HomeCase studyLarge VFD trips with power supply by transformer

Large VFD trips with power supply by transformer

There is a site that has a 5 winding 10kV / 1kV transformer for supplying a large VFD (variable frequency drive). The supply voltage is 11kV, however when the unit was installed (some years ago) the transformer was incorrectly specified with 10kV primary. The OEM assured site this would be fine, just at the upper limit of voltage tolerance.
The site now have issues when there is a sudden step down in load, the voltage rises before the OLTC can catch up, and the VFD is tripping on over voltage.

Site are considering putting in an 11kV / 10kV transformer to solve the problem. To my thinking this adds significant cost of not only the transformer, but civil works, protection systems, cabling etc.

The three other options I would consider are:
- Installing a chopper on the DC bus to regulate the voltage
- Removing the existing transformer and having it re-wound for 11kV
- Buying a new 5-winding transformer, spec'd for 11kV, and keep the old one as a spare

The cheapest solution would most likely be if you could adjust the over voltage trip level of the VFD, but check first with the VFD manufacturer. It is possible that this solution would require the intermediate capacitors to be upgraded, but I doubt it. Even if that were the case, I still would expect it to be the cheapest solution. There is no reason that the driven motor would be in any way affected by this change.

Second solution could be to install a small auto-transformer on the 10kV side of your main transformer. The dimensions and cost of such a transformer would be the same as for a (two-winding) transformer with a rating equal to 10% of your existing transformer. This solution should not require any additional cost for your protection system, but will obviously require cabling modifications, and perhaps some civil works to install the added piece of equipment. I believe it will be a lot cheaper than some chopper modifications for the DC bus.

If the OLTC can keep the voltage within limits after it has had time to catch up, the only solution will be to make the over voltage trip less sensitive, since you will still have the same variations in the main supply voltage.

Maybe you could also consider adjusting the "normal" tap of the OLTC to be a step lower than what it is at present. That will give you a standard DC bus voltage probably around 2.5% lower than at present, so you will have a little more leeway before the VFD trips.

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