Motor short circuit current / differential protection

[collies99]

Motor Data: 3500 HP SCIM, 95% efficiency, 4160V, Xd" = .18 or 1.133 ohm on 6.29 ohm base. Full voltage start cct.

CT Data: I have three zero seq CTs at the motor in standard differential config; T1-T4, T2-T5, T3-T6. It is a C50, ratio 100:5A.

My CT loop resistance measured at 2.3 ohms and CT sec winding resistance at .5 ohms.

I have doubt that the differential protection will work for high magnitude internal motor fault current. CT will saturate and unable to detect phase-to-phase faults in the group jumps or in the stator slots. I have a GE Multilin M60 which have filter and respond to fundamental frequency only. Low level faults I have no issues, just high magnitude phase-to-phase faultsfault even terminal faults downstream of the CTs.

 

[collies99]

correction: CT sec winding resistance is .05 ohms

 

[dpc]

The CTs do not saturate instantly. In many cases the relay will trip on instantaneous prior to saturation. This is similar to the application of zero sequence CTs on feeder protection.

Having said that your secondary burden is pretty high and the C50, 100/5 CTs do not much secondary voltage capability. What is the available fault current?

Presumably, you have standard phase CTs of a higher ratio. You should have a phase instantaneous (or nearly so) element set above the maximum inrush current.

 

[collies99]

Yes we have standard phase CTs and zero seq CT at the contactor end for the applicable motor curves with feeder 50/51 protection. Motor is 1000 feet away.

Fault current in my estimation is greater that 2.7 x FLA or about 1030 amps. We are asking the vendor to confirm and our e-tap PN department to calculate max motor internal phase fault current, that may take weeks yet to finalize.