Motor short circuit current / differential protection
Motor short circuit current / differential protection
(OP)
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.
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.






RE: Motor short circuit current / differential protection
RE: Motor short circuit current / differential 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.
RE: Motor short circuit current / differential protection
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.