failure protection - Stub protection: this is applied in switchyards with 1½ circuit breaker configuration. Operating an input by external contact when the line isolator opens allows to protect the line between the circuit breakers and the line isolator.

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Telecommunication systems used for differential protection Differential protection systems using pilot wires for 50/60Hz signals Pilot wires connect both ends electrically and establish a differential circuit where the secondary quantities may be in the form of current signals or voltage signals, which are proportional to the primary current. Accordingly, there are two basic methods of creating a differential circuit, current balance or voltage balance. Figure 3.1-3 shows a basic scheme of a current balanced system using three pilot wires.

ST Pilot wires

MCT

MCT TR

TR

ST

Evaluation circuit

Evaluation circuit

MCT = Mixing current transformer TR = Transformer for tripping ST = Transformer for stabilizing effect

Figure 3.1-3:

Basic scheme of a current balanced system using three pilot wires

In this case, the three-phase system is converted into a single AC current in the mixing transformer MCT (non-segregated). One differential system for each power phase (segregated) of the protected circuit can also be provided. If high resistance faults are expected or faults on which the value of earth fault current is relatively low, a fourth measuring system for the zero sequence component can be introduced. This however, increases the number of pilot wires and therefore the communication cost of the comparison information. In both methods, a replica of the vector difference is formed at each line end by means of a transformer ST for the stabilizing effect and a replica of the vector sum of the currents flowing at each end by means of a further transformer TR for the tripping effect. These values are evaluated separately at each line end in a measuring module and a tripping command is issued to the circuit-breaker when the fault current has exceeded a permanently adjusted threshold value. Where the voltage induced into the pilot cables during earth faults may exceed the rated values, the protective relays should be isolated from the pilot wires by isolating transformers, which can also be used to subdivide the total length of the pilot wires into two or three sections. This prevents the equipment from being subjected to excessive longitudinal voltage due to

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interference. In any case, the grounding conditions should be considered. The application of differential protection using pilot wires is restricted on lines up to 10-25 km depending upon the scheme used. So for longer lines, modulation techniques over other transmission media should be used. More details about differential protection using pilot wires and their limitations can be found in [1] and in chapter 4.3.1. Differential protection systems using modulation or coding techniques Modulation or coding techniques that are compatible with analog and digital telecommunication circuits are used to overcome some of the shortfalls experienced with direct pilot wire coupling.1 Typical techniques that are used: -

-

-

Frequency modulation (FM) for analog voice frequency (VF) channels. The instantaneous current values at each terminal are transmitted as analogue quantities to the other terminals in a voice frequency band (0.3 to 3.4 kHz) using frequency modulation. Whatever transmission media for analogue voice channels may be applied. Numerical coding for digital telecommunication systems The instantaneous current values at each end of the power line are sampled, converted to digital data and transmitted towards the other terminals through a digital telecommunication system. Sample rates ranging from 12 to 60 samples per cycle have been used. Normally, the telecommunication system is shared with other services like voice, telecontrol, etc. using Time Division Multiplexing techniques (see 4.4.1.2). The protection system is connected to the PCM) multiplexer through standard interfaces. The most commonly used electrical interfaces are those contained within the ITU-T or EIA recommendation and are described in 5.3.1 and in [2]. Dedicated optical fibres. Direct optical fibre links between protection terminals are also used. A higher reliability is achieved because intermediate devices are eliminated. However, when using dedicated fibres over long distances, the cost can be prohibitive beyond 10-20 km. See 4.3.4 for more information on optical fibres.

Multi-terminal configuration Transmission line protection based on a current differential scheme detects zone faults by using each terminal current and transmits the detection results of the zone fault to the other terminals. There are two types of multi-terminal current differential protection configurations; centralized and distributed configurations. As these configurations are applied to a single zone protection, they may be also applied to multi-zone and wide-area protections. 1

Note on pilot-wire replacement: The corrosion problems of buried copper wires, with the trend of telcos to replace copper-pair cables with fibre communication links, have put pressure on utilities to consider alternate means of connecting their extensive infrastructure of pilot-wire relays; this has created a market for specialized interface units which emulate these copper wires. The accuracy requirements of such interfaces depend on the accuracy requirements of the relay settings, the main parameters of concern are: The interfaces’ dynamic range. This should not limit on fault currents, whilst providing the required signal integrity during low line-current conditions. The end-to-end propagation delay. Since a 10% fault current error would be caused by the 5 degrees phase error accruing from 230µs on a 60Hz grid (280µs on a 50Hz grid), this delay is critical (this teleprotection application has the most stringent delay requirements of all teleprotection applications). In practice, up to 1ms may be manageable for the protection of 2-ended lines, but 500us or less may be required for 3-ended lines.

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Centralized configuration Figure 3.1-4 shows an example of line protection for a five-terminal EHV line [7]. Each terminal has a terminal unit that detects the current and transmits the data to the main unit terminal via a communication channel. This configuration simplifies the unit of each terminal and communication channel. Since the main unit has current data of all terminals, the fault locator function can be easily implemented by using these data.

Figure 3.1-4:

Centralized configuration

Distributed configuration Figure 3.1-5 shows a distributed configuration of five-terminal current differential line protection system. Each terminal has the current differential protection function as well as the signal transmitting function that multiplexes current data at each terminal into one communication signal. Master station A sends its own current data to slave station B. Slave stations B, C, D and E multiplex their own current data over communication signal. Slave station E turns back this signal toward slave station D. Now current data of all terminals are on the communication bus and available for protection. In addition, this system contains sampling synchronization function which enables the simultaneous sampling of current data at each terminal with high accuracy. Many installations were conducted using a 1.544-Mbit/s fiber-optic communications channel for HV double-circuit multi-terminal (up to ten terminals) or tapped lines [8]. In this network configuration where current differential calculation is usually carried out at each terminal, a centralized scheme where only master station conducts the calculation and sends the transfer trip signal to all slave stations is also available.

Figure 3.1-5:

Distributed configuration

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3.1.1.2 Phase comparison protection Operating principles Phase comparison protection is based on the comparison of the phase angle between currents of each end of the protected power line. Under normal load conditions or in case of an external fault, the angle measured between the local current and the current at the remote ends will be small. If the angle is large, it is due to an internal fault. The basic principle of all phase comparison systems is to measure the angle as above mentioned. However, the method of doing so can differ from manufacturer to manufacturer. A phase comparison system can be characterised by the following features: -

Comparison is made for each phase separately. A zero sequence circuit may also be included => Segregated protection. The currents of the three phases are mixed into one quantity for comparison => Nonsegregated protection. The measurement is made twice every period => Full-wave phase comparison. The measurement is made once every period => Half-wave phase comparison. The phase angle signal is transmitted to the remote end only when a starter has picked up. Measuring is carried out continuously and the signals are permanently transmitted. A phase comparison scheme can be designed for a blocking mode or for an unblocking mode of operation, similar to a distance protection system using telecommunication.

The current which is used in the comparison is converted into a square wave signal, so that the positive portion corresponds to the positive half-cycle and the zero portion corresponds to the negative half cycle. The square wave from the remote terminal is compared with the local square wave as shown in Figure 3.1-6.

A

SA

SQ

TX

iA

DEL

∆ϕ>θ

B

IB

IA

& iB

Telecommunication system

TX

iB

RX

RX

TPF

TPF

SA = Signal adapter (mixing circuit, filtering, etc.) SQ = Squarer TX = Transmitter RX = Receiver DEL = Delay compensation ∆ϕ = Coincidence angle θ = Stabilizing angle & = Logical AND TPF = Teleprotection Function

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a) External fault or normal load

b) Internal fault

IA

IA

IB

IB

iA

iA

iB

iB

iA & iB

iA

& iB

∆ϕ>θ

∆ϕ delayed tripping) or deblocking for external faults (=> unwanted tripping). In a full-wave comparison different frequencies for the two directions must be used. A FSK (frequency shift keying) signal is used, which can be transferred over pilot wires, power line, radio or fibre-optic link. The communication equipment continuously monitors itself and when a fault occurs, the local signal is compared with the remote for both positive and negative halfcycle in the protection relay. Phase-segregated technique In this case, the values of each phase are transmitted separately via independent channels. Most recent phase comparison systems usually operate in segregated mode and use digital communication systems. The square signals to compare are sampled and converted to digital data, which are transmitted serially to the opposite terminal by the telecommunication system. Data rates and electrical or optic interfaces are the same as those mentioned for differential protection. When starters are used to initiate the comparison, a sequence of “guard” bits is transmitted in

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normal state of operation, in order to monitor the channel availability and performance by the receiver. Some designs optionally include a modem to interconnect two terminals through a 4-wire audio channel. In this case, a data rate of 9'600 or 19'200 bit/s may be used. 3.1.1.3 Charge comparison protection Charge comparison is based on the principle of conservation of charge at a node. The charge entering one line terminal must be approximately the same as the charge leaving the other line terminal(s) of a healthy transmission line. This is also the principle from which Kirchoff’s Current Law (the theoretical basis of current differential relaying) is derived. To perform charge comparison, the waveform of each line terminal’s phase and residual current is sampled every ½ millisecond. The half-cycle area under each wave is measured by integrating current samples between zero-crossings. For each phase and ground, the resulting ampere-second area (i.e., coulombs of charge) is stored in local memory, along with polarity and start/finish time-tags. This storage operation occurs only if the magnitude exceeds 0.5 ampere r.m.s. equivalent and the half-cycle pulse width is equal to 6 ms or more.1 Every positive (negative 3Io) magnitude is also transmitted to the remote terminal, along with phase identification and some timing information related to pulse width and queuing time (if any) at the transmitting terminal. When the message is received at the remote terminal, it is immediately assigned a received time-tag. A time interval is then subtracted from the received time-tag. This interval represents the channel delay compensation (which does not have to be precisely equal to the actual channel delay time) and the timing information contained in the received message. The adjusted received time-tag (after subtraction) is then compared with the local start and finish time-tags, looking for a “nest”, per Figure 3.1-7 (shown for an external fault).

Remote current

Actual channel delay time Time adjusted in received message

Channel delay compensation

Time interval subtracted Local current

Start time-tag

Finish time-tag

Received time-tag

Adjusted received time-tag

Figure 3.1-7: 1

Operation of charge comparison, external fault

Magnitude is actually measured in terms of ampere-seconds (i.e., coulombs). However, all values are converted to amperes rms equivalent, based on a perfect 60 Hz (or 50 Hz) sine wave, without offset.

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A nest is achieved when the adjusted received time-tag is greater than the local start time-tag and smaller than the local finish time-tag, for a given half-cycle stored in memory. When the nesting operation is successful, the local and remote current magnitudes (actually charges converted to equivalent currents) are then added to create the scalar sum (sum of absolute magnitudes). The scalar sum becomes the effective restraint quantity and the arithmetic sum becomes the effective operate quantity, per the bias characteristic shown in Figure 3.1-8.

BIAS LEVEL

ARITHMETIC SUM

(TRIP)

(RESTRAINT) SCALAR SUM

Figure 3.1-8:

Bias characteristic of charge comparison

The bias level is an operate threshold which provides security in the presence of spurious operate current due to line charging current, current transformer mismatch, analog-to-digital conversion quantizing errors, etc. As shown in Figure 3.1-8, the bias level rises sharply after the scalar sum reaches a high value. This provides security for unequal CT saturation during high current external faults. At lower currents, the bias level has a slight upward slope. This takes care of the relatively minor non-communications-related errors that increase with current level, such as CT ratio errors. The operating characteristic of charge comparison, when plotted on a polar diagram, is the “ideal” rainbow-shape of Figure 3.1-9. Referring to Figure 3.1-7, if the adjusted received timetag nests with a local negative half-cycle, this is equivalent to the upper half of Figure 3.1-9. If the adjusted received time-tag nests with a local positive half-cycle, then the arithmetic sum and scalar sum are equal to each other, which describes a 45 degree line on the bias characteristic (well above the bias threshold for all except very small values of current). This is equivalent to the lower half of Figure 3.1-9.

