Digital Voltage Regulator Fails to Regulate once synchronised 1

[2Gunny]

We have proven the regulator to be fuctioning correctly by running the DVR in a running plant.
The auxiliary circuits (from the PLC) to the DVR are functioning correctly.
The problem seems to be between the DVR and the generator:
Voltage Sensing Circuit - 3 Phase?
PMG circuit - 3 phase?
Field - DC?
Whilst synchronised we have no excitation control. I understand we could be operating the generator in the wrong quadrant?

What are the affects of reversing the Sensing Voltages? PMG circuit? the field? and is this the issue - sensing / excitation is 120 degrees out of phase?

When the generator is not synchronised (No Load) to the utility we can fully regulate the voltage +/- 10% by varying excitation - as expected.

Should we excite the generator at 411Volts or below and then synchronise the utility pulls us down to 385V causing excessive VARs.

Or should we excite the generator to 412Volts or above and synchronise the utility pulls us up to 440V causing the sytem to trip on over voltage.

If you can help it would be much appreciated.

Cheers

PS When synchronised (and working correctly) the DVR initially excites in kVAr mode until the Real Generator Current is within 15% output current, then PF control takes priority - this is not the case as can been seen above, as we are trapped high or low.

 

[ScottyUK]

Where does the PF controller take its feedback from? Your description sounds like a closed-loop controller running open loop. Possible causes could be: dead transducer; incorrect polarity of transducer; incorrect connection of transducer.




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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[2Gunny]

Thanks Scotty, PF controller is integral to DVR (As first post, the DVR is fully functional as I've eliminated that by running it in a working plant). It takes its feedback from a CT designated to the "B" phase of the main generator terminals. To prove the polarity of this CT we preformed the battery 'flick test'.
The sensing voltage off the generator terminals is feed to the DVR via 2 No. potential transformers at a ratio of 2:1. Note the "B" phase here being 'commoned up' on the 400Vside (open V?) and as above dropped (ratio 2:1 = 200VAC) to a 3 wire sensing circuit into the DVR.

Any further ideas?

Cheers again
and go the mighty All Blacks on the forthcoming UK Tour!

 

[niallnz]

2Gunny
Have you checked the digital input/CB auxiliary contact that tells the DVR that it is now in parallel with the network and to go into PF control?

 

[rcw retired EE]

2gunny- sounds like the polarity of the reactive compensation circuit is incorrect. You could dial down the reactive droop to zero or short out the reactive droop CT (the one on B phase) to see if that is the problem. Are you trying to run in power factor or VAR mode as soon as you come on line? What happens if you set the control for zero VARS?

Also check the phasing of the B phase CT and the A & C phase voltage inputs. To make the reactive compensation work, the B phase current must be 90 degrees out of phase with the sensing voltage (Va-c).

The VAR controller setting may also be wrong. The Reactive droop control can be set to increase or decrease excitation as the machine picks up VAR load. It should only be set to increase esxcitation if you are trying to correct for voltage drop through a long T-Line or a transformer. Normally it is set to "droop" the voltage as the generator loads up on VARs.

 

[2Gunny]

Thanks Niallnz,
The utility Auxiliary contact is closing at the DVR as are all Auxiliary circuits from the PLC.
Cheers

 

[2Gunny]

Thanks rcwilson, you make some good points to check.
At present there are issues with the gas well and enough gas for us to run so we have not been back for two weeks.
From my investigations I think the outstanding problem is around the sensing voltage being out of phase with the terminal voltage of the generator.
Reactive voltage droop is currently set to 4% (range 0 to 10%). Are you suggesting taking this to zero may help? Initially we run in VAR mode until the regulator senses real current being within 10% of rated current. we are at about 90% with no chance of closing the gap to the 10% required prior to the regulator automatically switching to PF mode.
Your sesning voltage comment along with my other investigation (as I say) is my strongest lead / 'vib'.
The location of the transformer is within 15meters with a 100m 11kV cable to the overhead OCB. This does not vary to much from other installations of ours.

Thanks again, any other input is appreciated. Will let you now the outcome.


Ag

 

[ScottyUK]

Gunny,

Just a thought:

How are your instrument VTs grounded on the secondary side? UK practice is to ground yellow phase ('B' phase to the US readers). I've had a couple of instances where equipment has refused to operate properly with this kind of grounding; it worked perfectly with a grounded star point, a solution which usually cost us an interposing VT. I'd expect a digital regulator to have isolating VT's anyway to bring the 110V down to something more electronics-friendly so this may not be a problem but worth a look.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[2Gunny]

Howdie Gents,

Just a note of Thanks for your inputs.

Finally back from the deep dark south of NZ. The good news is the generator is know functioning correctly. The cause of the problem being the sensing voltage had a different rotation to the terminal voltage?? Not sure at this stage if this was wired incorrectly during generator manufacture or a label was removed at our end by an installation electrician (unlikely?). The phase rotation is definetly not as the drawings or manuals state.

Thanks again, appreciated.

Lee Gunn