Drive Isolation Transformers

[VicP]

I have always specified Delta/Wye connected transformers for DC Drive Isolation applications. All transformer manufacturers, that I am aware of, also specify Delta/Wye connected transformers for this application. I recall hearing that this connection cancels some of the harmonics generated by the drive in the Delta connected primary.

We have a customer who would like to use an existing Delta/Delta transformer for this type of an application. How would a Delta/Delta transformer performance compare to a Delta/Wye for drive isolation?

Thanks for any comments or info.

 

[jwerthman]

For a single drive, I don't think it really matters. The only real effect of either the delta-delta or the delta-wye isolation transformer is to provide additional source impedance. The delta-wye connection would provide some shunting of triplen harmonics except that the neutral is normally ungrounded in these applications so it is no better than the delta-delta in this regard.

If there are more than one drive, then the transformer connection can make a difference. By using a delta-delta and a delta-wye on two identical 6-pulse drives, the combination acts similar to a 12-pulse drive so most harmonics below the 11th are reduced or cancelled. In particular, the 5th and 7th harmonics for this arrangement would be substantially reduced. But that only works if both drives are running at about the same level.

 

[Skogsgurra]

There is one important issue: if the transformer has same or slightly higher kW as your drive has, then you might have a problem running full load or overload during start. The reason is that the impedance for triplenes gets quite high (a delta winding would short them out, the wye doesn't) and this will distort your line voltage to an extent that can make your firing circuits fail. It happened to me when starting a large pulper several years ago. Pulper and several other smaller drives just tripped. Had to change from wye/wye to delta/wye. Took some time and effort, but helped.