Generator - Transformer Connections 1

[mgtrp]

I have a somewhat unusual case of having several 2MVA generators that although star-connected, cannot have the neutral point earthed due to their unconventional construction.

The generators will feed into a common step-up transformer, with no loads (other than static excitation) fed directly. Another conventionally connected and earthed generator will feed into a tertiary winding on the step-up transformer. The step-up transformer then has some plant loads supplied from it, and feeds into a star-star utility transformer with delta tertiary.

The options that I am aware of for earthing the generator system are either to provide a dedicated earthing transformer (zig-zag or star-broken delta), or to use a step-up transformer with a star-connected LV winding. Either way, the earthing will be a HRG system.

If a earthing transformer is used, I see the arrangement as being to use delta-connected windings for the generator side of the step-up transformer and star-connected windings for the utility side and don’t believe that there are any great technical problems to solve.

However, it has been suggested that the equipment requirements could be simplified using a star-connected step-up transformer winding to earth the generators. A delta winding would still need to be provided, and this could be achieved in the tertiary winding (with the conventional generator connected), or on the high-voltage winding.

A delta winding would be needed somewhere, either the tertiary winding or the HV winding, and in either case I’m not sure where it would be better located (probably the HV winding?), or even if two delta windings should be used (probably not?).

Any suggestions, or advice of possible hooks to look out for would be much appreciated on the above (slightly long-winded, I apologize) story.

 

[davidbeach]

Define "unconventional construction". Either they are wye connected or they aren't.

 

[mgtrp]

Unconventional meaning a rotating star-connected armature and no means of accessing the neutral point for grounding (or protection CTs, but that's another issue...).

 

[davidbeach]

Ugh, bad product selection.

Yes, you always want the generator bus connected to a delta winding of the transformer that connects it to the rest of the system. Best to have no zero-sequence path between the generator bus and the rest of the system, so no Ydy transformer. The type of grounding transformer to use is generally a preference type of thing rather than there being strong technical reasons one way or another. A third option is wye-closed delta with the resistor in the neutral of the wye.

 

[mgtrp]

Bad product selection, but to be fair to the designers. these generators were built in the 19th century, and people were probably still trying to figure out what works best.


Re-reading my post, I was perhaps a little unclear. Please see the attached for the alternative arrangements that were suggested. I hadn't thought that a delta winding facing the grid would provide a zero sequence path.

 

[davidbeach]

I suppose that what you've shown in the sketch could work. It is rather different from what is usually seen, but then so are your generators.

No, I take that back, it won't work. What you are missing is your generator breakers and the only place they could go is between the generators and the transformer. With the generator breaker open you have an ungrounded generator. An ungrounded generator with an arcing ground fault becomes a real mess in a hurry, so you don't want to trip a generator and allow it to become ungrounded. So you will need a grounding transformer per generator located on the generator side of the generator breaker. With those grounding transformers you can then use a much more conventional Ydd transformer to connect the generators, one (or groups) per delta winding with the wye solidly grounded and connected to the rest of the system.

 

[mgtrp]

Yes, you're right. We are, however slightly cramped for room for additional equipment in the powerhouse.

One slightly leftfiled thought I've had is that static excitation transformers are often provided in a Yd arrangement - I wonder if we could use this for double duty by resistance grounding the neutral?

 

[davidbeach]

Wye on the generator side and delta on the excitation side?

Probably could work.

 

[mgtrp]

That's the one.

Thanks for your help.