VFD Failure on Generator Power

[LIM]

1.) Do generators supply any other loads possibly causing a Major VD at input?
2.) Any other related equipment on sources of power not on backup that could cause a fault during power loss?
3.)Also may not be a bad idea to look at ATS settings.

 

[smoothops10]

There may be power quality issues as you are investigating but I would recommend starting with the drive. The auatomatic restart is often implemented wrong and your issue may be as simple as this. Couple recommendations to check.

If you look in the altivar programming manual you will see that automatic restart only works in 2 wire control ie a dry contact to provide the 24VDC control terminal from the VFD to the LI1 input on the drive. Review how this contact closes. Many drives require this signal to be maintained the entire Max. restart time. I imagine you have a PLC or controller of some sort opening and closing this contact. Maybe that is UPS or battery backed however review where you have any interposing relays. These may not be UPS backed. If they drop out for just an instant on a resettable fault the restart feature is over at that point. Most drives, the Run signal to drive must be maintained the entire time through the retart attempts.

Next check what your 2 wire type is set to. Default is transition (edge) where it needs to see run command go from 0 to 1 or 1 to 0. This could be changed to Level where it runs or stops when bit is 0 or 1 regardless if drive saw the change or not. Assuming it is on default edge you may want to include a feature in controller to remove the run command and reapply it if a run is being sent but run status is not coming back so drive will see the edge.


If these are large HP VFDs (100+ say), you will see a big difference in time for VFD to power down on power outage based on whether the drive is running or not. If the drive is running at time of outage it will turn off in a few seconds or less as it has somewhere to dissipate its capacitor storage. If the drive is not running at the time of outage it will remain powered up for up to a minute+ and it will probably fault on undervolatage/overvoltage or some other resettable fault. In the former case you may have to toggle your run command as stated because controller may have called drive to run before it booted back up/was ready so it did not see the edge. In the latter case, (were not running at time of fault) you may be stuck . I'm not sure how the altivars do here but AB drives you'd need to send signal to the reset input on the drive. Alternatively you could consider a one shot relay to send reset command to drive input on power up.

 

[Besoeker]

Our group has been taking harmonic measurements for over 30 years and are puzzled at these recorded measurements.


At a quick glance ,it would appear there maybe something wrong with the harmonic measurement process and computations. For example the harmonics (assuming these are current harmonics being recorded and displayed in your chart) are significantly lower on generator power (which is also suprising) but the overall voltage THD is equal to or larger compared to when being supplied with utility power.Something does not add up !:?

I don't know why that would be a surprise. I commented on this in an earlier post. My take is that the higher impedance of the generator attenuates the current harmonics but a consequence is greater voltage distortion. That's why I suggested puting a scope on it.

 

[drktmplr12]

1.) Do generators supply any other loads possibly causing a Major VD at input?
2.) Any other related equipment on sources of power not on backup that could cause a fault during power loss?
3.)Also may not be a bad idea to look at ATS settings.

1. No other loads. The wells are remotely located.
2. No other equipment.
3. There is no ATS. These are portable generators with quick connect generator receptacles

 

[drktmplr12]

There may be power quality issues as you are investigating but I would recommend starting with the drive. The auatomatic restart is often implemented wrong and your issue may be as simple as this. Couple recommendations to check.

If you look in the altivar programming manual you will see that automatic restart only works in 2 wire control ie a dry contact to provide the 24VDC control terminal from the VFD to the LI1 input on the drive. Review how this contact closes. Many drives require this signal to be maintained the entire Max. restart time. I imagine you have a PLC or controller of some sort opening and closing this contact. Maybe that is UPS or battery backed however review where you have any interposing relays. These may not be UPS backed. If they drop out for just an instant on a resettable fault the restart feature is over at that point. Most drives, the Run signal to drive must be maintained the entire time through the retart attempts.

Next check what your 2 wire type is set to. Default is transition (edge) where it needs to see run command go from 0 to 1 or 1 to 0. This could be changed to Level where it runs or stops when bit is 0 or 1 regardless if drive saw the change or not. Assuming it is on default edge you may want to include a feature in controller to remove the run command and reapply it if a run is being sent but run status is not coming back so drive will see the edge.


If these are large HP VFDs (100+ say), you will see a big difference in time for VFD to power down on power outage based on whether the drive is running or not. If the drive is running at time of outage it will turn off in a few seconds or less as it has somewhere to dissipate its capacitor storage. If the drive is not running at the time of outage it will remain powered up for up to a minute+ and it will probably fault on undervolatage/overvoltage or some other resettable fault. In the former case you may have to toggle your run command as stated because controller may have called drive to run before it booted back up/was ready so it did not see the edge. In the latter case, (were not running at time of fault) you may be stuck . I'm not sure how the altivars do here but AB drives you'd need to send signal to the reset input on the drive. Alternatively you could consider a one shot relay to send reset command to drive input on power up.

This is good information. I don't have the control schematic, I should get my hands on it!

 

[drktmplr12]

Generator AVR's checked okay? Check with replacing the AVR's.

