7.2 Restricted earth fault protection
for a transformer with neutral
connection
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7.2.1. CT Requirements
Any difference between the CTs will give a misleading residual
current signal to the relay. Especially during heavy through
faults (i.e. the fault is outside the protected zone) the dissimilar
saturation of the CTs should not yield to a REF trip. On the
other hand a very high fault current causing an unselective
earth fault start or trip is not a fatal error.
Class X CT
In restricted earth fault protection the high and low side CTs
should give similar responses even for high over currents.
Class X CTs will fulfil this requirement. Their performance is
defined in terms of a knee-point voltage (V
KP
), the magnetizing
current at the knee point voltage and the resistance of the
secondary winding at +75 °C.
Knee point voltage (V
KP
) is the secondary voltage at which a 50
% increase of primary current is needed to increase the
secondary voltage by 10 %.
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