Protective Relaying - An Overview

7/30/2019 Protective Relaying - An Overview

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PROTECTIVERELAYING

Principles &Philosophies

FORTUNATO C. LEYNES, FIIEE

Chairman

Board of Electrical Engineering

Professional Regulation CommissionVice President

Manila Electric Company

15th IIEE Region 8 Conference

June 26, 2010


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Power System Protection - Meralco

The branch of electric power engineering concernedwith the principles of design, construction/

installation, operation and maintenance of

equipment (called relays or protective relays)which detect abnormal power system conditions,and initiate corrective action as quickly as possible

in order to return the power system to its normal

state.

Protective Relaying


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ARE PROTECTIVE RELAYING PRACTICES

BASED ON THE PROBABILITY OF FAILURE

protective relaying practices are based on the probability offailure to the extent that present-day practices are the resultof years of experience in which the frequency of failureundoubtedly has played a part;

the probability of failure, seldom if ever, enters directly intothe choice of a particular type of relaying equipment exceptwhen, for one reason or another, one finds it most difficult to

apply the type that otherwise would be used; more importantly, the probability of failure should be

considered only together with the consequences of failureshould it occur;

the justification for a given practice equals the likelihood of

trouble times the cost of the trouble; regardless of the probability of failure, no portion of a

system should be entirely without protection, even if it isonly back-up relaying.


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EVALUATION OF

PROTECTIVE RELAYING

the cost of repairing the damage.

the likelihood that the trouble may spread andinvolve other equipment.

the time that the equipment is out of service.

the loss in revenue and the strained public

relations while the equipment is out of service.

By expediting the equipments return toservice, protective relaying helps tominimize the amount of equipment reserve

required, since there is less likelihood ofanother failure before the first failure can berepaired.


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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 primaryprotection.

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

PROTECTION SYSTEM

OBJECTIVES


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FACTORS AFFECTING THE

PROTECTION SYSTEM

Economics

Personality of the relay engineer and

the characteristics of the power system

Location and availability ofdisconnecting and isolating devices

[circuit breakers, switches, and input

devices (CTs and VTs)]

Available fault indicators (fault studies

and such)


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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


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RELAY CLASSIFICATIONS

BY FUNCTION

1. Protective relays

2. Regulating relays

3. Reclosing, synchronism check, andsynchronizing relays

4. Monitoring relays

5. Auxiliary relays

6. Other relay classifications by operating principles by performance characteristics

etc.


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RELAY CLASSIFICATIONS

BY SPEED OF OPERATION

1. Instantaneous. These relays operate as soon as

a secure decision is made. No intentional timedelay is introduced to slow down the relayresponse.

2. Time delay. An intentional time delay is insertedbetween the relay decision time and the initiationof the trip action.

3. High speed. A relay that operates in less than aspecified time. The specified time in presentpractice 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 beoperation in 4 milliseconds or less.


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CLASSIFICATION OF

RELAY OPERATION

CORRECT TRIPPING

CORRECT TRIPPING BUT UNDESIRED

INCORRECT TRIPPING

F


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Primary Protection - Schemes that are designed tospecifically protect one equipment zone. In any locations, thisprimary relaying may overlap into other zone of protection,

providing additional protection for those zones.

Primary

A. Limited

B. Overlap

PRIMARY AND BACK-UP

PROTECTION


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Schemes that are designed to operate in place of or in

parallel with the primary protection. Back-up protection probablywill sense faults in more that one zone, is usually slower in

operation, and may isolate a larger portion of the system. Back-

up protection for a specific zone may be provided by a local

scheme or one located remotely.

Back-up

A. In Place of Primary

B. Overlap

C. Slower

D. Increase Coverage in IsolationE. Local/Remote

BACK-UP PROTECTION


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1. Sensor - Feeds system information to the relay,

e.g., currents and voltages

2. Relay - Makes a decision as to the need foraction, e.g., overcurrent relay, etc.

3. Switching or Controlling Device - Physically

isolates or control the problem, e.g.,

circuit breaker

THREE MEMBERS OF

PROTECTIVE SYSTEM


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Power Circuit Breaker

Relay

Feedback

Signals

Sensor

THREE MEMBERS OF

PROTECTIVE SYSTEM


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Power

System

Voltage

and

current

transformer

RelayCircuit

Breaker

These devices ChangeElectrical Quantities

to a Level low enough

for the relay to use i.e.

5A, 110 V

Decides whether system

quantities are normal or

abnormal

Opens and isolate

a faulty section ofthe system as sent

by the relay

FUNCTIONAL DIAGRAM

OF RELAYING


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Station

Battery

Transmission

Line

TripCoil

Relay Contacts

CB

CT

ELECTRICAL DIAGRAM OF

RELAYING


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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 requiredaction under the circumstances.


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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 closeswhen the relay resets


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Change the magnitudes, but not the nature of the

measured quantities

Provide isolation from the hostile environment of the

power system

Types

Current Transformers - CTsPotential Transformers - PTs

Voltage Transformers - VTsCoupling Capacitor Voltage Transformers - CCVTs

INSTRUMENT TRANSFORMERS

(Transducers)


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Secondary

Terminals

Iron Core

Secondary Winding

Primary Conductor

Rating:Specify continuous rating of secondary winding (1A, 5A)

Specify primary current which will nominally produce ratedsecondary current (e.g., 800A, 1,000A)

CURRENT TRANSFORMERS


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Current Ratio

100/5200/5

400/5500/5

600/5

800/51000/5

1200/52000/5

100/1200/1

400/1500/1

600/1

800/11000/1

1200/12000/1

Polarity:

- Indicated by dots (dot or

square) on drawings- Indicates instantaneous

relationship in the directions ofprimary and secondary currents.

Current entering the polarity mark on the primary

will cause a current to instantaneously leave thepolarity mark on the secondary

Ip

Is



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Core-Balanced or Ring type or Doughnut Type

Bushing or the Bar-Type

Wound Primary Type

Rogowsky Coil - Optical CT

MOST COMMON TYPES OF



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Is = Ip/N - Ie

Vs = Is * (Zb + 2Rw)

Ve = Vs + Is*Rct

Ip

IsIp/N

Ie

Rct Rw

Rw

ZbZm Ve Vs

N-turns

Ve

Ie

Rct - CT Winding

resistance in

ohms/turn

Rw - Lead (wiring)

Resistance

Zb - Burden Impedance

Zm - MagnetizingImpedance

CT EQUIVALENT CIRCUIT


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Ip

IsIp/N

Ie

Rct Rw

Rw

ZbZm Ve Vs

N-turns

Rct - CT Winding

resistance inohms/turn

Rw - Lead (wiring)

Resistance

Zb - Burden Impedance

Zm - Magnetizing

ImpedanceN - is the nominal ratio

of CT

CT EQUIVALENT CIRCUIT

At Saturation point: Is = Ip/NVe

Ie

Zm will be small which

result in Ie being large


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Given:

Primary Current , Ip

Total impedance burden on the CT, including leadwire resistance

CT Secondary Excitation Characteristics

Neglected Factor: CT transient characteristic

CT ERROR CALCULATION


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Ip

IsIp/N

Ie

Rct Rw

Rw

ZbZm Ve Vs

N-turns

Given :

Is, Zb, Secondary Excitation Characteristic curve

Steps :

1. From the Burden and Is, cal. Vs2. From Vs, Rct and Is, cal. Ve3. From Ve and Sec. Excitation curve, determine Ie

4. From Is and Ie, determine Ip/N5. From Ip/N and N, determine Ip

Ve

Ie


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Ip

IsIp/N

Ie

Rct Rw

Rw

ZbZm Ve Vs

N-turns

Ve

Ie

Given : Ip, Zb, Secondary Excitation Characteristic curve

Steps :

1. From Ip and N, det Ip/N

2. Calculate Ve to determine Ie from curve

3. From Ie, calculate Is, Vs and Ve4. From Secondary Excitation curve, determine

new value of Ie

5. Repeat step 3 and 4 until successive

iterations yields insignificant changes in Ie


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RelayIp

Is

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.

