Vk and Rct requirement

[Sargardani]

A certain protection relay has CT requirements of Vk > 50In (Rct + 2Rl + Rr). Where In is unity, and (2Rl + Rr) = 3. What would be the consequences if the Vk and Rct requirements are not 100 % met by the CT supplier? i.e Vk of the CT is lower than the calculated value (by the above formula).

Also I am not an expert with relays..... but is there normally a setting in the protection relays such that the relay can be compatable with the CT supplied.

Appreciate all your help.



Sarg

 

[Skogsgurra]

I think that you should post this question in the Electric power engineering forum. This forum is about SSR (Solid State Relays) and few people in here know about protection relays.

 

[RalphChristie]

Sargardani

Can you please provide more information? (Kind of protection scheme, relay type etc)

It seems like a kind of zone (differential) protection scheme. Since operation of this relays are dependant upon the magnitude and balance of currents , the performance requirments of the CTs are specified in terms of knee-point voltage. The knee-point voltage may be regarded as a practical limit beyond which a specified ratio may not be maintained. Beyond the knee-point the CT becomes saturated. Thus the scheme can become unstable with heavy through-faults.

Regards
Ralph

 

[Vegemite]

The best thing to do is ask the manufacturer of the relay if the spec for the CT is satisfactory given the particulars of the application.
You stated that (2Rl + Rr) = 3 Ohm. I assume that Rl is the one way resitance of the leads between the CT and the relay, but what is Rr?
Rl can be reduced by either shortening the length of the lead run or using conductor of larger cross section. Doing either of these will reduce the calculated minimum value of Vk.
What kind of relay is this, what are the details of the power system and what is it protecting?

 

[tlpelec]

1).What is the purpose of calculating knee point voltage for Current transformers.

2). How to choose a CT for a particular application.

 

[RalphChristie]

tlpelec

1).What is the purpose of calculating knee point voltage for Current transformers?

You have first to understand what is the knee-point voltage of a CT: It is the transition from the unsaturated to the saturation region of the open circuit excitation characteristic of the CT. (because it is a rather gradual process, is it difficult to define this transition and use is made of the knee-point voltage for this purpose.) Thus the knee-point is the point between the saturated and the unsaturated region of a CT. During big faults (depending on the connected burden) the CT can saturate and thus the specific turns ratio may not be maintained.

The purpose of this point is to determine if there is sufficient driving voltage during a big fault to push secondary current through the connected load or burden.

How to choose a CT for a particular application?

That would depend on the application.
For normal protective CTs you have to specify (IEC):
Rated primary current
Turns Ratio
Rated burden
Accuracy class
Accuracy limit factor
For special purpose applications, ClassX CTs you have to specify (IEC)
Rated primary current
Turns Ratio
Rated knee-point voltage at max. secondary turns
Max. exciting current at rated knee-point voltage
Max. resistance of secondary winding

To receive a more specific answer you'll have to provide more data/information.