Basic Power System Protection

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? Type of equipment

% of faults

Overhead line

45

Underground Cable

10

Busbar

15

Transformer

12

Instrument transformer

10

Control equipment

3

Other

5

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Why power system need the protection system? Type of faults

% of fault

One Phase to Earth

60

Two Phase to Earth

18

Two Phase Short Circuit

5

Three Phase Short Circuit

10

Series Fault

5

Other

2

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’ CT’s, CB’ CB’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’ CT’s, CB’ CB’s Batteries, etc.)

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Protective Relay Relay maybe classified according to the technology used. electromechanical static digital 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

ANSI / IEC Relay Symbols Description

ANSI

IEC60617

Description

ANSI IEC 60617

Overspeed relay

12

ω>

Inverse time earth fault overcurrent relay

51G

Underspeed relay

14

ω<

Definite time earth fault overcurrent relay

Distance relay

21

Z

Overtemperature relay

26

Undervoltage relay

27

Directional overpower relay

32

I

>

51N

I

>

Voltage restrained/controlled overcurrent relay

51V

U I>

θ>

Power factor relay

55

cos ϕ

U

<

Overvoltage relay

59

U

P

>

Neutral point displacement relay

59N

Ursd

<

>

> >

5

ANSI / IEC Relay Symbols Description

ANSI IEC 60617

Description

ANSI

Underpower relay

37

P

<

EarthEarth-fault relay

64

Undercurrent relay

37

I

<

Directional overcurrent relay

67 67N

IEC 60617 I

> I >

Negative sequence relay

46

I2

<

Directional earth fault relay

Negative sequence voltage relay

47

U2

<

Phase angle relay

78

Thermal relay

49

Autoreclose relay

79

Instantaneous overcurrent relay

50

Underfrequency relay

81U

f<

Inverse time overcurrent relay

51

Overfrequency relay

81O

f>

Differential relay

87

Id >

I

>> I>

I

>

ϕ< O

I

Zones of Protection To limit the extent of power system that is disconnected when a fault occur. Zone 1 Zone 2

G

Zone 3

Zone 4

Zone 6

Zone 5

Zone 4

6

Zones of Protection G

G

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’ CT’s.

Zones of Protection G

G

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.

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Principal of Protection Reliability „

Operate under all required condition, and refrain from operating when so required. Incorrect operation can be attributed to one of „

Incorrect 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 / testing Installation: The complexity of interconnections of many systems and their relationship to the remainder of the installation may make checking difficult. Deterioration 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.

8

Principal of Protection Selectivity „

To trip only those circuit breakers whose operation is required to isolate the fault. The property of selectivity tripping is also called ‘discrimination’ discrimination’ and is achieved by two general methods. Time Grading „

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 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 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 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 busbars 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.

10

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-graded systems. Generating plant and EHV systems require protection gear of highest attainable speed.

Principal of Protection Sensitivity „

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-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 ‘back-up protection’.

Back-Up Protection Local back-up protection „

This is achieved by protection which detect an un-cleared primary system fault at its own location and which then trip its own circuit breakers, e.g. time-graded over current relay.

12

Back-Up Protection C

A

D B E R1

R2 + +

Local backback-up protection

Back-Up Protection C

A

D B E R1

R2 +

-

+

Local backback-up protection ( Breaker failure relay )

13

Back-Up Protection Remote back-up protection „

This is provided by protection that detects an un-cleared primary system fault at a remote location and then issue a local trip command e.g. the second or third zones of distance relay.

Back-Up Protection 3

2

R3 Time

1

R2

R1

F

fault

T3 T2 T1

R R3 R2 1 Current Remote backback-up protection

14

Back-Up Protection The extent and type of back-up protection applied will naturally be related to the failure risks and relative economic importance of the system. „

„

For distribution systems where fault clearance times are not critical, time delayed remote backback-up protection maybe adequate. 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.

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