IR

IL

RESTRAINT REGION OF IR

Protected line

IL

Figure 3.1-9:

Ideal polar diagram characteristic

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The bias level of charge comparison is significantly more sensitive than that of conventional current differential relays for line protection. The conventional relay requires a gradually increasing bias to take care of increasing spurious operate current for a given assumed error in channel delay compensation (the biggest single source of spurious operate current). In contrast, charge comparison introduces no additional communications-related error as the currents get bigger, for a given error in channel delay compensation. Furthermore, for a given magnitude of through current, no operate error current is introduced, at all, for increasing channel delay compensation error (up to + 4 ms, at which point a total relay misoperation occurs – typical of a digital system). The + 4 ms misoperation threshold for charge comparison is almost three times the + 1.5 ms (approximately + 30 degrees on 60 Hz systems) misoperation threshold which is typical of conventional current differential schemes with circular polar diagram characteristics. Lit: [38]

3.1.2 State Comparison Schemes State comparison protection schemes use communication channels to share logical status information between protective relay schemes located at each end of a transmission line. This shared information permits high speed tripping for faults occurring on 100 percent of the protected line. The logical status information shared between the relay terminals typically relates to the direction of the fault, so the information content is very basic and requires very little communication bandwidth. Additional information may also be sent to provide additional control, such as transfer tripping and reclose blocking. For instance, breaker failure protection in ring bus and breaker and one-half bus configurations must transfer trip the remote terminal breaker(s) to isolate the failed breaker. Refer to chapter 3.2.2.2 for Bus Bar Protection/Breaker Failure Protection for more information on this subject. Overall, the communication requirements for state comparison protection schemes are considerably less stringent than for analog comparison protection schemes. Communication speed, or minimum delay, is always of utmost importance because the purpose for using communication is to improve the tripping speed of the scheme. Also, variations in communication speed are better tolerated in state comparison schemes than in the analog comparison protection schemes discussed in an earlier section. Communication channel security is essential to avoid false signals that could cause incorrect tripping, and communication channel dependability is important to ensure that the proper signals are communicated during power system faults, the most critical time during which the protection schemes must perform their tasks flawlessly. Comparing the direction to the fault at one terminal with the direction to the fault at the other terminal permits each relay scheme to determine if the fault is within the protected line section, requiring the scheme to trip, or external to the protected line section, requiring the scheme to block tripping. Directional distance and/or directional overcurrent relays are typically used at each line terminal to determine the fault direction. The relays used at each line terminal operate independent of the relays at other line terminals; some may even be set to provide time delayed tripping for faults outside the protected line section, hence the term “non-unit” protection, or “open system” protection is sometimes given to these types of schemes. If it were possible to set relays to see all faults on their protected line section, and to ignore faults outside of their protected line section, then there would be no need for communication schemes to assist the relays. However, distance and directional overcurrent relays cannot be set to “see” faults within a precise electrical distance from their line terminal. They are imprecise because of many factors, including voltage and current transformer errors, relay operating tolerance, line impedance measurement errors and calculation tolerance, and source

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impedance variations. The primary relay elements used to detect line faults are therefore set to see or reach either short of the remote line terminal (this is called under reaching), or to see or reach past the remote line terminal (this is called over reaching). Communication between line terminals at different electric power substations could be accomplished by simply extending a number of wires between the substations. Connecting a relay contact output from a relay scheme at one terminal to a relay scheme control input at the other line terminal with a pair of copper wires provides the communication necessary for one relay scheme to tell the other relay scheme that it has, or has not, seen a fault. Unfortunately, connecting communication wires directly between substations is not that simple and can even be hazardous. Voltage drop, induced voltages, and ground potential rise between substations during a fault make direct metallic wire connection between relay schemes unreliable, insecure, and hazardous. Communication for state comparison protection schemes must therefore be designed to provide safe, reliable, secure, and fast information transfer from one relay scheme to another. The communication scheme must also be able to transmit information in both directions at the same time. The amount of information required to transfer between relay schemes depends on the relay scheme logic. The basic and most common state comparison protection schemes are described in the following subsections. Their communication requirements are discussed within these subsections. The order in which they are presented does not imply their priority, relative importance, or usage. Other schemes and combinations of schemes may be designed to meet specific protection needs, however, they are typically all based on the basic schemes described in this document. The terminology used to describe these state comparison protection schemes may differ from utility to utility and country to country. State comparison schemes are basically defined according to the impedance zone which sends the protection signal to the remote end of the line. The following Table 3.1-1 shows the preferred CIGRE scheme names and alternate scheme names used elsewhere. CIGRE scheme names will be used throughout this document. CIGRE State Comparison Protection Scheme Name

Alternate State Comparison Protection Scheme Name

Intertripping underreach distance protection

Direct underreach transfer tripping

Permissive underreach distance protection

Permissive underreach transfer tripping

Permissive overreach distance protection

Permissive overreach transfer tripping

Accelerated underreach distance protection

Zone acceleration

Deblocking overreach distance protection

Directional comparison unblocking

Blocking overreach distance protection

Directional comparison blocking

Table 3.1-1:

State Comparison Protection Schemes

3.1.2.1 Intertripping Underreach Distance Protection The basic logic for a Intertripping Underreach Distance Protection scheme is shown in Figure 3.1-10. This scheme requires underreaching functions (RU) only, which are usually provided by phase and ground distance relay elements. The scheme is usually applied with an active channel that transmits a GUARD signal during quiescent, or unfaulted, conditions. The transmitter is keyed to a TRIP signal when the associated underreaching relay element detects a fault within its reach. The underreaching functions (RU) must overlap in reach to prevent a

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gap between the protection zones where faults would not be detected. RU RU

Bkr 1

Bkr 2 Protected Line Teleprotection Equipment

RU

OR

TX

TX

RX

RX

TRIP Bkr 2

TRIP Bkr 1

RU

OR

Protection Equipment RU - underreaching trip function, must be set to reach short of remote terminal and must overlap in reach with RU at remote terminal

Figure 3.1-10:

Intertripping Underreach Distance Protection Scheme Logic

For internal faults within the overlap zone, the underreaching functions at each end of the line operate and trip their associated line breaker directly. At the same time, the RU function keys its respective transmitter to send a direct transfer trip signal to the relay scheme at the remote line terminal. Receipt of the trip signal from the remote line terminal also initiates line breaker tripping. This scheme provides high speed tripping at both line terminals for all faults within the protected line section under most conditions. However, it will not provide tripping for faults beyond the reach of one of the RU functions if the remote breaker is open or if the remote channel is inoperative. If only one communications channel is used at each terminal, security may be jeopardized because any erroneous output from the channel initiates an instantaneous breaker trip. For this reason, this scheme is often applied with dual channels where both outputs must provide a TRIP signal to initiate a breaker trip. Or a slight delay may be added to a single channel output to ensure that the remote trip signal is valid before tripping the breaker. Time-delayed overreaching back-up tripping functions that do not interface with the communication scheme are usually added to trip the associated line breaker for faults beyond the reach of the RU functions when the remote breaker is open, or when the communication channel is inoperative. This scheme may use virtually any communication media that is not adversely affected by electrical interference from fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults.

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3.1.2.2 Permissive Underreach Distance Protection The Permissive Underreach Distance Protection scheme requires both overreaching (RO) and underreaching (RU) relay functions at both line terminals. This scheme is similar to the Intertripping Underreach Distance Protection scheme except that all communication assisted tripping is supervised by overreaching relay elements having what is often called a zone 2 reach. The scheme is usually applied with an active channel that transmits a GUARD signal during quiescent, or unfaulted, conditions. The transmitter is keyed to a TRIP signal when the associated underreaching relay element detects a fault within its reach. The underreaching functions (RU) must overlap in reach to prevent a gap between the protection zones where faults would not be detected. Basic logic for the Permissive Underreach Distance Protection scheme is shown in Figure 3.1-11. The relay functions and logic are easily performed with modern multi-zone phase and ground protective relays. Distance type relay elements are most often used for the underreaching functions (RU), and distance relay elements or directional overcurrent relay elements are used for the overreaching functions (RO). RO RU RU RO

Bkr 1

Bkr 2 Protected Line Teleprotection Equipment

RU

TX

RO

RX

&

OR

Duplex Communication Link

TRIP Bkr 1

TX

RU

RX

RO

TRIP Bkr 2

OR

&

Protection Equipment RU - underreaching trip function, must be set to reach short of remote terminal and must overlap in reach with RU at remote terminal RO - overreaching trip function, must be set to reach beyond remote end of line

Figure 3.1-11:

Permissive Underreach Distance Protection Scheme Logic

When the underreaching relay elements detect a fault, they trip the local breaker directly and key a TRIP signal to the remote line terminal. Unlike the Intertripping Underreach Distance Protection Scheme, the Permissive Underreach Distance Protection Scheme supervises the received trip signal with an overreaching relay element. Communication assisted tripping occurs only if the overreaching relay element detects a fault during the time that a trip signal is received from the remote line terminal via the communication channel. Because the received communication signal is supervised by the output from an overreaching relay element, there is less concern about a false signal causing an incorrect trip. This scheme is therefore typically applied with a single duplex communication channel. This scheme may use virtually any communication media that is not adversely affected by electrical interference from

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fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults. The overreaching (RO) relay elements often start a zone 2 timer to provide time delayed tripping for faults outside the reach of the underreaching (RU) relays elements if the communication channel is inoperative. 3.1.2.3 Permissive Overreach Distance Protection The Permissive Overreach Distance Protection scheme requires only overreaching relay functions. Phase distance functions are used almost exclusively for detection of multi-phase faults, whereas ground distance functions or directional ground overcurrent functions can be used for the detection of ground faults. The scheme is usually applied with an active duplex communication channel that transmits a GUARD signal during quiescent, or unfaulted, conditions. The transmitter is keyed to a TRIP signal when the associated overreaching relay element detects a fault within its reach. Basic logic for the Permissive Overreach Distance Protection scheme is shown in Figure 3.1-12. RO RO

Bkr 1

Bkr 2 Protected Line Teleprotection Equipment

RO

TX

Duplex Communication Link

RX

&

RO

TX RX

TRIP Bkr 1

TRIP Bkr 2

&

Protection Equipment RO - overreaching trip function, must be set to reach beyond remote end teminal

Figure 3.1-12:

Permissive Overreach Distance Protection Scheme Logic

For a fault anywhere on the protected line, both of the RO functions operate and assert one of the inputs to the logic AND (&) gate. At the same time, RO also keys the transmitter TRIP signal. Receipt of the TRIP signal at each terminal, and an output from the RO function, satisfies the logic AND (&) gate to produce a TRIP output to the breaker. For external faults, the RO functions at only one end of the line will operate, so communication assisted breaker tripping is not initiated at either terminal. The scheme is very secure in that it does not trip for any external fault if the channel is inoperative. Conversely, the scheme is lacking in dependability because it will not trip for any internal faults if the channel is inoperative. The scheme also will not trip for any fault if the fault is not detected at all terminals of the line. The scheme may not trip at high speed for close-in

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faults at the strong terminals because the fastest tripping time that can be expected is dependent on the slowest function to operate for an internal fault. Some means must be used to key the transmitter at an open breaker if tripping is to be initiated for faults seen at the other terminals. Breaker auxiliary contact switch keying with echo logic is commonly used to provide this requirement. Time-delayed back-up tripping can be provided because the scheme uses overreaching functions. Because the GUARD signal is transmitted continuously, the channel can be monitored on a continuous basis. This scheme may use virtually any communication media that is not adversely affected by electrical interference from fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults. 3.1.2.4 Accelerated Underreach Distance Protection Basic logic for the Accelerated Underreach Distance Protection scheme is shown in Figure 3.1-13. This scheme requires the use of underreaching relay element functions (RU) that can be extended in reach by the receipt of a TRIP signal from the relay scheme at the remote line terminal. The RU functions must be set to overlap in reach to avoid a gap in their fault detection. This generally requires the use of ground distance functions for the detection of ground faults, whereas phase distance functions are used for the detection of multi-phase faults. The scheme is often applied with an active communication channel that transmits a GUARD signal during quiescent, unfaulted conditions, and is keyed to a TRIP signal when the associated RU function detects a fault within its reach. Extended RU

RU RU

Extended RU

Bkr 1

Bkr 2 Protected Line Teleprotection Equipment

RU

Duplex Communication Link

TX

TX

RX

RU

RX

Extend RU

Extend RU

TRIP Bkr 2

TRIP Bkr 1 Protection Equipment

RU - underreaching trip function, must be set to reach short of remote terminal and must overlap in reach with RU at remote terminal. It must be capable of being switched in reach.

Figure 3.1-13:

Accelerated Underreach Distance Protection Scheme Logic

For an internal fault within the overlap zone of the RU functions, breaker tripping is initiated directly at both line terminals and each communication channel is keyed to the TRIP signal. Receipt of the TRIP signal extends (accelerates) the reach of the RU functions to beyond the remote line terminal. This reach extension has no further affect because breaker tripping has

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already occurred at each line terminal. For an internal fault near one terminal, the RU function at that terminal operates, tripping the breaker and keying its transmitter to the TRIP signal. Receipt of the TRIP signal at the other terminal extends the reach of that terminal’s RU function, which then detects the fault to initiate tripping. For external faults, none of the RU functions operate, therefore tripping does not occur at either line terminal. The scheme is more secure than the Direct Underreach Distance Protection scheme because it does not trip directly on receipt of a trip signal. Conversely, it is slower than the Permissive Underreach and Overreach Distance Protection schemes because it must wait for the extended RU function to detect the fault before tripping. As mentioned before, it also requires a special relay with zone extension capability. This scheme may use virtually any communication media that is not adversely affected by electrical interference from fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults. 3.1.2.5 Blocking Overreach Distance Protection Basic logic for a Blocking Overreach Distance Protection scheme is shown in Figure 3.1-14. The scheme requires overreaching tripping functions (RO) and blocking functions (B) as shown. Distance functions are used almost exclusively for multi-phase fault protection, but either ground distance functions or ground directional overcurrent functions are used for ground fault detection. A quiescent, or OFF/ON, communications channel is typically used with this type of scheme. The power line itself is often used as the communications medium because the communication channel is not required when the fault is on the protected line. The communication channel is only used to transmit a block trip signal when the fault is external to the protected line. Audio tone over leased phone lines, microwave radio, and fibre-optic media are also used. The transmitter is normally in the OFF state for quiescent conditions and is keyed to the ON state by operation of any one of the blocking functions. Receipt of a signal from the remote terminal applies the NOT or inverted input to BLOCK the trip output. The overreaching tripping functions (RO) must be set to reach beyond the remote terminal of the transmission line with margin so they will be able to detect a fault anywhere on the transmission line. The blocking functions (B) are used to detect any fault not on the protected line that the remote tripping functions are capable of detecting; so they must be set to reach further behind the terminal than the tripping function at the remote terminal. For a fault external to the protected line, one or more of the blocking functions operate to key its respective transmitter to send a blocking signal to the remote terminal. Receipt of the blocking signal blocks tripping in the event one of the tripping functions has operated for the remote fault. The coordinating timer, TL1, is required to allow time for a blocking signal to be received from the remote terminal. It is set to compensate for channel time, signal propagation time and for any difference in operating time that might result if the remote blocking function is slower than the local tripping function. For a fault anywhere on the transmission line, one or more of the tripping functions (RO) at each terminal will operate and apply an input to its respective AND gate (&). The blocking functions will not operate for an internal fault, therefore neither transmitter is keyed, so that there is no output from either receiver. The logic at each terminal produces an output that starts the TL1 timer. When the TL1 timer expires, the scheme produces an output to trip the breaker.

