Protective Relaying - An Overview

PROTECTIVE RELAYING Principles & Philosophies FORTUNATO C. LEYNES, FIIEE Chairman

Board of Electrical E ngineerin gineeri ng Professional Professional Regulatio Regulation n Commissi Commission on Vice President 

Manila Manila Electric Electric Company Compa ny 15th IIEE IIEE Region Re gion 8 Conference June June 26, 2010 2 010

Protectiv Prot ective e Relaying

The branch of electric power engineering concerned with wi th the princi prin cipl ples es of design, design, construction/ installation, instal lation, operation operation an and d mai maintenance ntenance of  equipment (called “relays or protective relays”) which detect abnormal abnormal power power system conditions, and initiate correctiv corrective e action action as quickly qu ickly as possible possible in order to return the power system to its normal state.

 Power System System Protec Protection tion - Me Meralc ralco o

Protectiv Prot ective e Relaying

The branch of electric power engineering concerned with wi th the princi prin cipl ples es of design, design, construction/ installation, instal lation, operation operation an and d mai maintenance ntenance of  equipment (called “relays or protective relays”) which detect abnormal abnormal power power system conditions, and initiate correctiv corrective e action action as quickly qu ickly as possible possible in order to return the power system to its normal state.

 Power System System Protec Protection tion - Me Meralc ralco o

ARE PROTECTIVE PRO TECTIVE RELAYING PRACTICES BASED ON THE PROBABILITY OF FAILURE •

•

• • •

protectiv protecti ve relay relayin ing g practi practices ces are base based d on on th the e proba probabil bility ity of  failure to the extent that present-day practices are the result of years of experience in which the frequency of failure undoubtedly has played a part; the th e proba probabil biliity of fail failure, seldo seldom m if ev ever, enters enters directl directly y into into the choice of a particular type of relaying equipment except when, for one reason or another, one finds it most difficult to apply the type that otherwise would be used; more im impo portan rtantl tly, y, the the proba probabi bilility ty of fai failu lure re shoul should d be considered only together with the consequences of failure should it occur; the th e justi justific ficati ation on for for a giv given practi practice ce equ equal als s the the likelih likelihoo ood d of  trouble times the cost of the trouble; regardl reg ardless ess of the the proba probabi billity of failu failure, re, no no porti portion on of a system should be entirely without protection, even if it is only back-up relaying.  Power System System Protec Protection tion - Me Meralc ralco o

EVALUATION OF PROTECTIVE RELAYING • the co cos st of of re repa paiiri ring ng the the da dama mage ge.. • the like likeliho lihoo od that that the tro trou uble may may spre sprea ad and and involve other equipment. • the time time that the equipme equipment nt is out out of se serv rvic ice e. • the los loss s in in rev revenue an and the the str stra aine ined d public relations while the equipment is out of service. By expedit expediting ing the equip equipmen ment’s t’s retur retu rn to service, protective relaying helps to minimize the amount of equipment reserve required, si since nce there is less l ess likelih likelihoo ood d of  another fail f ailure ure before b efore the the first f irst failure failure can be repaired.  Power System System Protec Protection tion - Me Meralc ralco o

PROTECTION SYSTEM OBJECTIVES 1. To remove the faulty device from the power system to prevent or minimize hazards to people, equipment damage, and adverse effect upon the normal operation of the remaining system. 2. To provide alternate means for removing the faulty device, for  the same reason as in 1, when there is a protective equipment failure such as a breaker or any primary protection. 3. Prevent operation of protective system for heavy load surges and power swings or other conditions that will not cause damage or adversely affect operation of the system. 4. Recognize when a catastrophic system failure is imminent or  has occurred and take necessary steps to minimize the disturbance and facilitate the speedy restoration to normal  Power System System Protec Protection tion - Me Meralc ralco o

FACTORS AFFECTING THE PROTECTION SYSTEM •

Economics

•

‘‘Personality’’ of the relay engineer and the characteristics of the power system

•

Location and availability of  disconnecting and isolating devices [circuit breakers, switches, and input devices (CTs and VTs)]

•

Available fault indicators (fault studies and such)  Power System Protection - Meralco

HOW DO PROTECTIVE RELAYS OPERATE? These are the parameters that may cause the protective relays to operate:  – magnitude (voltage, current, power)  – frequency  – phase angle  – duration  – rate of change  – direction or order of change  – harmonics or wave shape  Power System Protection - Meralco

