CT specification

[Sargardani]

How important is it to know that knee point voltage of a CT? Does it impact the design of the protection and metering systems? Is the formala; Vk > 50(Rct =3) where Rct is the resistance of the CT at 75C and Vk is the minimum knee point voltage, correct?

Any and all help shall be highly appreciated.

Sarg

 

[DiscoP]

If the Vk of a CT is too low, protection relays will either not operate or if you are lucky operate very slowly.

The minimum Vk required depends on a number of factors - eg system X/R, max fault level, lead lengths etc.

This all depends on the type of protection being applied.

It is not possible to determine if Vk > 50 is OK with the information you have supplied

 

[RalphChristie]

In my opinion:

1. The best way to find the kneepoint of a CT is:
From the nameplate (Class-X CT) or
from the manufacturer or
to do a mag-curve on the CT.

2. The knee-point voltage is extremely important in differential schemes, especially in schemes like high impedance REF-protection.

3. The knee-point is also important in normal O/C and E/F protection schemes, especially in the older types of electromechanical relays, because the relays can have large burdens. But I think with the newer type of electronic relays, with their very low burdens, it is not that important anymore. (if the CT is rated for the max short-circuit current) Maybe someone with more knowledge of CTs can correct me?

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

Regards
Ralph

 

[DiscoP]

I would suggest that the knee point voltage is important in all protection schemes.

It is just that the required knee point voltage is lower on modern overcurrent relays compared to high impedance diff schemes.