1_introduction to power system protection

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INTRODUCTION TO POWER SYSTEM PROTECTION

P. KaliappanRelay Testing Laboratory

Power Systems Division

Central Power Research InstituteBangalore

[email protected]

1.0 INTRODUCTION:

The Protective relay is to protect the system and its equipment from unnecessary

damage due to faults, such as short circuits; it is obvious that speed is important. The

damage caused by a short circuit is certainly minimized by clearing the fault quickly

and it is essential to maintaining system stability. It is to detect and locate a fault and

issue a command to the circuit breaker to disconnect the faulty element. They are in

effect, a form of insurance designed to maintain a high degree of service continuity

and limit equipment damage. They are also called as Silent Sentinels.

2.0 CLASSIFICATION OF RELAYS:

Relays can be divided into five functional categories:

a. Protective relays, which detect defective lines, defective apparatus, or other

dangerous or intolerable conditions. These relays can either initiate or permit

switching or provide an alarm.

b. Monitoring Relays, which verify conditions on the power system or in the

protection. These relays include fault detectors, alarm units, channel- monitoring

relays, synchronism verification, and network phasing. Power system conditions

that do not involve opening circuit breakers during faults can be monitored by

these relays.

c. Programming Relays, which establish or detect electrical sequences.

Programming relays are used for reclosing and synchronizing.

d. Regulating Relays, which are activated when an operating parameter deviates

from predetermined limits. Regulating relays function through supplementaryequipment to restore the quantity to the prescribed limits.

e. Auxiliary Relays, which operate in response to the opening or closing of the

operating, circuit to supplement another relay or device. These include timers,

contact-multiplier relays, sealing units, receiver relays, lock-out relays, closing

relays and trip relays.

In addition to these functional categories, relays may be classified by input, operating

principles or structure and performance characteristic:

mailto:[email protected]





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i) Input

Current

Voltage

Power

Pressure

Frequency Temperature

Flow

Vibration

ii) Operating principle of structure

Percentage

Multi-restraint

Product

Solid state

Electromechanical

Thermal

The above classification and definitions are based on the ANSI Standard 37.90

(IEEE 313)

3.0 PROTECTIVE RELAYING SYSTEMS AND THEIR DESIGN:

Any protection scheme is required to safeguard the power system components. The

protection scheme basically consists of current transformers, voltage transformers,

Protective relay and circuit breaker. The Protective relay functions as a sensing

device. It senses the fault through Current/Voltage transformers determine its location

and finally it sends the command to the circuit breaker by closing its trip coil. The

circuit breaker after getting the trip command from protective relay disconnects the

faulty sections from the rest of the power system. Thus it is seen that the protective

relay, which is primarily the brain behind the whole scheme, plays a very important

role. Therefore proper care should be taken in selecting an appropriate protective

relay, which is reliable, efficient and fast in operation.

4.0 DESIGN CRITERIA:

4.1 Reliability:

A protective system must operate reliably when a fault occurs in its zone of

protection. The reliability of the protective relay system has two aspects:

dependability of operation and security from false operation. Dependability means

that each relay sends a trip signal when a fault is present in its zone. Security means

that no relay sends a trip signal if no fault is present in its zone. Since no human

invention is perfect, and the protective relay system is no exception, compromise

between dependability and security are inevitable. To achieve a high degree of

reliability, greater attention should be given to the design, installation, maintenance

and testing of the various elements of the protective system.



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4.2 Selectivity:

Selectivity is the quality of a protective relay by which it is able to discriminate

between a fault in the protected section and the normal condition. Also, it should be

able to distinguish whether a fault lies within its zone of protection or outside the

zone. Sometimes, this quality of the relay is also called discrimination.

4.3 Sensitivity:

Sensitivity refers to the minimum level of fault current at which operation occurs. In

other words, it is fault setting. There is a difference between the sensitivity of a relay

and the sensitivity of a protection system. The sensitivity of a relay is expressed as the

apparent power in VA required to cause operation; thus 1 VA relay is more sensitive

than 3 VA relay.