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RO B B RO

Bkr 1

Bkr 2 Protected Line Teleprotection Equipment

B

TX

RO

RX

TL1 C 0.0

&

Simplex or Duplex Communication Link

TRIP Bkr 1

TX

B

RX

RO

TRIP Bkr 2

&

TL1 C 0.0

Protection Equipment

RO - overreaching trip function, must be set to reach beyond remote end of line B - blocking function, must be set to reach beyond overreaching trip function at remote end of line C - Coordinating time, required to allow time for blocking signal to be received (set equal to channel time plus propogation time plus margin)

Figure 3.1-14:

Blocking Overreach Distance Scheme Logic

The scheme is very dependable because it will operate for faults anywhere on the protected line even if the communication channel is out of service. Conversely, it is less secure than permissive schemes because it will trip for external faults within reach of the tripping functions (RO) if the channel is out of service. This scheme does not require breaker auxiliary contact or echo logic keying when the remote breaker is open to permit tripping for faults anywhere on the line. It provides relatively fast tripping (dependent on coordinating time delay) for most source and line conditions. However, it may not trip weak terminals of the transmission line, if fault levels are below the sensitivity of the tripping relays. If quiescent (OFF/ON) communication channels are used there is no way to monitor the channel continuously because the channel is only keyed on during external faults. A communication channel check-back scheme is often used to periodically key a momentary block signal to check the channel status. Some check-back schemes echo a signal back to verify that the channel is operational in both directions. Other schemes must receive a signal within a preset time period to declare the channel in service. The overreaching functions can be used to drive timers so that time-delayed back-up tripping can be provided for faults within reach of the overreaching functions. 3.1.2.6 Deblocking Overreach Distance Protection As mentioned in some previous sections, metallic communication paths adversely affected by fault generated noise may not be suitable for some teleprotection schemes that rely on a signal transmitted during a protected line fault. With power line carrier, for example, the communication

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signal may be attenuated by the fault, especially when the fault is close to a line terminal, thereby disabling the communication channel. Multi-phase power line carrier coupling schemes can be used to minimize this problem. The Deblocking Overreach Distance Protection scheme includes logic specifically designed to accommodate a loss of communication signal during the protected line fault. The Deblocking Overreach Distance Protection scheme, like the Permissive Overreach Distance Protection scheme, uses overreaching phase distance functions almost exclusively for multi-phase fault detection, and ground distance or directional ground overcurrent functions for ground fault detection. The logic requires the use of an active communication channel that transmits a GUARD signal during quiescent, or unfaulted, conditions, and is keyed to a TRIP signal when the associated overreaching relay element detects a fault within its reach. To overcome the loss of signal caused by the internal line fault, deblocking logic permits a TRIP output if the loss of signal occurs at nearly the same time the overreaching relay function(s) detect a fault. A tripping period is controlled by a timer that is typically set between 150 and 300 milliseconds. Basic logic for the Deblocking Overreach Distance Protection scheme is shown in Figure 3.1-15. RO RO

Bkr 1

Bkr 2 Protected Line Frequency Shift Power Line Carrier Communication Link

GUARD OR TRIP TX

RO

RX TRIP

LOG

GUARD OR TRIP

}

{

&

OR

TRIP Bkr 1

TRIP Bkr 2

OR

T

& T

0.0

&

&

TX

RO

RX TRIP

LOG

0.0

RO - overreaching trip function, must be set to reach beyond remote end teminal LOG - Loss of GUARD detection from receiver, RX T - deblocking time delay, typically set for 150 to 300 milliseconds.

Figure 3.1-15:

Deblocking Overreach Distance Protection Scheme Logic

If the signal loss is due to a fault on the protected line, at least one of the overreaching trip functions (RO) will be picked up. Thus, tripping will be initiated when the deblocking output is produced. If none of the permissive trip functions are picked up, the channel will lock itself out 150 - 300 milliseconds after the signal is lost and will stay locked out until the GUARD signal returns for a pre-set amount of time. It is important to understand that this logic requires that the loss of signal associated with the operation of an overreaching relay element must only be caused by a fault on the protected line. Loss of signal due to external line faults will cause false trips. Therefore, the Deblocking Overreach Distance Protection Scheme Logic is used almost exclusively with power line carrier communication.

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3.2 BUSBAR PROTECTION Very often fault clearing criteria for a power system specify that busbar faults must be cleared in the order of 5 cycles, and that only a few feeders are allowed to be tripped. This may be the maximum allowed disturbance for a power system, in order to maintain stability of the remaining power system after fault clearing. Therefore phase to phase faults and phase to ground faults should be cleared within 5 cycles. Typical power system busbar configurations are shown on Figure 3.2-1, Figure 3.2-2, and Figure 3.2-3. Busbar protection is typically based on differential current principles. Busbar protections are mostly configured with zones, one or more zones for bus A and one or more zones for bus B. The busbar protection very often includes breaker failure protection, time delayed typically 5 to 9 power frequency cycles.

3.2.1 Two-breaker busbar configuration Two-breaker power system busbar configuration is shown on Figure 3.2-1. With two current transformers in each bay, busbar protection functions (measuring and trip actions) are independent of isolator positions. Breaker failure protection is started from busbar protection, line protection and transformer protection. Bus B

Bus A

BP-A Id-A

CBFP -A-L1

BP-B CB-A-L1

d

CB-B-L1

CBFP -B-L1

Id-B

Line 1

Notation : Bus-A is section A of the bus.

c

CB-A-L1 is circuit breaker A for line 1. CBFP -A-L2

CB-A-L2

CB-B-L2

CBFP -B-L2

Line 2 b

a

CBFP -A-T

CB-A-T

CB-B-T

BP-A is bus protetion for bus zone A. CBFP-A-F1 is circuit breaker failure protection for breaker A on feeder 1. Feeders may be lines, transformers or any other feeder. Id-A is current differential protection for bus zone A.

CBFP -B-T

Transf c

Figure 3.2-1:

Two breaker busbar configuration

3.2.1.1 Normal fault clearing For improving dependability or security, combinations of protection systems may be applied. The protection system has to detect faults and initiate actions on following faults:

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Fault location a (b) : CBFP-A (CBFP-B) trips bus A (B), and the fault is cleared. There is no need for telecommunication. Fault location c : This is a fault for line protection or transformer protection, see Chapter 3.1 and 3.3.2. Fault location between CB and CT, exemplified with fault location d : Busbar protection zone A trips bus A. But the fault is not yet cleared - there is still infeed from bus B and Line 1. To obtain fast fault clearing, the breaker failure protection 'CBFP-A-L1' trips breaker B on Line 1 and must initiate tripping of the remote breaker(s) on Line 1. This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1 to trip the line at least at the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection executes the tripping, it should be three phase without initiation of automatic reclosing. Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this fault is very seldom. If the line protection is performed by distance relays, transmitting a carrier signal to accelerate the 2nd zone of the line protection, at the remote line end, would provide a good solution. 3.2.1.2 Breaker failure The following fault clearing procedures apply in case of a breaker failure. Fault location a (b): For fault location a, if breaker CB-A-L1 is stuck, CBFP-A has to trip CB-B-L1 and initiate tripping of remote breaker(s) on Line 1. This can only be done by means of telecommunication as described in chapter 3.2.1.1 for fault location d. Fault location c: If breaker CB-A-L1 (CB-B-L1) is stuck, CBFP-A-L1 (CBFP-B-L1) has to trip bus A (B). There is no need for telecommunication in this case. Fault location between CB and CT, exemplified with fault location d : The scenario is the same as described in 3.2.1.1.

3.2.2 One- and a half breaker busbar configuration One- and a half breaker busbar configuration is shown on Figure 3.2-2. Busbar protection functions (measuring and trip actions) are independent of isolator positions. Breaker failure protection is started from busbar protection, line protection and transformer protection.

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Bus A

Bus B

BP-A Id-A

CBFP -A-L1

CB-A-L1 e

g

h

Line 1

CBFP -A-L2

CBFP -A-T1

CBFP -B-L1

CB-L3-L4

CB-A-L3

CB-B-L4

Line 4

a

b

CB-T1-T2

CB-B-T2

T2 d

Bus-A is section A of the bus.

CB-A-L1 is circuit breaker A for line 1.

Line 3

CB-A-T1

Id-B

T1 is transformer 1.

c

T1

Figure 3.2-2:

CB-B-L2

Line 2 c

Notation :

BP-B

CB-L1-L2 f

d

CBFP -B-L2

CB_L1-L2 is circuit breaker between line 1 and line 2. BP-A is bus protetion for bus zone A.

CBFP -B-T2

CBFP-A-F1 is circuit breaker failure protection for breaker A on feeder 1. Feeders may be lines, transformers or any other feeder. Id-A is current differential protection for bus zone A.

1½ breaker busbar configuration

3.2.2.1 Normal fault clearing For improving dependability or security, combinations of protection systems may be applied. The protection system has to detect faults and initiate actions on following faults : Fault location a (b) : CBFP-A (CBFP-B) trips Bus A (B), and the fault is cleared. There is no need for telecommunication. Fault location c and d : This is a fault for line protection or transformer protection, see Chapters 3.1 and 3.3.2. Fault location between CB and CT, exemplified with fault location e (h) : Busbar protection zone A trips Bus A. But the fault is not yet cleared - there is still infeed from bus B and Line 1. To obtain fast fault clearing, the breaker failure protection 'CBFP-A-L1' trips breaker CB-L1-L2 and must initiate tripping of remote breaker(s) on Line 1. This remote tripping can be executed either by direct intertripping of breakers, or by ’commanding’ or helping line protection systems on Line 1 to trip the line at least at the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection execute the trip, it should be three phase without initiation of automatic reclosing. Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this type of fault is rare in practice. If the line protection is performed by distance relays, transmitting a carrier signal to accelerate the 2nd zone of the line protection, at the remote line end, would provide a good solution.

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Fault location f and g: If current measurement for line protection of Line 1 and Line 2 crosses, this is a fault for line protection or transformer protection. See ’Fault location c and d’ above. 3.2.2.2 Breaker failure The following fault clearing procedures apply in case of a breaker failure. Fault location a (b): For fault location a, if breaker CB-A-L1 is stuck, CBFP-A-L1 has to trip CB-L1-L2 and initiate tripping of remote breaker(s) on Line 1. This can only be done by means of telecommunication as described in Chapter 3.2.1.1 for fault location d. Fault location c : If breaker CB-A-L1 (CB-B-L1) is stuck, CBFP-A (B) has to trip Bus A (B). There is no need for telecommunication in this case. If breaker CB-L1-L2 is stuck, the breaker failure protection of that breaker has to initiate tripping of remote breaker(s) of Line 1 (2). This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1 (2) to trip the line at least in the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection execute the trip, it should be three phase without initiation of automatic reclosing. Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this type of fault is rare in practice. Fault location f or g : If current measurement for line protection of Line 1 and Line 2 ’crosses’, this is similar to ’Fault location c’ above. Fault location between CB and CT, exemplified with fault location e or h : The probability of this fault location in combination with stuck breakers is very low. Normally no breaker failure protection is applied.

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3.2.3 Two zones / one breaker configuration Bus B

Bus A

BP-A

BP-B I-A-L1

Id-A

I-B-L1

CB-L1

Id-B

Line 1

Notation : Bus-A is section A of the bus.

c

T1 is transformer 1. CB-L1 is circuit breaker for line 1.

Coupler-A-B

CB-T1 is circuit breaker for transformer 1. Coupler is coupler between bus section A and B.

d

b

a

CB-T1

Figure 3.2-3:

I-A-L1 is isolator A for line 1. Id-A is current differential protection for bus zone A.

