Voltage drop during power transfer

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tanwirenut

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We are experiencing a voltage drop upon transfer from utility power to generator power from 480 volts to as low as 320 volts. The generator is only a year old and we have increased our load to about 750 amps within the past couple months due to the warmer weather and some building expansion. Lately we have had lights and equipment dropping out due to this voltage drop. Under low amperage (500 amps) we do not experience much of a voltage drop- the lights may flicker a little bit. The problem seems to be getting progressively worse.
We have had the generator looked over and the manufacturer is saying it is running and operating like it should. It is a 4 pole ATS with a 2000 amp main, generator has a 1600 amp main breaker.
Any suggestions?
 
Re: Voltage drop during power transfer

You never said what the actual generator output rating was. But assume that it must be sufficient to require 1600A main breaker ands a 2000a transfer switch.
Think about it. The
manufacturer is saying it is running and operating like it should.
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You didn't say if you measured the voltage at the generator while drawing the 750a load to confirm that it was 480v. One assumes that you measured the voltage at the load somewhere and measured 320v. If the gen. mfg. "says" that the gen.set is operating correctly are they saying that it is putting out 480v @ a 750a load? And you are also saying that you have only 320v at the load. That's a 160V drop between the gen. set and the load. Keeping it very simplistic 160v x 750a= 120kw of heating that is occurring somewhere as this voltage drop is going to end up as heat which I find very hard to believe. Personally I don't think you're getting the voltage output from the gen.set in the first place.
 
Re: Voltage drop during power transfer

Personally I don't think you're getting the voltage output from the gen.set in the first place.
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I agree. Two reasons for that - voltage regulator or the load (RPM) regulator. Most likely it is the voltage regulator. Check the RPM and the voltage at high loads to determine where to look into it.
 
Re: Voltage drop during power transfer

This drop in voltage (down to 320 volts) only occurs for a second or so after transfer- causing lights to dim, AC units to shut down, and HID fixtures to drop out. After a second the voltage goes back to 480. This generator and transfer switch have been operating fine for the past year until just recently as our load has increased. The output rating of the generator is 1250 kva.
The transfer switch will not transfer power if it does not see 480 volts from the generator. I guess I have never checked the output voltage on the generator with my multimeter prior to transfer, but the screen on the genset has shown 480V on the display.
 
Re: Voltage drop during power transfer

What are the increased loads?

How many motors are coming online at the same time?

It would probably be worth some coordination studies to come up with some load sequencing after transfer.

Roger
 
Re: Voltage drop during power transfer

I think that is normal at least to some extent. I remember from a generator course I took years ago that the voltage requlation circuit in the gen set is a feedback system. Thus you have to have an deviation from setpoint for it to act. If memory serves me, this amount of voltage drop is dependant on the load. But it can be like 30% - 40%. Therefore, since you have added load, you may have made the drop worse and more noticeable.

Also, I know that fire pumps are only allowed a 15% voltage drop (695.7). This requires the genset to be sized much larger than would normally be the case.

So what can you do? Can you load the genset in steps. i.e. arrange the loads such that they are not all dropped on the genset at once. This may require some rework of your distribution system or upgrade of your ATS but it should help you out.

Also, If you have a mechanical governor now you may want to consider an electronic isochronus governor. This should reduce the droop that can be the cause of the voltage dip.

Good luck
 
Re: Voltage drop during power transfer

Do you have HVAC units on cooling or other large inductive loads running when you transfer? The startup of several large loads at transfer can pull the generator down below limits. Suggest delay timers on the control circuits of any large loads starting up with transfer. I ran into a similer problem at a large retirement home with a large number of window units. The generator was more than sufficient for the running load but the startup of all the units at transfer was too much. A simple timer relay on the compressor control circuit of each unit solved the problem.
 
Re: Voltage drop during power transfer

If you have motor loads and A/C loads I would suggest you add a time delay on to some or all of them and stagger the time delays such that the loads are added in a sequence. When it comes down to it does it really matter if a motors and A/C unit turns on instantly? What you appear to be sacrificing if those loads that drop out because of a low voltage condition for those that really don't need to be instantaneously started all at the same time upon power transfer.
 
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