4.4 Stability:

The term stability is often used to describe the quality of protective system by virtue

of which it remains inoperative under specified conditions usually associated with

high values of fault currents. Strictly speaking, it is a quality that only unit systems

can possess because they are required to remain inoperative under all conditions

associated with faults outside their own zone.

4.5 Speed:

A protective system should be fast enough to isolate the faulty element of the system

as quickly as possible to minimize damage to the equipment and to maintain system

stability. For modern power system, the stability criterion is very important and

hence, the operating time of the protective system should not exceed the critical

clearing time to avoid the loss of synchronism.

4.6 Cost:

Maximum protection at the lowest cost possible.

5.0 FACTORS INFLUENCING RELAY PERFORMANCE:

Relay performance is generally classified as:

Correct

Incorrect

No conclusion

Incorrect operation may be either failure to trip or false tripping. The cause of

incorrect operation may be, a) Wrong application, b) Incorrect settings, c) A

personnel error, or d) Equipment ma-function. Equipment that can cause an incorrect

operation includes CTs, VTs, circuit breakers, cable and wiring, relays, channels or

station batteries.



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Incorrect tripping of circuit breakers not associated with the trouble area is often as

disastrous as a failure to trip. Hence, special care must be taken in both application

and installation to ensure against the possibility of incorrect tripping.

“No conclusion” is the last resort when no evidence is available for a correct or

incorrect operation. Quite often this is a personnel involvement.

6.0 ZONES OF PROTECTION:

A power system contains generators, transformers, bus bars, transmission and

distribution lines, etc. There is a separate protective scheme for each piece of

equipment or element of the power system, such as generator protection, transformer

protection, transmission line protection, bus bar protection, etc. Thus, a power system

is divided into a number of zones for protection. A protective zone covers one or at

the most two elements of a power system. The protective zones are planned in such a

way that they collectively cover the entire power system, and thus, no part of the

system is left unprotected. The different zone of protection is as given figure 1.

LINE ZONE

BUS ZONE BUS ZONE

LINE ZONE

BUS ZONE BUS ZONE

TRANSF ZONE

BUS ZONE

BUS

ZONES

TRANSF

ZONES

Figure 1.

7.0 PRIMARY AND BACK UP PROTECTION:

If a fault occurs in a particular, it is the duty of primary relays of that zone to isolate

the faulty element. The primary relay is the first line of defence. If due to any reason,the primary relay fails to operate, there is a back-up protective scheme to clear the



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fault as a second line of defence. With proper design, installation and maintenance of

the relays, circuit breakers, trip mechanisms, ac and dc wiring, etc. a very high degree

of reliability can be achieved.

The back-up relays are made independent of those factors, which might cause primary

relays to fail. A back-up relay operates after a time delay to give the primary relaysufficient time to operate. When back-up relay operates, a larger part of the power

system is disconnected from the power source, but this is unavoidable. There are three

types of back-up relays:

Relay back-up

Breaker back-up

Remote back-up

Relay Backup: In this relaying scheme, the main relays, their CTs, PTs etc. are

duplicated

Breaker Backup: When a feeder breaker fails to trip on a fault, the feeder fault

becomes virtually a bus bar fault. In breaker back-up scheme, a time-delay relay is

operated by the main relay and it is connected to trip all other breakers on bus if the

proper breaker has not tripped within pre-set time.

Remote Backup: Remote back-up is provided by a relay on the next station towards

the source. This remote relay will trip in a delayed time if the breaker in the faulty

section has not tripped due to some reason. This is the most widely used form of

back-up protection.

8.0 ELECTRICAL POWER SYSTEM DEVICE NUMBERS AND FUNCTIONS:

Numbers, with appropriate suffix letters when necessary, refers to the device

switching equipment, according to the functions they perform.

These numbers are based on a system adopted as standard for automatic switchgear

by IEEE and incorporated in American standard C37.2-1970. This system is used in

connection diagrams, in instruction books and in specifications.

