Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 87 (504
)
A996A
most probably directional earth fault may not even pick-up and if it picks up it will surely
release before set operating time. Residual voltage still stays on for longer period and
most probably in this case it will also release before the set tripping time. This situation
may last for long time and cause stress to the network unnecessarily and if let to last long
may cause insulator breakdown in other parts of the network.
In the under- / overcompensated network case the residual voltage stays on almost
maximum level all the time. In addition to that the current flashover spikes are seen
constantly in every power cycle. In this case normal FFT based directional earth fault
protection algorithms lose the direction sensing due to FFT processed input signal
expects power cycle long stabile data for accurate directional output. In this case the zero
crossings during the power cycle are multiple so FFT result may be anything in between
of 0 – 180 degrees. When analyzing the situation on the normal directional earth fault
protection point of view the result may be no trip at all or trip to non faulted feeder in
addition to faulted feeder expected trip. All these three scenarios are equally probable.
3.2.6.1 I
NTERMITTENT ALGORITHM DESCRIPTION
Patent pending. Further information by request.
3.2.6.2 S
ETTING PRINCIPLES
The intermittent earth fault protection shall be coordinated with bus bar residual voltage
protection in such a way that in case of intermittent earth fault the faulty feeder will be in
all cases tripped by intermittent earth-fault protection function prior to residual voltage
protection function considering a sufficient safety margin as well. On the other hand, since
an intermittent earth fault causes significant network stress the protection trip should be
performed as fast as possible.
The strike through time of an intermittent earth fault in a close to resonance tuned network
sets the limit for the minimum operate time of an intermittent earth fault protection stage.
To ensure a correct protection operation in all cases, the reset time of intermittent earth
fault stage shall be set according to the network in question, to such a level that ensures
that fault has disappeared and no new strike through is expected after set reset time.
The size of the network is a dominant factor in defining the strike through time interval. In
larger network in amperes a less frequent strike troughs can be expected. The following
can be presented as a rule of a thumb: in a small / medium size networks (<60A)
approximately. 250 - 350ms strike through interval and in a large network (~100A)
approximately. 500ms strike through interval is expected
. As a recommended practice it