1-Power System Protective Relaying-Part One

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Power System Protective Relaying

Introduction The purpose equipment is to 

of

network

protection

min inim imiz izee th thee ef effe fect ctss of fau faults lts,, wh whic ich h ca can n never be entirely avoided, on an electrical powerr syst powe system em..

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Power Systems-Fault Free Not Possible  Many Reasons 

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Power system Protection Engineering is an imp important ortant component component of the electrical electrical plant system and of decisive significance for the reliable and safe operation of a power pow er sys syste tem m.

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Power Systems-Fault Free Not Possible  Many Reasons 

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Power system Protection Engineering is an imp important ortant component component of the electrical electrical plant system and of decisive significance for the reliable and safe operation of a power pow er sys syste tem m.

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The cause of electric power system faults is insulat insulation ion break breakdown down

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This breakdown can be due to a variety of  differen diffe rentt facto factors: rs:

– Lightning. – wire wiress blow blowing ing toge togethe therr in the the wind. wind. – animals or plants coming in contact with the wires. – salt spr spray ay or pollu pollution tion on insulat insulators. ors. 5

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Tree limbs falling on the line

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Insulation deterioration

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Vandalism

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Nature Cause

Introduction 

Equipment Failure

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Substation Failure

The Aftermath

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Results of Transformer Fire

Generator Fault

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Expensive Consequences for Protection Failure

Introduction 

Human Error

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Introduction 

Relay:an electric device that is designed to respond to input conditions in a prescribed manner and , after specified conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits. (IEEE)

Introduction 

Protective Relay:A relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action. (IEEE)

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What is Relaying  Detects abnormal conditions  Initiate Corrective actions  Has fast response  Disrupts only essential and minimum area

Requirements  Define the undesirable conditions  Remove only the faulted equipment and to maintain the un-faulted portion of the system.

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Power System Protection  “Science, skill and art of applying and setting relays &fuses to maximum sensitivity to detect disturbances and to avoid the operation on all permissible tolerable conditions”

Define and understand Power System faults and Monitor system  Measure parameters V, I, f through transducers( Current, potential transformers)  Detection schemes Relays  Action Equipment Circuit Breakers 

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Effects of faults on power system 

Flow of excessive current

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Abnormal voltages

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Voltage elevation of system neutral

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Induce over voltages on neighbouring equipments .

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Hazards animals.

to

human,

equipment

and

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Need for fault analysis 

Design of protection system requires the knowledge of fault current.

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The information obtained from the fault studies are used: to select the sizes of circuit breaker, fuse and characteristic, setting of relays.

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Types of fault Symmetrical fault : 

Usually three phase to ground fault

Unsymmetrical (Asymmetrical) fault 

The fault is unbalanced in nature

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Sources of Asymmetrical fault are 

One phase open circuit

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Unbalanced in load mainly the arc loads

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One phase open circuit

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SLG fault

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Sources of Asymmetrical fault are:

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Typical Protective Relays

Portable Protective Relay Test Equipment

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Relays A relay is a low-powered device used to activate a high-powered device. are used to trigger circuit  Relays breakers and other switches in substations and transmission and distribution systems. 

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Relay Characteristics Reliability  Probability to function properly. ability to operate for  The disturbances within zone (dependability)  The ability to refrain from operation for external abnormalities( security)

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Sensitivity  Ability to detect abnormal quantities

Selectivity to obtain the desired  Ability operation to avoid unnecessary tripping Speed to initiate operation in the  Ability shortest time possible

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Economy  The ability to function at minimum cost Simplicity function and to operate  Ability to with minimum equipment and circuitry

Substation control panel relays 36

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Relay and control panel 37

High Voltage Fuses High voltage fuses are used to protect the electrical system in a substation from power transformer faults.  They are switched for maintenance and safety. 

