Basic Power System Protection

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POWER SYSTEM

PROTECTION

Fundamentals of

Protection Practice

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Why power system need the

protection system?

Severe disruption to normal routine of modern

society is likely if power outages are frequent or

prolonged.

Many items of equipment are very expensive,

and the complete power system represent a very

large capital investment.

Fault may represent a risk to life and/or property.

Why power system need the

protection system?

5 Other 3 Control equipment 10 Instrument transformer 12 Transformer 15 Busbar 10 Underground Cable 45 Overhead line % of faults % of faults Type of equipment Type of equipment

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Why power system need the

protection system?

2 Other 5 Series Fault 10 Three Phase Short

Circuit

5 Two Phase Short Circuit

18 Two Phase to Earth

60 One Phase to Earth

% of fault

Type of faults

What can the protection system do?

Increase emphasis on reliability and

security of supply.

Prevent the diverse items of equipment

from the severe damage by detect and

disconnect elements of the power system.

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More fundamental

However, is the power system should operate

in a safe manner at all times. No matter how well

designed, faults will always occur on a power

system.

The provision of adequate protection is

therefore on integral part of power system

design.

As requirements of reliability and economic are

largely opposed, power system design is

inevitable a compromise

Protection System

Protection system is a complete arrangement of

protection equipment and other devices required to

achieve a specified function based on a protection

principal.

Protection equipment is a collection of protection device

( relay, fuse, etc.) excluded are device such as CT

( relay, fuse, etc.) excluded are device such as CT’’s, s, CB

CB’’s, contactor, etc.s, contactor, etc.

Protection scheme is a collection of protection

equipment providing a defined function and including all

equipment required to make the scheme work (i.e.

relays, CT

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Protective Relay

Relay maybe classified according to the

technology used. technology used. electromechanical electromechanical static static digital digital numerical numerical

The different type have somewhat different

capabilities due to limitations of technology

used.

Type of protective relay

a relay that responds to single quantity

a relay that responds to several quantities

a single relay containing several elements, each responding

independently to a different quantity

I > I >

ANSI / IEC Relay Symbols

59N 59N Neutral point Neutral point displacement relay displacement relay 32 32 Directional Directional overpower relay overpower relay 59 59 Overvoltage Overvoltagerelayrelay

27 27 Undervoltage Undervoltage relay relay 55 55

Power factor relay Power factor relay

26 26 Overtemperature Overtemperature relay relay 51V 51V Voltage Voltage restrained/controlled restrained/controlled overcurrent overcurrentrelayrelay

21 21 Distance relay Distance relay 51N 51N

Definite time earth Definite time earth

fault

fault overcurrentovercurrent relay relay

14 14

Underspeed Underspeedrelayrelay

51G 51G

Inverse time earth Inverse time earth

fault

fault overcurrentovercurrent relay relay

12 12

Overspeed Overspeedrelayrelay

IEC 60617 IEC 60617 ANSI ANSI Description Description IEC60617 IEC60617 ANSI ANSI Description Description ω > ω < Z< θ > U< P> U I> cosϕ > U> Ursd>

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O I

ANSI / IEC Relay Symbols

87 87 Differential relay Differential relay 81O 81O Overfrequency Overfrequencyrelayrelay

51 51

Inverse time Inverse time overcurrent overcurrentrelayrelay

81U 81U Underfrequency Underfrequency relay relay 50 50 Instantaneous Instantaneous overcurrent overcurrentrelay relay

79 79

Autoreclose Autorecloserelayrelay

49 49 Thermal relay Thermal relay 78 78

Phase angle relay Phase angle relay

47 47 Negative sequence Negative sequence voltage relay voltage relay 67N 67N Directional earth Directional earth fault relay fault relay 46 46 Negative sequence Negative sequence relay relay 67 67 Directional Directional overcurrent overcurrentrelayrelay

37 37 Undercurrent relay Undercurrent relay 64 64 Earth Earth--fault relayfault relay

37 37

Underpower Underpowerrelayrelay

IEC 60617 IEC 60617 ANSI ANSI Description Description IEC 60617 IEC 60617 ANSI ANSI Description Description P< I< I2< U2< I>> I > I > I > I > ϕ< f< f> Id>

Zones of Protection

To limit the extent of power system that is

disconnected when a fault occur.

G

Zone 1 Zone 2 _{Zone 3} Zone 4

Zone 4 Zone 6

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G

Zones of Protection

Ideally the zones of protection should overlap. So that no part

of the power system is left un protected.

The point of connection of the protection with the power

system usually defines the zone and corresponds to the

location of CT

location of CT’’s.s.

Zones of Protection

Unit protection will result in the boundary being a clearly

define close loop.

Zone maybe un restricted, the extent or reach will depend on

measurement of the system quantities.

G

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Principal of Protection

Reliability

Operate under all required condition, and refrain from Operate under all required condition, and refrain from

operating when so required.

