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POWER SYSTEM PROTECTION

J.B. Ekanayake

PROTECTION IS INSTALLED TO :Detect fault occurrence and isolate the faulted equipment.

How do you detect a fault?

How do you minimise the disruption to adjacent loads?

What are the risks associated with a fault?

Synchronous Generator

33kV

T1

CB1 (Auto Reclose)

CB2

Fuse

T3

11kV

CB4CB3 (Auto Reclose)

T2

Induction Generator

FAULT

Fire in a 500 MVA transformer Fault current contribution from Synchronous generators

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Fault current contribution from Induction generators

Fault current contribution from Power electronically connected

generatorsPower electronically controlled generators are now emerging.If the power electronic converter is:

‘Thyristor’ based converters can contribute to 2 -3 times rated current during a fault.The fault current contribution of an ‘IGBT based converter is equal to full load current. Can design with higher rating, but cost will go up.

Types of Fault

3Øabc

abc

e

Ø/E

Ø/Øabc

Ø/Ø/E

e

abc

3Ø/E

e

abc

Generator

Transformer

CB

RelayCT

VT

Relay is a logical device, which detects the faults and instructs CB operation.

A relay takes a decision based on the measurements received from current transformer (CT) and voltage transformer (VT).

CTs and VTs reduce primary current and voltage to a low level suitable to input to relay

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Zones of Protection

CB1 provides protection for respective lines and back-up protection for down steam lines - Called Non-unit protection

Generators and transformers have their own protection - Called unit protection

GENERATION ZONE

BUSBARZONE

TRANSF-ORMERZONE

BUSBARZONE

FEEDERZONE

BUSBARZONE FEEDER

ZONE

OVERLAP ZONE

CB1

Main types of protection• Over current

• For distribution feeders and transmission and sub-transmission circuits

• Differential• For feeders, busbars, transformers, generators,

etc.

• Distance• For distribution feeders and transmission and

sub-transmission circuits

Typical relays according to IEEE/ANSI designation

50 Three-phase instantaneous51 Three-phase time-delayed overcurrent (IDMT)

51V Three-phase voltage controlled time-delayed o/c51N Earth fault time-delayed overcurrent

32 Reverse power40 Loss of excitation87 Phase differential

87N Earth fault deferential27/59 Under or over voltage

81U/81O Under or over frequency46 Negative phase sequence

67N Sensitive directional earth fault64 Rotor field protection

Cost

DISTRIBUTION SYSTEMS• Large numbers of switching and distribution

points, transformers and feeders

• Economics often overrides technical issues

• Protection may be the minimum consistent with statutory safety regulations

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CostTRANSMISSION SYSTEMS• Emphasis is on technical considerations rather

than economics

• Economics cannot be ignored but is of secondary importance compared with the need for highly reliable, fully discriminative high speed protection

• Higher protection costs justifiable by high capital cost of power system elements protected

• Risk of security of supply should be reduced to lowest practical levels