T1

Two protection zones / one breaker busbar configuration

3.2.3.1 Normal fault clearing Fault location a and b: The busbar protection trips the bus, and the fault is cleared. If a line breaker fails, the second zone of the line protection ( Z< ) at the opposite line end serves as back-up protection. Fault location c: Busbar protection zone A and/or B trips bus A and/or B dependent of isolator positions. But the fault is on the line side of the breaker. Therefore, the fault is not cleared. To achieve fast fault clearing, trip command from busbar protection - dependent of isolator position - must initiate tripping of remote breaker(s) of Line 1(n). This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1(n) to trip the line at least in the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection executes the trip, it should be three phase without initiation of automatic reclosing. Fault clearing time will not necessarily exceed 5 cycles. Fault location d: The protection initiates a trip command, but the fault is not yet cleared. In order to clear the fault busbar protection zone B is designed to trip bus B if receiving a signal from zone A for more than 5 cycles.

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3.3 OTHER PROTECTION SCHEMES The following protection schemes may require telecommunication for intertripping.

3.3.1 Generator protection Generator and step-up transformer protection is normally designed to detect all faults and abnormal conditions dangerous for generators and step-up transformers. If action is needed, a stop signal is issued to the generator and a trip command is issued to a breaker interfacing the power grid. Telecommunication is normally not needed. As indicated on Figure 3.3-1, telecommunication is needed to trip a remote breaker if, for instance, the breaker interfacing the power grid is stuck is stuck or has not been installed to reduce capital expenditure. Substation Intertripping (telecommunication)

Short or long overhead power line

Step-up transformer Stop

Generator

Figure 3.3-1:

Transformer protection & Generator protection

~ Generator protection

3.3.2 Transformer protection The transformer protection normally consists of differential protection, overpressure protection and residual current protection. Overcurrent and impedance protection are often used as backup protection. The absence of a circuit breaker on the high voltage side in order to economize on circuit breakers requires an intertripping system to the adjacent station. In the event of an internal fault a lock out signal is recommended in order to block the closing of the connected circuit breakers.

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Lockout Intertripping

Telecommunication system

Power line protection

Power line protection Transformer protection

Trip Trip

Figure 3.3-2:

Transformer protection

3.3.3 Reactor protection Reactors are used to regulate the network voltage. These reactors are placed on the high voltage line to compensate capacitive generation. Normally the reactors have no circuit breakers, hence the reactor protection must send a trip and intertrip signal to the circuit breakers to both ends of the power line.

Intertrip

Telecommunication system

Intertrip

Reactor

Reactor

Reactor protection

Reactor protection

Figure 3.3-3:

Reactor protection

3.4 SYSTEM PROTECTION Figure 3.4-1 shows the relationship between protected zones/areas and operate times for various protection schemes. Main protection systems operate to clear faults at the very

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beginning in power transmission lines, busbars, transformers, and so on. If faults cannot be cleared by a main protection, however, successive operations are needed in the forms of backup, multi-zone and/or system-wide protections. Time after fault occurrence

Local backup protection

System stabilising protection Remote backup protection

Main protection #2 (Redundant or backup)

Wide-area or system-wide protection Multi-zone protection

Main protection #1 Protected zones/area

Figure 3.4-1:

Relationship between protected area and operate time with respect to protection schemes

3.4.1 Back-up protection Back-up protection [3] is a protection system that operates independently of specified components in the fault clearing system. It may duplicate the main protection system or may have the task to operate only when the main protection fails to operate or when the main protection is temporarily out of service. Back-up protections are usually categorized into circuit local back-up protections, substation local back-up protections, and remote back-up protections. On EHV networks it is common practice to use duplicated line protections as circuit local backup protection; a main protection (#1) and another redundant main protection (#2), taking account of maintenance or failures of one of the two main protections. A substation local back-up protection including a circuit-breaker failure protection is energized from instrument transformers located within the same substation as the corresponding main protection and is not associated with the same primary circuit. For example, when a circuit breaker failure occurs after a power line fault and a main protection operation, the breaker failure protection trips all the circuit breakers connected to the same busbar in the substation, if it is confirmed that the main protection has operated and the fault is not cleared. A remote back-up protection is located in a substation remote from that substation in which the corresponding main protection is located. The conventional remote back-up protections employ distance relays and utilize local electrical data for operating in zone 2 or wider zones. Figure 3.4-2 shows a network protected by distance protections without telecommunications. The distance protection uses current and voltage measured at one end of the power line. The protection uses these measurements to decide if the fault lies within the zones of the distance protection. A zone of the distance protection is open at the remote end. Zone-1 of the distance protection covers only about 85% of the power line. Zone-2 of the distance protection at A reaches beyond the remote terminal B. Zone-1 of the distance protections at B and zone-2 of the distance protection at A both detect fault close to B on the power line from B to C. To obtain rapid fault clearing, distance protections operate instantaneously when the fault occurs within zone-1. To obtain selectivity we have to delay the tripping for faults within zones-2 and 3. This

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co-ordination delay is usually about 0.4 seconds.

Figure 3.4-2:

Distance protection providing remote backup

Zone-2 of the distance protection at A must cover the entire power line from A to B. Zone-2 of the distance protection at A must not reach beyond zone-1 of the distance protection at B. Zone-2 of the distance protection at A backs up the distance protection at B. However, this is true for only one part of the power line from B to C. Zone-3 of the distance protection at A provides back-up for the rest of the power line from B to C. We have to delay the tripping from zone-3 of the distance protection at A more than the tripping from zone-2 of the distance protection at B, direction C. Splitting protection for busbar using communication for multi-circuit multi-terminal line For the configuration of double busbar and double circuit transmission lines, if a fault persists due to a CB failure or main protection failure, separation of the busbar by using splitting protection before remote back-up operation is effective to prevent interruption. However, for multi-terminal lines the splitting protection is done by sequential tripping and the operation time may not be coordinated with remote back-up operation. Figure 3.4-3 gives a sample application for three-terminal transmission lines.

Figure 3.4-3:

Splitting protection (BD) using telecommunications for multi-circuit and multiterminal line. Ry, CB and Td denote operating times of relay (30 ms) and CB (40 ms) and time delay for coordination, respectively.

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Splitting protections operate sequentially from (1) to (2), and then to (3). Therefore, it cannot be coordinated with the operation time (340 ms) of remote back-up protection (zone 2 of distance relay) of substation A, and remote back-up tripping occurs at the substation A end on both lines, which results in the isolation of substation C. To prevent such isolation, splitting protection at substation C performs transfer tripping of busbar CBs at substations A and B using communication channels. Busbars at substations A and B are separated within 260 ms which allows coordinating of the remote back-up protection. Lit.: [4]. Coordination time control using communication For the configuration where long distance transmission lines adjoin short distance transmission lines, coordination between remote back-up protections may not be achieved. Figure 3.4-4 gives an example where there is a long distance transmission line between substations A and B and a short distance transmission line between substations B and C. If fault F1 occurs at the busbar in substation B, zone 2 of distance relay of substation A may operate. Zone 2 of substation A cannot be coordinated in the standard zone-2 time setting of 270 ms. In this case, the time setting needs to be changed from 270 ms to 370 ms, which is equivalent to the operation time of zone 3. However, there is another problem that remote back-up operation (zone 2) of the substation B is delayed for the busbar fault F2 at substation B. In order to accelerate the operate time, the splitting protection operation signal is sent from substation B to substation A by a communication channel, and the operation time of zone 2 in substation A is shortened to 270 ms. Lit.: [4].

Figure 3.4-4:

Coordination time control using telecommunications. Ry, CB and Td denote operating times of relay (30 ms) and CB (40 ms) and time delay for coordination, respectively.

Wide-area current differential back-up protection To cope with such complexity of coordinating operate times and reaches and obtaining necessary selectivity in remote back-up protection employing distance relays, wide-area backup protection based on a current differential algorithm which utilize electrical data at remote stations employing wide-area telecommunication networks among substations is proposed as shown in Figure 3.4-5. The wide-area back-up protection system covering multi-zones consists of central equipment (CE) and terminal equipment (TE) which are connected by telecommunication networks. The terminal equipment samples all the currents from instrument transformers installed at a busbar and at power transmission lines and power transformers

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connected to that busbar. The data are transmitted by the terminal equipment to the central equipment through data communication channels. As this protection scheme has to be installed for every busbar, one central equipment may cover two or more protected areas or busbars. For the double-busbar and double-circuit configuration shown in the figure, the conventional backup protection firstly performs busbar splitting protection to prevent the interruption of a sound circuit or isolation of transmission lines, which leads to longer operate time, wider outage area or isolated transmission lines. When a fault occurs at F in the busbar in substation B, for example, and if the busbar protection fails to operate, the conventional remote back-up protections or distance relays of substation A and C operate in zone-2 after the bus-tie splitting protection operates to prevent disruption of the sound circuits. The wide-area back-up protection operates to minimize the outage area, which is the same as the main protection in this case. Therefore, the operate time is 140 ms shorter than the conventional protection and the outage area is smaller. Lit.: [6].

Figure 3.4-5:

Wide-area current differential back-up protection employing telecommunications

The wide-area current differential protection system requires wide-area timing synchronization for simultaneous current sampling. As some current differential multi-terminal line protections employ centralized timing synchronization scheme in their telecommunication circuits, a similar scheme may be applied to such wide-area protections. More terminals, however, lead to the complexity of achieving total synchronization among the terminals using telecommunication circuits. Satellite-based wide-area timing synchronization such as GPS may be an alternative solution. Since back-up protections are initiated after a main protection operated, delays for transmitting current data and tripping signals are not necessarily crucial, while timing synchronization and data integrity and reliability are still important.

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Since this kind of wide-area protection system employing telecommunications can collect various kinds of power system data simultaneously sampled throughout the area, further sophisticated power system monitoring, control, protection, and restoration could be achieved.

3.4.2 System-wide protection System stabilizing protection operates in a wider area than that for power line protections or in a system-wide area to prevent power system disturbance. For example, when severe faults such as double faults in a double-circuit transmission line occur, and even if main and back-up protections operate properly, it may result in a power system disturbance such as overload, power swing, abnormal frequency or voltage. Operations of such protection are load shedding, generator shedding or system separation, which in many cases requires wide-area telecommunications. Some adaptive protections, defined as a philosophy that permits and makes adjustments to protection functions automatically for making the protection more attuned to the prevailing power system conditions, require wide-area on-line telecommunication channels [9]. A predictive out-of-step protection [10] operates for preventing total system collapse caused by step-out between large-capacity generator groups due to a serious fault in the trunk power transmission line as shown in Figure 3.4-6. When a double-fault occurs along both circuits of a double-circuit line forming one route, the substations at both ends of the line are disconnected and power transmission capability is interrupted. If a successive fault occurs after reclosing, a slow cyclic power swing develops between the western generator group and the bulk power system. The same situation occurs in case of failure of a busbar protection to operate during a busbar fault. Over time, the phase difference of the generator groups thus undergoes oscillating divergence. If this condition is not corrected, an out-of-step situation will begin to occur in various parts of the power system and may lead to total collapse of the power system. Taking account of this characteristic of the power system, the western area can be isolated from the bulk power system before an out-of-step situation occurs and then be operated independently. This eliminates power swing between the generator groups of the two systems and restores stability. This separation of the western area is performed in a manner to preserve the power supply and demand. The separation point is selected based on the power flow at pre-determined points for separation before the fault. Adjustment of the supply/demand balance of each area after separation is performed by governor control of the corresponding generator groups. The western generator group, however, may under certain conditions becomes overloaded. In this case, load shedding via under-frequency relays is relied upon to correct the unbalance. This protection is accomplished by using on-line voltage data, or busbar voltage waveforms, collected from the generator group by central equipment to predict step-out based on the measured voltage phases and then issuing a system separation command. This system consists of central equipment and RTUs, and requires sampling synchronization for the voltage phase measurement. The telecommunication requirements from this protection are almost the same as the wide-area current differential protection described above. The required overall operating time is less than a few hundreds of milliseconds, where transmission time including initiation and processing of frame-formatted cyclic data transmission (Tac in Figure A1-5) should be less than several tens of milliseconds, and propagation delay requirement including media and equipment delay be at most several milliseconds.

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Figure 3.4-6:

A system-wide protection; predictive out-of-step protection

Another system stabilizing protection, which consists of central processing unit, fault detecting unit, and transfer trip unit, operates for stabilizing power frequency or transient instability [11]. The central processing unit collects data on generated energy and load as controllable quantities from power stations and substations, automatically recognizes electric power system connections, performs calculations in advance to prepare for faults, and automatically determines control quantity and objects for each pattern of separation. If a fault occurs, the central processing unit sends a trip signal to the transfer trip unit based on the calculation results. The fault detecting unit detects a route disconnection fault in any of the EHV lines, and calculates power flow through the main lines, frequency, and voltage drops, and transmits these data to the central equipment. If faults occur, the transfer trip unit receives a transfer trip signal from the central processing unit, and sheds the generators and/or loads as controllable quantities based on the received information to stabilize the frequency of each separated part. A means of high-speed multiplexed data transmission of large volume of information is essential to a power stabilizing system that provides adaptive approach at high speed. A dedicated transmission unit is used for the important information such as fault or route-off detection and transfer trip signals to ensure high-speed and reliable transmissions, while relatively large volumes of information that do not necessarily require high speed are transmitted by an EMS/SCADA communication network. The overall operate time and transmission time requirements are similar to the previous system stabilizing protection.

