3rd harmonic question

[Pafi]

I am looking for a basic explanation related to the presence (and necessity of filtering) of the 3rd harmonic current in the secondary of an 25 MVA, 110/ 6kV, star/ delta, ungrounded secondary transformer.
This harmonic can be related to the presence of arc furnaces as a load and be of zero sequence ? If so, is it necessary to be filtered out if you intend to install a PFC + harmonic filtering installation? How can the L-C ungrounded star connected filter to filter out the zero sequence harmonic?
I have measured the voltage and current harmonics but my scope had not the possibility to measure also the fase of harmonics.
I was reading some articles related to the arc furnace reactive compensation but none is so basic.
Thanks in advance for any comment!

 

[jghrist]

Third harmonics are zero-sequence only if they are balanced. This is generally not the case with an arc furnace so significant third harmonic (and other triplen harmonic) currents can flow through an ungrounded transformer connection.

 

[waross]

A wye delta transformer or bank will have large circulating currents in the secondary if the primary voltages are unbalanced and the primary neutral is connected. The first indication is often transformers running hotter than justified by the load.
If you are feeding multiple electrodes you may be able to run broken delta.
The wye delta transformer bank will try to balance the primary voltages so the voltage unbalance may be worse than it appears. You will find the primary line currents disproportionately high compared to the secondary load currents if the primary voltages are unbalanced.
respectfully