Damaged Capacitors in VFD 2

[rockman7892]

I have a 30hp 480V Siemens MM440 VFD that recently failed. When pulling the VFD out of service to investigate, it was obvious that something has been smoked for there was black soot all over the enclosure and the control boards.

Upon taking all the boards apart and looking at them one by one we found that it appeared that only two of the capacitors were all that were damaged. These capacitors appeared to be burnt up and was obvious that they were the cause of the failure. Everything else seemed to be fine, and it appeared that these capicitors smoking were the cause of the soot elsewhere.

What would cause capacitors to fail suddenly like this? Is it possible that they were subject to overvoltage? What is the common cause of capacitor damage in VFD's?

I should mention that this VFD is on a HRG system limited to 5A.

We have suspected possible voltage transients in this area for a while. In some cases overnight when the VFD was not in use the drive would trip out on a DC bus overvoltage. This led us to belive that there were some transenits occuring in the system subjuecting the DC bus to overvoltage.

I'm going to put a fluke 1735 power meter down in the area to monitor voltge. The meter will only pick up events lasting for 1/2 cycle or more. Do you think the meter would pick up a voltage transient or are typical voltage transients shorter than 1/2 cycle?

 

[itsmoked]

Your theory that:

was obvious that they were the cause of the failure

is not a good theory.

But it is a possible theory.

The caps can go due to age or transients. I would probably give replacing them a try, on a drive that expensive.

If it were me I would;
Check the front end diodes as one of them failing could have caused the problem or one could easily fail when the caps went.

Hunt down replacement caps/diodes and order them. Then wash the sooted circuit boards in very warm water with some 409. Use a tooth brush. Then rinse them with warm water. Follow that by a rinse of distilled, de-ionized, or distilled water. Then stand them on a corner and let them dry for a day or two.

Reassemble the whole thing, roll the dice, and power it up from a distance.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ozmosis]

It is unusual that overvoltage transients would cause the damage that you describe in the way the caps have failed.
A few possibilities:
1) What is the ambient temperature? Is it possible the drive is subjected to high temps over a period of time?
True, the VFD has protection over this but if this is a constant problem and the drive is stopping over periods and cooling down, the damage could be done and electrolytic caps 'dry out' and could fail.
2) Is the MM440 mounted in a place where there could be high vibration?
The caps on the MM440 are soldered directly to a power board and vibration could affect the solder junction point and cause failure. I've seen this before.
If the damage to the cap is at the 'solder' end then this is a possibility.
3) If you cannot power up the VFD due to the state of the VFD. Try and get hold of a Profibus module- not for profibus but this has a 24Vdc power supply that allows you to power the control board up. Apply 24Vdc and then check the fault and alarm history. This is a good indication of what was going on before the VFD went kerrboom.

What's a HRG?

 

[Drivesrock]

Rockman7892

Thinking about your problem I have the following questions;

How long has the drive been in service?
Siemens drives can be bought either with the DC link earthed - centre point of the DC link capacitors - or without. Which version have you got?
Are there any power factor correction capacitors on the same or connected 480V busbars?
Is the neutral really earthed - ie not been disconnected?
Have you got an input line reactor?
Are there any varistors across the input rectifier - if so are they still OK?

 

[jraef]

Just to pile on a bit, my experience has been that when the caps go, most likely the transistors are now bad if the motor was running at the time. If it was not running when the damage happened, you have a chance.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ozmosis]

Drivesrock
On the MM440 the Y cap is disabled by the user, not at the factory. A wire is clipped out. Once clipped then it cannot be reconnected easily.

 

[LionelHutz]

I think you need to provide more details. Are you talking about the main buss capacitors or some small capacitors on one of the PC boards?

 

[rockman7892]

Management wanted to go ahead and purchase a new drive at the cost of $5,000.00. Apparently they told me that Siemens does not sell replacement boards or parts and that we would have to ship the drive out to get repaired.

To answer some of the questions:

HRG stands for High Resistance Ground, which is the type of system this drive is connected to.

The drive has been in service for about a year now. The drive is actually mounted in an outdoor MCC so I agree that there could be some temperature issues. The front of the MCC bucket which it is located in has a vent fan which is in operation when the drive is powered up. Over the past year this drive has been sitting in its location and seen pretty high temperatures without ever having a problem. This problem occured early in the morning when temperatures were somewhat still cool.

I'm not sure weather I have a drive that has the DC linked or not. Should there be a physical connection? How can I tell?

There are no input line reactors.

I mentioned earlier that this system is HRG limited to 5A, so the neutral of the system transformer goes throguh a ground package before it is earthed. We have had some problems getting the ground fault test indicator on this package to work and I'm begining to wonder if maybe there is something wrong with the ground system and there is maybe a high impedence connection somewhere between neutral and ground.

I'm not sure if there are varistors across the input rectifier. The manual that I have for the drive is very vauge about what components are in the drive and their layout.

I may try to see if we can power up the drive with 24V and see any fault alarms.

There are no power factor caps anywhere on this 480V system.

 

[rockman7892]

LionelHutz

Unfortunately like mentioned I cannot find anything in the manual that reference or show what boards are what and what parts are what.

The capacitors that I am referencing where fairly large capacitors in size and there were about (8) of them.

 

[davidbeach]

My guess is that the VFD was designed for a solidly grounded system, a ground fault happened somewhere on the system, voltages shifted, and the caps didn't respond well to the sustained overvoltage. Hopefully the ground fault is repaired before the new VFD is installed and toasted.