Device Number Definition Function

1 Master Element It is an initiating device, such as a Control switch,

voltage relay, float Switch etc which serves either

Directly or through such permissive Devices as

protective and time delay relays .to place an equipment

in or out of operation

2 Time Delay Starting

Or Closing Relay

It is a device which functions to give a desired amount

of time delay before of after any point of operation in a

switching sequence or protective relaying system,

except as specially provided by device function 48, 62

and 79 described later.



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3 Checking or

Interlocking Relay

It is device which operates in response to the position

of a number of other devices ( or to a number of

predetermined conditions), in an equipment, to allow

an operating sequence to proceed , to stop , or to

provide a check of the position of these devices or of

these conditions for any purpose.4 Master Contactor It is a device, generally controlled by The device No.1

or equivalent, and the required permissive and

protective devices that serve to make and break the

necessary control circuits to place equipment into

operation under the desired conditions and to take it out

of operation under other or abnormal conditions.

5 Stopping Device It is a control device used primarily To shut down an

equipment and hold it out of operation. This device

may be manually or electrically actuated, but excludes

the function of electrical lockout (see device function

86) on abnormal conditions.

6 Starting Circuit

Breaker

It is a device whose principal Function is to connect a

machine to Its source of starting voltage

7 Anode Circuit Breaker It is one used in the anode circuits of a power rectifier

for the primary purpose of interrupting the rectifier

circuit if an arc back should occur.

8 Control Power

Disconnecting Device

It is a disconnecting device – such As a knife switch,

circuit breaker or pullout fuse block, used for the

purpose of connecting and disconnecting the source of

control power to and from the control bus or

equipment. Note: control power is considered toinclude auxiliary power, which supplies such apparatus

as small motors and heaters.

9 Reversing Device It is a used for the purpose of reversing A machine

field or for performing any other reversing function

10 Unit Sequence switch It is used to change the sequence in Which units may

be placed in and Out of service in multiples – unit

Equipment.

11 Reserved for future

Application

12 Over Speed Device It is usually direct connected Speed switch which

functions on Machine over-speed13 Synchronized

Speed Device

A Centrifugal speed switch, a slip Frequency relay, a

voltage relay, an Undercurrent relay or any type of

device that operates at approximately synchronous

speed of a machine

14 Under Speed Device It is device which functions when The speed of a

machine falls below a predetermined value

15 Speed or Frequency

Matching device

It is a device that functions to match and hold the speed

or the Frequency of a machine or of a system equal to,

or approximately equal to, that of another machine,

source or system.


Application



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17 Shunting or

Discharge switch

It serves to open or to close a shunting circuit around

any piece of apparatus (except a resistor), such as a

machine field, a machine armature, a capacitor or a

reactor. Note: this excludes devices which perform

such shunting operations as may be necessary in the

process of starting a machine by devices 6 or 42 orthey’re equivalent, and also exclude device 73 function

which serves for switching of resistors

18 Accelerating or

Decelerating Device

It is used to close or to cause the closing of circuits

,which are used to increase or to decrease the speed of

a machine

19 Starting – to-

Running transition

contactor

It is a device which operates to initiate or cause the

automatic transfer of machine from the starting the

running power connection.

20 Distance Relay It is a electrically operated, Controlled or monitored

valve in a fluid line. Note: The function of the valve may be indicated by

the use of the suffixes.

21 Distance Relay It is a device, which functions when the circuit

admittance impedance or reactance increases or

decreases beyond predetermined limits.

22 Equalizer circuit

breaker

It is a device to serves to control or to make and break

the equalizer or the current balancing connections for a

machine field, or for regulating equipment, in a

multiple-unit of the installation

23 Temperature

Control device

It is a device to raise or lower the temperature of

machine or other apparatus or any medium, when itstemperature falls below, or rises above, a

predetermined value.