Incorrect operation can be attributed to one of

Incorrect design / settingIncorrect design / setting

Design: Due consideration must be given to the nature,

frequency and duration of fault, all relevant parameters of

the power system and type of protection equipment used.

Setting: The setting are chosen for protection relays and

system which take in to account the primary system, fault,

load levels etc. The characteristic of power system

changes with time change in load etc. Therefore, setting

value of relay may need to be checked at suitable intervals

to ensure that are still appropriate.

Principal of Protection

Incorrect installation / testingIncorrect installation / testing

Installation: The complexity of interconnections of

many systems and their relationship to the remainder

of the installation may make checking difficult.

Deterioration in serviceDeterioration in service

The time between operations of protection relays

maybe years rather than days. During this period

defects may have developed unnoticed until revealed

by the failure of the protection to respond to a power

system fault. For this reason, relays should be

regularly tested in order to check for correct

functioning.

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Principal of Protection

Selectivity

To trip only those circuit breakers whose operation is To trip only those circuit breakers whose operation is

required to isolate the fault. The property of selectivity

tripping is also called

tripping is also called ‘‘discriminationdiscrimination’’and is achieved and is achieved by two general methods.

by two general methods.

Time Grading

Protection systems in successive zones are arranged to Protection systems in successive zones are arranged to

operate in times that are graded through the sequence of

equipments so that upon the occurrence of a fault although a

number of protection equipments respond, only those relevant

to the faulty zone complete the tripping function. The others

make incomplete operations and then reset.

Principal of Protection

Unit Systems

The protection systems that respond only fault conditions The protection systems that respond only fault conditions

occurring with in a clearly defined zone, it does not

involve time grading, is relatively fast in operation. The

speed of response is substantially independent of fault

severity.

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Principal of Protection

Stability

The ability of protection system to remain The ability of protection system to remain

unaffected by conditions external to the

protected zone, for example through load

current and external fault conditions.

Principal of Protection

Speed

The function of protection systems is to isolate faults The function of protection systems is to isolate faults

on the power system as rapidly as possible.

The main objective is to safeguard continuity of supply by

removing each disturbance before it lead to widespread loss

of synchronism and consequent collapse of power system.

As the loading on a power system increase the phase shift

between voltages and different

between voltages and different busbarsbusbarson the system also on the system also

increases, and therefore so does the probability that

synchronism will be lost when system is disturbed by a fault,

protection must thus operate as quickly as possible.

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Principal of Protection

However speed of operation must be

weighed against economy.

Distribution circuits which do not

normally require a fast fault clearance, are

usually protected by time

usually protected by time--graded systems. graded systems. Generating plant and EHV systems

Generating plant and EHV systems

require protection gear of highest attainable

speed. speed.

Principal of Protection

Sensitivity

This is a term frequently used when referring This is a term frequently used when referring

to the minimum operating level ( current,

voltage, power etc.) of relays or complete

protection scheme.

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Primary and Back

-

Up Protection

The reliability of a power system has been

discussed earlier, including the use of more than

primary ( or main ) protection system operating

in parallel.

In the event of failure or non

In the event of failure or non--availability of availability of

the primary protection some other means of

ensuring that the fault is isolated must be

provided. These secondary systems are referred

to as

to as ‘‘backback--up protectionup protection’’..

Back

-

Up Protection

Local back

-

up protection

This is achieved by protection which detect an This is achieved by protection which detect an

un

un--cleared primary system fault at its own cleared primary system fault at its own location and which then trip its own circuit

location and which then trip its own circuit

breakers, e.g. time

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Back

-

Up Protection

R1 R2 A B D E C + + Local back

Local back--up protectionup protection

Back

-

Up Protection

R1 R2 A B C D E + + -Local back

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Back

-

Up Protection

Remote back

-

up protection

This is provided by protection that detects an This is provided by protection that detects an

un

un--cleared primary system fault at a remote cleared primary system fault at a remote location and then issue a local trip command

location and then issue a local trip command

e.g. the second or third zones of distance

relay.

Back

-

Up Protection

F R1 R2 R3 1 2 3 Current Time R 1 R 2 R 3 fault T1 T2 T3

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Back

-

Up Protection

The extent and type of back

The extent and type of back--up protection up protection applied will naturally be related to the failure

applied will naturally be related to the failure

risks and relative economic importance of the

system.

For distribution systems where fault clearance times For distribution systems where fault clearance times

are not critical, time delayed remote back

are not critical, time delayed remote back--up up protection maybe adequate.

protection maybe adequate.

For EHV systems, where system stability is at risk For EHV systems, where system stability is at risk

unless a fault is cleared quickly, multiple primary

protection systems, operating in parallel and possibly

of different type ( e.g. distance and unit protection )

will be used to ensure fast and reliable tripping.