2250HP Motor Start with 2500KVA Transformer 2

[ProjEngr]

We have a 2250 HP 4160V 3P 60Hz 1.15SF 270FLA Induction motor on a large compressor. There are surge protectors and capacitors (GE 1.5uF, 3 Pole, 4160V) on each phase at the motor. The motor dropped out one day in the middle of unit operation - no alarms on the Multilin, no obvious signs of any issues except that the starter cubicle was hot inside. So we replaced the starter with a new Eaton Ampgard MVC and Multilin 369. The transformer is a 13.8kV/4.16kV Delta-Delta, 2500KVA, that feeds only this motor. When trying to start, the motor would start to turn then everything would drop out. We get an under voltage alarm on the Multilin. The control power voltage dropped to 89V and the incoming line voltage dropped to 3225V before tripping. We tested the motor (high pot & surge), we tested all the feeders (megger), we tested the transformer oil (no signs of degradation - no dissolved gases) we tested the caps. We found one of the capacitors to be bad, so we changed all three (it's really one assembly with (3) caps in it). We checked the Schweitzer relay in the substation and found no issues with voltage drop or amp draw on the 13.8kV side. To get it started, we supplied 110v control power to the starter from a nearby UPS. This move helped keep the starter coil pulled in and ultimately allowed restart of the motor. This motor has been in service for 15 years with no issues - it's generally started twice a year. We once installed a soft start, but the machine started blowing out couplings - this was attributed to having critical speeds (machine natural frequency) that were between 0 and operating RPM - so we went back to an across the line type starter. So I have a few questions for those who would like to take a stab...and hopefully I have supplied enough information!

To me, all signs point to the transformer and it's voltage drop during inrush.
1. Can a transformer degrade in performance without showing signs of this in the oil?
2. Is a Delta-Delta the proper transformer for this application?
3. Does this type of setup need the starter capacitors? Should their sizing be reviewed?
4. Should I get a transformer with a different inductance? It's a 5.6% IZ @ 85C.

Your thoughts are appreciated!!

 

[electricpete]

and look at 3 different frequencies of electromagnetic torque oscillation that are predicted in motors in various circumstances:

I didn't mean to imply these are the only three (Non-sinusoidal power supply can create others)



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(2B)+(2B)' ?

 

[crthompson]

That is a large motor and the inrush current would be pretty high. When the current suddenly increases, you will have a corresponding voltage drop. Your problem may be that you don't have your undervoltage setting on your multilin programed to ride through the voltage drop until the motor comes up to speed...