CT response to DC current

[HiSet]

Recently numerical Trf Diff relay of an out of service transformer picked up when it’s HV dead tank circuit breaker was being tested for contact resistance by injecting 200A of DC current.I am bit curious to find out CT behaviour or response to pure DC current. Please share your views

 

[wcaseyharman]

My understanding is the application of DC current would cause a voltage spike in the secondary, but as the primary current approached steady state the secondary current would approach zero.
Typically I think CTs are shorted during testing for this purpose.

 

[thermionic1]

"Typically I think CTs are shorted during testing for this purpose."

That typically requires the relay group to go out there and short the CT's for the maintenance / doble crew. High Impedance Bus dif CT's are a different challenge.

I commission HV breakers among other things. Some new, some are used and moved around in the system. We have a modern, pure DC, computer controlled 100A unit, with switchable ranges. I can test pole #1 in 100A, test again in 10A with a negligible difference. Start pole #2 on 10A, repeat on 10A with similar results. I think some of the standards were written at a time where the technology of the test equipment was limited. Most modern test sets have the ability to test with both sides of the breaker grounded, by using a clamp on that substracts the ground current.

I had a conversation with a construction manager that was proposing that every crew purchase an equalpotential grounding mat to be used when timing circuit breakers. I suggested to him that instead of having each crew spend $5k for the mat, they stop specifying a breaker timer that is not able to test with both sides grounded. I showed him a presentation of a variety of vendors that make this technology. I was met with deafening silence.

 

[stevenal]

HiSet,
Like using a battery for checking polarity, you get a sharp spike as wcaseyharman stated. You are likely to have left CTs magnetized. I suggest de-magnetizing them.

 

[dfdt]

Thanks for your valuable comments and yes CT magnetisation/demagnetisation is another issue. Could you please share the test equipment details which can do contact resistance measurement with both side grounded as mentioned by thermionic1?

 

[thermionic1]

https://us.megger.com/200-a-micro-ohmmeter-with-dualground-safety-mjolner200-2

there are others as well