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High voltage fuses in a switch box 39

External switch for high voltage fuses 40

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Typical Power Circuit Breakers

Air circuit breaker 42

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Vacuum circuit breaker, inside 43

Vacuum circuit breaker, outside 44

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Oil circuit breakers in a 41 kV circuit 45

Oil circuit breakers in a distribution

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SF6 gas power circuit breaker 47

SF6 gas power circuit breaker 48

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Typical Power Circuit Breakers

Sample Device Numbers         

Master element: 1 Time-delay starting or closing relay: 2 Distance relay: 21 Directional power relay: 32 Instantaneous overcurrent relay: 50 AC time overcurrent relay: 51 AC directional overcurrent relay: 67 Frequency relay: 81 Differential protective relay: 87

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IEEE Device Numbers & Functions

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Device Description Legend 49 Thermal 50/51 Instantaneous & Time Overcurrent 51 Time Overcurrent 51N-1 Ground Time Overcurrent 51N-2 Neutral Time Overcurrent 51N-3 Ground Time Overcurrent 63 Sudden Pressure 67 Directional Overcurrent 67N Directional Ground Overcurrent 86 Lockout Auxiliary 87T Phase Differential, 3 Phase 87N Ground Differential OP Operating Coil Pol Polarizing Coil N.C. Normally Closed Transformer Bushing CS Circuit Switcher Rg Grounding Resistor 52

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Typical Relay and Circuit Breaker Connections 

Typical single line AC connection

Typical Relay and Circuit Breaker Connections 

Typical three-phase AC connection

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Basic Objectives of System Protection Reliability  Selectivity  Speed of Operation  Simplicity  Economics 

Factors Affecting the Protection System Economics  Personality  Location of Disconnecting and Input Devices  Available Fault Indicators 

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Philosophy of Protection 

Both an art and a science

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Well-designed system will use: – Overlapping primary zones of protection – Some form of backup protection

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Qualities Required of Protection  Selectivity (Discrimination): Effectiveness in isolating only the faulty part of the system. The property of remaining  Stability: inoperative with fault occurring out side the protected zone (called external faults).

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Speed of operation:  The time between the incidence of a fault and the trip command being issued to the circuit breaker by the protection.  The speed of operation must be as fast as possible. modern power systems the relay  In operates in one or fraction of a period of  the power system frequency.

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Sensitivity: The level of magnitude of  the fault current at which the operation of protective device occurs.  Reliability: The ability of a protective system to fulfill its purpose throughout its operation life.  It should not mal-operate at any time. 

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 Economic of consideration: In distribution system it is very important and it overrides the technical consideration provided basic safety requirements are met. transmission systems the technical  In aspects are more important. 

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Zones of protection Transf  Zone

Gen Zone

Line 1 Zone

Bus 1 Zone

  e   n   o    Z    2   s   u    B

Line 3 Zone

Line 2 Zone

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Overlapping zones Zone A

CB Zone B CT B CT A

Trip all zone A

Trip all zone B

– Zones A and B overlap at circuit breaker CB • Faults in overlap trip both zones • No gaps in protection

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Classification of Relays 

Protective Relays

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Regulating Relays Reclosing, Synchronism Check, and Synchronizing Relays Monitoring Relays

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Auxiliary Relays

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Others

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Protective Relay Performance 

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Since many relays near the trouble area may begin to operate for any given fault, it is difficult to completely evaluate an individual relay’s performance. Performance can be categorized as follows: Correct: (a) As planned or (b) Not as planned or expected.  –  Incorrect: (a) Fail to trip or (b) False tripping  –  No conclusion  – 

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Principles of Relay Application 

The power system is divided into protection zones defined by the equipment and available circuit breakers. Six possible protection zones are listed below:  –   –   –   –   –   – 

Generators and generator-transformer units Transformers Buses Lines (Transmission, subtransmission, and distribution) Utilization equipment Capacitor or reactor banks

Principles of Relay Application 

Typical relay primary protection zones

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Principles of Relay Application 

Overlapping protection zones

Information for Application 

One line diagram and system configuration

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Impedance and connection of the power equipment, system frequency, system voltage, and system phase sequence

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Existing protection and problems Operating procedure and Practices

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Importance of the system equipment being protected

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