RELAY CLASSIFICATIONS BY FUNCTION

1. Protective relays 2. Regulating relays 3. Reclosing, synchronism check, and synchronizing relays 4. Monitoring relays 5. Auxiliary relays 6. Other relay classifications • by operating principles • by performance characteristics

RELAY CLASSIFICATIONS BY SPEED OF OPERATION 1. Instantaneous. These relays operate as soon as a secure decision is made. No intentional time delay is introduced to slow down the relay response. 2. Time delay. An intentional time delay is inserted between the relay decision time and the initiation of the trip action. 3. High speed. A relay that operates in less than a specified time. The specified time in present practice is 50 milliseconds (3 cycles on a 60 Hz system). 4. Ultra high speed. This term is not included in the Relay Standards but is commonly considered to be operation in 4 milliseconds or less.  Power System Protection - Meralco

CLASSIFICATION OF RELAY OPERATION • Correct • Correct but undesired • Incorrect • No conclusion

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CLASSIFICATION OF RELAY OPERATION CORRECT TRIPPING

CORRECT TRIPPING BUT UNDESIRED F INCORRECT TRIPPING

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PRIMARY AND BACK-UP PROTECTION Primary Protection - Schemes that are designed to specifically protect one equipment zone. In any locations, this primary relaying may overlap into other zone of protection, providing additional protection for those zones.

Primary A. Limited B. Overlap

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BACK-UP PROTECTION Schemes that are designed to operate in place of or in parallel with the primary protection. Back-up protection probably will sense faults in more that one zone, is usually slower in operation, and may isolate a larger portion of the system. Backup protection for a specific zone may be provided by a local scheme or one located remotely. Back-up A. B. C. D. E.

In Place of Primary Overlap Slower   Increase Coverage in Isolation Local/Remote

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THREE MEMBERS OF PROTECTIVE SYSTEM 1. Sensor - Feeds system information to the relay, e.g., currents and voltages 2. Relay - Makes a decision as to the need for  action, e.g., overcurrent relay, etc. 3. Switching or Controlling Device - Physically isolates or control the problem, e.g., circuit breaker 

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THREE MEMBERS OF PROTECTIVE SYSTEM Sensor

Feedback  Signals Relay Power Circuit Breaker  Power System Protection - Meralco

FUNCTIONAL DIAGRAM OF RELAYING Decides whether system quantities are normal or  abnormal Power  System

Voltage and current transformer 

Relay

These devices Change Electrical Quantities to a Level low enough for the relay to use i.e. 5A, 110 V  Power System Protection - Meralco

Circuit Breaker 

Opens and isolate a faulty section of  the system as sent  by the relay

ELECTRICAL DIAGRAM OF RELAYING CB CT

Transmission Line Trip Coil

Station Battery

Relay Contacts

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TYPICAL CONTROL CIRCUIT

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DEFINITION OF OPERATION Mechanical movement of the operating mechanism is imparted to a contact structure to close or to open contacts  – we say that a relay "operates," we mean that it either closes or opens its contacts - whichever is the required action under the circumstances.  Power System Protection - Meralco

RELAY CONTACTS “a” contact - normally open contact, it closes when the relay operates and opens when the relay resets “b” contact - normally closed contact, it opens when the relay operates and closes when the relay resets  Power System Protection - Meralco

INSTRUMENT TRANSFORMERS (Transducers) Change the magnitudes, but not the nature of the measured quantities Provide isolation from the hostile environment of the power system Types Current Transformers - CTs Potential Transformers - PTs Voltage Transformers - VTs Coupling Capacitor Voltage Transformers - CCVT’s  Power System Protection - Meralco

CURRENT TRANSFORMERS Secondary Winding

Iron Core Primary Conductor  Secondary Terminals Rating: Specify continuous rating of secondary winding (1A, 5A) Specify primary current which will nominally produce rated secondary current (e.g., 800A, 1,000A)  Power System Protection - Meralco

CURRENT TRANSFORMERS Current Ratio

Polarity: - Indicated by dots (dot or  square) on drawings - Indicates instantaneous relationship in the directions of  primary and secondary currents.

