Standby generator RPM hunting

[RKing]

Ok guys I've got a strange one. We have a remote pumping facility with a generator backup. For years this unit operated with no problems supplying power for two three phase 40hp vfd and control panel. Lately the generator will start hunting when under load from either vfd not alot of rpm difference maybe 50 rpm or so. The control panel is single phase with a small battery backup. When we start one of the pumps the controls start acting strangely the battery backup still shows 117 volts but it causes the PLC to act erratically there has been control wiring changes over the years. We have had the local CAT generator people out who brought out their engineers and could not find any problem other than a leading pf on utility and a lagging pf on generator when the vfd is running. Keep in mind that it worked fine for the last 10+ years. And one of the vfd's is new. The generator will get a vars fault if you try to run both pumps at the same time, something you were able to do in the past. Any ideas?

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[GoldDigger]

If you have installed power factor correction capacitors, causing a leading power factor, that can cause voltage regulation problems with many generators. That in turn could cause speed hunting.
Unless the VFDs have an active front end for regeneration or harmonic control, the rectifier circuit could cause a large distortion power factor in addition to a capacitive reactive power factor.

 

[Hv&Lv]

Any chance the utility did a recent capacitor study and added a new bank close to you?

 

[synchro]

Perhaps a ground connection has gotten loose or degraded and that's allowing VFD common mode noise to enter sensor and communication ports on the PLC and cause the erratic operation that was observed. Also such common mode noise might be interfering with the operation of the generator controls and cause hunting and perhaps faults to be triggered.

 

[zbang]

it worked fine for the last 10+ years.

And one of the vfd's is new.

For troubleshooting purposes, those sentences conflict; the setup may have worked fine for a long time with the old VFD, but when was the VFD replaced and when did the problems start? I'm willing to bet the new VFD is not the same as the old one and it may have some new features (like PF correction?) that change the overall behavior.

If you can, please post the model numbers.

 

[RKing]

For troubleshooting purposes, those sentences conflict; the setup may have worked fine for a long time with the old VFD, but when was the VFD replaced and when did the problems start? I'm willing to bet the new VFD is not the same as the old one and it may have some new features (like PF correction?) that change the overall behavior.

If you can, please post the model numbers.

The problem started before installing the new vfd. Generator alarms on reverse vars. When the pumps are running.

 

[RKing]

UPDATE. Today I had a chance to put a Fluke 1738 logger on the single phase supply line to the enclosure with the PLC and on the output from the generator at the transfer switch. When the generator was running without the vfd's starting the pumps the sine wave looked good very smooth lines. But when I started either vfd the lines on the sine wave became very jagged and the PLC started acting strangely flashing all the input and output lights. The vfd's have no output filters and are less than 100 feet from the motors. The only filtering on the input side would have to be inside the vfd's. Definitely some kind of undesired feedback on the lines. Any suggestions?

 

[synchro]

The generator waveform will normally get somewhat distorted because of the nonlinear load from the rectifiers in VFDs, and particularly since generators have a relatively high output impedance compared to typical a POCO source. However, you might be getting excessive waveform distortion.

The paper below describes how sometimes generator voltage waveforms get "flat-topped" by the high current from rectifiers around peaks of the input voltage. If the generator is regulating and maintaining the RMS voltage and not the peak, then the peak voltage available to the rectifiers and therefore VFD DC bus will be reduced. The paper on pg. 5 describes how this may trigger the VFD into pre-charge mode until the DC bus voltage goes above the threshold, and then the VFD regains operation and the cycle repeats. The resulting cyclic change in generator loading might cause it to hunt.

Do your VFDs have SCRs in the rectifier bridge? If so, when they are being phase controlled in pre-charge they would cause a lagging power factor which is what you mentioned earlier.

The paper mentions some options that may mitigate the problem above if it's occurring.

https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/drives-wp014_-en-p.pdf

 

[RKing]

The generator waveform will normally get somewhat distorted because of the nonlinear load from the rectifiers in VFDs, and particularly since generators have a relatively high output impedance compared to typical a POCO source. However, you might be getting excessive waveform distortion.

The paper below describes how sometimes generator voltage waveforms get "flat-topped" by the high current from rectifiers around peaks of the input voltage. If the generator is regulating and maintaining the RMS voltage and not the peak, then the peak voltage available to the rectifiers and therefore VFD DC bus will be reduced. The paper on pg. 5 describes how this may trigger the VFD into pre-charge mode until the DC bus voltage goes above the threshold, and then the VFD regains operation and the cycle repeats. The resulting cyclic change in generator loading might cause it to hunt.

Do your VFDs have SCRs in the rectifier bridge? If so, when they are being phase controlled in pre-charge they would cause a lagging power factor which is what you mentioned earlier.

The paper mentions some options that may mitigate the problem above if it's occurring.

https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/drives-wp014_-en-p.pdf

It was suggested to me today by the local ABB rep that it sounds like a ground problem as you previously suggested in another post. Monday we are going to check the pump station ground grid to eliminate that possibility.