Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 88 (504
)
A996A
can be stated that the reset time of an intermittent earth fault stage should not be set
lower than 450ms to obtain a network independent setting.
Using this reset set value one
can be sure that function will not reset too early even in resonance tuned network.
Typically the maximum operate time of the intermittent earth-fault function is dictated by
bus bar residual voltage protection. If the residual voltage protection is set to very fast
tripping it may be necessary to prolong the set value of the same.
As a recommended
practice it can be stated that the operation time of an intermittent earth fault stage should
be 500ms counting from the first strike through.
In this case the protection tripping
requires minimum two strike troughs even in resonance tuned network where strike
through happens less frequently. If the residual voltage protection is set to very fast
tripping (<1s) it may be necessary to verify the reset value of the residual voltage
protection. In no case shall the residual voltage protection operate time be faster than set
intermittent earth fault operate time plus circuit breaker operate time added with reset time
of 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 for both healthy and faulty
feeder relays. In general, if intermittent earth fault protection is not used to block
directional earth fault protection, it shall be verified that the operate time of regular
directional earth fault protection is longer than set intermittent earth fault protection
operate time. It is recommended to block regular directional earth fault protection only to
avoid start events of 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 faster operate time than intermittent earth fault protection.
If the intermittent earth fault protection should be set for optimal operation, too 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 xI0n
FWD reset time 0.250 s
REV reset time 0.250 s
Definite operating time delay 0.500 s
Spikes to trip > 2
Best verification for the settings is field test with intermittent earth fault capable test
system. Each network characteristics may vary significantly from each other. By following
basic rules presented in this chapter the correct setting range should be easier to define.