CT CONNECTIONDelta Connection

lineremaningin3*Ip/NIslinesin two2/3*Ip/NIs

:faultphase-to-phaseFor

3*Ip/NIs

:faultphase-3balanceFor

=

=

=


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Relay

Ip1

Is1

Ir

Is is in phase with Ip

Wye connection will detect all kinds of fault and loads

With the saturation of any one CT, a fake residualcurrent will be produced

Is1 = Ip1/N

Ir= Is1 + Is2 + Is3

CT CONNECTIONWye Connection

Is2Is3

Ip2Ip3


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Power Cables

Induced Current is a function of:

Ia + Ib + Ic = 3Io

Will not respond to 3-phase and

phase-to-phase faults

Normally used for low voltage groundfault applications

CT CONNECTIONCore Balance CT


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100

200

300

400

500

600

700800

10010 20 30 40 50 60 70 80 90

8

4

2

1

C400

C800

C200

C100

Secondary Amperes

S

econdaryTerminalVoltage

ANSI C57.13

Class C - Indicates thatthe transformer ratio can

be calculated

Class T - Indicates thatthe transformer ratio

must be determine bytest

Errors will not exceed 10%

for secondary voltage equal

to or less than value described

by curve

CT ACCURACY CLASS


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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 elementto be considered in power system design is system

protection.

System Protection is accomplished via the coordinated

application of protective devices including fuses, circuitbreakers, 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. Awell designed protective system can limit damage to

equipment, as well as minimize the extent ofassociated service interruption.


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Factors Which Influence Design of a Protective System

Sensitivity

Selectivity

Reliability

Dependability

Security

Speed

Economics

Experience

Industry Standards

PHILOSOPHY OF

PROTECTIVE RELAYING


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Sensitivity - the minimum signal required to produce an

output. A more sensitive relay will be able todiscern 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

PHILOSOPHY OF

PROTECTIVE RELAYING


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Reliability - the level of assurance that the relay will

function as intended. Reliability is

considered in two parts, dependability andsecurity

Dependability - the ability of the relay to trip for all faults

and conditions for which operationtripping is desired.

Security - the ability of the relay to not operate trip

for any fault or condition for which tripping

is undesired.

PHILOSOPHY OF

PROTECTIVE RELAYING


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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 aspossible.

Economics - The cost of installation, operation, and maintenance

of the protection system which must be weighted

against potential losses due to equipment damageor service interruption.

Experience - Those problems which experience has shown to bemost likely are given highest priority. Larger,

critical systems are protected from less probableevents.

PHILOSOPHY OF

PROTECTIVE RELAYING


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The Institute of Electrical and Electronic Engineers (IEEE)

and other organizations provide industry standards throughANSI or IEC. These include specific standards for many

applications.

ANSI-C37.90-1989 - Relays and Relay System

Associated with Electric PowerApparatus

IEEE STD 242-1975 - Recommended Practice for

Protection and Coordination of

Industrial and Commercial PowerSystem

PROTECTIVE RELAYING

Industry Standards

FAULTS VERSUS


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One important concept in protective relaying is thedifference 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 undesirableevents, 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.

FAULTS VERSUS

ABNORMAL CONDITIONS


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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 isindividually 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 equipmentdamage, thus, permitting more rapid restoration of the system,

and, minimizes the extent and duration of the interference with theoperation 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 aspecific element. When a fault occurs, the zone including thefailed equipment is isolated from the rest of the system.

ZONE OF PROTECTION


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The boundaries of the zone of protection are defined by thecurrent 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 circuitbreaker will be in two zones. This is necessary to ensure

that blind spots cannot exist, and that all the portions ofthe 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.

