Basic power system protection

Cơ bản về bảo vệ hệ thống điện 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’s, 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’s, CB’s Batteries, etc.)

POWER SYSTEM

PROTECTION

Fundamentals of

Protection Practice

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 Type of equipment

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.

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’s,

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’s, CB’s Batteries, etc.)

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

I >

I >

ANSI / IEC Relay Symbols

59N Neutral point

displacement relay 32

Directional

overpower relay

59 Overvoltage relay 27

Undervoltage

relay

55 Power factor relay 26

Overtemperature

relay

51V

Voltage restrained/controlled overcurrent relay 21

Distance relay

51N

Definite time earth fault overcurrent relay 14

Underspeed relay

51G

Inverse time earth fault overcurrent relay 12

Overspeed relay

IEC 60617 ANSI

Description IEC60617

ANSI Description

ω >

ω <

Z <

θ >

U<

P>

U I>

cos ϕ >

U>

U rsd>

O I

ANSI / IEC Relay Symbols

87 Differential relay

81O Overfrequency relay 51

Inverse time

overcurrent relay

81U Underfrequency relay 50

Instantaneous

overcurrent relay

79 Autoreclose relay 49

Thermal relay

78 Phase angle relay 47

Negative sequence

voltage relay

67N Directional earth fault relay 46

Negative sequence

relay

67 Directional

overcurrent relay 37

Undercurrent relay

64 Earth-fault relay 37

Underpower relay

IEC 60617 ANSI

Description IEC 60617

ANSI Description

P<

I<

I 2<

U 2<

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

Zone 5

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’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

G

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

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’ 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.

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.

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

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

Back-Up Protection

A B D

E

C

+ +

Local back-up protection

Back-Up Protection

A B

C

D

E

+ +

-Local back-up protection ( Breaker failure relay )

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

F

R1 R2

R3

1 2

3

Current

Time

R 1

R 2

R 3

fault

T1

T2

T3

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 back-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.