How can you calculate in detail the VA burden of current
transformer used for Restricted earth fault protection?
Please understand that the CT required here is Class PX or
X. Also please show the calculations for knee point vtg,
stabilizing resistor, magnetising current.
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Answer Posted / williamlobo
Restricted earth fault protection is a differential scheme,
with CTs on the transformer neutral and phase leads. The
phase CTs are connected together in parallel & then
paralleled with the neutral CT. The resticted earth fault
(REF) relay is connected across the paralled CT circuits.
The relay is either a voltage operated device (eg Alstom
MFAC14)or a current relay with an external setting resistor
(eg Alstom MCAG14). In either case, the scheme is set at a
voltage based on the maximum voltage that is developed at
the relay for an external fault. A practical setting is
50% of the CT knee point voltage, if full info is not
available.
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