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Transient Voltages Causing Drives to Trip on Overvoltage 2

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rockman7892

rockman7892

Electrical
Apr 7, 2008
1,156

I have 2 480V Siemens MM440 VFD's which I have discussed on this forumn previously. I have an issue with both of these drives tripping on a DC Bus Overvoltage when these drives are not running but rather are just sitting idle. The only thing that can cause these drives to trip on overvoltage while sitting idle would be a voltage spike or fluculation on the line side of the drive. I'm assuming that a voltage spike of a certain amount will cause the DC bus link to go high and thus trip the drive on an overvoltage condition.

I have set up a power monitor to try and capture some of these events. However when I see that drive is tripped and then check power meter I see nothing on the power meter. I suspect that since the power meter (Fluke 1735) is only capable of capturing events longer than a half cycle (8.3ms) then it is possible the voltage spikes are happening too quickly for the meter to capture. I have done some research and saw that these voltage spikes are usually very quick and in the order of .5m - 2ms which would be much too quick to capture wih my meter. Would a voltage spike this quick be enough to cause the DC bus to go high and trip the drive?

I would suspect that if these drives were tripping on an overvoltage condition as a result of a transient then I would see other drives througout the plant on the same distribution system trip as well for the same reason. I do not see this happening. Of course this is assuming the transient is coming from the utiltiy or somwhere up in the plant. These drives are by far located at the furtherst point on the distribution system. Does this furthest distance have anything to do with only these drives tripping?
 
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When my generators are lightly loaded, and I put too many harmonic filters on line (far leading power factor), I get door drives that trip on high DC link voltage while idle. The voltage monitoring I have done did not show any abnormalities on the AC input. I have many other VFDs, and only these door drives seem to trip. We added a automatic reset with a limited number of resets (3?) in certain time frame, and it has never exceeded those few reset attempts.

Our VFDs are Reliance.
 
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Yes rockman, there are narrow transients and then very narrow ones. The transients most likely to be energetic enough to pump up the DC link voltage are transients caused by power factor correcting capacitors being switched to the system. One such case is shown in the attached pdf.

"mellanled" = DC link
"nät" = mains voltage

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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rockman
A couple of things:
1) When the VFd is not running the DC bus voltage will be high anyway and so any instability has even less tolerance before it trips. The trip voltage on the DC bus should be (depending on the rating) between 820 and 840Vdc. If you are on the end of line and therefore a weak supply, and maybe you have AC line reacotrs fitted then this can have the effect of adding a high level of resonance onto the DC bus causing increased ripple. The overvoltage DC bus trip circuit takes the top of one ripple, plus any transient (big or small) and can trip the drive.
2) I think the latest FW version of the MM440 should have a facility to ignore transients when the drive is not running. I understand this was discussed. The reality is that fast transients are virtually impossible to protect against with a VFD and so if you get a shoot through, it would have already passed through the rectifier before it is picked up. So if it is high energy then it would damage prior to any protection in SW picking it up. MOV's on the input would have little chance to protect this as well.
On this basis, it was looked at to simple ignore fast transients when the drive is not running but this was a decision pending.
 
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ozmosis / skogs,

Any chance of the braking resistor being used to pull the DC bus down before it rises to a dangerous level, or are the transients you're thinking of too fast and too energetic to allow this method to be used? I'm not sure how fast the code controlling this function executes, and whether it could be made to execute more quickly. In the case of the example shown by Gunnar it looks plausible provided the brake resistor can dissipate the energy faster than the the transient can inject it.


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Not so sure about that, Scotty.

The transient shown was a result of around 40 Mvar PF correction capacitors being connected to a 130 kV grid. Lots of power behind that. I think that a braking resistor stands very little chance against that.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Since brake choppers and resistors are relatively inexpensive, I would suggest installing them with the intent of swamping the transient. This would be even more effective if there is some reactance in the input power leads.

Choose a resistor that has the lowest resistance for the rating of the chopper circuit. Choose the resistor wattage at the low end of the scale to save on costs. A resistor can be expected to handle 10 times its rated wattage for a short period if it is fully cold at the beginning of the event.

While the points mentioned above about pf caps and the expectations about high energy in the pulse are all valid, they are speculations and if may well be that the brake snubber system will end the faulting. In most cases, it has done that for me.
 
