Motor Protection Self Balancing Differential Current Transformer

[elmatador]

Hello All,

We have a several 4kV motors, horsepower range is anywhere from 2000HP to 3500HP. These motors come with 50:5 self balancing differential CT. The accuracy class of these CT's is C10. The worst case distance of the motor from the Multilin 469 Motor Protection Relay is 180 meters. I am waiting on the CT curve for this specific CT. For a preliminary cable size we were going to run a #10AWG. However the cable lead resistance exceeds the 0.1Ohm burden rating of the CT. I spoke to the motor vendor and they said that they are only able to supply C10 CT's. What issues can you see with this setup where the cable leads already exceed the rating of the CT. I don't have the excitation curves yet to check saturation. Instantaenous and overcurrent protection will be handled by phase CT's. We also have a zero sequence CT for ground fault protection.

BTW Differential Protection requirement was part of our client specification.

 

[elmatador]

Anyone?

 

[oldfieldguy]

I just looked up AC resistance and reactance for #10AWG and got 1.1 ohms per thousand feet. Your 180 meter loop would be 1180 feet, or 1.3 ohms. The listed burden for a GE SR-469 relay's current input is <0.2 VA. Typical setpoints for current balance CT's in motor differential service run in the 0.10 x CT range. While a massive winding fault might push your CT into saturation, the initial effect is that the differential element of the relay will operate as expected. It's an instantaneous element. It cares little about waveform shape, only that the magnitude exceeds the trip setpoint.

I have seen current balance CT's in motor differential service in the 300:5 range, but that was when the differential relay was an electromechanical type that offered a much higher burden.

old field guy

 

[elmatador]

Thank you kindly for your response! Much appreciated!