Note: An example is a thermostat which switched on a

space heater in a switchgear assembly when the

temperature falls to a desired value as distinguished

from a device which is used to provide automatic

temperature regulation between close limits and would

be designated as 90T


Application

25 Synchronizing or

Synchronism-device

It is a device that operates when two as circuits are

within the desired limits of frequency, phase angle and

Voltage to permit or to cause the paralleling of these

two circuits.

26 Apparatus

Thermal device

It is a device which functions when the temperature of

the shunt field or the armature winding of a machine,

or that of a load limiting or load shifting resistor or of a

liquid or other medium exceeds a predetermined value;

or if the temperature of the protected apparatus, such as

a power rectifier, or of any medium decreases below a

predetermined value.



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27 Under voltage relay It is device which functions on a given value of under

voltage.

28 Flame detector It is a device that monitors the presence of the pilot or

main flame in such apparatus as a gas turbine or a

system boiler.

29 Isolating contactor It is a device used for disconnecting one circuit fromanother for the purpose of emergency operation,

maintenance, or test.

30 Annunciator relay It is a non-automatically reset device that gives a

number of separate visual indications upon the

functioning of protective devices and which may also

be arranged to perform a lockout function.

31 Directional Power relay It is a device which functions on a desired value of

power flow in a given direction, or upon reverse

power, like resulting from arc back in the anode or

cathode circuits of a power rectifier.

32 Separate

Excitation device

It connects a circuit such as the shunt field of a

synchronous converter during the starting sequence ,or

one which energises the excitation and ignition circuits

of a power rectifier

33 Position switch It makes or breaks a contact when the main device or

the piece of apparatus, which has no device function

Number, reaches a given position

34 Master Sequence

device

It is a device such as a motor-Operated multi-contact

switch, or the equivalent, or a programming device,

such as a computer, that establishes or determines the

operating sequence of the major devices in anequipment during starting and stopping or during other

sequential operations.

35 Brush-operating

Or slip-ring-short

Circuiting device

It is used for raising, lowering, or shifting, the brushes

of a machine, or for short circuiting its slip ring, or for

engaging or disengaging the contacts of a mechanical

rectifier.

36 Polarity or

Polarizing voltage

device

It operates or permits the operation of another device

on a predetermined polarity only or verifies the

presence of a polarising voltage in an equipment

37 Undercurrent or

Under power relay

It functions when the current or power flow decreases

below a predetermined value.38 Bearing protective

device

It functions on excessive bearing temperature, or on

other abnormal mechanical conditions, such as

Undue wear, which may eventually result in excessive

bearing temperature

39 Mechanical Condition

monitor

It is device that functions upon the occurrence of

abnormal mechanical conditions,(except that associated

with bearing as covered under device function 38),

such as excessive vibration , eccentricity, expansion,

shock, tilting or seal failure



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40 Field relay It functions on a given or abnormally low value or

failure of mechanical field current, or an excessive

value of the reactive component of armature current in

an ac machine indicating abnormally low field

excitation.

41 Field circuit breaker It is a device which functions to apply or to remove thefield excitation of a machine

42 Running circuit breaker It is a device whose principal function is to connect a

machine to its source of running or operating voltage.

This function may also be used for a device, such as a

contactor, that is used in series with a circuit breaker or

other fault protecting means, primarily for frequent

opening and closing of the circuit.

43 Manual transfer

Or selector

Device transfer

It transfer the control circuits so as to modify the plan

of operation of the switching equipment or of some of

the devices

44 Unit sequence

Starting relay

It is a device, which functions to start the next available

unit in multiple-unit equipment on the failure or on the

non-availability of the normally preceding unit.

45 Atmospheric

Condition monitor

It is a device that functions upon the occurrence of an

abnormal atmospheric condition, such as damaging

fumes, explosive mixture smoke or fire.

