Exploding capacitor in 40HP drive 1

[csgasbm]

I recently had the pleasure of being 2ft away from an enclosure which housed a 40HP VFD when an internal capacitor blew. The events leading up this are as follows:

Power supply: 100kVa 480V Generator
VFD: 40HP Yaskawa G5 (Current rating 51A)
Load: submersible pump consisting of a 20HP 3450rpm motor @ 1800ft down a well. FLA: 28A

1. Attempted to start the pump, only to have it trip on DC bus overvolts twice, then start on the third attempt (restarts where automatic and in quick succession)

2. Appart from the problems at startup, the pump seemed to operate OK (current and torque readings back to the drive were good) and we had pumped water to the surface for some time.

3. I then attempted to switch the controller from manual to auto speed (the VFD changes speed to maintain a set water level), which due to the water level I expected the pump to increase speed to the frequency upper limit (which was set to 90% of the full speed).

4. Instead the VFD tripped on Overcurrent (again, the drive attempted to autorestart but this time stayed down on the third attempt). It was then that a burning smell was noticed from the unit, followed very quickly by an explosion.

5. After a change of underwear and a lot of fingerpointing at the drive (by everyone but me), we installed an identical drive in it's place, only to have the same overvoltage trip occur on a manual speed startup.

6. I then convinced the techs to wire up a small 5HP motor we happened to have nearby to see if it will run it without any probs, which worked.

My questions are (never really having to much experience with hardware probs)
1. Is it safe to say the downhole motor is the problem? (it seems to run OK in manual speed; apart from the startup problems, and the electricians tell me that it meggars fine)
2. What actaully goes on in the drive to cause the bus capacitor to explode?
3. What other things could/should I be looking at to determine the prob?

Thanks for any help

 

[Marke]

Hello csgasbm

One possible problem is the length of the cable between the drive and the motor. Was there any form of output filter or reactor connected to the output of the drive?
Was the cable between the drive and the motor screened?
With very long cables, there can be a number of issues due to cable capacitance, and/or standing waves with the high frequency carrier frequency and fast switching of the output devices.
You may need to fit a sinusoidal filter at the output of the drive.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[csgasbm]

Yes Marke, the company which built the system has put in place an output reactor and a long lead filter (which they place on all systems where the motor lead to the motor is longer than 40-50m). I'm not sure what the specs of these devices are but I assume they are fit for purpose seeing as they where installed in the first place (but it's something to check up on, thanks).

Could the problem lie with one of these components? It should be noted however that when we successfully ran the 5HP motor, these devices remained in the system.

 

[thewellguy]

Did you shut off the drive to switch from manual to auto? If so, you stated that the pump was set 1000' are there check valves in the pipe string or does the water drain back? How soon after the shutoff did you restart it? If the system was draining back this causes the motor and pump to spin backward. If you tried to start the motr while it was in backspin this would explain what happened.

 

[waross]

How is the incoming voltage? Do you have a large bank of capacitors connected that may over-correcting and driving the system voltage up?
respectfully

 

[resqcapt19]

I have had high bus voltage trips that occured when a drive that had not been online for a few months was placed into service. I was told by tech support that I needed to condition the capacitors before using the drive if it has not had power on in for a few months. They told me to just energize the line side, but not run the motor for at least 24 hours. That got rid of the high bus voltage promblem for me.
Don

 

[csgasbm]

thewellguy - i didn't shut the well down when switching from auto to manual.

waross - We checked the incoming voltage several times while it was running and it checked out OK. There are no added capacitors in the system.

resqcapt19 - this is very interesting, i believe the drives were sitting around for quite a few months before being put into service. I have had other installs where the drives had not been used for months (up to 6) and everything started OK, however, the drives in question were stored in a different part of the country with very different weather conditions, i wonder if this would play a part? It is usually a lot colder where these drives are, do you think this would exacerbate this particular problem?

 

[Skogsgurra]

I think that you had regeneration from water flowing back down when you switched modes. Is there a check valve? Thewellguy obviously also thinks about that possibility.

Gunnar Englund

http://www.gke.org

 

[jraef]

I'm with thewellguy. I have experienced this exact situation, minus the explosion (luckily). Caps were damaged however.