 

[ozmosis]

As it is a HRG (sorry, being a bit duh there), you should have removed the connector linking the Y capacitor circuit. It is also recommended to fit an output choke. Here is a section from the Siemens guide:
The purpose of the choke is to limit high frequency circulating currents that can flow during earth
faults. These would cause high currents in the output devices, leading to excessive dissipation and
possible damage.
Further Considerations:
1. Operation on ungrounded power supplies is only possible with unfiltered units. Filtered
units (or units which do not have their 'Y' capacitors disconnected) will be damaged
after running for a short time.
2. An output choke is recommended to limit the fault current during an ground fault. All
qualification tests have been carried out using an output choke.
3. It is often the case that ungrounded power supplies have high levels of transients and
interference. Supplies should be checked for damaging transients before fitting MM4. If
in doubt, an input choke should be installed.

 

[DickDV]

Imagine! Having a nice safe grounded wye power network and then ruining it by adding High Resistance Grounding. What a shame.

And now, as has been reported here numerous times, it has cost the life of a drive only one year old. Hopefully, when disconnects are opened, the disconnected conductors are ground jumpered before anyone comes near them or it may well cost the life of a human being, as well.

Why, on earth, would anybody choose High Resistance Grounding. Low resistance grounding to limit ground fault current maybe, but high resistance grounding? I'll never understand it.

 

[davidbeach]

DickDV, lots of good reasons for an HRG system provided they are maintained by knowledgeable staff. Wouldn't consider anything else for a generator over 1kV. Some processes need the continuity of service provided by HRG (or ungrounded, but there is no excuse for ungrounded). Obviously in the case at hand the "maintained by knowledgeable staff" part was violated. Absent the knowledgeable staff, I'm not sure that even a low resistance grounded system should be attempted, only solidly grounded.

 

[ScottyUK]

Oz,

Try and get hold of a Profibus module- not for profibus but this has a 24Vdc power supply that allows you to power the control board up. Apply 24Vdc and then check the fault and alarm history. This is a good indication of what was going on before the VFD went kerrboom.

That is a fantastic little snippet of knowledge - thanks. Could be very useful.


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

If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

Davidbeach

I'm assuming that when you mention the voltges shifting during ground fault, you are referring to the fact that when one of the phases is grounded the other two L-G voltages become the system L-L voltages. If some of the Vfd components are not rated for this L-L voltage then they can be damaged during a ground fault on a HRG or ungrounded system? How can you find out weather or not the drive is rated for use on a HRG system?

We did pull the drive out and megger the motor leads from the MCC out to the motor and did not appear to have anything grounded and all voltages looked ok.

Ozmosis

What is the Y capacitor you are referencing used for? I saw instructions for the removal of this capacitor in the operating manual but no reference to what it was used for. I also cannot find anything showing what the individual components of the Vfd are. Where can I find such information? What Siemens guide did you reference above?

I have set up a power meter on the MCC to record voltages. Like I mentioned above there are times whre overnight when this drive is not running that the drive will trip on a DC bus overvoltage. Could there be something strange going on with the HRG system causing these events?

 

[ozmosis]

Page 22/23 of this:

http://cache.automation.siemens.com/dnl/Tg1NTE1MwAA_19484693_HB/Quick Start Guide 1_1 Low Res.pdf

Siemens will not allow repair by individuals. This needs to be carried out by authorised persons. When a cap goes like this it is important to check other components such as IGBT's and maybe even the gate drive board. Not tests for someone who has not had the the training.
It is quite possible for the drive to trip on overvoltage when it is not running. Under no load, the DC link voltage will be high and if you get a disturbance on the AC supply, it is more likely to trip the drive due to the limited headroom of over-voltage protection.

It was discussed within Siemens to disable the overvoltage protection when the drive is not running as it actually serves no purpose other than annoyance. However, I don't think this really got implemented.

 

[rockman7892]

Ozmosis

Thanks for the document it is helpful. It does not say exactly what the Y capacitor does or what its purpose is. Do you know the technical purpose of this capacitor?

It would appear that in the document it referenceds a connection of the drive to an HRG system so therefore the drive would be rated for use on such a system if the Y cap was removed. What problems would exist if Y cap is not removed on HRG system.

What is this document referring to as a "station ground"? It appears that it is simply showing a case where the ground connection on the VFD is not bonded to the neutral of the transformer but is connected to some other earth point?

 

[LionelHutz]

That sounds like the main DC capacitors. Some electrolytic caps aren't rated for below freezing temperatures. Basically, the electrolyte freezes. Maybe the drive enclosure was getting too cold in the winter?

 

[rockman7892]

I set up a fluke 1735 power logger on this MCC over the weekend and all through yesterday. I set the logger up to trigger on voltages with the trigger thresholds being +/- 10% nominal voltage.

Attached are plots of three of the events I picked up. It appears that on this MCC several times yesterday we saw L-N voltages as high as 371V. This would equate to roughly a L-L voltage of 641 which is extremely high. Like I mentioned above the system is a 480/277V wye HRG system so nominal voltage should be 277V as shown for periods in the plot.

Ignore the voltage magnitude for the L1 phase for this probe lead appears to be damaged and only reads 12V L-N when connected. You can see however that it rises with the other phases.

The connections were made with each of the three voltage probes on a phase of the MCC, and the neutral probe connected to the ground bus in the MCC. So technically we were reading L-G voltages with the ground tied back to the neutral of the transormer through the HRG package.

What do you guys make of these high voltage swells?

 

[davidbeach]

You've got a ground fault on L1, that's all. I don't see anything that suggests that you ever had more than 480V line-line.