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I have encountered this phenomenon (bulk PF capacitor switching ahead of the drives) on Cutler Hammer/Vacon drives and Toshiba G3s. It's annoying. After seeing it in the Vacons and making it go away by turning off the bulk PFC system, I proposed this as the issue when I encountered in again with the G3s. In that case the plant had 20+ drives, all Toshiba G3s (which was a very good drive at the time) and ALL of them would trip off-line on high DC bus voltage at the same time. Recording instruments never showed anything (at least the one's we had access to). There was no PFC cap bank in the plant but they were out in the desert, 10 miles from the nearest town so it was a long run to the nearest HV transmission line. The utility swore up and down that they were not using caps to boost the line, but we drove down the single access road to the highway where the HV line was tapped off to feed this plant and took pictures of the cap bank on the feeder structure. They came clean about them and installed damping (series) reactors at the cap bank, the problem went away.

To ScottyUKs point, it would seem to me that there must be some sort of operating code in the drive's mP that is telling it to trip on high DC bus. So if you inserted an interrupt right before it to fire the braking transistor first, the only remaining delay would be the firing time of the transistor. Wouldn't that work? I'm not saying it would change the nature of the transient, but maybe change the nature of the drive's RESPONSE to the transient. Essentially a shunt resistor?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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How about when the drive is OFF have the input relayed to single phase. It could reduce the surge seen to perhaps 1/2 what it normally reaches.

Or, insert a line filter followed by MOVs. The filter could even be too small to support any form or running operation. Have a shunt contactor short the line filter as the drive starts.

Keith Cress
kcress -
 
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Scotty
I would say no to the brake resistor. The speed transients come through would never be picked up by the brake chopper circuit and then diverted to the brake resistor.
I'm not sure there would be any point either. Once it gets to the DC section of the VFD then high energy transients would have already done the damage. MOV's and maybe line reactors would take some steam out of the transient but these passive solutions are for slow acting surges.
I have also, as with jraef, seen the main cause to be PFC's switching on and off and causing these transients.
 
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Would a faster surge suppressor, like silicon avanlanche, work in this case?
 
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Patick,
I agree with you about the issue of damage to the DC bus components being unavoidable if the energy in the transient is high enough. But if the drive is tripping on OVDC then it is obviously surviving the transients, yet the energy buildup is causing the nuisance trip. Having the energy shunted to the resistors could at least avoid the nuisances.


"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
 
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Dangit, saw that squiggly red line under your name the split second after clicking Submit...


"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
 
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jraef has captured my thoughts perfectly.


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I agree; but firstly the speed for a brake chopper would be too slow in mho and secondly, the VSD should not be tripping anyway. It serves no purpose if the drive is not running. Ok, it trips, but for what reason? Once a drive trips it doesn't mean the rectifier or DC bus circuit is any less protected. Only when a drive is running do you want over-voltage protection.
It would be a lot of money in brake resistors for the odd transient that passess through.

It could also be one of those insects that get left in the firmware and the reason to check parameter r0018 to ensure you have the latest version. But I cannot say too much, especially as jraef has divulged my mispelt name..
;-)
 
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While the pulse itself may be very fast, the DC bus response would be much slower due to the DC bus capacitors. For that reason, I believe that the snubber resistor would be effective in preventing the tripping. My own experience confirms that.
 
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If I recll correctly, I believe I heard or saw somewhere that the braking choppers were only active when the drive is in the run mode. If that is the case, then the chopper whould not even be active when the drive was sitting idle, and therefore the braking resistor would do nothing to help with these line disturbances while sitting idle.
 
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Do you think a logical first step would be to get ahold of a high speed meter to try and capture these transients. Once we know the speed of the transient we can the use this information to size or specify reactors and or supressors?

Any truth the the drives being at the end of the line, as to why we only see this problemm on these particular drives?
 
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I agree with the answer DickDV gave. Install a braking resistor and a line reactor.

Looking at Gunnar's curve, I see no reason why the brake chopper could not turn on quicker than the rising DC buss voltage. The IGBT's in a VFD are capable of switching much quicker than the DC buss is rising in that graph.

 
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Good question as to whether the DC chopper is even active when the drive is not in Run mode. One that only a specific drive mfr could answer. I'll see if I can extract one from Siemens. I no lonmger work there, but I still have connections... (albeit at a lower level that ozmosis', at least I am in the same hemisphere [wink]).


"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
 
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