Is Ip

Current entering the polarity mark on the pri mary will cause a current to instantaneously leave the polarity mark on the secondary

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100/5 200/5 400/5 500/5 600/5 800/5 1000/5 1200/5 2000/5

100/1 200/1 400/1 500/1 600/1 800/1 1000/1 1200/1 2000/1

MOST COMMON TYPES OF CURRENT TRANSFORMERS Core-Balanced or Ring type or Doughnut Type

Bushing or the Bar-Type

Wound Primary Type

Rogowsky Coil - Optical CT  Power System Protection - Meralco

CT EQUIVALENT CIRCUIT Rct

Ip/N

Rw

Is

Ie Zm

Ve

Vs

Ip

Zb

Rw N-turns

Rct - CT Winding resistance in ohms/turn Rw - Lead (wiring) Resistance Zb - Burden Impedance Zm - Magnetizing Impedance

Ve Is = Ip/N - Ie Vs = Is * (Zb + 2Rw) Ve = Vs + Is*Rct  Power System Protection - Meralco

Ie

CT EQUIVALENT CIRCUIT Rct

Ip/N

Rw

Is

Ie Zm

Ve

Ip

Vs

Zb

Rw N-turns

Rct - CT Winding resistance in ohms/turn Rw - Lead (wiring) Resistance Zb - Burden Impedance Zm - Magnetizing Impedance N - is the “nominal” ratio of CT

 At Saturation point: Is = Ip/N

Ve Zm will be small which result in Ie being large  Power System Protection - Meralco

Ie

CT ERROR CALCULATION

Given: • Primary Current , Ip • Total impedance burden on the CT, including lead wire resistance • CT Secondary Excitation Characteristics Neglected Factor: CT transient characteristic

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CT ERROR CALCULATION Rct

Ip/N

Rw

Is

Ve

Ie Zm

Ve

Vs

Zb

Rw

Ip N-turns

Ie Given : Is, Zb, Secondary Excitation Characteristic curve Steps : 1. 2. 3. 4. 5.

From the Burden and Is, cal. Vs From Vs, Rct and Is, cal. Ve From Ve and Sec. Excitation curve, determine Ie From Is and Ie, determine Ip/N From Ip/N and N, determine Ip  Power System Protection - Meralco

CT ERROR CALCULATION Rct

Ip/N

Rw

Is

Ve

Ie Zm

Ve

Vs

Zb

Rw

Ip N-turns

Ie

Given :

Ip, Zb, Secondary Excitation Characteristic curve Steps : 1. 2. 3. 4.

From Ip and N, det Ip/N Calculate Ve to determine Ie from curve From Ie, calculate Is, Vs and Ve From Secondary Excitation curve, determine new value of Ie 5. Repeat step 3 and 4 until successive iterations yields insignificant changes in Ie  Power System Protection - Meralco

CT CONNECTION Delta Connection

For  balance 3 - phase fault : Is = Ip/N * 3

Relay

Ip

For  phase - to - phase fault : Is = Ip/N * 3 / 2 in two lines

Is

Is = Ip/N * 3

in remaning line

Is is 30 degrees phase shifted relative to Ip. Delta-connected CT will not produce Zero-sequence currents. Zero-sequence currents will be “trapped” inside the delta and cannot be measured by the relays in the CT secondary.  Power System Protection - Meralco

CT CONNECTION Wye Connection Is1 Is2 Is3 Ir  I p1

I p2

Relay

Is1 = I p1/N Ir  = Is1 + Is2 + Is3

I p3

Is is in phase with I p Wye connection will detect all kinds of fault and loads With the saturation of any one CT, a fake residual current will be produced  Power System Protection - Meralco

CT CONNECTION Core Balance CT

Induced Current is a function of: Ia + I b + Ic = 3Io

Will not respond to 3-phase and phase-to-phase faults Power Cables Normally used for low voltage ground fault applications

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CT ACCURACY CLASS  ANSI C57.13   e 800   g   a    t    l 700   o    V 600    l   a   n 500    i   m   r   e 400    T   y   r 300   a    d   n 200   o   c   e    S 100

Errors will not exceed 10% for secondary voltage equal to or less than value described  by curve

C800

8Ω C400 4Ω

C200

2Ω 1Ω

C100

Class C - Indicates that the transformer ratio can be calculated Class T - Indicates that the transformer ratio must be determine by test

