Overvoltage conditions damaging VFD's

[mtsgeorge]

I would like to see some opinions on whether a line reactor will really help in this situation. 1 300hp VFD and aprrox. 30 5hp VFD's fed by a 1500kva transformer within 30 feet. All of these drives have a history of overvoltage faults and many have been repaired or replace more than once. The facility is very close to a steel mill and the line voltage is usually around 500 volts. The VFD's will fault on OV at 544 volts. Thanks in advance for any enlightening discussion.

 

[electricuwe]

Are you sure the failure are caused by overvoltage?
Are there capacitors, e.g. for power factor correction, involved?

 

[mtsgeorge]

I am not sure that the overvoltage is the cause, but the majority of the drives have faulted on overvoltage several times. No PF capacitors involved.

 

[CHDean]

You may want to look at transient voltage surge suppression if you're sure it's voltage related. Do you have any impedance in front of the drives now? Also, are the 5HP units rated 440 or 460V?

 

[Skogsgurra]

Sounds very familiar. But there are so many things unknown that a guess could do more harm than good. I think that you have to apply standard procedures: Hook up a transient recorder and have it triggered by transient overvoltage from any (I mean all) of the phases. Put a filter with something like 10 us before it (or activate if built-in) so you do not trig on EMI from the inverters. Then wait and see.

When the next fault occurs, check the transient recorder for transient overvoltage. If it is there and if it is not in the milliseconds region, then a line reactor with 3 or 4 percent voltage drop will probably do the job.

If you have longer transients - well, I do not know what to do, really. More inductance than 4 percent might cause problems with the inverters since their input voltage drops too much when loaded. I once used a large DC choke in the DC link to make the inverter immune to PF capacitor switching, but that's a hell of a job.

 

[jghrist]

By fault on OV, do you mean fail by a flashover or trip on overvoltage protection (probably high voltage on the dc bus)? This could be caused by a transient surge that is not clearly related to the elevated line voltage (nominal 480v?). Does the 1500 kVA transformer have taps?

 

[busbar]

I second the potential excess capacitors causing momentary or sustained overvoltage, but put main-transformer tap checks on the top of the list. Remember that—almost without exception—a primary-side powerdown must be scheduled before tap change.

It may be that the serving utility has local capacitors on their distribution system that are not disabled during light-load/low-var-demand periods. The capacitors may be unswitched, or the associated control scheme is not operating correctly.

The 544V-trip feature should be verified as indeed happening at the intended time and voltage level.

508 volts steady-state AC is often the consensus upper limit at 480V building service entrance. It may be worth experimenting with installing buck-connected transformers on a few of the smaller “problem” drives. Stock ratings are typically in the range of 12-16-24-32-48 volts. Check the drytype-transformer literature: for a smaller 3ø 3W load, two open-delta-connected buck/boost transformers can be used, or for bulk loads where a neutral conductor is available, three wye-connected units may be more cost effective. Double check the “new” voltage on the buck-transformer load sides before powering drives.

 

[rbulsara]

I have the same questions as jgrist, before venturing any guess.

544 is less than +10% variance the utility supply will normally have on a 500V bus. I would look at the tranformer tap and adjust to 480 or even 475V first.

Some swithing suirges caused by the adjacent still mill may be involved? Is there a time delay on OV proteciton on the drives?

 

[jbartos]

Suggestion: The AC-DC conversion is producing a harmonic content on the AC side. If there are many AC-DC converters working simultaneously, i.e. attached to the same bus, there will be harmonic interactions that will cause higher voltages, e.g. voltage spikes, or swells. This can impact the equipment attached to the bus insulation and lead to equipment damages or malfunctions. Some companies limit amount of drives to about eight per bus to avoid detrimental effects of harmonic interactions or magnitude additions.

 

[i8flipper]

What about the load side? How come no one is considering what is going on with the load side. Over voltage faults can occur from motor regeneration. You might want to check what the decel. time parameter is of the VFD's. Just another point of view.

 

[jbartos]

Suggestion: Please, check the immediate upstream transformer system grounding. The transformer should not have ungrounded secondary windings.