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?

 

[jraef]

Wow, to quote Lewis Carrol, curiouser and curiouser. I can't think of anything that a voltage probe would do to affect an over voltage transient.

Coincidence perhaps?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[ScottyUK]

Well, I suppose the better multimeters do have front end protection against transients to a greater or lesser degree. Not 100% certain on the type of protection but almost certainly MOVs and possibly a spark gap as protection for a Cat III or Cat IV rated meter. If adding the meter really makes a difference then possibly the transient protection on the meter is acting to calm down the transients to a level which doesn't upset the drive. Seems a long shot though. Would be a cheap experiement to try a handful of MOVs connected across the lines and line-ground.


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

 

[ozmosis]

The Siemens MM440 already has a number of MOV's protecting the input rectifier.
Sounds like a little inductance on the input might solve the problem. Fit a line reactor, it's a lot cheaper than leaving your fluke connected.

 

[rockman7892]

O.k. sounds like all roads point to the line reactor and it may be best just to fit a line reactor rather than try to capture any transients.

I know you typically fit these reactors to 3-5% of the circuit impedence but I would like to become a little more educated on fitting these reactors. Can anyone suggest anywhere where I can read up on this application?

 

[rockman7892]

I understand how line reactors can protect the drive by creating a high impedence for high freuency transients and thus limit the current to the DC bus.

However can the reactor actually attenuate the magnitude of the voltage transients itself to prevent it from damaging components in the drive. I would think that with these high frequency transients the impedence of the reactor goes to a high value and alot of the transient voltge is therefore dropped across this impedenc thus diminishing the votlage that would appear at the terminals of the drive.

 

[PPC800]

Thank you everybody, I really enjoyed reading this thread!

 

[ScottyUK]

MOVs being devices with a finite lifetime, is there any possibility that the MOVs fitted to the drive are knackered? How easy are they to replace?


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

 

[ozmosis]

scotty
good point. In this drive they are bloody difficult to take out.

rockman
As the drives are not actually being damaged currently, they are tripping on overvoltage, the transient may not have that much energy to cause real damage, just trips. The added inductance may just eliminate the trips.
You are right in your thoughts about high frequency transients and line reactors though-sometimes they may just pass straight through.
The overvoltage circuit is on the DC link of the VFD so measuring means the transient has already passed through the rectifier. hence the comment I made a while back that trying to solve the problem of "what to do with this voltage once the VFD has measured it" may be seen as a little too late if it is high energy.

 

[rockman7892]

I spoke with Siemens regarding this issue. They were more inclined to recommend a TVSS on the line side of the drive as opposed to a line reactor for any line side disturbances. They said that most likely a reactor would not be a good solution for any line side disturbances, suggested a TVSS of some sort as a solution.

They recomended using a TVSS from a company called Rayvoss whith whom they claim they work closely with on these type of issues. I've attached a brochure for one of the type of models they recommended.

Has anyone ever heard of this company or their modules? I know many of you have said that reactors would be a viable solution, but do you agree with Siemens on their recommendation?

 

[controlsdude]

One manufacturer recommends line reactors

http://www.transcoil.com/Applications/InputApps/NuisanceTripping.nws

another says line reactor

http://www.mtecorp.com/lineload.html

You can dig thru most vfd manufacturers and they say line reactors on input for nuiscense? (cant spell) overvoltage tripping.

To me there selling the next new toy. maybe it works better, probably pay premium on the product line i am sure.

 

[ScottyUK]

Hmm, Transcoil recommends the products that they make... that's amazing!

MTE recommends the products that they make too... there's a pattern emerging here!


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

 

[ozmosis]

Rockman
Yes, Rayvoss are a well established company and their products are good. A TVSS would typically be recommended where high energy surges are a possibility, like lightening.
From your description, and the fact that the Siemens MM4's rectifier is still intact, I don't think you are getting high energy surges but low energy fast transients.
I'm sure the Rayvoss would be ok but it could be seen as 'belts and braces' i.e. overkill. Your choice, your money I guess

 

[LionelHutz]

I'd say the Rayvoss surge arrestors are the best available. They have some amazing ratings and will not fail open at the end of their life. Read the rating of 4000A fuse on a 200kA fault current. They will short and blow the fuse without exploding. Most other arrestors are composed of a bunch of small disk MOV's and they will fail open which is a fairly useless mode for a transient surge surpressor if you want it to be there to protect the equipment.

