I have always wondered about the advantages of a zig-zag grounding transformer versus a wye-delta bank.
The first time that I needed a grounding transformer it was in the third world and I couldn’t find a supplier who even knew what a zig-zag transformer was.
Then and since then I have used a wye-delta connection for an artificial neutral or grounding.
The advantages of the wye-delta solution are:
Usually no lead time.
Easy to find suitable transformers off the shelf.
Suitable replacements may be readily available “off the shelf”.
What are the advantages of a zig-zag transformer, if any?
The advantage is that you can keep the flux balance in a simple three leg transformer if you have an unbalanced load or o highly non-linear load. You can achieve the same thing with separate unconnected, shorted winding.
But you don’t have the problem at all if one of the windings (primary or secondary or both) is Delta connected, so no Z winding needed there.
Our 400 V grids (50 Hz, 400/230 V with Neutral brought out) has that problem in many places where the feeding transformer 6 or 3 kV primary is Wye connected.
One saw mill was running their own steam turbine/generator and needed a pony motor to bring it up to speed. Old installation with a Ward-Leonard MG set and DC pony motor. The MG set was replaced and a six pulse SCR installed instead. Didn’t work very well.
I got there to see what happened and found this Y/Y transformer. We also found a Delta primary transformer (needed Wye to bring out the Neutral). Got that one installed. Worked wonderfully 'cause the harmonics now had a path for their flux components. With Y/Y the transformer impedance for harmonics was so high that they never got up to 3000 RPM (50 Hz grid) and then the interlock didn´t allow the generator breaker to close.
But, as already said, if you have one or two Delta windings, there is no need for a Z winding. Not to my knowledge.
Thank you Gunnar.
My application was grounding a delta wound generator.
As a grounding transformer for a delta system, what are the differences between the zig-zag and the wye:delta?
The Generator Step-up Transformer was a 460 V to 13,200 V delta wye transformer bank.
The delta side facing the generator.
That´s worse. I guess that there was a concern about ground fault on the 13 kV side and that you didn’t want any voltage capacitively coupled from the HV side to the generator? Or some mishaps in the transformer itself. Then, it is not possible to ground the LV side - no Neutral available there.
In this case, it is seems not to be so much about non-linear load but more about how to handle faults in the system. Is that a correct assumption?
How did you solve it?
One could, perhaps, have the primary re-arranged (Z winding) to get a floating neutral and ground it. But that would not be easy. So, I’m sorry. I see no easy way out here.
Driving ground rods to eliminate high voltage on the genset would save some lives. But I guess that has already been done.
I hope that someone out there can help.
BR
Gunnar
The 13.2 kV side was a grounded wye.
We had three 600 KW, wye sets and two 350 KW delta generators.
The wye generators were grounded at the wye point.
The issue was the synchroscope.
Originally, because of the delta generators, the synchroscope reference voltages were taken line to line.
The bus reference was taken from line to neutral.
They had been running for years with a 30 degree error hard wired into the syncroscope circuit.
When I became involved I was exploring options to correct the error.
The nature of the operators varied from quite alert to slow and deliberate.
The most alert operators tried to close the breaker exactly at 12 o-clock.
They had frequent breaker trips when trying to bring a set on-line.
When the “quite slow” operator was bringing a set online, his hesitation allowed the set to come into closer synchronization and he almost never had a breaker trip when synchronizing.
After installing a small lighting transformer line-to-line to develop a bus reference voltage all the problems and trips ceased.
Thank you for your advice.
So, that was the issue? Nothing else?
Good to know.
Now, what was the question?
The question was the pros and cons of w zig-zag versus a wye:delta when an artificial neutral or a resistance grounding transformer is desired.
OK. So, the question was more general.
Wiki is my first resource when I can’t express myself clearly enough. I even pay a monthly fee to support them to keep them kicking, Wiki is one of the most reliable sources I know. Not commercial, not academic - just giving enough information (with references) for the subject. The references give more detailed information, if needed.
The Steinmetz symmetrical components are a little hard, but definitely worth getting acquainted with. After some time, they will be second nature and help a lot.
Getting late here, we are already in Monday. So I will hit the sack. And dream of something entirely different.
Like steam engines and such…