46 Reverse – phase

Phase-balance

Current relay

It is a relay which functions when the poly-phase

currents are of reverse phase sequence, or when the

poly-phase currents are unbalanced or Contain negative

phase-sequence component above a given amount

47 Phase sequenceVoltage relay It functions upon a predetermined value of poly phasevoltage in the desired phase sequence

48 Incomplete

Sequence relay

It is a relay that generally returns the equipment to the

normal of off position and locks it out of the normal

starting , operating or stopping sequence is not properly

completed within a predetermined time, if the device is

used for alarm purpose only, it should preferably be

designated as 48A(alarm)

49 Machine or

Transformer,

Thermal relay

It is a relay that functions when the temperature of a

machine armature or other load carrying winding or

element of a machine or the temperature of a power

rectifier or power transformer (including a powerrectifier transformer) exceeds a predetermined value.

50 Instantaneous

Over current or

Rate of rise relay

It is a relay that functions instantaneously on an

excessive value of current, or on an excessive current

rise, thus indicating a fault in The apparatus of circuit

being protected.

51 AC time

Over current relay

It is a relay with either a definite or an inverse time

characteristic that functions when the current in an ac

circuit exceeds a predetermined value.

52 AC circuit breaker It is 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.



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53 Exciter or

Generator relay

It is relay that forces the dc machine field excitation to

build up during starting or which functions When the

machine voltage has build up to a given value.

54 Reserved for

Future application

55 Power factor relay It is relay that operates when the power factor in an accircuit rises above or below a predetermined value.

56 Field application relay It is a relay that automatically controls the application

of the field excitation to an ac motor at some

predetermined point in the slip cycle.

57 Short circuiting relay It is primary circuit switching device that functions to

short circuit or to ground a circuit in response to

automatic or manual means.

58 Rectification

Failure relay

It is a device that functions if one or more anodes of a

power rectifier fail to fire, or to detect an arc-break or

failure of a diode to conduct or Block properly.

59 Over voltage relay It is relay that functions on a given value of over

voltage

60 Voltage or

Current balance relay

It is relay that operates on a given difference in voltage,

or current input or output of two circuits.

61 Reserved for

future application

62 Time – delay Stopping

or opening relay

It is a time delay relay that serves in conjunction with

the device that initiates the shutdown, stopping, or

opening operation in an automatic

63 Pressure switch It is switch , which operates on given values or on a

given rate of change of pressure64 Ground

Protective relay

It is a relay that functions on failure of the insulation of

a machine, transformer or of other apparatus to ground

m or on flashover of a dc machine to ground.

Note: this functions is assigned only to a relay which

detects the flow of current from the frame of am

machine or enclosing case or structure of a piece of

apparatus to ground, or detects a ground on a normally

ungrounded winding or circuit. it is not applied to

device connected in the secondary circuit or secondary

neutral of a current transformer, connected in the

power circuit of an normally grounded system65 Governor It is the assembly of fluid, electrical, or mechanical

control, equipment used for regulation the flow of

water, stream, or other medium to the prime mover for

such purpose as starting, holding or stopping speed

load.

66 Notching or

Jogging device

It functions to allow only a specified device, or

equipment, or a specified number of successive

operations within a given time of each other. It also

functions to energise a circuit periodically or for

fractions of specified time intervals , or that is used to

permit intermittent acceleration or jogging of a

machine at low speeds for mechanical positioning



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67 AC directional

Overcurrent relay

It is a relay that functions on a desired value of ac over

current flowing in a predetermined direction.

68 Blocking relay It is relay that initiates a pilot signal for blocking of

tripping on external faults in a transmission line or in

other apparatus under predetermined conditions, or co-

ordinates with other devices to block tripping or to block re-closing on an out-step condition or on power

swings.

