VFD overvoltage probems 1

[jraef]

This is the result of a side-track in thread237-96331 posted by skogsgurra regarding flashovers in VFDs. I too am having repeated difficulties with that, all in the same general geographical area. Although a forensic analysis is ongoing, I received a "theory" from a consultant hired by my customer that I want to run up the flagpole here for comentary.

A brief (although that is difficult for me) description of the problem: we have had a relatively high failure rate in one geographic location for VFDs of a particular frame (25-40HP). In most cases, the users have tried many other brands, all with worse MTBF than ours. Most begin having problems with tripping on overvoltage even though the supply is well within nominal ranges. Eventually they "blow up". Our units lasted longer, probably due to a higher bus voltager tolerance than most, but now we are succumbing to the problem as well. This happens in more than one plant, but all withing a 100 mile radius and on the same utility.

Here is the intersting part. One of the users hired a consultant on their own since this seems to cross over manufacturers lines. This consultant has put forth the theory that the utility has, somewhere in their grid, poorly separated power lines which are occasionally touching each other in the wind, sending very short duration but repeated spikes down the lines. Without much detail I find this a bit difficult to swallow, but not being a utility guy I thought I would throw it out here for those who may have heard of this.

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

Line slap huh? That does not sound like a likely cause at all to me. Especially with the fast acting protective relays around now days.

I think the best bet is to get a high speed recorder on one of the feeders to get an idea of what kind of voltages your dealing with. Many utilities will provide this type of device for you too use temporarily. Once the magnitude of the voltage is know and the duration, it may help get you in the right direction or at least, help identify a short term fix (voltage suppression).

There could be many different things going on here but given the info it sounds like it is something on the utility side for certain. Start with a recorder to see what you have. Discussions with the utility may shed some light on it as well (then again maybe not since they generally do not like to point the finger at themselves).

 

[jraef]

Tahnks buzzp.
You hit the nail on the head about the utility not wanting to point the finger at themselves. Their official response is "... there are no voltage disturbance or capacitor issues in our distributuion system that could be deemed responsible for the failures encountered." (the capacitor issue was one brought up by another manufacturer). They calimed to have applied a recorder and monitored it for 3 weeks, seeing no anomallies. I personally find that fact alone to be dubious ;-p

 

[Skogsgurra]

I agree fully. We had a Dranetz 4300 attached to the grid in "my" problem. Did not show any anomalies. But it is a must, for starters.

jraef. It is very considerate of you to start a new thread. But not at all necessary. At least not in my mind.

 

[buzzp]

So they claim they put one on there and did not see anything? I would request a copy of the results, push hard for this. Otherwise, I would look at renting/buying one. Also, if you see something suspicious at one sight you should move it to another to see if the magnitude goes down or up, as this may help find the origination of the problem, up or down stream of the two facilities.

What types of transient protection are there on the front end of your drives? Are these in a high humidity or high altitude environment(you probably already scoped this out)?

 

[jraef]

The drive is brand-labeled from another manufacturer so one of my problems is that I don NOT know exactly what components are inside, but being that this problem crosses manufacturers I do not feel that it is a standard industry design issue. As I said, we actually had a better MTBF than most. There is an LC filter built in to the front end, mainly as an RFI suppressor, and that may also have contributed to our survivability. But maybe that is contributing to the problem as well. The reason I suspect that is the LC filter design being one of the the few variables in our product between frame sizes, and this is the only frame size that is having this problem for us. Other manufacturers seem to be having problems in all sizes. As I said, we have an internal evaluation going on.

By the way, it was my suggestion that they do an independant study that promted the customer to hire this guy with the line slap theory. So you really think that issue is a red herring eh? Funny how that can backfire on you; I didn't mean for them to hire a GOOFBALL!

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

BTW, roughly 2000' elevation, arid desert area.

 

[DickDV]

Just a little comment on transient recorders. It is equally important to monitor line to ground transients as line to line.

This is particularly important if the substation is using an ungrounded delta secondary. Theoretically, the "float" voltage can go anywhere until some insulation or protective device conducts and then, if there is power behind the transient, bang!

In my view, it is even worse to operate continuously with one leg deliberately grounded on a delta secondary substation. Most electronic equipment is not really designed to see full voltage to ground on the phase legs and the result is more insulation stress than necessary.

 

[ozmosis]

There was a particular interesting case in S.Africa where a sugar cane plant would have power supply problems at certain times of the year and the drives all started tripping on OV. Analysis on the incoming to the plant showed serious transients, surges and generally dreadful supply conditions that we were quite thankful the drives were just tripping and not popping. The supply authority investigated and came back with the conclusion that due to the burning of the sugar cane, the smoke and debris created such high density around the overhead cables that this was what caused the problems.
You wouldn't happen to be in a sugar cane area?

 

[jraef]

No but they do have dust storms quite frequently. This is outside of Las Vegas in the industrial areas (not the casinos, which have their own set of problems. I don't know why I didn't say that earlier). One of the reasons why nobody has given much thought to that is because the VFDs need to be protected from heat, so they are in realtively clean areas protected from dust as well. The effects on the distribution system however might be another issue. Thanks.

