Figure. 5.3.7 - 55. Undercompensated medium size network intermittent earth fault seen by healthy feeder relay.
As can be seen from the figures above, the residual voltage is high both in the network tuned close to
resonance and in the undercompensated network. In the case of a normal directional earth fault
protection, a network tuned close to resonance would probably not even pick up on the fault, and if it
did it would release before the set operating time. The residual voltage stays on for a longer period of
time. Although the release would most likely come before the set tripping time, this situation could last
for quite some time and put a lot of unnecessary stress on the network, possibly causing an insulator
breakdown in another part of the network.
In undercompensated and overcompensated networks the residual voltage stays near the maximum
level all the time, and current flashover spikes occur every power cycle. In this case, normal FFT-based
directional earth fault protection algorithms lose their directional sense because an FFT-processed
input signal expects the power cycle to provide long, stable data for accurate directional output. There
are multiple zero crossings during a normal power cycle and therefore the FFT result may be anything
from 0 to 180 degrees. When analyzing the situation from the point of view of normal directional earth
fault protection, the result may be an expected trip in a faulty feeder, a false trip in a healthy feeder, or
no trip whatsoever, all equally probable.
Setting principles
The intermittent earth fault protection will be coordinated with bus bar residual voltage protection. This
way, during an intermittent earth fault, a faulty feeder's protection function will trip in all three previously
described scenarios. Also, an intermittent earth fault protection function tripping before the residual
voltage protection function results in a sufficient safety margin. However, since an intermittent earth
fault causes significant network stress the protection trip should be performed as fast as possible.
A
AQ
Q-C255
-C255
Instruction manual
Version: 2.06
© Arcteq Relays Ltd
IM00035
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