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Generators Voltage Drop

02/22/2017 12:20 PM

I'm dealing with three 450 volts, 3 phase, 60 hertz, 174 kva generators equipped with auto voltage regulator. while running separately, no load voltage is 450 volts. Then putting on load voltage drops up to 435 volts at full load. Two generators show the same behavior and runs good in separately and also in parallel configuration.

Whereas the third generator does not drop its voltage while running separately and maintains 450 volts at no load and at full load while having the same avr settings as the others. when synchronizing with others, load sharing is having problem.

what is the voltage drop limit proportional to the no load to full load?

what could be the reasons that third generator not droping voltage with the load?

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#1

Re: GENERATORS VOLTAGE DROP

02/22/2017 12:44 PM

Why do you think this is happening?

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Active Contributor

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#3
In reply to #1

Re: GENERATORS VOLTAGE DROP

02/22/2017 1:04 PM

In my view voltage should drop as we go from no load to full load with reactive load. But one that is not dropping the voltage is confusing.

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#2

Re: GENERATORS VOLTAGE DROP

02/22/2017 1:02 PM

I don't think the third generator avr has the same settings as the other. Check again.

Decreasing voltage with increase in current (load) is called voltage droop. Voltage droop helps with transient control.

https://en.wikipedia.org/wiki/Voltage_droop

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#4

Re: Generators Voltage Drop

02/22/2017 1:08 PM

Seems like you have spelled out the problem in your question. AVR is not be having the same for one generator as the other two.

The generator not showing any voltage drop probably requires less AVR gain. The other two might require a little more AVR gain.

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#5

Re: Generators Voltage Drop

02/22/2017 1:32 PM

First, this is only a (3+1/3)% change in output voltage. I suspect that this is well within the tolerance window for these generators and their AVR. I suspect that the the root of the modest performance difference is which phase is being monitored for the output voltage, which phase is the feedback for the voltage regulation and lastly which phase has the heaviest load.

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#6

Re: Generators Voltage Drop

02/22/2017 11:31 PM

The voltage drop can occur even if the speed of the generator falls below 1800 rpm.

Check individual machines for voltage and frequency for the same load.

If there is variation in speed (frequency), check governor of the machine.

If there is variation in voltage alone check AVR .

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#7

Re: Generators Voltage Drop

02/23/2017 9:09 AM

Are the sets identical?The droop characteristics of AVR should match.

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#8
In reply to #7

Re: Generators Voltage Drop

02/23/2017 10:10 AM

All three sets are identical and droop charactestic's are same. I interchanged the AVR but the results are same.The AVR of the third generator working good on other sets and the AVR of the 1 &2 behaving like the third one.

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#9

Re: Generators Voltage Drop

02/23/2017 10:48 AM

In my opinion, the third AVR settings are correct.

By the way, voltage drop of 450-435=15 V[=15*100/450=3.33%] it is about 7 times usually voltage drop [+/-0.5%].

So, the former 2 generators could be of an old generation AVR with limit +/-3-4%.The third is advanced one and +/-0.5% is now the standard.

See [for instance]:

http://www.northern-lights.com/media/PDFs/brochure_pdfs/spec_sheets/Marine/B155%20M1305.pdf

“Automatic voltage regulator gives fast response to electrical load changes. Voltage is regulated to ±0.5% (one half of one percent) over the entire range from no load to full load.”

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#10

Re: Generators Voltage Drop

02/25/2017 5:52 PM

For parallel operation, generators have a voltage "droop" behaviour achieved by feedback from a current transformer [CT] in each generator output. Typical value 3-5% droop zero to rated current. Without this instability is likely when paralleled with other genarators.

Possible CT is not fitted/shorted out by standard terminal shorting link [CT must never be open circuit - extreme voltage damages its insulation - always shipped with link in place]/ AVR droop circuit wrong setting or defective.

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