Harmonics with VFDs and Wye-Wye Transformer

[irmalcol]

Hello,

I am looking for some opinions regarding a plant distribution system I am investigating with the following configuration:

There is a 1500kVA 25kV/480V wye-wye transformer feeding the plant. The bulk of the load is comprised of 2 600HP VFDs. The primary neutral of the TX is solidly grounded, and the secondary neutral has a 2A NGR. I am not able at this time to determine the exact construction of the transformer core, but I do know there is no tertiary delta winding. Large harmonic distortion has been measured on the utility side of the transformer, and 5th harmonic filters installed at the VFDs have failed due to overheating.

I am trying to determine whether the transformer is causing any problems or if the VFDs are solely to blame for the harmonic mess here. A couple of questions I have:
1. Could the transformer be contributing to or exacerbating the harmonic issues with the VFDs? Further, would the transformer only cause 3rd harmonic issues, or would it play a role in higher-order harmonics as well?
2. Am I right in thinking that harmonic distortion/zero sequence currents are passed easily to the utility with a wye-wye configuration?
3. Does the 2A NGR have any bearing on these harmonic issues?

Any thoughts anyone has on this problem would be appreciated.

Thanks.

 

[sibeen]

Kiribanda, that may be true theoretically, but as I stated earlier, that only applies if the input voltage is perfectly balanced and the firing of the rectifier is ideal. In reality you generally see a small amount of third harmonics generated.