emergency generator load issues

[afjohn]

I need some help with something. We are having trouble starting 3 ea. 60 hp booster pumps with PFC (Capacitors) and then starting a 300 hp well pump with solid state starter.

The generator is an 800 kW.

It is tripping out for under frequency when we start the well pump after the booster pumps, but it is not actually dropping under load.

If we remove the capacitors from the booster pumps, everything works fine.

If we start the well pump first, then start the booster pumps, everything is fine.

There appears to be a problem with the capacitors and solid state starter.

The engineer has to start the booster pumps first, and then the well pump.

He also has to keep the capacitors on the booster pumps to help with the utility side, something to do with useage.

It is a Newage Alternator.

Have you got any suggestions on how we can make this work in the sequence they require?

Thanks for any help.

 

[waross]

Switch out the capacitors when you are on the gen-set. You probably don't need them with the generator anyway.
Contactors for the capacitors are probably cheaper than a damaged or destroyed soft starter.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Maybe move the PFC correction to a sequential controller on the bus, and enabled only when on utility (should easy enough to provide a permissive contact from the utility breaker). I can't imagine you need capacitors to aid motor starting with a big set relative to the motor size. As Bill says, the alternator is unlikely to be bothered by the lower power factor when the caps are out.

Under-frequency trip sounds like something is either dragging the engine down through overload or poor governor response, or the controller is being confused by a distorted current caused by the soft start interacting with the capacitors. Given the size of the generator relative to the loads I'd suspect the latter. Can you get a scope on the generator? Ideally you'd like to see voltage and current waveforms, but voltage distortion would be the main concern when the soft start is ramping as this would affect other the loads.


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If we learn from our mistakes I'm getting a great education!

 

[afjohn]

We put a scope on this and the waveforms are all over the place. I know the voltage distortion is what is causing the generator to get a false reading and shut down. These capacitors are only there for the PF on the utility side. I have tried to convince the engineer that we need a way around these capacitors when on emergency generator. I need some strong supporting documentation explaining the capacitors are reacting with the soft start. Do you know if a filter could be placed in the circuit to clean this up or if adding some other load ahead of the booster pumps might help?

 

[rasevskii]

It can b that the distorted waveform is causing the excitation system on the generator to see a too-high AC voltage, and this results in a partial loss of excitation, in other words the voltage (not the frequency) partly collapses. The protection may be reporting underfrequency but likely this is incorrect, as you said that the speed does not change.

We had a case on an isolated system feeding rectifiers that were for radio transmitter plate supplies, in that the severe harmonics caused the generator AVRs to drop the voltage several per cent when the transmitters were operating. There was no effective solution for this at that time.

As said by others, remove the caps when on generator.

What sort of control and protection does the genset have? Our expert =catserveng= may show up here with some specific advice for your equipment.

rasevskii

 

[rasevskii]

BTW if you are using ordinary DVMs (a rectifier type instrument) to measure the voltage, the distorted waveform will give false readings of the voltage. Use a moving iron meter (a rare beast today) and you will see what the voltage is really doing when the softstart is ramping up.

Or one of Edison's famous lightbulbs...

rasevskii

 

[jraef]

To rasevskii's point:
This used to be a very common problem with soft starters being run behind portable generators (and VFDs for that matter). The high harmonic content can mess with the AVR in the generator, making it think the voltage is too high when it isn't. Most of the generator mfrs have long ago added more filtering to their AVR sensing circuit because of this. It could be that your's, despite being "new age", is getting some additional interaction with the caps that are on-line as they are getting ravaged by the soft starter switching. Normally it's a good practice to isolate caps off-line until a soft starter is done ramping. It isn't 100% necessary, as it is to avoid having caps on the LOAD side of the soft starter when ramping, but that doesn't mean there are no issues. It could just be that the caps, in this case, are the straw breaking the camel's back so to speak.

I'd put each cap on it's own contactor and only close them when everything is up and running already, or not at all when on generator power as waross suggested (although technically they might save an eyedropper of fuel per hour).

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