What kind of test for bushing CTs on 12kv breaker

[cgetty]

Hi

We had a 12kv (vacuum) breaker in our sub replaced due to a fault on the primary bushing.

The damaged breaker was replaced with a new one. Unfortunately the current return for the 3 relaying Cts were open.
The new breaker was in service like this (open circuited) for about two months.

My question is what kind of test need to be done to confirm that these Cts are working correctly?

Clark

 

[scottf]

Ratio Check
Excitation test
Secondary resistance test
Secondary hi-pot test
Megger Test...secondary winding to ground

If the CT has a protection class rating and the current throught the breaker bushing was anywhere close to rated current for any significant amount of time, I would seriously consider getting new CTs. Even if they test ok now, their life was undoubtedly severely shortened.

 

[cgetty]

Hi Thanks for the fast responce.

I agree with you. Looking for new Cts

Thanks
Clark

 

[amps21]

Scottf said it all, just a suggestion, if u test CTs in field always demagnetize before and after any / all tests. Use sec. AC excitation.

 

[waross]

De-magnetizing. I was baffled by some posts in a previous thread that recommended using DC to de-magnetize. My old Timex, years ago, would get magnetized whenever I serviced a DC motor and run slow. I soon learned that when I left a DC motor I must go immediately to a large AC motor and hold the Timex close to the windings.
Question. If you used a Variac and a transformer to push a CT into saturation and then reduced the current to zero would that effectively demagnetize it. Do you have to go all the way to saturation or just exceed the hysterisis. I understand that you apply the demagnetizing current to the secondary winding.
Thanks

 

[scottf]

Certainly DC will not demagnatize.

The use of a variac as described is the most common method. Theoretically, it's best to drive the CT in to saturation. However, in practice, driving the CT up to its maximum current rating (rated current x rating factor) and then lowering it slowly back to zero will do the trick.

 

[amps21]

There have been experiments done with DC which claim effective demagnetization with DC. I dont remember the exact paper by IEEE on this.
Although i have been successful with Sec. AC excitation.