Transient Voltages Causing Drives to Trip on Overvoltage 2

[rockman7892]

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?

 

[DickDV]

rockman, I've been selling and specifying reactors for 20 years mostly for MTE and the best explanation of reactor % ratings is simply that a 5% reactor conducting its nameplate current will drop 5% of its input side voltage. A 3% reactor will drop 3% of its voltage, etc.

Why they are rated that way baffles me but that's the way it is. Wouldn't it make more sense if they were rated in microhenries and amps.

I've never been able to get a better explanation than the above and have, after all these years, stopped asking.

I suggest you take it and run with it too!!

 

[rockman7892]

DickDV

You are right, somthings are better as just being accepted however I cannot help but be curious

I found the attached article somewhat interesting, as it goes into a small calculation about how these reactor values are calculated.

The article also goes on to say how the effect on the drive is a function of source impedance and transient magnigude. I guess the more source impedance the lesser effect the magnitude will have on the drive?

The article also says that a reactor may not have any effect when the drive is sitting idle. I have also heard this from others, simply due to the fact that current is not flowing when idle. However it sounds like others have said that even without current flow the drive trips as a function of the current resuting from the transient voltage so I would not think it mattered in this regard wheather or not the drive was pulling current. The current surge that would charge the bus cap would be dampaned by the inductance of the reactor.

I could see however that an idle drive would not have a load contributing to reducing the bus voltage at all which would not help things.

I do plan on taking all advice above and installing both 5% reactor and TVSS however I cant help but be curious to learn certain details. I appreciate all of the help with this.

 

[rockman7892]

Forgot Attachment. Here it is.

 

[Brokendiode]

rockman7892

I have a great deal of experience with the MM series drives
And there is a setting or two in the setup that may help with this problem. However line reactors are possibly the best. In fact if you look in the install instructions you will see that above a certain KVA feed that line reactors are required.

 

[rawelk]

Out of curiosity, would you happen to have any large DC drives (or some other high power electronics that uses phase angle fired thyristors) on the same AC bus as the drives that are tripping?

I've seen various AC inverter drives that are susceptible to the 'notching' DC drives produce, and which fault on DC bus overvoltage due to them.

A thought - it probably won't help, but is fairly cheap to implement - try adding suitably sized ferrite toroids around each AC line wire. If the observation about drives not tripping when an AC clamp was used wasn't a coincidence then this might do the trick.

 

[MAGTiger]

MAGTIGER

I am curious about how to determine the impedance ratings you discussed above. For instance I looked at a 2hp VFD that had an input rating of 3.8A at 480V. On the line side of this drive I noticed a reactor with the following info:

L= 12mH
If = 4 Amps
Ith = 6 Amps

I was told that this is a 5% impedance reactor. How do you determine from this information what % impedance this presents in the circuit? Can you simply take the drive rated voltage and divide it by the current rating to come up with a rough aproximation of the drive impedance and then use 2*w*L for the reactor to figure out its impedance and compare the two?

Can you explain about the different % impedances and their current values on different drive sizes as you mentioned above?

Well, it depends on your understanding of the terminology. If and Ith are not industry accepted terminology but may be understood to mean fundamental amps (If) and thermal amps (Ith). Which one applies to the 3.8 amps of drive rating. Take your pick. Neither results in 5% Z.
.012 * 2 * pi * 60 * 1.73 * 4/480 = 1.6%
.012 * 2 * pi * 60 * 1.73 * 6/480 = 3.2%
Hopefully you didn't pay for a 5% reactor.
In most cases 3% is enough anyway. But for your drive this reactor looks like about 1.5%, marginal.

Sizing reactors on percent impedance is nothing more than a convenient way for buyers to compare price in a standard format. I agree that Engineers would probably be more comfortable just using inductance but then every reactor rating would have a different L. With %Z, the buyer just buys reactors on amps and for volume contracts, typically would buy all 3% or 5% reactors. It's part of the comoditization of the product.



Neil

 

[rockman7892]

MAGTIGER

Thanks for the information. I am having a little trouble following your calculation to arrive at the % impedances.

It looks like you are using:

L *2*pi*f*1.73*A / V

So it looks like you are taking the overall impedance of the reactor times the current and dividing it by the voltage? Is this correct for determining %Z? I dont quite follow it.

 

[MAGTiger]

L * 2 *PI *f = ohms of reactance = X.
then multiply the X ohms times amps to get the voltage dropped across the coil (phase). then times 1.73 to get the line to line voltage drop. Finally take the line to line voltage drop just calculated and divide by the system rated voltage (typically 480). This gives the per unit voltage drop which is by definition, the impedance (of course we are ignorring the very small effect of resistance). Now multiply times 100 to get %impedance.