10 20 30 40 50 60 70 80 90 100 Secondary Amperes  Power System Protection - Meralco

CT SATURATION CURVE

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PHILOSOPHY OF PROTECTIVE RELAYING A critical factor in the success of any nation is electric power. Providing, operating and maintaining an effective power system is an important challenge. One key element to be considered in power system design is system protection. System Protection is accomplished via the coordinated application of protective devices including fuses, circuit breakers, reclosers, sectionalizers and other relays. Protective relays are devices which monitor power  system conditions and operate to quickly and accurately isolate faults or dangerous conditions. A well designed protective system can limit damage to equipment, as well as minimize the extent of  associated service interruption.  Power System Protection - Meralco

PHILOSOPHY OF PROTECTIVE RELAYING Factors Which Influence Design of a Protective System •Sensitivity •Selectivity •Reliability •Dependability •Security •Speed •Economics •Experience •Industry Standards  Power System Protection - Meralco

PHILOSOPHY OF PROTECTIVE RELAYING Sensitivity - the minimum signal required to produce an output. A more sensitive relay will be able to discern a “smaller” condition. Sensitivity is very important when the input quantities are very small Selectivity - the ability of the relay to recognize a fault or  abnormal system condition, and to discriminate between those upon which it should and should not operate or at a slightly delayed manner 

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PHILOSOPHY OF PROTECTIVE RELAYING Reliability -

the level of assurance that the relay will function as intended. Reliability is considered in two parts, dependability and security

Dependability - the ability of the relay to trip for all faults and conditions for which operation “tripping” is desired. Security -

the ability of the relay to not operate “trip” for any fault or condition for which tripping is undesired.

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PHILOSOPHY OF PROTECTIVE RELAYING Speed -

The ability of the relay to operate in the required time period. The ultimate goal of the protective equipment is to isolate the fault as quickly as possible.

Economics - The cost of installation, operation, and maintenance of the protection system which must be weighted against potential losses due to equipment damage or service interruption. Experience - Those problems which experience has shown to be most likely are given highest priority. Larger, critical systems are protected from less probable events.

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PROTECTIVE RELAYING Industry Standards

The Institute of Electrical and Electronic Engineers (IEEE) and other organizations provide industry standards through ANSI or IEC. These include specific standards for many applications. ANSI-C37.90-1989 - Relays and Relay System Associated with Electric Power  Apparatus IEEE STD 242-1975 - Recommended Practice for  Protection and Coordination of  Industrial and Commercial Power  System

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FAULTS VERSUS ABNORMAL CONDITIONS

One important concept in protective relaying is the difference between faults and abnormal conditions. Faults are short circuits or arcs, actual system failures. Abnormal conditions are such as overvoltage, undervoltage, or  overexcitation. Abnormal conditions are undesirable events, and can often lead to faults or equipment failure. Most relays are applied to protect the system or equipment from either faults or abnormal conditions. This will govern the philosophy of protection.

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ZONE OF PROTECTION Relay schemes are designed to protect specific areas or  equipment. The electric grid is divided into zones which can be isolated via circuit breakers, fuses or sectionalizers. Each zone is individually protected, and is defined as a ZONE of Protection. Protective relay schemes are designed to isolate a given zone for  any tripping condition. This minimizes or prevents equipment damage, thus, permitting more rapid restoration of the system, and, minimizes the extent and duration of the interference with the operation of the whole system (overtrip). Zones are established encompassing certain system elements such as generators, busses, transformers, and lines. This allows protective relaying schemes to be tailored to the equipment of a specific element. When a fault occurs, the zone including the failed equipment is isolated from the rest of the system.  Power System Protection - Meralco

ZONE OF PROTECTION

The boundaries of the zone of protection are defined by the current and voltage transformers, which provide the system information to the relays. • Each zone of protection includes the isolating circuit breakers, as well as the protected equipment. • Each zone overlaps the adjacent zone, and the circuit breaker will be in two zones. This is necessary to ensure that “blind spots” cannot exist, and that all the portions of  the power system are protected. • A fault in the overlap area will trip both zones. This especially desirable in the case of a circuit breaker failure.

