Power systems Protection

course

Department of Electrical Energy Engineering

Dr Audih alfaoury 1

Al-Balqa Applied University

Dr Audih alfaoury 2

Part 4

Zones of Protection

Zones of protection

Is the place or the distance that the relay can protect

easily.Fig.1 shows an example of three protection zones for

three circuit breakers. It is to be noted that the protection

zones are overlapped.

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Fig.1

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Nota:-In power systems, all power system elements must be

covered by at least one zone

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The important elements must

be included two zones or

more.

Zones must overlap to prevent

any element from unprotected.

The overlap must be finite or

limited, but small to minimize

the probability of outcome

fault from inside of this region.

A zone boundary is usually

defined by a CT and a CB.

The CT provides the ability

to detect a fault inside the

zone The CBs provide the ability to

isolate the fault.

The protection zone may be

open or close zone. Dr Audih alfaoury

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CLOSED AND OPEN PROTECTION ZONES

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Backup protection

It is an alternative protection that work when primary

protection fails to operate.

Primary and backup protection are independent ( relay,

breaker , CT,PT) .

Backup is slower time than primary.

Sometimes backup protection opens more circuit

breakers (that net necessary) to clear the fault.

backup protection become primary protection when usual

primary equipment out of service.

Back up relaying may be installed locally, in the same

substation (local), or remote in other substations(Clears

fault on station away from where the failure has

occurred).

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Primary and Back up

protection at the same

location ( Local Backup)

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Local backup

protection at

different locations

VT

CT CB

VT

CT

CB

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Remote back up protection

Selectivity and zones of protection

Selectivity is defined in terms of regions of a power

system (zones of protection) for which a given relay is

responsible.

The relay will be considered secure if it responds only to

faults within its zone of protection. Dr Audih alfaoury

CT

VT VT

CT

CB CB

Primary Back up

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Examples of Backup Protection:

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Example 2

The tie circuit breaker (T) is assumed to work normally closed.

For a fault at CD, Line Breakers 5 and 6 should operate as the

primary protection. If Protection 5 fails to operate, with existing

technology, we have two possibilities for cutting the fault current

contribution from A, B, and F: open Breakers 1, 3 and 8; or open

Breakers 2 and T. In any case, backup protection needs time

delay. The primary protection needs to be given an opportunity to

operate before using the decision of a backup operation Dr Audih alfaoury

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Local Backup( in same substation) Breakers 2 and T represent local backup protection, which is located in the same substation as the primary protection. Local backup protection is more expensive than remote backup because additional equipment is needed. Advantages of local backup over remote backup are greater sensitivity, greater selectivity, and faster operation speed.

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Remote Backup ( in same substation)

Breakers 1, 3 and 8 are located in a remote substation. This is the remote

backup protection. An advantage of remote backup protection is low cost:

the remote backup protection comes from protection equipment that is

needed for primary protection functions of adjacent system elements.

Therefore, there is no need for additional investment.

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Example 2: Consider the following simple power system:

Discuss the local and remote backup protection for two fault locations as follows:

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Case 1: Local Backup

For fault at F1: Suppose that breaker J operate and breaker H failed to operate. Therefore: G and I must operate as local backup protection

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For fault at F2: Suppose that breakers P and Q operate and breaker M failed

to operate. Therefore:

L must operate as local backup protection and it should tripped by

communication signal (Transfer Trip).

Case 2: Local Backup

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Case 3: Remote Backup

For fault at F1: Suppose that breaker J operate and breaker H failed to

operate. Therefore:

E, F, L and M must operate as Remot backup protection

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Case 4: Remote Backup

For fault at F1: Suppose that breakers P & Q operate and breaker M failed to

operate. Therefore:

I, B, and C must operate as Remot backup protection Dr Audih alfaoury