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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 / 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.
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