Now, after saying that you would use a 277V arrestor for the 480V system. It appears this arrestor has a let-thoutgh voltage greater than 1000V during a fault, which, to me, appears too high to keep the drive from tripping on over voltage. However, if Siemens has a history of success with these then you have to decide if their claimed successes mean you have success.

Every drive we sell has a braking resistor installed and we also push for a line reactor on every drive. We really don't have to deal with very many overvoltage trip issues. The last one was creating a huge puzzle until I had them turn the resistor switch back on. It was working and suddenly stopped and no-one could figure out why. On power down, the electrician had turned off every disconnect switch he could reach.

 

[controlsdude]

Line reactors manufacturers - there are many of them and you can easily second source them. Have problem with brand a you can go to brand b.

Siemens is recommending some particular manufacturers product.

Other VFD manufacturers still recommend line reactors on line input transisents.

I guess the real question you have to ask can you second source this product? Yes, it has great features, but to be tied to them at the hip is risking realibility on a brand new product, can you get replacement parts in time, and at a reasonable price?

 

[digitrex]

I am not good at VFD but I have seen similar problem many years ago in ABB MV drives.

ABB solved the problem by using a relay to connect the 5kV DC bus through discharge resistors when the drive is on idle/standby mode. The relay disconnects the discharge resistor when the drive starts.

The discharge resistor is cooled by the drive's cooling water system.

I think without the discharge resistors, the DC link voltage is being built up accumulatively by the spikes or surges in the line. When the drive is running, there will not be any high voltage built up in the DC link because the inverter is drawing the current off from the DC bus to the load (motor).

 

[DickDV]

Without a reasonably complete explanation of why Siemens is recommending this product over reactors, I believe I'd stay with reactors.

Probably wouldn't hurt to call Siemens back and press them for an explanation on their choice.

 

[rockman7892]

O.K. Here is a follow up on this.

In total I had (3) drives that were having these overvoltage issues. Drives were a 5hp, 10hp, and 70hp.

I purchased a line reactor specified by Siemens for the 70hp drive and this did not solve the transient problem. This reactor however was only a 2% reactor.

For the two smaller drives I purchased 5% reactors and installed them on these drives and they have not tripped since after having been tripped every morning.

Do you think that increasing to a 5% reactor on the 70hp drive will be better than the 2% and help eliminate the problem?

Is it true that it is the impedance matching of the drive to the source (transformer) that leads these transients to be of more effect in some cases? Thus installing the imput impedance to help match more to the source?

 

[MAGTiger]

A voltage step function causes a burst of current into the VFD front end because the capes drawing current by I=C*dv/dt. So a pf cap switching even or other large load shut down can cause this voltage step. The current pulse into the bus raises the bus voltage and you have a ov trip. The line reactor slows the rate of change of that current pulse thus reducing the likelihood of a bus voltage boost. But, you have to be careful of the line reactor ratings. %impedance is just a convenient way of comparing sizes. What's important is uh. So if you have a 5% reactor rated 100 amps on a 50 amp drive, then you are only getting the uh of a 2.5% reactor correctly rated at 50 amps. 3% X usually results in enough uh if the amps are sized correctly. 5% is better if you can afford the extra cost and watts.

Neil

 

[PQ101]

Do you mind explaning the overvoltage issue. On your 480V incoming service the voltage should be well within the nominal-continueous. Same as on your VFD panel. You may have power factor correction capacitors on your main switchgear or elsewhere on the system. Either way your line voltage should be maintenained within 480volts. If the VFD internal harmonic filter is damaged, then the VFD controls may see it as overvoltage conditions- but not necessarily may be the case. Is this experienced all over or on specific part of the building or on one machine? have you set Dranetz 658 power analyzer or PowerVisa (or equal) on panel serving the VFD or at the VFS terminal to see what VFD is experiencing (or verify voltageconditions at the affected equipment). I solve all my VFD problems with power analyzer because each case may be different and VFD control display the parameter thay can be little messy at times.

 

[rockman7892]

I have put monitoring equipment on the line side of the 70hp vfd and have not seen any issues with the voltage here. It stays pretty steady at 480V. I have also set up the monitoring equipmnet to look for any transients, however the monitoring equipment is only capable of capturing transients of 1/2 cycle or greater. I suspect that whatever transient is occuring is less than 1/2 cycle and therefore is not being detected by the monitor. All of the drives in question are located throughout the plant in different areas of the plant. We do have power factor capacitors inside the plant although they are not switched.

For the 70hp drive I am going to get a 5% reactor and also a TVSS to hopefully eliminate this issue.

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?