Answer / william lobo

Knee Point voltage is nothing but a point at which the CT
saturates. Thats why its always better to design the CTs
which can operate without saturation even at a voltage
appears in the CT during fault condition. Pls find below
the following formula for calculating the Knee point
voltage.

Vk=2If(Rct+2Rl).

where, If is the current refected to the CT secondary
during fault condition,

Rct is the resistance of the CT which can be adopted by
means of a thumb rule considering the CT ratio,

Rl is the lead resistance i.e. sum of the resistance of the
wire connected between the relay & CT and Impedance offered
by the internal circuitary of the relay

For PS class CT's designed considering the above,burden is
not applicable.

Answer / 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.

Answer / william lobo

For PX class CTs , the burden is not normally mentioned .
There the CT is defiened based on the CT kneepoint
voltage , magnetising current etc . The burden is mentioned
in class P CTs

In PX class CTs the kneepoint voltage is normally based on
the max fault current and a diamensioning factor based on
system X/R ratio

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