The only explosive cap scenario I had was from energizing a 40HP drive that was on a mountian side at 20 degrees F and fed by a portable generator. Because power was off at night, when they powered the genset and closed the breaker feeding the drive, the caps exploded almost immediately. It had nothing to do with running, just energizing them when frozen was enough to blow them apart. I doubt that is your problem in this case though. Was the 2nd drive frozen as well? Probably not, yet it saw the OV scenario. The 5HP motor was a teat only to see if the VFD worked at all, it would not be simulating the conditions in the well.

What can happen with drives is that unlike Across-the-Line starters, the drive can energize a backspinning motor without creating enough torque to overcome the backspin and go the right direction. It also starts a speed search where it is attempting to match the output waveform with the motor speed, but it isn't expecting the motor to be spinning the wrong way. This just excites the motor field and turns it into a generator. The usual fix is a backspin timer or a delayed restart to allow the motor to come to a stop first.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[csgasbm]

thewellguy, skogsgurra and jraef - There is a check valve installed in the water line, and the drive was switched from manual to auto without shutting it down, so it would be unlikely that the motor would be spinning backwards in this case. It's something to note for the future anyway thanks (i haven't had any experience with submersible pumps, just PCP's where the motor is at the surface)

 

[jraef]

Are you sure the check valve is operating correctly, i.e. closing under head pressure and not opening until there is flow?

VFD caps don't just blow up (barring a defective part somewhere) without some help. Being frozen or being subjected to a voltage surge are about the only types of help I have seen in the real world. The generator (assuming island mode) would eliminate outside sources of spikes and surges. I assume that you would have mentioned any other significant loads on that same generator, right?

Here's another random thought. Any chance that in your automatic mode programming, someone inadvertently inverted the speed reference signal compared to being in manual mode? Some drives will reverse if the speed command is of the opposite polarity or the reference of the signal is inverted in software. That would explain the occurrence right when you changed over. Still, I doubt that would cause the caps to blow though, but you never know.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

Okay lets list the problems.

A major difference that is now (over previous history) occurring is [red]over voltage[/red], [red]over voltage[/red], then start.

When the motor was running it was shifted to manual.

This resulted in a step speed change which instead of speeding up the motor resulted in a [blue]over current[/blue].

Next start attempt resulted in an [red]over voltage[/red], , followed by an [red]over voltage[/red], followed by an [red]over voltage[/red].

Next something began overheating.
Presumably that something was the capacitor which lost integrity while still being stoked by the rectifiers and suffered rapid dis-assembly.

A new drive which is entirely unrelated to the first is installed.
It trips on [red]over voltage[/red], [red]over voltage[/red], [red]over voltage[/red].

This drive happily runs a motor that is 1/4 the size and unloaded, flawlessly.

Other facts: The pump has just started and run successfully and meggars fine.

What causes over voltage trips? Over running motors and supplies out of spec.

Either that pump is spinning by gravity and it doesn't really seem to be in your experienced opinion csgasbm, or your generator is putting out more than 480Vac. Many drives are very particular about frequency and voltage.

Is it possible that your islanded generator is not speed or voltage regulating well, poor governor? When hit with a 50-60KVA load does it stagger then briefly over speed? This could be blasting the DC bus.

This could explain the [red]over voltage[/red], [blue]over current[/blue], and even a blown cap. It would also explain the 5Hp start since that motor is a far smaller starting load and won't disturb the generator as much.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Laplacian]

What mode are you operating this drive; V/F, sensorless vector, flux vector, etc..

For whatever mode you are operating, did you let the VFD tune itself to the motor?

 

[Laplacian]

Reading the troubleshooting flowhchart for you problem given information already given in this post leads to the path of power quality issues on the line side of the drive. Have you checked power quality of the generator output?

 

[csgasbm]

I've had a few people outside this thread leaning towards the incoming power, and that seems to be where this is going also. I have a few questions/comments on this:

1. How would I check the power quality? Is there some sort of instrument/logger which can accurately give me the output of the genny? Going through the fault trace history on the drive itself doesn't show any abnormalities with incoming voltage.

2. If the incoming supply was not regulated well, would we have problems with the hut running off the same genny? When the caps blew, we must have got a pretty big hit, would you expect some sort of damage at the hut as well (eg blown light bulbs)?

