Motor Diff with Variable Speed Drive

[cesgibis]

Hi
We are specifying medium volatge induction motors (6.6kV) with biased differential protection above 1500kW. However some of these motors will be supplied by Varibale Speed Drives. A Supplier is saying that motors driven by VSDs should not be protected by differential protection because at low speed, the CTs will saturate and the diff protection would trip the motor spuriously. Does anyone has experience in this or ever installed Diff prot on VSD motors ?
Thanks

 

[Skogsgurra]

I have lots of experience with the first part of the problem. It is not possible to measure motor current with normal CT:s - you have to use current transducers that can measure down to DC. There are many possible technologies; Hall effect, Magneto Resistors, Shunts with isolation amplifiers, Faraday effect (optical) and Magamps (so called Transductors) and a few other that I haven't been in touch with.

But standard CT:s won't work well. That is true.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[cesgibis]

thanks Skogsgurra. What about voltage measuremnents at the output of the VFD ? I assume we cannot use voltage transformers neither ?

 

[Skogsgurra]

Voltage transformers are possible to use.

The reason is that the volt-seconds area that the motor sees is quite constant, independent of motor speed. That does not apply to CT:s since you can have high current at low frequencies, which saturates the core. That does not happen in voltage transformers.

Then, another question is: is it current or voltage unbalance you want to check?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Are you buying a MV drive that is incapable of providing you with this information, or are you insisting on not trusting it to do its job? Remember, once you have a drive, EVERYTHING that gets to the motor comes FROM the drive, it is essentially a new dedicated power source. So the drive will know exactly what it is sending to the motor, and in the case of modern drives with vector control capabilities, the information is sliced up into very small time slices. This tends to make them as good, if not better, than many external protection relays because the same information that the drive needs to make it function as a drive is available for protective purposes at the same time. Granted, differential protection is not done, and I understand the desire to attempt to prevent collateral damage. But honestly, in my experiences whenever a motor fails that HAS had differential protection on it, there was collateral damage anyway. I personally feel that the paradigm for adding diff protection hardens back to the days of electro-mechanical numeric relay protection systems, and the speed and accuracy of modern electronic protection schemes supplants the need. Just an opinion though.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington