overcurrent relays2011-2
TRANSCRIPT
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Over current protection
• Protection against excess current was
naturally the earliest protection system to
evolve
• Over current protection, on the other hand, is
directed entirely to the clearance of faults,
although with the settings usually adopted
some measure of overload protection may beobtained
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Over current relays are the most common form of
protection used to deal with excessive currents due to
Faults in power systems Over current relays use the current level to detect the
presence of faults and operate circuit breakers
Over current relays are primarily intended to operate
under faults They should not be installed purely as a means of
protecting systems against overloads
The setting of an over current relay is usually a
compromise in order to cope with both over currents and
overloads
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The operating characteristic of an over current relay can be presented as a
plot of the operating time vs. the current.
Over-current relays
This figure represents the operating time
for an independent delay time over
current relay.
It will operate always at the same time for
currents over the pick up setting This relays are defined by the pick up
current, as number of times the normal
current, and the operating time
Coordination of different protections of
this type is achieved by time delaying andpick up setting
It must be a minimum of 0,3 sec. to permit
operating of the first breaker
t
i In n*In
t 0
Relé tiempo independ.
50 (ANSI)
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Types of over current relays
Instantaneous
Definite time
Inverse time
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Definite-Current Relay
Definite-current relay operateinstantaneously when the current
reaches a predetermined value
Coordination of definite-current relays is
based on the fact that the fault current
varies with the position of the faultbecause of the difference in the impedance
between the fault and the source
The relay located furthest from the
source operate for a low current value
The operating currents are progressively
increased for the other relays when
moving towards the source
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Relay Basics
• Electromechanical Relays – Magnets, coils, transformers, springs, etc.
• Static or Electronic –
Transistors, diodes, opamps• Also magnets, coils, transformers, etc.
– Can duplicate E/M relay functions
• Digital or Microprocessor
– Computers!• Also magnets, coils, transformers, etc.
– Can duplicate E/M relay functions
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Relay Basics
•
Non-directional – Detect fault in any direction
– Operate when quantity exceeds pickup value
• Directional
– Use voltages, currents, and angles to determine
fault direction
• Trip for faults in front of relay
•Block for faults behind relay
– Operate when quantities exceed pickup value and
correct direction is determined
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T i m e ,
S e c
o n d s
Multiples of PU Current
1 21 10
Inverse
Definite Time
Modern Relay
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T i m e ,
S e c
o n d s
Multiples of PU Current
1 21 10
Inverse
Definite Time
Modern Relay
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Definite time over current relay
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Feeder Protection
1) Over Current Protectiona) Time delayed non directional (51)
b) Time delayed directional (67)
c) Instantaneous (50)
2) Ground Over Current Protection
a) Time delayed non directional (51N)
b) Time delayed directional (67N)
c) Instantaneous (50N)
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Instantaneous (50) protection
• Used for detecting high magnitude fault current
• Same time delay regardless of fault magnitude or distance
•
Co-ordination with down stream section cannot be maintained.
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Protection for 3 Phase System
a) Three Over Current One Earth
Current relayb) Two Over Current One Earth
Current relay
Ia
Ib
Ic
In=Ia+Ib+Ic
Ia
Ib
Ic
In=Ia+Ib+Ic
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E/F O/C O/C O/C
Earth Fault
E/F O/C O/C
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E/F O/C O/C
Phase Fault
O/CE/F O/C O/C
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Overcurrent Protection (50,51)
A
B
C
132 kV /33 kVA
B
C
Relay(s)Relay(s)
50/5150/51
50/51N51GN
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Earth fault Protection (50N,51N)
A
B
C
132 kV /33 kVA
B
C
Relay(s)Relay(s)
50/5150/51
50/51N51GN
Grading between 51GN & 51N necessary
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