Basics of Power System Protection

7/31/2019 Basics of Power System Protection

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Power System Protection

Why do we need system protection: Detect fault

Isolate faulted component

Restore faulted component

Aims:

Continued supply for rest of system

Protect faulted part from damage


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3GE Consumer & Industrial

Multilin

1. Generator or Generator-Transformer Units

2. Transformers

3. Buses

4. Lines (transmission and distribution)

5. Utilization equipment (motors, static loads, etc.)

6. Capacitor or reactor (when separately protected)

Unit Generator-Tx zone

Bus zone

Line zone

Bus zone

Transformer zoneTransformer zone

Bus zone

Generator

~

XFMR Bus Line Bus XFMR Bus Motor

Motor zone

ProtectionZones


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Zones of Protection

Zones are defined for:

Generators

Transformers

Buses

Transmission and distribution lines

Motors


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Zones of Protection

For fault anyway within the zone, the

protection system responsible to isolate

everything within the zone from the rest of

the system.

Isolation done by CB

Must isolate only the faulty equipment or

section


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Zones of Protection


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Overlapped of Protection

No blind spot:

Neighboring zones are overlapped to avoid the

possibility of unprotected areas

Use overlapping CTs:

Isolation done by CB. Thus, it must be inserted in

each overlap region to identify the boundary of

protective zones.


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Overlapped of Protection

Overlap accomplish by having 2 sets of

instrument transformers and relays for each CB.

Achieved by the arrangement of CT and CB.


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9GE Consumer & Industrial

Multilin

1. Overlap is accomplished by the locations of CTs, the key source for protective

relays.

2. In some cases a fault might involve a CT or a circuit breaker itself, which

means it can not be cleared until adjacent breakers (local or remote) are

opened.

Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at both sides of CB-faultbetween CTs is cleared from both remote sides

Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at one side of CB-fault between CTs is sensed by both relays,

remote right side operate only.

Zone Overlap


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Primary & Back-up Protection

Primary protection is the protection provided

by each zone to its elements.

However, some component of a zone

protection scheme fail to operate.

Back-up protection is provided which take

over only in the event of primary protection

failure.


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11

GE Consumer & IndustrialMultilin

Selection of protective relays requires compromises: Maximum and Reliable protection at minimum equipment

cost

High Sensitivity to faults and insensitivity to maximum load

currents

High-speed fault clearance with correct selectivity

Selectivity in isolating small faulty area

Ability to operate correctly under all predictable power

system conditions



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12

GE Consumer & IndustrialMultilin

Cost of protective relays should be balancedagainst risks involved if protection is not sufficient

and not enough redundancy.

Primary objectives is to have faulted zonesprimary protection operate first, but if there are

protective relays failures, some form of backup

protection is provided.

Backup protection is local (if local primary

protection fails to clear fault) and remote (if remote

protection fails to operate to clear fault)



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Simplicity

Economy

Speed

Sensitivity

Selectivity

Reliability


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Reliability Operate dependably and in healthy operating condition

when fault conditions occur, even after remaining idle formonths or years.

Selectivity Clearly discriminate between normal and abnormal system

condition to avoid unnecessary, false trips.

Sensitivity Ability to distinguish the fault condition, although the

different between fault and normal condition is small.


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Speed Fault at any point in the system must be detected and

isolated rapidly to minimize fault duration and equipmentdamage. Any intentional time delays should be precise.

Economy Provide maximum protection at minimum cost

Simplicity Minimize protection equipment and circuitry


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Basics of Power System Protection

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