69 Permissive Control

device

It is generally a two position, manually operated switch

that in one position permits. The closing of an circuit

breaker, of an equipment into position prevents the

circuit breaker or the equipment from being operated

70 rheostat It is a variable resistance device used in an electrical

circuit which is electrically operated or has other

electrical accessories such as auxiliary position of limit

switches

71 Level switch It is a switch which operates on given values, or on a

given rate of change of level

72 DC circuit breaker It is used to close and interrupt a dc power circuit under

normal conditions or to interrupt this circuit under fault

or emergency conditions

73 Load resistor contactor It is used to shunt or insert a step of load limiting,

shifting , or indicating resistance in a power circuit , or

to switch a space heater in circuit , or to switch a light

,or regenerative load resistor of a power rectifier other

machine in and out of circuit

74 Alarm relay It consists a device other than the annuciator, ascovered under Device no.30 which is used to operate in

connection with a visual or audible alarm

75 Position Changing

mechanism

It is a mechanism that us used for moving a main

device from one position to another in an equipment as

for example , shifting a removable circuit breaker unit

to and from the connected ,disconnected and test

positions

76 DC over current relay It is a relay that functions when the current in a dc

circuit exceeds a given value

77 Pulse transformer It sis used to generate and transmit pulse over a

telemetering or pilot-wire circuit to the remoteindicating or receiving device.

78 Phase angle Measuring

or Out-of-step

Protective relay

It is a relay that functions at a predetermined phase

angle between two voltages of between two currents or

between voltage and current

79 AC re-closing relay It is a relay that controls the automatic reclosing and

locking out of an ac circuit interrupter

80 Flow switch It is a switch , which operates on given values , or on a

given rate of change, of flow

81 Frequency relay It is a relay that functions on a predetermined value of

frequency, either under/over on normal system

frequency or rate of change of frequency



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82 DC re-closing It is a relay that controls the automatic closing and

reclosing of a DC circuit interrupter, generally in

response to load circuit conditions

83 Automatic Selective

Control or Transfer

relay

It is relay that operates to select automatically between

certain sources or conditions in an equipment, or

performs a transfer operation automatically84 Operating mechanism It is the complete electrical mechanism or servo-

mechanism, Including the operating motor, solenoids,

position switches, etc, for a tap change , induction

regulator or any similar piece of apparatus which has

no device function number

85 Carrier or pilot

Wire receiver relay

It is a rely that is operated or restrained by a signal used

in connection with carrier-current or dc pilot-wire fault

directional relaying

86 Locking out relay It is an electrically operated, hand or electrically reset,

relay that functions to shut down and hold an

equipment out of service on the occurrence of

abnormal conditions.

87 Differential

Protective relay

It is protective relay that functions on a percentage or

phase angle or other quantitative difference of two

currents or of some other electrical quantities

88 Auxiliary motor

Or motor generator

It is one used for operating auxiliary equipment such as

pumps, blowers, exciters , rotating magnetic amplifiers

, etc.

89 Line switch It is used as a disconnecting load interrupter , or

isolator switch in an as or dc power circuit in an ac or

dc power circuit when this device is electricallyoperated or has accessories , such as an auxiliary

switch, magnetic lock, etc

90 Regulating device It functions to regulate a quantity , or quantities , such

as voltage, current, power , speed , frequency,

temperature and load , at a certain value or between

certain (generally close) limits for machines, toe lines

or other apparatus.

91 Voltage Directional

relay

It is relay that operates when the voltage across an

open circuit breaker or contractor Exceeds a given

value in an given direction

92 Voltage and Powerdirectional relay

It is a relay that permits or causes the connection oftwo circuits when the voltage difference between them

exceeds a given value in an predetermined direction

and causes these two circuits to be disconnected from

each other when the power flowing between them

exceeds a given value in the opposite direction

93 Field changing

contactor

It functions to increase or decrease in one step the

value of field excitation on a machine

94 Tripping or trip

Free relay

It functions to trip a circuit breaker contactor, or

equipment or to permit immediate tripping by other

devices, or to prevent immediate re-closure of a circuit

interrupter in case it should open automatically even

though its closing circuit is maintained closed



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95 Used only for specific

applications on

individual installations

where none of the

assigned numbered

functions from 1 to 94is suitable

1_introduction to power system protection

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