3. When the caps blew, the genny actually shut itself down, i.e. it didn't just close the main breaker, it actually stopped the deisel engine. I'm told by the manufacturer that this happens when the genny detects an irregularity.

4. The manufacturer also says that there is a safety curcuit in the genset which only lets it produce a tolerable voltage.

 

[csgasbm]

Another possibly critical issue which I realise I have failed to mention in previous posts, is the noise coming from the drive. When running the motor, the drive outputs a very high pitched "squealing" noise. I have noticed this on drives with long lead filter before, but not to this magnitude. The system designers told me that by lowering the carrier frequency should reduce/eliminate this, but it didn't seem to make much difference to the noise volume, just the pitch (It was higher at higher frequencies and lower at the lower ones). I did notice however that the DC Bus Voltage would alter when I changed the carrier frequency, and it was actually at the lowest level (around 650V; trip level is 800V) when the carrier frequency was at it's highest (14Hz).
Another side note: when the motor was running, I had my laptop plugged into the hut and noticed that the transformer on the cable was singing the same tune as the drive. I immediately unplugged it (it wasn't connected when the caps blew), but the cable tranny died the next day.
Perhaps this sheds some light on the problem.

 

[Marke]

Hello csgasbm

There is always the possiblity of a disturbance on the input supply causing this problem, but there are several things that lead me to look at the output.
1. The capacitors in the drive will be rated at lease 400VDC and possibly 450VDC and connected as series pairs. This means that the continuous voltage rating of the capacitors is going to be in the order of 750VDC - 900VDC. The capacitors should be able to withstand that continuously. You would need to have a continuous input voltage in the order of 550 volts to get near to this voltage and to cause an immediate failure, the voltage would need to be considerably higher. I would expect that you would have many other problems if this was the case.
2. The problem manifested itself the moment you tried to start the drive. If it was a supply problem, it would be random. You had repeated problems when you tried to restart the drive. I expect that it was coincident with the drive starting rather than coincidence.
3. Issues on the supply usually show up when the drive is connected and not drawing current rather than on load.
4. If you have some sort of resonance on the output of the drive, you can get very high voltages generated and these will lift the DC bus voltage, especially when the load is light. - initial start up and decelleration.
5. Your last posting describes an elevated bus voltage that was dependant on the carrier frequency. This can only be due to energy comming back in from the output.

Was the output filter a sinusoidal filter, or just an output reactor? I suspect the latter and this will reduce capacitive current issues, but will not necesarily correct standing waves etc.

I am concerned that replacing the drive without addressing the output filter etc will result in the same problem recurring.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

OT, but just had to laugh at Keith's (itsmoked) comment,
"...suffered rapid dis-assembly".
I'm going to use that on my next failure analysis.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[coastpumpguy]

A couple of things come to mind.

The caps on a vfd tend to be quite intolerant of ripple on the dc bus beyond the design point. Have a look at the incoming supply and dc bus on scope.
Usually ripple should be less than 13-10% depending on manuf.
Does the drive have an integral input reactor or was one incorporated in the design? Without an input reactor you can get nasty harmonic voltages depending on the site wiring.
Are there any other capacitors on the power supply system ( PF correction?). The switching frequency of the rectifier stage can interact with these and cause the bus caps to fail.
A very good,cheap and readily available trouble shoot tool for VFD startup is a cheap AM radio. Run your 5 Hp motor and sweep around the gen set, cables ect while going across the dial. If you are hearing the tell tale vfd squeal out at the genset or along the circuit feeder cable there is likely some type of unwanted couple happenning.

I have seen dc bus caps fail in both ac and dc drives due to variations on the harmonic voltage at the input theme.

I have also seen brand new gensets with out put waveforms so full of noise they are really suitable to run a kettle and thats about it.

I really don't think the load side is causing the problem but is the motor cable flat jacketed or twisted? The flat stuff with parallel conductors can give capacitive coupling but this will ussually show up as output side problems.

It has been about 15 years since I worked on an yaskawa drive so I don't know if they have a dc bus charge circuit or not but most of the better drives do. This basically is a contactor on the line in with a high K resistor around it. When you enegize the drive the line contactor is open and the dc bus charges through the resistor limiting inrush and cap heating. If this circuit has gone wrong (if present) the caps can have a reduced life.

Good luck and I hope you pump is fine.