Knee Point Voltage and Current Transformers

Dear friend,

Knee Point voltage is nothing but a point at which the CT saturates. Thatsy 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.

Pls revert back if you have any further queries.

Regards,

Manimaran.V

dear friend,

thank you for your guidence, how i calculate the fault current limit in primary side with knee point voltage

Dear Friend,

Pls find below the formula for calculating the fault current,

Fault current(in KA) = Rated Curent/(% Impedance*1000)

% Impedence shall be calculated for the system from the point of source to the point of occurence of the fault.

Regards,

Manimaran.V

Dear Manimaran,

Can you please explain, how to calculate Ia and what is its significance?

dear friend,

thankyou friend

Dear member

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

Ayub

Understand from above formula to calculate knee point voltage. Can you please explain how to calculate magnetising current.

There is no formulae as such for calculating the magnetising current . However some a application demands minimum value of mag current for relay setting ( eg; REF protection ). Iin any case a magnetising current of less than 30 mA would be sufficient for most of the application

ya.. thats right.. there is no formula for calculating magnetising current. however, lesser the magnetising current, better is your CT performance.

Regards,

Manimaran.V

The formula for calulating the required magnetising current for the CT =

[(IOP / CT Ratio) - IR] / n

Where,

IOP = Relay operating current (absolute current value in terms of CT primary)

IR = Relay setting current (in terms of CT secondary)

n = Number of parallel CTs forming (87) protected zone