I don't suspect the AVR's are to blame. This was happening on 7 different generators. The only common element is the VFD, imo.

A side note, we are recommending adding tuned passive filters to the drives in the future. In general, I don't want the capacitors connected while on generator as to avoid supplying reactive power to whichever portal gen happens to be connected.

 

[drktmplr12]

I don't know why that would be a surprise. I commented on this in an earlier post. My take is that the higher impedance of the generator attenuates the current harmonics but a consequence is greater voltage distortion. That's why I suggested puting a scope on it.

Any thoughts on how this could trip a VFD.

 

[Sahib]

In general, I don't want the capacitors connected while on generator as to avoid supplying reactive power to whichever portal gen happens to be connected.

That may be the problem. When the capacitor on the output side of a generator is large enough, the AVR would lose control and voltage will rise. Check by disconnecting the capacitor.

 

[drktmplr12]

That may be the problem. When the capacitor on the output side of a generator is large enough, the AVR would lose control and voltage will rise. Check by disconnecting the capacitor.

I agree with everything you said. To be clear, the filters are not currently installed. We are recommending to add them to comply with IEEE 519. They don't want a notice "encouraging" them to fix it.

 

[Besoeker]

Any thoughts on how this could trip a VFD.

As others have suggested, it may upset the AVR or maybe the voltage sensing on the VFD.
If I had been called to a similar problem , my first action would be to put my scope on it and go from there,

 

[Sahib]

drktmplr12: Some more suggested checks. Are the generators really big enough to take up the loads? Is it possible to run the generators to take up the loads bypassing VFD?

 

[Jraef]

The HP seems low for this, but by any chance are these Active Front End drives, aka "Low Harmonic" versions? Some designs have issues with being run off of portable generators because of the high impedance. By the way if these are that type, you can't add the passive filters to them.

If they are standard 6 pulse drives and you do want to run them through passive filters, most of them can be ordered with a contractor that disconnects the capacitors when the drive is not running, you can also use that to disconnect them when needed for other reasons.

 

[Besoeker]

The HP seems low for this, but by any chance are these Active Front End drives, aka "Low Harmonic" versions?

The harmonic data on the first page of the thread strongly indicates that it is not.

 

[JFletcher]

ssitdb (super shot in the dark back) here, but would the lower barometric pressure that a hurricane brings cause engine problems with the generators? Possible it sucked in enough water to choke the air filter? Those particular conditions would not be easily duplicated after the fact. and those 7 different generators do have something in common: the conditions in which they were running at the time.

 

[Sahib]

The harmonic data on the first page of the thread strongly indicates that it is not.

The generator itself will contribute to harmonics due to higher internal impedance and so no certainty to what you say.

 

[__dan]

I would be looking for two additional information factors, the drive manufacturer's consultation or instructions about running on generator power. It's possible the drive's trip settings can be relaxed for generator power. And the generator manufacturer's consultation or instructions about running straight into a 100% front end rectifier load. What problem that may cause that the manufacturer may know about (voltage regulation, distortion).

For example, if half of the load was straight running line connected three phase motors, they would provide some buffering or rotating line conditioning to mitigate the line effects of the big non linear rectifier load. Without that there was no buffering or line conditioning.

Are you assured the generator was always on and connected and it was the drive that tripped. Is it the sense that the drive tripped from detecting a distorted supply that itself caused.

 

[Besoeker]

The generator itself will contribute to harmonics due to higher internal impedance and so no certainty to what you say.

Not certainty but blindingly obvious I would say.
An active front end would not draw 60% of 5th harmonic. That's an example of exactly what it is designed to not do.
From the figures presented, it is a six-pulse plain rectifier input and no DC choke. If it's a problem that still needs a solution that's the first thing I would do - fit a DC link choke. Based on my limited experience in these matters. You may have a different opinion and that's fine - it's what discussion forums are about. Discussion.

 

[Ingenieur]

As I noted the smaller gen is marginally sized for starting
this can be mitigated by drive settings
the larger should be no issue

I would slow down the pid or hz/sec rate of the drive
and Inwould ramp up much greater than 1 sec

assuming once started the drive runs continuously and only varies speed via pid modulation these steps may help

 

[Jraef]

The harmonic data on the first page of the thread strongly indicates that it is not.

Good point. Never mind then.

I just keep thinking of a way in which the failure took place during the storm, but conditions can't be duplicated without the storm. I don't know where I would have gone if that AFE track had been valid, but since it isn't, lets drop it.

So even going down the high harmonics track, why would the harmonics be greater during the storm and not now, running from the SAME generators without the storm in play? Or did I misinterpret something here?

 

[Ingenieur]

Good point. Never mind then.

I just keep thinking of a way in which the failure took place during the storm, but conditions can't be duplicated without the storm. I don't know where I would have gone if that AFE track had been valid, but since it isn't, lets drop it.

So even going down the high harmonics track, why would the harmonics be greater during the storm and not now, running from the SAME generators without the storm in play? Or did I misinterpret something here?

We only know what they are now, after the storm, correct?

honestly imo it is likely just a drive set-up issue exacerbated by the storm