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4 TELECOMMUNICATION SYSTEMS FOR PROTECTION The purpose of a telecommunication system in conjunction with protection systems is to transfer a protection signal in due time from the protection equipment at one station to a similar equipment at the remote station. A secure and dependable point-to-point communication is normally required for this purpose. Possible transmission media are: -

Pilot wires / copper wires Power line carrier (PLC) links Microwave radio links Fibre optic links Satellite links

The telecommunication link should have a high degree of availability and should transmit the protection signal as fast as necessary to the remote station with the highest possible reliability. The actual requirements on transmission speed, dependability for wanted operation and security against unwanted operation may vary for different protection schemes and line configurations. Practical and economical reasons may define which type of transmission medium has to be used. All communication systems are subject in varying degrees to interference and noise of various forms. These can corrupt the information arriving at the receiver, either by simulating a signal when no such signal has been transmitted, or by delaying or blocking a true signal. In analog systems, there are many ways in which transmission can be degraded. For example, the signalto-noise ratio may be poor, or the signal may suffer distortion or crosstalk from one user to another, or the system may clip the input signal. In comparison, a digital system has the parameters: bit rate, error rate, delay, and delay variation. Channel impairments may result in bad messages, no messages, excessive message delay, excessive message delay variation and/or excessive delay difference in the transmit and receive direction. The quantity of information per unit time (bits per second) which a communication channel can transfer depends on its bandwidth and on the received signal quality1 (normally expressed as Signal-to-Noise Ratio, SNR). The signal transfer delay introduced by the medium is normally low for terrestrial links, since in most media the signals propagate typically at speeds between 60% and almost 100% of the speed of light in vacuum. The propagation delay is for example about 3.3µs/km for open-wire (e.g. Power Line Carrier) and microwave radio links, about 5µs/km for optical fibres and 5 .... 10µs/km for pilot wires. The significant part of the overall operating time of a teleprotection system is normally introduced by the terminal equipment including their interfaces to the protection, by intermediate repeater stations and network node devices with channel routing functions. See also Figure A1-5 in ANNEX A1. Transmission time delay, bandwidth and signal quality are important parameters when considering the design of a telecommunication system used for protection. The criteria apply equally for both analogue and digital communication systems. For digital systems it is however 1

The maximum information flow that a communication channel can transfer without errors is called its capacity. According to Shannon's law the channel capacity is given by the formula C = B x ld(1 + SNR), with C = Capacity in bits/s, B = Channel bandwidth in Hertz, ld = logarithm to the base of 2, and SNR = Signal-to-Noise Ratio of the received signal. The channel capacity is a theoretical value that can only be approximated at the cost of excessive signal transfer delay due to infinite coding- and decoding efforts. The formula also indicates that a bandwidth related data rate increase is compromized by the bandwidth related SNR deterioration (=> the wider the bandwidth is, the more noise is captured).

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more convenient to use the data rate instead of the bandwidth and to express the transmission quality in terms of bit errors (e.g. bit error rate, errored seconds etc.) rather than signal-to-noise ratio.

4.1 TELECOMMUNICATION CIRCUITS The term "circuit" may be used to characterize legal aspects or physical properties of a communication service. Some examples of circuits are described below.

4.1.1 Private and rented circuits Communication circuits may be utility owned or rented from third parties. Security, dependability and availability of rented circuits do not always satisfy the requirements from protection. Some typical threats and risks are: -

Rented circuits are beyond the control of the power utility Rented circuits may be re-routed for operational reasons. This can change the transmission characteristic, e.g. the signal transfer delay, which may cause problems to the protection function Signals may be injected into the circuit for routine tests or maintenance reasons which may prevent protection from operating or may cause unwanted operation The medium (wires, fibres, radio etc.) and hence its associated typical risks may not be known to the user

Circuit or service providers however may offer circuits or services with guaranteed performance, which seem to be applicable to protection.

4.1.2 Analogue and digital circuits All physical transmission media are analogue by nature. The distinction between analogue circuits and digital circuits is defined solely by the communication equipment technology. The term "analogue" or "digital" circuits thus mainly relates to the physical properties of the communication interface, see also interfaces (a) and (b) in Figures A1-1 to A1-5 in 9. If an interface accommodates waveforms that vary continuously with time and amplitude, that interface provides an analogue circuit. If an interface accommodates signals that may change between few (normally 2 or 3) amplitudes at certain instants of time only, that interface provides a digital circuit. Analogue communication systems have enhanced protection systems for many years. Their advantage is their efficient use of bandwidth, especially for the transmission of analogue signals such as voice. Historically, analogue communication systems provided analogue circuits to the user. The situation has changed with the advances in digital electronics and signal processing, with the development of bandwidth efficient digital modulation principles and with the breakthrough in optical fibre technology. Due to the availability and the advances in digital communications, it is increasingly being used for the protection of power systems. Digital communication systems may provide both analogue circuits (e.g. for voice, telefax and modems) and digital circuits (for data) to the user. The relatively simple characterization of a digital communication system is an important advantage over analog systems, where there are many parameters and ways in which a transmission can be degraded.

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Important parameters for analogue circuits are the bandwidth, the frequency response ( = attenuation and group delay) and the signal-to-noise ratio at the receiver input. Analogue circuits are characterized by their “graceful degradation” under disturbed channel conditions, i.e. the quality of the received signal deteriorates gradually with increasing disturbance and noise. Important parameters for digital circuits are the bit rate, the data transfer delay and delay variation (timing jitter) and the error rate of the received data. The impairments of the communication medium, which may be quite severe, are hidden from the user. Error rates can be bounded to very low values by placing regenerative repeaters at periodic intervals (intermediate stations) along the physical medium. Digital circuits are characterized by their “threshold behavior” under disturbed channel conditions. Simply speaking, they are either very good or not available. The protection of power systems normally imposes very stringent demands on the communication system regarding its real-time properties. The signal transfer time and transfer time variation is for example much more critical for protection signal transmission than for general data or voice communication. Voice frequency circuits The term voice frequency (VF) circuit is used for analogue circuits that pass frequencies between approximately 300 Hz and 3400 Hz and block frequencies outside this range. Historically this frequency range has been defined for the transmission of speech signals. Today, analogue voice frequency circuits are provided by both analogue and digital telecommunication systems and may be used by voiceband modems for data transmission up to approximately 33 kbit/s. Voice frequency circuits may further be characterized according to the number of wires that are required: 2-wire circuits employ the same wire pair for transmitting and receiving, whilst with 4wire circuits one wire pair is used for transmitting and the other wire pair is used for receiving.

4.2 TELECOMMUNICATION NETWORKS The requirements from protection on communication have traditionally been met with simple point-to-point links. The introduction of high capacity digital networks is therefore hardly justified by its exclusive use for power system protection. The deployment of digital networks is primarily motivated by the need for enhanced power network control and increasing data traffic in distributed systems, and particularly by new telecom business opportunities in deregulated markets. Protection may however technically and economically benefit from modern communication networks if some inherent network problems and their impact on protection operating performance are carefully analyzed. Finally, properly designed networks are a prerequisite for the emerging wide-area protection systems that will require the exchange of information between many sites rather than isolated point-to-point links. Networks can enhance the availability of a protection system when the network inherent redundancy and route diversity is exploited. Measures have however to be taken to ensure that automatic re-routing is prevented from re-using the same bearer (e.g. the same fibre cable) when attempting to re-direct a channel which has failed, thus destroying the diversity concept. Pre-definition of a primary and an alternate path with ensured diversity and guaranteed signal transfer delay is suggested. Special attention has to be paid to networks where the protection information may pass through network nodes with switching, routing and loop-back facilities, or when the protection signal

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shares an aggregate with other services. Switching, routing and multiplexing techniques bear a certain risk that a transmitted signal is directed to a receiver which it was not originally intended for (channel cross-over or signal loop-back). The consequences of signal misdirections on protection are different for analog comparison and state comparison protection schemes. For example, signal misdirection is typically less critical for state comparison protection systems that are normally in the guard (non-operate) state, than for analog comparison protection systems, which depend on continuous exchange of information between line ends and may immediately trip the line if signals from the wrong transmitter are received.1 In addition to inadvertent signal misdirection there exist some other network-related risks that are new or have a different impact when compared to traditional ‘hard-wired’ point-to-point links, for example: -

Automatic re-route to some non-defined alternate path with inadequate performance for protection Automatic re-route to a pre-defined non-preemptible (dedicated) path Excessive outage time until re-route completed Different propagation time delays between the various paths selected Possibility of different go and return propagation time delays Protection circuits may be bumped at the expense of others when re-routing after a link failure, unless prevented by adequate circuit priority rating mechanisms Channel may not revert back to its original path unless manually optimized, eg on a least cost basis Unacceptable signal transfer delays due to queuing mechanisms in networks with dynamic bandwidth allocation Availability may be less than expected due to the particular definition of "Available Time" for telecom ISDN circuits according to ITU-T G.821

Power system protection performance may be unacceptably jeopardized unless appropriate measures are taken regarding the control and management of the network, and unless the protection system is designed to cope with typical network related risks. More on networks is found in Chapters 4.4, 4.5 and 4.6.

4.3 TRANSMISSION MEDIA 4.3.1 Pilot wires / Copper wires Pilot wires consist of a pair of metallic wires normally embedded in an aerial or underground cable. They have historically been widely used for transmitting protection signals. Although the 1

State comparison schemes and command-based protection systems: When an inadvertent channel mix-up or loop-back occurs in a command based protection system, normally only the guard signal (‘do not trip’) is misdirected or looped back, as the system is normally in the guard state. A residual risk for a missed tripping or unwanted tripping exists for the unlikely case when the channel-misdirection would coincide with protection operation. Analog comparison schemes: Signal misdirection is more critical for analog comparison schemes like current differential protection. A channel cross-over or signal loop-back would simulate a differential signal, which may immediately produce an unwanted tripping. Should a channel cross-over coincide with a line fault, an unwanted tripping for the wrong line may be produced. Terminal equipment addressing with address validation times of less than the protection relay’s operating time is therefore a prerequisite. Any measure for improving the security has however to be weighted against its adverse impact on dependability.

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tendency is to replace pilot wires by optical fibres which are free from electromagnetic interference, their use may still be justified for economical reasons. Pilot wire circuits may be utility owned or rented from telecommunication companies. Utility owned pilot wires often follow the same route as power cables. Since pilot wires may be subject to dangerous induced voltages during faults in the primary system, appropriate installation precautions must be taken in order to prevent maloperation, hazards to personnel and damage to equipment. The electrical parameters of pilot wires such as signal attenuation and signal delay per unit length depend on their mechanical parameters like wire diameter, insulation material and cable construction, as well as on the signal frequency. Values for the attenuation coefficient between 0.5 dB/km and 3 dB/km in the audio frequency range are typical. The signals, which are transmitted over pilot wires are historically sometimes DC signals or signals at power frequency (50 or 60 Hz) e.g. from pilot wire differential relays. Transmission of DC or AC signals at power frequency is hardly used any more due to the pronounced susceptibility to interference from the primary system. Normally the information is modulated onto a carrier which “shifts” the information from the power frequency range into the audio frequency range for transmission. At the receiving end, the information signal can be separated from the power frequency by means of filtering. This function is usually performed by means of a teleprotection equipment operating over a 2- or 4-wire circuit. 2-wire circuits use the same pair of wires for transmitting and receiving. Transmit and receive signals are normally separated by their respective frequencies. With 4-wire circuits, a pair of wires is allocated to the transmitter and a pair of wires is allocated to the receiver. The same frequency is normally used for transmitting and receiving. A typical application for pilot wires is the transmission of binary on/off protection commands using dedicated teleprotection equipment in conjunction with distance or directional comparison relays. The protection command is modulated onto an audio frequency carrier somewhere in the 0.3 kHz to 3.4 kHz range, which makes the transmission less susceptible to power frequency interference and high frequency noise. The teleprotection equipment may also multiplex several commands from different relays onto the same wires. Internet access and multimedia services had a tremendous impact on the development of new high speed transmission principles for copper wires. High-speed modems would allow the use of pilot wires for higher data rates, ranging from several tens of kbit/s for voiceband modems to up to 10 Mbit/s over short distances for wideband (xDSL) modems. However, due to their inherent high signal transfer delay (latency) and their sensitivity to channel disturbances, the use of high-speed modems is not recommended for the transmission of protection signals. Electromagnetic interference, power frequency harmonics and wideband noise produced by faults in the power system are likely to block the modem receiver just in that moment when the communication is truly needed.

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Summary Advantages and disadvantages of Pilot Wires as related to protection signal transmission are: Advantages

Disadvantages

•

High availability and reliability with MTBF in the order of 200’000 to 500’000 hours

•

Wide deployment

•

Low cost, especially when also used for other purposes

•

Little interference from power lines if separate routing is used

Table 4.3-1:

•

High sensitivity to induced voltages in the event of power line faults and lightning strokes

•

Problems with potential barriers at the station entrance

•

Crosstalk between circuits in the same cable deteriorates performance and reduces link lengths

•

Buried cables my be damaged or broken by civil works

•

Short to medium link lengths only, due to the attenuation, bandwidth, crosstalk and interference constraints

•

High cost of new cables, e.g. when civil works are required.

