l2-fundamentals of protection
Post on 19-Jul-2016
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1
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
2
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
3
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
4
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
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