Protection of Motor-Transformer block. 1

[slavag]

Dear All.
Im locking for MV ( about 5MW ) block Motor-Transformer protection scheme solution.
On the motor side a system is ungrounded.
Issue is additional capcitor banks for the PF correction.
Capacitor bank will be installed in the 87T zone.
What is better: used 3-winding 87T functionality or connect motor and capacitor CT's in parallel?

Next point. Motor will be ungrounded,Trafo on the motor side is delta. Are capacitor bank should be grounded?

Are someone have expirience with such configuration?
Could you please send some link to examples.

Thanks in advance.
Best Regards.
Slava

 

[rbulsara]

I would think, you want to keep the capacitors out of the transformer diff. zone, unless you work out a way to cancel the effect of two currents of different power factor. Summation could work. ( I assume the caps are on the motor side).

Grounding the caps will make the delta side grounded through the capacitive reactance. Not sure it poses any issues, especially if the bank is wye-connected. If the original idea was to keep the motor side ungrounded for higher availability, you do not want to ground the caps.
I would think there is a means to detect the first ground fault.



Rafiq Bulsara

http://www.srengineersct.com

 

[slavag]

Hi Rafiq.
Yep, you understand right.
Capacitor will be out of diff zone and caps on the motor side.
From the other hand, summation could work, no extarnal/outside SC.
We will install CT with two cores, one for 87T and one for the cpas protection ( unbalnce, 50, etc).

Thnaks a lot.
Best Regards.
Slava