Advantages and disadvantages of pilot wires

4.3.2 Power Line Carrier (PLC) A PLC system uses the high voltage power line as a transmission medium. Both overhead lines and buried high voltage cables can be used. Lines with mixed overhead line sections and cable sections are also possible, but each case has to be carefully investigated. PLC systems have been extensively used for more than 60 years on HV and EHV lines for the transmission of voice, control data and protection signals. PLC links are entirely under the control of the power utility. They normally provide the shortest and most direct connection between line ends, power stations and substations and are in many cases justified by the transmission of protection signals, where PLC links have proven to perform very effectively. Continued operation has even been reported for power lines, which were broken down after an earthquake. Due to their reliability PLC links are often the preferred back-up medium for selected important channels of wideband communication systems. This is especially true for protection signals. The carrier frequency range which can be used by PLC systems is normally between 40 kHz and 500 kHz. It is sometimes subject to national regulations to prevent interference with other systems operating in the same frequency band. The carrier frequency range between 40 kHz and 500 kHz is subdivided into slots of 4 kHz bandwidth. A PLC link may typically use one to four such slots for transmitting and receiving, depending on the number of channels and on the technology used. Traditional Analogue PLC transmitters translate a voice frequency band of 4 kHz gross bandwidth into one of the 4 kHz carrier frequency slots using single sideband (SSB) modulation. The voice frequency baseband may contain speech, superimposed data and protection signals which share the channel by means of frequency division multiplexing (FDM, see 4.4.1.1). During

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the transmission of protection commands the speech and data signals may be switched off such that the maximum transmit power is available to the protection signal. This "boosting" of the protection signals compensates the additional signal attenuation, which is introduced by the fault on the protected power line when signalling over faulty lines. As voice and data are interrupted during protection signalling, boosting is not recommended for the transmission of persistent commands, as might be the case for reactor protection for example. Emerging Digital PLC systems translate serial digital data into one or several 4 kHz slots at carrier frequency range using bandwidth efficient digital modulation techniques, such as quadrature amplitude modulation (QAM) or multicarrier (MCM) modulation. The serial aggregate data may accommodate digitized speech and digital data by time-division-multiplexing (TDM, see 4.4.1.2). Protection command transmission is usually accomplished by means of a dedicated subsystem in order to achieve the required dependability and minimum signal transfer delays under faulty line conditions. For both analogue and digital PLC systems the signal at carrier frequency is amplified to typically 5 to 100 Watts output power (PEP, peak envelope power) and coupled to the power line via an impedance matching device and a high voltage coupling capacitor. For optimum transmission performance under faulty line conditions coupling onto two phases in “push-pull” mode is normally preferred. Line traps in series to the power line prevent the carrier signals from being shunted by the local busbar and prevent signal leakage to adjacent lines. Several PLC terminals may share a common coupling equipment. The propagation of the signal along a multi-conductor power line may be explained by the combined transmission of independent modes whose number is equal to the number of nonearthed conductors above ground. Each mode propagates with its specific attenuation and velocity. The signal attenuation depends on the construction of the power line, the line condition and on the carrier frequency used. It is typically in the range of 0.02 dB/km to 0.2 dB/km, increasing with frequency. The signal quality may be impaired by various noise sources. Corona noise results from electric impulse discharges along the surface of the phase conductors. Its spectrum extends well into the carrier frequency range. Corona noise is always present on an energised line and is perceived as background noise in a PLC receiver. Its level depends on the power system voltage and design, the climatic conditions and the altitude above sea level. It normally does not constitute a problem to protection signal transmission since its level is less than other channel impairments caused by line faults for example. Isolator operation creates high frequency noise of high amplitudes which cause poor signal-tonoise ratios in the PLC receiver. Its duration may last some seconds depending on the isolator design. The signal quality degradation depends largely on the method of coupling and on the characteristics of the equipment. The interference produced by isolator noise is most severe in comparison with other noise sources. Because it occurs under healthy line conditions it may cause unwanted operation of the protection system. Operation of breakers produces disturbance similar to isolator operation. Its duration is however limited to the operating period of the circuit breaker which is typically less than 20 ms. During line faults the PLC channel is subject to strong transient noise at the onset of the line fault until the arc has established, followed by an immediate increase in signal attenuation due

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to the short circuit of the faulty phase(s). During the interruption of the fault current, noise is produced again by the operation of the circuit breakers. Interference produced by power system faults occurs during the time when the protection is in active operation. It may therefore prevent operation of the protection system. Channel impairments during line faults are the primary reason why PLC links have so far been restricted for two applications for protection signal transmission, where they have however proven to perform most effectively: - Transmission of binary status information in conjunction with distance protection or directional comparison relays in state comparison protection schemes - Transmission of phase comparison signals in conjunction with phase comparison relays in analog comparison protection schemes. Summary Advantages and disadvantages of PLC links as related to protection signal transmission are: Advantages

Disadvantages

•

The overhead power line constitutes a very reliable transmission medium

•

The power line is normally the shortest and "fastest" link between line ends, power stations and substations

•

PLC teleprotection links are normally “hard-wired” point-to-point links with little risk of unwanted re-routing, switching or tampering

•

Channel is subject to increased disturbance during faults in the primary system

•

Application for protection signal transmission is limited to the transmission of binary commands and non-segregated phase comparison signals

•

Not applicable for current differential protection

•

The equipment is situated at the power station, giving easy access for control and maintenance

•

The narrow bandwidth (few kHz) constrains the number of signals that can be transferred and the signal transfer time

•

The medium (power line) and terminal equipment are under the full control of the utility

•

Limited frequency band available, limiting the number of PLC links that can work in a given network (frequency congestion)

•

Very long distances of many hundred kilometers may be covered without intermediate repeaters.

•

No earth potential rise problems since the transmitter and receiver as well as coupling equipment are normally situated within the station earth network

Table 4.3-2:

Advantages and disadvantages of power line carrier links

4.3.3 Microwave Radio Microwave radio links have been extensively used by many electric power utilities mainly to satisfy the increasing demand for more communication capacity. Until the introduction of fibreoptic links they represented the only true wideband medium which could accommodate a large number of voice channels. Another reason for selecting microwave radio links is their relative immunity against electromagnetic interference produced by the high-voltage power network.

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From a legal point of view terrestrial microwave radio systems may be broadly categorized into licensed radio and unlicensed radio systems: Licensed radio systems operate in a "protected" frequency range that has been licensed to the utility by national authorities. Licensed radio systems typically constitute point-to-point multichannel links that are used in the backbone of the communication network. Unlicensed radio systems operate in an "unprotected" frequency range that is open to the public. Unlicensed radio systems usually share a common frequency band and support point-tomultipoint communication. Access to the shared medium (i.e. the common frequency band) is accomplished either through TDMA (Time Division Multiplex Access) or CDMA (Code Division Multiple Access), or a combination of the two, to prevent mutual interference between users. See also chapters 4.4.1.2 and 4.4.1.3.1 4.3.3.1 Multichannel radio Although the microwave equipment may be owned and operated by the power utility, the frequency bands for its use have to be licensed from national authorities. The frequency bands for microwave radio systems are typically between 400 MHz and 40 GHz. In legacy analog microwave systems a number of voice baseband channels with 4 kHz bandwidth each are combined onto a single aggregate signal by frequency division multiplexing (FDM, see 4.4.1.1). One or several of the 4 kHz baseband channels may be used individually or collectively for the transmission of protection signals. Earlier analogue microwave systems used frequency modulation (FM) where the analogue aggregate FDM signal varies the frequency of the emitted carrier. Analogue microwave systems are mainly of historical interest since they have been gradually replaced by digital systems. In digital microwave systems a number of digital data channels of typically 64 kbit/s each are combined onto an aggregate data stream using time division multiplexing (TDM, see 4.4.1.2). Analogue signals such as speech are converted into digital data prior to multiplexing. One or several of the digital 64 kbit/s channels may be used either individually or collectively for the transmission of protection signals. In digital microwave systems frequency modulation has been replaced by phase shift keying (PSK) modulation or combined phase-amplitude shift keying, which is also called quadrature amplitude modulation (QAM), with 16-QAM being widely used today. Higher level QAM like 64QAM or 128-QAM provide a higher bandwidth efficiency, i.e. they allow to transmit more bits per second in a given bandwidth, however at the expense of an increasing susceptibility against interference and noise. Licensed microwave radio links are normally point-to-point with maximum distances between 40 and 100 km. The distances that can be covered depend on the transmitter output power, on the frequency band used, on atmospheric conditions, on the topography and on obstacles, which may impede signal propagation or cause signal reflections. Signal reflections may lead to multipath propagation which causes a certain additional attenuation or signal extinction when the direct wave and the reflected wave are opposite in phase at the receiver. Waves reflected by the ionized part of the atmosphere or by a changing refractive index due to temperature or humidity variations have the same effect, but since the degree of reflection is subject to a random process, the received signal varies statistically with time. This phenomenon is called “fading” as the received signal can fade or disappear on a statistical basis. 1

TDMA and CDMA are general media access technologies that are for example typically used in point-to-multipoint radio systems where many users share a common frequency band.

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These drawbacks can be overcome by careful link planning, by the positioning of intermediate repeaters, by the selection of the transmitted power and by the antenna design. Diversity is commonly used to improve the availability of microwave links by adding some degree of redundancy. Space diversity is obtained if different antennas are located in different positions on the antenna tower. Frequency diversity is used when the same signal is transmitted using different frequencies. At the receiving side the signals coming from different antennas are combined to achieve the best possible signal at the output, permitting to limit the outage time of the link during the worst period of the year. 4.3.3.2 Single channel radio Unlicensed radio systems normally constitute single-channel point-to-multipoint short-haul links that can be set up on the fly at moderate cost. Single-channel point-to-multipoint microwave radio links have been used at MV level with less stringent demand on signal transfer times. Access to the common medium - i.e. the shared radio frequency band - is accomplished by means of TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access) to prevent mutual interference between transmitters operating simultaneously, see also 4.4.1.3. Successful operation of such systems has been reported from South Africa. Both intertripping as well as differential protection signals are transferred over point-to-point TDMA-based singlechannel radio links between outstations, with typical signal propagation delays (outstation to outstation) in the range of 14ms to 22ms1 at data rates of 19.2kbit/s and 64kbit/s. The use of unlicensed spread spectrum radio for the transmission of state indication in a state comparison scheme for a 138 kV line has been reported from the USA. Summary Advantages and disadvantages of microwave radio links as related to protection signal transmission are: Advantages

1

Disadvantages

•

Wideband medium, with scalable capacity (number of channels)

•

Little interference from the primary system

•

No earth potential rise problems when the transmitter and receiver are situated within the station earth network

•

Fast setting up, especially when towers are existing or when roof top installations are possible, or when unlicensed radio systems can be used

•

Frequency bands constitute a limited resource and may not be available as desired

•

Influence of atmospheric conditions such as rain, fog, snowfall, sandstorms. Unless a high signal margin is provided, the link may be temporarily lost due to fading

•

Correlation / coincidence between poor weather conditions, line faults and poor link performance exists

•

Problem of getting line-of-sight both for single-hop and multi-hop links

•

Multiple hops introduce extra cost, reduce reliability and cause additional signal transfer delays

Caution: It is most important to mention that the “upstream” and “downstream” time delays between masterstation and outstation are different with certain types of TDMA equipment – this can lead to difficulties with differential protection systems: An additional “dummy” outstation may have to be installed at the master-station site for the sole purpose of equalizing the go and return time delays!

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Table 4.3-3:

•

Microwave antenna towers are subject to lightning strokes

•

Potential barrier problems when the transmitter and receiver are located outside the station earth network

•

Many channels are lost when a high capacity microwave link fails

•

Unequal upstream and downstream signal propagation delays of certain TDMA or CDMA radio systems may cause serious problems for differential current protection relays

Advantages and disadvantages of radio links

4.3.4 Optical fibres The deployment of optical fibres for signal transmission started in the seventies with a few shorthaul links and has made tremendous progress since then regarding fibre and terminal equipment technology. The unique advantage of optical fibres is their immunity to electromagnetic interference, their isolating quality and their extremely wide bandwidth, all making the introduction of optical fibre links very attractive for electric power utilities. Optical fibres are normally used in pairs, i.e. one fibre is used for transmitting and one for receiving. Communication over one fibre in both directions is technically possible, for example using time-shared multiplexing or wavelength division multiplexing (WDM, see chapter 4.4.1.1) techniques. It has however been rarely used for long distance telecommunication systems so far. A number of optical fibres (10 … 50 … 100) are normally embedded in an underground or aerial cable. The immunity against electromagnetic disturbance allows installing fibre-optic cables along the same route as power cables. They may also be integrated into power cables or ground wires of HV power lines. The latter design which is called OPGW (OPtical Ground Wire) is preferably used by electric power utilities. Other popular techniques are the mounting of ADSS (All Dielectric Self-Supporting) Cables along the towers, or the Helical Wrapping of a fiberoptic cable around the ground wire or phase wire, which may be advantageous for refurbishing existing lines. Lashed aerial cable techniques are also used whereby an alldielectric cable is lashed to a messenger (e.g. earth wire) by means of a tape or cords. In all cases the mechanical strength of the towers has to be examined regarding the additional load introduced by the optical cables, especially when extra loads due to snow and ice are to be expected. Care must be taken with ADSS and Helical Wrap cables to avoid surface erosion caused by dry-band arcing in high field strength locations. For long distance links, Multimode Step-Index fibres and multimode Graded Index fibres are of historical interest only. They have been almost completely superseded by Single Mode fibres which provide a very large bandwidth over a long distance. The transmission properties of optical fibres are characterised by their attenuation per unit length (dB/km) and by their chromatic dispersion (ps/nm∗km). Chromatic dispersion means that

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lightwaves of different wavelengths (= "colours") propagate with different velocities. An injected light impulse, which is for technical and physical reasons composed of several wavelengths, thus tends to “broaden” when it propagates. The impulse broadening limits the useful bandwidth of the link because the individual impulses can no longer be discriminated by the receiver when they overlap significantly. Using laser emitters with a narrow emission spectrum is therefore mandatory for long-haul high bitrate links. At 1300 nm optical fibres naturally exhibit minimum dispersion which introduces minimum pulse distortion at high data rates. At 1550 nm the attenuation is lowest, however the dispersion is higher than at 1300 nm. Special fibre designs called Dispersion Shifted Fibres minimize the chromatic dispersion at 1550 nm, however at the expense of a higher attenuation due to mechanical stress combined with certain penalties when used in WDM systems [12]. The maximum length of an optical fibre link may therefore be either attenuation limited or dispersion (bandwidth) limited which is an important system planning issue for high capacity long-haul links. Very long distances can be overcome by means of optical boosters and amplifiers which inject more light power into the fibre at the transmit side and amplify the received signal on an optical basis at the receiving end. Laser Diodes (LD) or Light Emitting Diodes (LED) may be used as optical transmitters. Laser diodes are required for long repeater spans (up to about 100 to 200 kilometres, depending on the bit rate) and high bitrates (up to some Gigabits per second), whereas LEDs are cost efficient for shorter distances and lower data rates. The optical power injected by a LD into a single mode fibre is in the order of 1 Milliwatt, that of an LED is around 10 to 20 Microwatts. The emitted wavelength of both LDs and LEDs is in the infrared range at either around 850nm, 1300 nm or 1550 nm. Special optical transmit- and receive devices such as Optical Boosters and Erbium Doped Fibre Amplifiers (EDFA) may be used for bridging extra long distances of several hundred kilometers without intermediate repeater stations. Wavelength Division Multiplexing (WDM) may be used to further exploit the huge transmission capacity of optical fibres, or simply to use the same fibre for different communication systems by “stacking” their optical transmitters onto the same fibre, each transmitter using a different wavelength. More on the subject is found in chapter 4.4.1.1 and in [12]. As applied to protection signal transmission, either dedicated optical fibres from relay to relay may be used, or the protection signal may be electrically or optically multiplexed with other services, as shown in ANNEX A1, Figures A1-2 and A1-3. Whilst the installation of dedicated optical fibre cables for the transmission of protection information would match the traditional point-to-point approach and guarantee minimum signal transfer delays, it might not be easily justified for cost reasons. However, the use of dedicated fibres is facilitated when the incremental cost of extra fibres in a cable are low, or when “spare” fibres can be used. A more economical means to achieve a certain isolation of the protection from other services and/or systems is to perform the multiplexing at the optical level using WDM (Wavelength Division Multiplexing), where only the optical fibre but not the terminal equipment is shared between individual systems. More on the subject of WDM is found in chapter 4.4.1.1 and in [12]. Fibre optic communication systems are - with very few exceptions - realised as digital systems. Since the optical fibre represents a wideband medium, a large number of channels and services are usually combined into an aggregate by some form of time-division-multiplexing (TDM, see Chapter 4.4.1.2). The aggregate digital bitstream finally modulates the optical transmitter (Laser diode or LED) by switching it on and off in accordance with the data to be transmitted.

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Summary Advantages and disadvantages of optical fibre links as related to protection signal transmission are: Advantages

Disadvantages

•

Wideband medium, supports extremely high data rates

•

Many channels are lost when a high capacity fiberoptic link fails

•

Immune against electromagnetic interference from the primary system (at the optical level)

•

Repair is difficult when fibres are integrated into high voltage cables or OPGWs

•

Immune against athmospheric interference

•

•

Perfect electrical isolation between link ends and between high-voltage euipment and telecom equipment

High installation cost when only moderate data rates are needed.

•

•

No crosstalk between fibres

Dedicated fibres for protection signal transmission may not be justified for cost reasons

•

Normally extraordinarily low bit error rate

•

•

No earth potential rise problems

For long distances (> 200km) repeater stations have to be used

•

Little influence by atmospheric conditions

•

Fairly long repeaterless distances possible (…..200km)

Table 4.3-4:

Advantages and disadvantages of optical fibre links

4.3.5 Satellites The race for satellite communication has been on ever since the announcement of pocket-sized ground terminals to provide a truly global mobile telephone service. At present, there are many different satellite systems that have been proposed to complement terrestrial communication networks, all at varying developmental stages. Narrowband satellite systems which carry many voice or low speed data channels - up to 9'600 bits per second - are more advanced in terms of development than wideband systems supporting SDH and ATM (see Chapters 4.4.2.2, 4.5.1 and 4.6.2.2 on SDH and ATM). The reason is mainly due to new or more acute issues related to creating broadband satellite links with QoS (Quality of Service) guarantees (ANNEX A3). Projects have been launched worldwide to investigate the integration of terrestrial wideband networks with satellite networks. Satellites are usually classified according to the type of orbit they are in. 4.3.5.1 GEO - Geosynchronous Earth Orbit Satellites GEO satellites are placed in the orbit such that their period of rotation exactly matches the Earth’s rotation, i.e. they appear stationary from earth. Earth station antennas do therefore not need to move once they have been properly aimed at a target satellite in the sky. Today, the majority of satellites in orbit around the earth are positioned in GEO at 36’000 km orbital height. It is at the precise distance of 36’000 km that a satellite can maintain an orbit with a period of rotation exactly equal to 24 hours. Due to the long distance of 36’000 km GEO satellites experience long up-down signal propagation delays of about 250 ... 280 ms which normally excludes them from being used as a communication medium for protection signal transmission, with perhaps few exceptions for wide-area protection applications.

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4.3.5.2 MEO - Medium Earth Orbit Satellites Technological innovations in space communications have led to new satellite system designs over the past few years. MEO satellite systems have been proposed that will orbit at distances of about 10’350 km. The lower distance as compared to GEO systems means improved signal strength at the receiving antenna, which allows for smaller receiving terminals. The lower distance also translates into less signal transmission delay of about 120 ms which leads to a significant performance improvement for certain real-time applications such as voice communication. As applied to protection signal transmission the delay appears however still unacceptable for most applications, with perhaps the exception of wide-area protection where requirements on signal transfer times to remote locations that are distributed over a geographically widespread area may be less stringent. Another problem is that signal interruptions of approximately 25 milliseconds duration are expected about every 2 hours when the signal is switched from one satellite to the next: As the satellite descends towards the horizon, the traffic being serviced by that satellite must be handed over to the satellite just ascending from the opposing horizon. 4.3.5.3 LEO - Low Earth Orbit Satellites Proposed LEO satellite systems are divided into three categories: Little LEOs operating in the 800 MHz range, big LEOs operating in the 2 GHz or above range, and mega LEOs operating in the 20 - 30 GHz range. The higher frequencies associated with mega LEOs translate into more communication capacity and better performance for real-time applications. Present systems support moderate data rates of up to 9’600 bit/s yet, with much higher data rates being targeted for the near future. The orbital distance of LEO satellite systems is between 750 and 1500 km, giving rise to signal up-down propagation delays of about 20 to 30 ms. As applied to protection signal transmission the delay introduced by a single LEO satellite updown link may be acceptable for certain protection applications, provided that the extra delay possibly introduced by relaying the signal between satellites plus the delay introduced by the terrestrial section can be kept sufficiently low. It is noted that signal interruptions of 3 to 9 milliseconds duration are expected about every 8 to 12 minutes when the signal is switched from one satellite to the next (roaming): as the satellite descends towards the horizon, the traffic being serviced by that satellite must be handed over to the satellite just ascending from the opposing horizon. Moreover, the signal propagation delay variation as the signals are routed dynamically from satellite to satellite before reaching the terrestrial destination will require further detailed investigation, before MEO and LEO satellite channels may eventually be used for conveying protection signals. LEO satellite systems may eventually become a communication alternative for certain protection applications when signal transfer delay and reliability requirements are not very demanding. Little experience seems to exist today in this area. There are still many open research issues that need to be addressed before such systems can be used.

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Summary Advantages and disadvantages of satellite links as related to protection signal transmission are: Advantages

Disadvantages

•

Coverage of geographically widely spread areas

•

Easy and fast deployment of ground terminal stations

•

Electrical isolation between terminals

Table 4.3-5:

•

High signal propagation delay

•

Availability and reliability may not be adequate for protection

•

Subject to adverse atmospheric influence, including lightning strokes and snow and ice covering satellite dishes

•

prohibitive costs for permanent connections and/or high bandwidth

Advantages and disadvantages of satellite links

4.4 MULTIPLEXING TECHNIQUES AND DIGITAL HIERARCHIES 4.4.1 Multiplexing Techniques Because of the installation cost of telecommunication systems, such as microwave radio or optical fibre links, it is desirable to share the communication medium among multiple users or multiple services. Multiplexing is the sharing of a communications medium through local combining of signals at a common point. Multiplexing is thus a technique that is used to transmit two or more signals over a shared medium. The reverse action of extracting the individual signals from the aggregate at the receiving end is called demultiplexing. Three basic types of multiplexing are commonly employed: frequency-division multiplexing (FDM), time-division multiplexing (TDM) and code-division multiplexing (CDM).1 As there is a certain - although low - risk of accidental channel cross-over in multiplexed systems, it is recommended that precautions are taken at the teleprotection side to prevent unwanted operation of the protection. Robust synchronization procedures and/or terminal equipment addressing2 may be used. The benefits of measures for improving the security have however to be carefully balanced against their adverse influence on dependability. 4.4.1.1 Frequency Division Multiplexing (FDM) With FDM, multiple channels or multiple services are combined onto a single aggregate by frequency translating, or modulating, each of the individual signals onto a different carrier frequency for transmission. The individual channels are thus separated in the aggregate by their frequencies, i.e. each channel has its dedicated frequency slot. At the receiving end, the reverse action of extracting the individual signals is accomplished by filtering. While each user's 1

Note on 'Multiplexing' and 'Multiple Access': Both techniques deal with the sharing of a communication channel or a transmission medium among communication users. The term 'multiplexing' is relevant for the sharing of a communication channel or medium through the local combining of signals at a common point (signal aggregation or signal concentration). The three main multiplexing techniques are FDM, TDM, CDM. 'Multiple access' deals with the sharing of a common medium among terminal stations that are located at physically different locations by mastering the medium access procedures. Similar to multiplexing, the three multiple access technologies are FDMA, TDMA and CDMA respectively, which are widely used in radio communications.

2

Terminal addressing will also protect against protection maloperation when signals are (inadvertently) looped back for testing or maintenance reasons.

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information signal may be either analog or digital, the combined FDM signal is inherently an analog waveform. FDM is therefore primarily used with analogue transmission systems. Wavelength Division Multiplexing (WDM) With optical fibre systems, a special form of FDM called WDM (Wavelength Division Multiplexing) is increasingly being introduced to further exploit the huge capacity of optical fibres. Several transmission systems, each using a different wavelength or 'colour', may be stacked onto the same fibre using WDM. In its simplest form, WDM uses different optical windows for the multiplexing, e.g. the windows centred around 1300 nm and 1550 nm wavelength. More sophisticated systems multiplex a number of optical channels (e.g. 4, 16, 32 or 64) within the same optical window centred around 1550 nm wavelength. As the spacing between the different wavelengths becomes very narrow in this case, the technology is called Dense Wavelength Division Multiplexing (DWDM). An in-depth treatment of WDM technology is found in [12]. As WDM actually creates 'virtual fibres' it may also be employed for the de-coupling of transmission systems from each other. Dedicated teleprotection links that operate quasi-isolated from other telecom services could be realized using WDM for example: In Figure 4.4-1, system 1 consists of a protection relay with internal or external teleprotection function plus a fibre-optic transmitter/receiver operating at wavelength λ1. System 2 could be any other fibre-optic communication system operating at wavelength λ2 and carrying other services such as data and voice. A failure or maloperation of System 2 should not adversely affect System 1, as the only common parts of the two systems are the optical fibre and the passive optical wave-division multiplexer / demultiplexer. Although the isolation of the teleprotection from other services by means of WDM appears attractive from an operational point of view, it may not be easily justified for cost reasons.

