Clarification on Zigzag Transformer

The answer to your question requires knowledge of your protective relaying scheme and the local codes in effect where this will be operated. Asking the question implies you lack both. Hire a competent engineering consultant to guide you.

In a previous thread started by our OP he stated he was a junior engineer in a power plant. It would be reasonable to assume there are more senior engineers that should really be giving guidance to a junior.

So why ask a group of random guys?

Am I being cynical?

Dear All,

Than you very much Gentlemen for sparing time to answer my question, though not in a way I expected.

Mr. Tony was right, I was working as a JE in a TPP. But I find myself in an unfortunate position where I cannot seek support of my Seniors for getting my doubts clarified. I have to rely on books/ online support by myself and Technical forums like CR4 and others to get it clarified by Unknown faces and I am happy that in the past many concerned and generous Engineers have helped me. I may not have a strong Design Background as my experience is in site activites and PM, but I do look upon 'EXPERTS' like you guys in the forums to know more. I am not gonna stop posting question I don't know the answer, If you know the answer please help me out, otherwise please dont mock others in public forum. CR4 is not only for experts but also for budding engineers like me I believe...

Thanks once again..

Your descrioption is a bit run-on, and I can see that English is probably not your native language, but I respect that you have tried to give a detailed description of your system. A single-line diagram with nameplate info on the various pieces of apparatus could help, but based on what you described, I would say the following:

1. If the 11kV portion of the system has ONLY delta windings connected to it, then you need the zigzag transformer to provide a ground source so that ground fault relays can operate. GF relays will not look through a transformer, so a grounded neutral on the other side of the windings (such as the 415V side of the station service transformer) does not give you a ground reference.

2. If there is a Wye winding (having a grounded neutral) on the 11kV bus, then you don't need the zigzag when the wye is connected. If the breaker for the wye winding transformer is open, then the zigzag is needed.

3. You didn't give the winding configuration of the 11/132kV transformer, but I assume it's Delta on the 11kV side and wye on the 132kV. That is the standard approach on a GSU transformer. If the 11kV is wye, then you don't need the zigzag. However, I assume you have the zigzag just because the main transformer is delta-wye.

I hope this helps.

Dear Sir,

Thank you very much for that update. As correctly stated by you the GSU trf is YNd1 winding trafo.

My sincere thanks to you once again for helping me out.