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ZONE OF PROTECTION 3

6

Zone of Protection

5

1

52 87B 50/51

2

4 G

CT REQUIREMENTS FOR OVERLAPPING ZONES

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PROTECTION COORDINATION In order to increase dependability, and insure that all faults will be cleared, protective relays from a given zone of protection will usually operate as backup devices for faults in the adjacent zones. Utilities generally design their systems for  single contingency, meaning, that the system can survive the loss of any single device (including protective relays). In order  to provide this backup function while still isolating the minimum amount of equipment, the protective relays must be coordinated. That is, if the relays in the faulted zone fail to operate (single contingency), the relays in the adjacent zone(s), will operate after a time delay. In this means, dependability is increased with only a small risk to security.

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PROTECTION COORDINATION 51 LOADS 50/51 TO SOURCE

R 

51

LOADS  Power System Protection - Meralco

DEVELOPMENT OF PROTECTIVE RELAYS

•Electro-mechanical relay •Solid-state relay •Digital relay

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ELECTRO-MECHANICAL RELAYS • The most commonly used • Uses the induction disc principle (watthour meter) • Provides individual phase protection

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SOLID-STATE RELAYS • Characteristic curve is obtained through use of RC  timing circuits • No moving parts • Used to retrofit electromechanical relays • Fast reset • Less maintenance

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DIGITAL RELAYS • Selectable characteristic curves and protection functions • Metering and control functions • Event and/or disturbance recording • Remote communication • Self-monitoring • “All in”

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DIGITAL RELAYS

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DEVICE FUNCTION NUMBERS 52 59 64

67 68 69

79 81 86 87 94

Device ac circuit breaker

Description A device that is used to close and interrupt an ac power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions. overvoltage relay A device that operates when its input voltage exceeds a predetermined value. ground detector relay A device that operates upon failure of machine or other apparatus insulation to ground. NOTE T his function is not applied to a device connected in the s econdary circuit of current transformers in a normally grounded power system where other overcur rent device numbers with the suffix G or N should be used; for example, 51N for an ac time over  ac directional overcurrent  A device that functions at a desired value of ac overcurrent flowing in a predetermined relay direction. blocking or "out-of-step"  A device that initiates a pilot signal for blocking of tripping on external faults in a transmission relay line or in other apparatus under predetermined conditions, or coo perates with other devices to permissive control device A device with two-positions that in one position permits the closing of a circuit breaker, or the placing of a piece of equipment into operation, and in the other position, prevents the circuit breaker or the e ui ment from bein o erated. reclosing relay A device that controls the automatic reclosing and locking out of an ac circuit interrupter. frequency relay A device that responds to the frequency of an electrical quantity, operating when the frequency or rate of change of frequency exceeds or is less than a predetermined value. lockout relay A device that trips and maintains the associated equipment or devices inoperative until it is reset by an operator, either locally or remotely. differential protective  A device that operates on a percentage, phase angle, or other quantitative difference of two rela or more currents or other electrical uantities. tripping or trip-free relay A device that functions to trip a circuit breaker, contactor, or equipment; to permit immediate tripping by other devices; or to prevent immediate reclosing of a circuit interrupter if it shou ld

95-99 used only for specific applications

These device numbers are used in individual specific installations if none of the functions assigned to the n umbers from 1 through 94 are suitable.

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DEVICE FUNCTION NUMBERS (Suffixes) Suffix Letter   A B G GS L M N T V U X Y Z

Relay Application

Alarm only or automatic Bus protection Ground -fault or generator Ground -fault protection Line protection Motor protection Ground -fault protection Transformer protection Voltage Unit protection Auxiliary relay Auxiliary relay Auxiliary relay

Amplifying Information

System neut ral type protect ion Tor oidal or ground sensor type

Relay coil connected in residual CT circuit

Generator and transformer  

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BASIC STEPS FOR RELAY SETTING & COORDINATION STUDY • Data collection • Fault current calculation • Equipment performance • Special requirements • Selection and plotting of preliminary settings • Check final settings  Power System Protection - Meralco

SETTING & COORDINATION • Organized time-current study of  all devices in series from the utilization device to the source. • Comparison of the time it takes the individual devices to operate.

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SETTING & COORDINATION • Determine the characteristics, ratings and settings of overcurrent protective devices against a fault • Provide protection against overloads on equipment • Data useful for selection of instrument transformer ratios, fuse ratings, CB ratings and settings  Power System Protection - Meralco

QUESTIONS?

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