Can somebody explain why earthing transformers are usually zig zag connected? Is it always necessary to use earthing transformer in delta connected power transformers?

 

Cephas, I disagree with your statement, "earthing transfomers are usually zig-zag connected!"

It may seem so in in your part of the world, or indeed, the basic practice of power system designers with whom you are acquainted. I believe their use is dictated by two factors... economics and/or operational requirements.

For the former, they are smaller, while for the latter, tripping time can be extended to insure a safe and orderly shutdown.

Paraphrasing your second statement, "Is it always necessary to use earthing transformers in unearthed systems?" Essentially, the answer depends on two factors: 1) safety to operating personnel; and 2) exposure to overvoltage transients.

Let me know if additional detail is required.

Regards,
Phil Corso (cepsicon [at] aol.com)

 

Thanks Phil,

Would you also mind explaining the main use of earthing transformers?

 

Cephas... the simple answer is that it's an alternative to NGR and NGT methods!

However, it has a decided advantage over the other methods. That is, internal losses, resulting from earth-current flow, are equally distributed amongst six wibdings!

Regards, Phil

 

A second advantage is that its size is about 10% of that of a typical 3-phase transformer having the same kVA capacity.

Phil Corso