Folks,
Zigzag transformers are used all over the world for earthing (grounding) and unearthed bus, and in voltage ranges from LV to MV. I think the original question is along the lines "why would you bother earthing a bus with a separate zigzag when star-points (wye-point) are available?" I think it mainly comes down to cost - if there is a need to Earth a bus via several separate star-connected transformers, each with associated neutral bushing and full voltage insulation at the netra end, the lower cost with a single zigzag transformer comes into consideration. I have seen them on a set of 33 kV busses where one of the incomers was delta, so there was a possibility of an ungrounded bus if that incomer was in service with all other star-connected feeders and incomers out of service - a dedicated earthing transformer then makes sense and zigzag construction is a very cost-effective way of achieving that. If all incomers on a bus were star connected and are able to be earthed at tehe star-point, then there is no need for a separate earthing transformer.
Earthing transformers do not have to be zigzag - it is just one method - they can also be a more normal star delta with the star-point earthed and the delta either used for aux power or not used.
I'm curious if we use a direct wye connection instead of zigzag in case of earthing transformer.. i know that by merely looking if we use wye connection in earthing specially in medium voltage you have to have enough reactance for your exciting voltage.which will cost more in construction.also should there be shorts you directly ground the system which is very dangerous.does my opinion correct? any opinion might help..thanks