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Question
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.
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
 
Answer
# 1
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.
 
Is This Answer Correct ?    6 Yes 0 No
William Lobo
 
Answer
# 2
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
 
Is This Answer Correct ?    3 Yes 2 No
William Lobo
 
 
 
Answer
# 3
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.
 
Is This Answer Correct ?    3 Yes 3 No
Williamlobo
 

 
 
 
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