"Venditori de oleum-vipera non vigere excordis populi"

 

[GGOSS]

Hello jraef,

I can sympathise with you. Not too long ago we too experienced repeated failures at 3 sites within in a common geographical area. At each installation a number of soft starters and motors had failed over a period of about 12 months, further inspection strongly suggesting transient over-voltage damage in each and every case. The 'events' however were not captured by the recording equipment installed by the supply authorities and for some strange reason, we were discouraged from installing our own, thereby eliminating any possibility for us to confirm one way or the other.

On further investigation we found that the HV supply comprised an automatic tap changer that we believed (but could not confirm due to a lack of available information) was open transition by design.

Although I cannot confirm the automatic tap changer to be cause of those failures, you might be interested to note (as we were) that the sites have been trouble-free since we outlined our suspicions (10 months ago) to the 3 clients involved.

Regards,
GGOSS

 

[GGOSS]

PS: Don't discount the possibility of 'line slap', I have actually witnessed this occurring.

PPS: Surge protection may provide benefits in some installtions but bear in mind we are talking about transients and that subject to rise time, they may actually penetrate any installed protection. It's all about dv/dt !

 

[Marke]

Hi Jraef

I have seen a similar situation happening on a MEN supply system where the HT transformer 22KV / 415 volt was delta primary and star secondary. The eath neutral link was loose resulting in a high impedance between the earth and neutral.
When HT power factor correction was switched in and out of circuit, it caused transients on the ht line. This was cpcacitively coupled through the transformer causing the whole secondary to leap above earth momentarily. The 415 volt three phase appeared normal phase to phase, but very high fast transients relative to ground on all phases at the same time. This lifted the DC bus away from ground and effectivley injected a very fast high voltage transient into the output of the drive (due to the capacitance of the cable and motor to earth). This caused overvoltage trips, instantaeous over current trips and very ocasionaly, caused output device failure. Have you uswed a Drantez to check all phases to ground??

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[BrianR]

I have seen a number of instances where electrical control equipment was failing the way you describe this. Voltage transients were suspected however connecting a recorder showed nothing wrong with the supply.

Convinced that voltage transients were the cause we connected surge suppressors and/or line filters that filtered common mode and differential and in most cases cured the problem.

Perhaps this is worth a trial in your situation. The voltage spikes were very short duration and I think the recorders missed them.

 

[Skogsgurra]

I had a similar problem in the west coast of Sweden several years ago. Paper machine drives tripped and lots of lesser equipment had problems. The utility connected a recorder - and this is where semantics starts to count - but "could not see anything".

I went there to find out what was going on and first of all, I inspected the recordings. Sure enough, they had used an analog pen and ink scribbler. It was set to 10 mm/h so the chances of seeing any transients in the ms range was not directly good. To say the least.

I interviewed some of the industries down there and found that they all had this problem around half past six in the mornings. A fast HIOKI memory recorder hooked to the mains and set to trigger on wave-form deviations caught the transient the next morning.

The transient was seen on all three phases, but was more prominent on the phase that happened to be closest to its peak value. The transient started with a rather deep dip, I would say from 700 V down to about 300 V in a short time, about 50 microseconds. The voltage then swung back and overshot up to about 900 V. It died out ringing at around 800 Hz and was over in half a period.

It was clear that the transients was caused by the switching of a phase compensation capacitor bank. But it was not easy to find it. It was only after having discussed with the regional power company that we found out that they had just installed a 39 Mvar capacitor bank on their 130 kV line. It so happened that they used to switch it in just before demand started to grow in the morning.

We managed to make them refrain from switching and they later built a syncronisation device into the switch control so that all capacitors were switched close to the zero crossing of the phase-phase voltages. The breaker had one common shaft that closed all three poles and it was possible to adjust the poles individually to close at 120 degrees difference in time. It was only the start of the closing that had to be synchronised with the mains voltage. A very neat solution. It even adjusted itself by sensing if the closure was spot on or a little too early or late. It then adjusted the delay accordingly.

There are no problems with the capacitor bank any more. As a side effect, the breaker does not need to have their poles changed as often as before. They are always switching low currents, which they did not do before.

To conclude: Capacitor banks can, theoretically, make your voltage dip almost 100 % for about 100 microsecond and then swing back to 70 - 80 % above the mains peak value. That can produce a problem with many drives. An RFI filter will usually not take care of that, but line reactors and a DC link reactor will. But first of all: Do not believe the utility guy when he says that "nothing can be seen on the recorder". He is invariably talking RMS and minutes/hours, not instantaneous voltage and split milliseconds.

 

[Laplacian]

We had a small VFD tripping frequently on bus overvoltage during run, decel and accel. I researched potential causes for the nuissance tripping and went back and re-read the manual word by word. It placed particular emphasis on grounding.

I had the ground checked at the drive gound terminal to a known good ground. There was 15-19 ohms resistance. We re-grounded the VFD to a known good ground and started the drive...phase-ground fault.

Megger checks from the VFD to the field did not pick up anything even at 1000VDC on the 480V system. We should have megged from the motor back, but that was a difficult location to get into.

The bad ground at the VFD was not allowing the fault to blow through insulstion, but it was enough for the drive protection circuitry to pick up and trip on high DC bus voltage.