ZONE OF PROTECTION


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Zone of Protection

52

87B

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CT REQUIREMENTS FOR

OVERLAPPING ZONES

ZONE OF PROTECTION

G

1

3

5

6

42

PROTECTION


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In order to increase dependability, and insure that all faults will

be cleared, protective relays from a given zone of protectionwill 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 minimumamount 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.

PROTECTION

COORDINATION

PROTECTION


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51

51

LOADS

LOADS

TO SOURCE

R

PROTECTION

COORDINATION

DEVELOPMENT OF


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Electro-mechanical relay

Solid-state relay

Digital relay

DEVELOPMENT OF

PROTECTIVE RELAYS

ELECTRO MECHANICAL


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ELECTRO-MECHANICAL

RELAYS

The most commonlyused

Uses the induction disc

principle(watthour meter)

Provides individual phase

protection


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SOLID-STATE RELAYS

Characteristic curve is

obtained through use ofRC

timing circuits

No moving parts

Used to retrofit electro-

mechanical relays Fast reset

Less maintenance


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DIGITAL RELAYS

Selectable characteristic

curves and protectionfunctions Metering and control

functions

Event and/or disturbancerecording

Remote communication

Self-monitoring

All in

DIGITAL RELAYS


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DIGITAL RELAYS


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DEVICE FUNCTION


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DEVICE FUNCTION

NUMBERSDevice Description

52 ac circuit breaker 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.59 overvoltage relay A device that operates when its input voltage exceeds a predetermined value.

64 ground detector relay A device that operates upon failure of machine or other apparatus insulation to ground.NOTE This function is not applied to a device connected in the secondary circuit of current

transformers in a normally grounded power system where other overcurrent device numbers

with the suffix G or N should be used; for example, 51N for an ac time over67 ac directional overcurrent

relayA device that functions at a desired value of ac overcurrent flowing in a predetermineddirection.

68 blocking or "out-of-step"relay

A device that initiates a pilot signal for blocking of tripping on external faults in a transmissionline or in other apparatus under predetermined conditions, or cooperates with other devices to

69 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 circuitbreaker or the e ui ment from bein o erated.

79 reclosing relay A device that controls the automatic reclosing and locking out of an ac circuit interrupter.81 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.86 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.87 differential protective

rela

A device that operates on a percentage, phase angle, or other quantitative difference of two

or more currents or other electrical uantities.94 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 should

95-99 used only for specificapplications

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

DEVICE FUNCTION


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NUMBERS(Suffixes)

Suffix

Letter

Relay Application Amplifying Information

A Alarm only or automaticB Bus protection

G Ground -fault or generator System neutral type protect ion

GS Ground -fault protection Toroidal or ground sensor type

L Line protection

M Motor protectionN Ground -fault protection Relay coil connected in residual CT circuit

T Transformer protection

V Voltage

U Unit protection Generator and transformer

X Auxiliary relay

Y Auxiliary relayZ Auxiliary relay

BASIC STEPS FOR RELAY


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SETTING &

COORDINATION STUDY

Data collection

Fault current calculation

Equipment performance

Special requirements

Selection and plotting of preliminary

settings Check final settings

SETTING & COORDINATION


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Organized time-current study of

all devices in series from theutilization device to the source.

Comparison of the time it takesthe individual devices to operate.



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Determine the characteristics, ratings

and settings of overcurrent protectivedevices against a fault

Provide protection against overloadson equipment

Data useful for selection of instrument

transformer ratios, fuse ratings, CBratings and settings

QUESTIONS?


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QUESTIONS?


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PROTECTIVERELAYING

Principles &Philosophies

FORTUNATO C. LEYNES, FIIEE

Chairman

Board of Electrical Engineering

Professional Regulation Commission

Vice President

Manila Electric Company

15th IIEE Region 8 Conference

June 26, 2010

Documents

Protective Relaying - An Overview