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The strike-through time of an intermittent earth fault in a network tuned close to resonance sets the
limit for the minimum operating time for an intermittent earth fault protection stage. To ensure a correct
protection operation in all cases, the reset time of an intermittent earth fault stage will be set according
to the network in question, to such a level that ensures that the fault has disappeared and no new
strike-throughs are expected after a prescribed reset time.
The size of the network is a dominant factor in defining the time interval of a strike-through. One can
expect less frequent strike-throughs in larger (in amperes) networks. The following can be presented
as a rule of a thumb: in a small or medium size network (<60 A) the strike-through interval is
appr. 250...350 ms, in a large network (~100 A) it is appr. 500 ms. It is recommended that the reset
time of an intermittent earth fault stage should not be set lower than 450 ms in order to obtain a
network independent setting. Using this recommended value one can ensure that the function will not
reset too early even in resonance tuned networks.
Usually the maximum operating time of an intermittent earth fault function is dictated by the residual
voltage protection of the bus bar. If the residual voltage protection is set to very fast tripping, it may be
necessary to also prolong its set value. It is recommended that the operating time of an intermittent
earth fault stage should be 500 ms counting from the first strike-through. Using this recommended
value the protection tripping requires a minimum of two strike-troughs even in resonance tuned
networks in which strike-throughs occur less frequently. If the residual voltage protection is set to very
fast tripping (<1 s), it may be necessary to verify the reset value of the residual voltage protection. The
residual voltage protection operating time will never be faster than the sum of the following: the
prescribed intermittent earth fault operating time, the circuit breaker operating time, and the reset time
of the residual voltage protection stage.
If an intermittent earth fault protection start is used to block regular non-intermittent directional earth
fault protection, the blocking should be applied to both healthy and faulty feeder relays. In general, if
intermittent earth fault protection is not used to block directional earth fault protection, it should be
verified that the operating time of regular directional earth fault protection is longer than the set
intermittent earth fault protection operating time. It is recommended to block regular directional earth
fault protection to avoid start events in directional earth fault protection during intermittent earth faults
(if start events are considered disturbing), or if directional non-intermittent earth fault protection is set
to a faster operating time than intermittent earth fault protection.
If intermittent earth fault protection would be set for optimal operation, sensitive pick-up settings should
be avoided. General setting parameter values are presented below.
Setting parameter
Value
U0 Detect spike >
60 %
I0 Detect spike >
0.5 x
I0
n
FWD reset time
0.250 s
REV reset time
0.250 s
Definite operating time delay
0.500 s
Spikes to trip >
2
The best verification for the settings is a field test with a test system capable of intermittent earth faults.
One network characteristic may vary significantly from another. By following the basic rules presented
in this chapter it should be easier to define the correct setting range.
It is also important to check that the reset time settings are never set longer than the desired operating
time delay setting.
A
AQ
Q-C255
-C255
Instruction manual
Version: 2.06
116
© Arcteq Relays Ltd
IM00035
Содержание AQ-C255
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Страница 363: ...Figure 7 4 180 Example block scheme A AQ Q C255 C255 Instruction manual Version 2 06 Arcteq Relays Ltd IM00035 361 ...
Страница 384: ...Figure 8 14 202 Device installation A AQ Q C255 C255 Instruction manual Version 2 06 382 Arcteq Relays Ltd IM00035 ...