System 1

λ1

λ1 System 1

λ1+λ2

System 2

Figure 4.4-1:

λ2

Optical fibre

λ2

WDM

WDM

System 2

Principle of Wavelength Division Multiplexing for 2 wavelengths, 1

4.4.1.2 Time Division Multiplexing (TDM) Multiplexing may also be conducted through the interleaving of time segments from different signals onto a single shared transmission path. With TDM, multiple channels thus share the common aggregate based on time. While TDM may be applied to either analog or digital signals, in practice it is applied almost always to digital signals. The digital signals may be interleaved bit-by-bit (bit interleaving), byte-by-byte (byte interleaving) or cell-based where data is broken up into “cells” consisting of a number of bytes. 1

Figure 4.4-1 shows a simplex (i.e. unidirectional) communication for simplicity reasons. Full-duplex (i.e. bi-directional) operation would require either a second fibre, or a 3rd and a 4th wavelength (λ3 and λ4 respectively) on the same fibre.

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Most modern telecommunication systems employ some form of TDM for transmission over longdistance routes. The multiplexed signal may be sent 'directly' (called 'baseband' transmission) over optical fibres, or it may be modulated onto a carrier signal for transmission over analogue media, such as microwave radio or coaxial cables for example. TDM can be split into various subclasses. The most important are Fixed TDM and Statistical TDM. Fixed TDM In fixed TDM - sometimes also called synchronous TDM - each channel has its assigned timeslot which sustains a fixed data rate and uses aggregate bandwidth irrespective of actual user data being transmitted or not. The number of channels is normally equal to the number of timeslots in a frame. Due to the fixed allocation of channels and timeslots, data can always be transmitted. Buffering and flow control are not required. Continuous data flow at a fixed bit rate without delay variations is ensured, a condition which is a prerequisite for protection signal transmission. Statistical TDM Statistical - sometimes also called asynchronous TDM - multiplexers rely on the ‘bursty’ traffic characteristics of certain information sources. Data may be transmitted in any timeslot as long as there are free slots available. Relying on the statistics of the data, the number of channels or the peak data rate which is supported by the statistical multiplexer may be larger than the total number of timeslots or the aggregate data rate in a frame. Data buffering and flow control is employed to store and withhold data until a free timeslot or free cells become available. Buffering and flow control introduce extra delay as well as delay variations, and data may be discarded in case of overload. Loss of information is normally not acceptable for protection signal transmission. Statistical multiplexing has therefore to be avoided unless the required quality of service is explicitly guaranteed. A multiplexing technology which was originally intimately bound up with the emerging SDH (Synchronous Digital Hierarchy, see 4.4.2.2) standards is ATM (Asynchronous Transfer Mode, see also 4.5.2.3 and 4.6.2.2) which was conceived as a way in which arbitrary-bandwidth communication channels could be provided within a multiplexing hierarchy consisting of a defined set of fixed bandwidth channels. ATM multiplexers support both constant bit rate (CBR) and variable bit rate (VBR) traffic, where CBR which basically emulates fixed TDM is a prerequisite for today’s protection systems using telecommunication. 4.4.1.3 Code Division Multiplexing (CDM) In CDM, several signals share a common medium (copper wires or radiowaves) using the same frequency band simultaneously. Multiplexing of different channels is achieved by utilizing different pseudorandom binary sequence codes that modulate a carrier. The process of modulating the signal by the code sequence causes the power of the transmitted signal to be spread over a larger bandwidth. Systems based on CDM are therefore sometimes also referred to as 'Spread Spectrum' (SS) systems. The spreading of the spectrum enhances the noise immunity of such systems. CDM and in particular CDMA (code division multiple access) is mainly used with unlicensed spread spectrum radio where many simultaneous users have to share the same frequency band. CDM/SS techniques may also be used with wire-based systems to enhance the transmission capacity and noise immunity. Its application for inter-substation communication would however need to be further examined with respect to cost efficiency and transmission performance.

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The use of unlicensed single channel CDMA/SS radio for the transmission of protection commands for a 138 kV line has been reported from the USA. However, no practical installations of CDM/SS technology for the transmission of protection signals over copper wires have been reported. A final conclusion on CDM is not possible at the time of writing.

4.4.2 Digital Hierarchies Digital transport systems form the backbone of modern telecommunication networks or widearea networks (WAN). As the demand for information transmission increased and levels of traffic grew higher it became evident that larger number of channels need to be bundled in order to avoid having to use excessively large number of individual physical links. Thus, it was necessary to define further levels of multiplexing which are structured in Digital Hierarchies. 4.4.2.1 PDH - Plesiochronous Digital Hierarchy Multiplexing structure Digital telecommunication systems have historically been based on the plesiochronous digital hierarchy (PDH). PDH systems accommodate "almost synchronous" channels in multiples of 64 kbit/s. The base rate of 64 kbit/s represents the digital equivalent of an analogue telephone channel using traditional, uncompressed PCM speech coding techniques. The PDH hierarchy levels and transmission rates are given in Table 4.4-1 below. Hierarchical level

Europe

North America

Japan

0

64 kbit/s

64 kbit/s1

64 kbit/s1

1

2’048 kbit/s

1’544 kbit/s

1’544 kbit/s

2

8’448 kbit/s

6’312 kbit/s

6’312 kbit/s

3

34’368 kbit/s

44’736 kbit/s

32’064 kbit/s

4

139’264 kbit/s

139’264 kbit/s

97’728 kbit/s

Table 4.4-1:

PDH - Plesiochronous Digital Hierarchy levels

When multiplexing a number of digital signals with the same nominal bitrate they are likely to have been created by different pieces of equipment each generating a slightly different bitrate due to their independent internal clocks. A technique called “bit stuffing” is used for bringing the individual signals up to the same rate prior to multiplexing. Dummy bits or justification bits are inserted at the transmit side and discarded by the demultiplexer at the receiving end, leaving the original signal. The same problem with rate alignment occurs at every level of the multiplexing hierarchy. The process of multiplexing “almost synchronous” signals is called “plesiochronous”, from Greek. The use of plesiochronous operation throughout the hierarchy has led to the adoption of the term “Plesiochronous Digital Hierarchy”. Plesiochronous operation does not allow extracting and inserting individual channels from the aggregate without prior demultiplexing and subsequent re-multiplexing, leaving towers of multiplexers. With the exception of vendor specific solutions, network management and performance monitoring throughout the hierarchy is not adequately supported with PDH systems either, as PDH systems have developed over time with insufficient provision for standardized management. These disadvantages have - amongst others - finally led to the definition of a new digital transmission hierarchy: the Synchronous Digital Hierarchy. 1

Some (legacy) systems may provide only 56 kbit/s to the user.

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4.4.2.2 SDH - Synchronous Digital Hierarchy Multiplexing structure The rapid growth of digital networks and the convergence of telephone and high-speed data networks have enforced the development of new standards, which would facilitate the deployment of complex networks with new services and comprehensive network management options. Proposals in ITU-T for a Broadband Integrated Services Digital Network (B-ISDN) opened the door for a new synchronous multiplexing standard that would better support switched broadband services. The new standard appeared first as SONET (Synchronous Optical Network) in the United States. Initially, the objective of the SONET standard was to establish a North American standard that would permit interworking of equipment from multiple vendors (1985 …1987). Subsequently, the ITU-T (former CCITT) was approached with the goal of migrating this proposal to a worldwide standard. Despite the considerable difficulties arising from the historical differences between the North American and European digital hierarchies, this goal was achieved with the adoption of the SDH (Synchronous Digital Hierarchy) standards (1988). In synchronous networks, all multiplexing functions operate synchronously using clocks derived from a common source. SDH embraces most of SONET and is an international standard, but is often mistakenly regarded an European standard, because most of its suppliers carry only the European PDH bit rates specified by ETSI (European Telecommunication Standards Institute). While there are commonalities between SDH and SONET, particularly at the higher rates, there are significant differences at the lower multiplexing levels, in order to accommodate the requirement of interworking the differing regional digital hierarchies. Through an appropriate choice of options, a subset of SDH is compatible with a subset of SONET; therefore, traffic interworking is possible. Interworking for alarms and performance management is however generally not possible between SDH and SONET systems. The ITU-T recommendations define a number of basic transmission rates within the SDH and SONET, see table below, with further levels proposed for study. SDH Synchronous Transport Module level

STM-1 STM-4

SONET Aggregate Rate

Optical Carrier level

Synchronous Transport Signal level

Aggregate Rate

Max. number of simultaneous voice channels (informative)

OC-1

STS-1

51.840 Mbit/s

783

155.520 Mbit/s

OC-3

STS-3

155.520 Mbit/s

2'349

622.080 Mbit/s

OC-12

STS-12

622.080 Mbit/s

9'396

STM-16

2’488.320 Mbit/s

OC-48

STS-48

2’488.320 Mbit/s

37'584

STM-64

9’953.280 Mbit/s

OC-192

STS-192

9’953.280 Mbit/s

150'336

Table 4.4-2:

SDH - Synchronous Digital Hierarchy levels

The recommendations also define a multiplexing structure whereby an STM-N (Synchronous Transport Module level N) or STS-N (Synchronous Transport Signal level N) aggregate can carry a number of lower bitrate signals as payload, in order to facilitate the transport of legacy PDH tributaries. SDH / SONET are expected to dominate transmission for decades to come, as the multiplexing structure has been designed to carry not only current services but also emerging ones using ATM and/or IP framing structures for example.

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Although SONET and SDH were conceived originally for optical fibre transmission, SDH radio systems exist at rates compatible with both SONET and SDH. SDH/SONET network topologies and network resilience A synchronous network will be more reliable than PDH due to both the increased reliability of individual elements, and the more resilient structure of the whole network. SDH will allow development of network topologies which will be able to achieve 'network protection', that is to survive failures in the network by reconfiguring and maintaining service by alternate means. Network protection can be accomplished by the use of cross-connect functionality to achieve restoration, or through the use of self-healing ring architectures. Two main types of synchronous ring architectures have been defined: - The Dedicated Protection Ring - This is a dedicated path switched ring which sends traffic both ways around the ring, and uses a protection switch mechanism to select the alternate signal at the receive end upon failure detection. - The Shared Protection Ring - This is a shared switched ring which is able to provide 'shared' protection capacity which is reserved all the way around the ring. In the event of a failure, protection switches operate on both sides of the failure to route traffic through the reserved spare capacity. The ability to share protection capacity in shared protection rings can in many instances offer a significant capacity advantage over dedicated protection rings. This means, in economic terms, less equipment, lower cost and less operation efforts. However, this is at the expense of a slower restoration time than a path switched ring. Protection switching in a ring topology can be either "uni-directional" or "bi-directional". Unidirectional means that only the faulted path is reverted along the ring by selecting the healthy fibre at the receiving end, whilst the non-faulted path follows the original route. With bi-directional protection both the go and return path are switched to follow the opposite direction along the ring. It is noted that only bi-directional protection will maintain equal signal propagation delays for the go and return path, whilst uni-directional protection may introduce unequal propagation delays that may cause severe difficulties for current differential protection relays! The synchronous ring structure, with its inherent resilience, is a powerful building block from which survivable networks can be built: A typical power system control network has a radial (star) topology, with point-to-point links connecting a central control station with associated substations. SDH/SONET network implementations may connect the substations in rings. The logical star connection is achieved by configuring the channels within the SDH/SONET network in order to provide the required logical point-to-point links. In case of a path interruption, signal flow may be reversed along the ring such that communication is sustained. More about SDH network design and -operation is found in chapter 4.6.2.1 of this document. SDH/SONET for power system protection Since SDH/SONET networks provide a set of fixed bandwidth channels with a deterministic transmission characteristic, they are well suited for applications that rely on the transmission of a sustained fixed data rate and short signal transfer times, as needed by differential current protection for example. As SDH/SONET signals follow a fixed physical path through the network, SDH/SONET channels will exhibit a fixed transmission delay with low delay variations

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or "jitter" unless paths are re-routed automatically or manually due to network failures. Transmit and receive directions may however still experience different signal propagation times when the physical route does not follow the same path. Provisions to accommodate the non-equal signal transfer times have to be built into the protection relay in this case, see also Chapter 6.3.1.1. In conclusion, transport networks based on SDH/SONET technology can be designed to meet the stringent requirements of legacy and future protection systems regarding signal transfer times and error characteristics. Propagation velocity of the light pulses in optical fibres is around 200 km/ms, signal transfer delays between ports of an SDH/SONET node are typically well below 1ms, and networks are designed to produce very low error rates ( 1ms

No data available

No data available >> 1ms

No data available

< 1ms if the same route < 1ms if the same route

< 1ms

Propagation time symmetry (Differential delay) < 1ms if the same route

No data available

64kbit/s

> 64kbit/s

< 10-5

< 10-5

Cell misinsertion ratio (CMR) < 1/day < 10-6 (ITU-T I.356)

< 10-6

> 64kbit/s

> 64kbit/s

> 64kbit/s

< 10

< 10-3

No data available No data available

No data available

No data available

> 64kbit/s

> 64kbit/s

< 10-6

> 64kbit/s

> 64kbit/s

few kHz;