Neil

 

[rockman7892]

MAGTiger

Ah ha! Makes perfect sense now. Thanks for clearing this up!

 

[cjhut]

I have had these same exact problems on Allen Bradley Power flex drives, and also SLC 500 pwer supples. I started to notice that it was occuring around the same time everyday. Eventualyy I contacted the local utility company and found out that they were closing a capacitor bank up the street every time I experienced the transients. I spoke with an Engineer specializing in power transmission and distribution and he felt hands down that they were causing the problem. I would sometimes have 2 or 3 drives go down and PLC power supplies blow fuses all at once. Once I informed the Utility of our situation and the downtime it was causing, they made a change in their procedure and the problem went away.

 

[fraserjim]

I just joined today, but wanted to add my two cents.

I have been troubleshooting power quality porblems for 27 years. We first used the BMI PQ monitors and then switched to the RPM 1650s. RPM was bought by Fluke and their latest version is the 1750s and 1760s. We still like the 1650s best.

Our experience is that most drive trips on overvoltage are caps switching. The facility may have switching caps for power factor correction or the utility will have voltage support capacitors.

The utilities will normally switch the capacitors in the 6AM region and then at 10-11PM. All depends on the local grid and loads.

The switching causes ringing on the line, which can cause an overvoltage trip. They usually last about 1/4 cycle and do not have any voltage rise, so they don't trigger a transient capture on a PQ meter. They can trigger a waveshape fault.

Usually line reactors are the best bet.

As far as MOVs, the normal failure mode is to fail short and explode. If there is limited fault current avaiable, they smoke and smolder. If there is sufficent fault current available they will short and then blow open. If the problem really is voltage spikes MOVs are fast enough to protect you.

Make sure that you get TVSS with multiple, individually fused MOVs. This way, a single MOV failing will not take out the TVSS main overcurrent protection and leave you unprotected. Also, UL has just come out with a new standard UL1449-3rd Edition. Make sure that the TVSS (UL now calls them Surge Protection Devices SPDs)has a UL1449-3rd edition label from UL or a Nationally Recognized Test Lab (NRTL).

We have had limited success in applying TVSS for overvoltage drive trips.

We have had a lot of success applying high-end TVSS protecting drives from catastrophic utility hits.

Fraser Jim

 

[jjmoab]

I have also seen this overvoltage effect in our plant. In our case we have an IT configuraction. A phase to ground fault, while starting a "big" motor, was creating the same problem you are having.

 

[rockman7892]

Let me ask this question out of shear curiosity. Will the magnitude of the transient voltage waveform seen at the drive be different depending on the source impedance the drive was connected to?

So for example for a given VFD (lets just say 75hp) and the same utility transient, would the transient magnitude seen at the drive be different if the drive was connected to a 1000kVA transformer as opposed to a 1500kVA transformer?

If source impedance does have an effect, then for the same transient on lets say a 1000kVA transformer will there be any differnce of the magnitude seen at the drive on a 10hp and a 75hp power drive side by side? In other words does impedance mataching have an effect?

 

[LionelHutz]

The caps in the drive has a fairly low impedance. The caps charge each time the line voltage is higher than the capacitor voltage. At these times, the source is feeding current to an impedance.

So, the source impedance will have an effect and the size of the drive will have an effect on the same source.

Looking at it another way, the 5% reactor sized for a 75hp drive would not have much effect if it was installed on a 10hp drive.

Look up 3-phase rectifiers and get a better feeling for how the caps charge then consider what happens when you place an impedance in the AC lines feeding the rectifier.

 

[PQ101]

To rockman7892-

It is possible that the panel serving the VFDs is experiencing momentary voltage sag (within 2-3cycles let's say), that would cause a havoc, and the way the VFD display/parameters are setup they will not necessary indicate the actual cause of problem.

You will hunt it down, just make sure your fluke setup is done properly. I suggest connecting phase-to-neutral (set the low threshold at 255volts hight to 305volts) so you can see each phase seperate. If supplied with 3-phase delta connection w/ground, you have no other choice but to go Delta (try to set the low at 456-460volts and high to 508volts or above, to 528volts dependig on your situation)

Make sure your pre-trigger and post triggers are set properly so you can see what VFD is experiencing prior to the shudown event, etc. Small problems can cause big head-aches.

Based on experience I'd say incoming voltage is a good place to start your investigation to identify the cause. I assume you have checked both the system grounding & equipment grounding, and they all verified/checked OK.



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[HamidEle]

Recently, we experienced unbalance voltages at 4160V level while switching the capaciotr bank to the system, which tripped on phase reversal on Relays, which further tripped 600V VFDs due the transient voltages in the system