Step down generator output with autotransformer

[mls1]

We have an application in which a 13.8kV, 20 MVA generator is connected to a transmission system via a delta/wye step up transformer. The generator is high impedance grounded and that is the only ground point in the generator zone. The auxiliaries for this plant are served at 12.47kV via a different utility connection. It is desired to be able to switch the auxiliary loads over to the generator output. We're looking at doing so with an autotransformer to step the 13.8kV to 12.47kV. The auxiliary system consists of a number of delta/wye transformers so there would also be no ground points in the auxiliary system primary.

What impact will the grounded autotransformer have on the system, in particular with regards to the neutral point during ground faults? Since the auxiliary system is all delta connected, is it correct that there is no point in adding a buried tertiary delta to the autotransformer (i.e. there is no path for 3rd harmonics anyways)? Lastly, should consideration be given to not grounding the autotransformer neutral? Thanks!

 

[waross]

First thing, get to know the difference between a neutral and a ground. Often they are the same physical point but with impedance grounding, they are not. The wye point of your generator is a neutral. The ground connection on the other side of the grounding impedance is the ground. Do not ground the auto-transformer neutral; connect it to the generator neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mls1]

Thanks, Bill. Yes, I do understand the difference between the neutral point and ground. As shown in several examples in the Westinghouse T&D book, the neutral of the autotransformer does not necessarily need to be at the same point as the generator neutral.

The autotransformer should function satisfactorily with the neutral floating but there are overvoltage concerns. I believe the delta tertiary also has influence on the overvoltage issue and may be needed.

Autotransformers used for motor starting do not have their neutrals tied to ground and there are cases of the same method used for power step up autotransformers. Connecting the generator neutral to the autotransformer is really not an option in this application so I'm really looking for the pros and cons of operating the autotransformer floating or solidly grounded.

Marty

 

[waross]

The auto-transformers that I have seen used for motor starting were connected in open delta and there is no true neutral in an open delta.
Three phase transformers with a three legged core develop a phantom delta which will aid stability.
Grounding the auto-transformer neutral may lead to protection issues in the event of a ground fault.
Back to motor starting. Have you considered two auto transformers in open delta configuration?
I have used this configuration many times and have seen it used a number of times to convert from 480V to 600V and from 600V to 480V.
This connection has no neutral and should avoid grounding and ground fault coordination issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mls1]

Bill - I have heard of this configuration but I've not seen it in practice for large power autotransformers. Do you have a good reference you could refer me to? For the floating wye configuration there is pretty good information in the old GE GET-3388B "The Whys of the Wyes" publication.

One clarification I should make is that the step down autotransformer will be needed to supply the distribution transformers from the utility source with the generator off line. This will be necessary during start-up and in the event of a unit trip. So with the unit on-line, the system is impedance grounded and with it off-line the system is ungrounded.

I think at this point, we can say the use of an autotransformer is feasible but the exact configuration will need further analysis. I do appreciate the input.

Marty

 

[waross]

The auto-transformers will have three terminals; Zero kV, 14.47 kV and 13.8 kV.
Transformer #1 will be connected:- Zero Volts to A phase, 13.8 kVolts to B phase and load to 12.47 kVolts.
Transformer #2 will be connected:- Zero Volts to A phase, 13.8 kVolts to C phase and load to 12.47 kVolts.
This does need a neutral and so can be physically located where it is most convenient.


Bill
--------------------
"Why not the best?"
Jimmy Carter