2400V startup of long idle motors

[bentov]

A customer needs to start using some pumps that have been offline for several years, typical unit is 900hp 2400V with 200kVar power factor correction capacitors connected on load side of contactor. We'll test the motor windings from the panel with our Baker ADX12 (Resistance, IR/PI, Surge, etc. - DC Hipot at reduced level, only if desired by customer).

How to test the old Westinghouse capacitors prior to energizing though? Might they need reforming? Do we need to figure out how to energize them separate from the motors?

 

[waross]

This is one of the few decisions that I have never had to make in the field.
But disclaimer aside. I would consider one of two courses of action.
Disconnect from the motor in any event.
1. Test with a rising DC voltage that is current limited (Series resistor?) to see if reforming is needed.
2. Just set-up and reform all capacitors.
You may wish to check the cost of replacement capacitors against the cost of testing to help with your decision.
I would also consider flooding any ball or roller bearings with grease until new grease is discharged and then letting the bearings drain when the motors are started.

 

[Gr8blu]

If you're going to be testing the machine windings from the panel, you're going to have to disconnect the caps anyway. So, they are already a separate entity.
Test caps (in groups, or as individuals) per the steps outlined by Bill (waross). And regrease the bearings.

If these machines have basically been sitting outside on the pump stand all this time, unprotected from weather and local pests (and probably without anticondensation heaters wired up and powered), get them into a reliable local shop for a once-over.

Also ensure that whoever is responsible checks the condition of the pumps and impellers as well.

 

[PHovnanian]

How to test the old Westinghouse capacitors prior to energizing though? Might they need reforming? Do we need to figure out how to energize them separate from the motors?

Testing the caps might be a good idea. A capacitance bridge can be used to see if the nominal values and loss factors have changed from their spec (if the latter is even published somewhere). I'm not sure about reforming. That's usually an issue for electrolytics (polarized caps). And those would be unusual for power factor correction.

If you are going to use a capacitance bridge, they will need to be taken out of circuit.

 

[bentov]

Thanks for your replies. I've also run this question through EASA (and their new AI, "Eddy") and Baker/Megger, been digesting a fair amount of info. Consensus is these oil filled PFCCs must be film, not electrolytic, so no need to reform. Installed 3 phase PFCC banks with external fuses must be either wye or delta connected internally, and must have discharge resistors (typically internal also). After verifying complete discharge, plan is to test as follows:

1. With the Megger ADX in RLC Configuration we'll do a Resistance Test (amounts to connecting a DLRO to each phase pair), record value and unbalance %

2. Using a multimeter we'll do a Capacitance Test of each phase pair, record values.

3. Using the ADX DC Test we'll do an IR test at 2400V.

If IR passes and all else seems balanced and near expected values, and assuming the motor windings test OK, we'll reconnect, close doors and test run, ideally following up with thermographic inspection when possible.

Note: "expected" values for a typical 200kVar 2400V would be 31uF per phase pair if delta (most likely), or 92uF if wye. The internal discharge resistors will be in parallel with the caps, cannot access/disconnect for testing so measured values will really be meaningful only in terms of balance, and as compared to known good units tested with the same instruments.

Any thoughts or suggestions?

 

[edison123]

Without replacing the motor and pump bearings? Good luck.

 

[MintJulep]

Where are these? What else is around that's be in operation?

False Brinelling bearing damage is a real risk.

At the very least, get some acoustic and acceleration sensors on the bearing housings.

 

[bentov]

Thanks again for the replies - edison123 & MintJulep you are correct about the likelihood of mechanical issues. We will certainly inspect as thoroughly as we can and follow waross's lubrication strategy, only actually test run if it seems safe to proceed (and then measure vibration & db at bearing locations). District staff are pretty careful, may have monitored oil levels over the years but I doubt anyone's done the periodic shaft rotation to mitigate false brinelling, which will be very likely if the mothballed units live on functioning lift stations (typically with 2-10 large pumps on common decks, plenty of ambient vibration impacting stationary ball and roller bearings). I have not seen these yet, will certainly offer a PM recondition in our motor shop if it seems prudent, and suggest a pump company evaluation of the turbines.

My initial concern is switchgear assessment - nothing quite like 50+ year old MV panels to give one pause. A recent PFCC fuse failure "blew the door open", the door being the outer one where the operator controls are mounted (didn't pop the cover of the actual capacitor compartment located directly behind it - expanding gas goes where it will, I guess). Lucky no one was standing there.

I figure we inspect and test as thoroughly possible, then verify control and contactor sequence/operation with mains disconnected (special attention to start delay, adding some if not already present) . . . eventually flip the switch and stand way back, right? Maybe rig a control station on a long extension cord?