we have 132kv transmission lines and it connected to our 132/33Kv substations. The lines are connected to 132/33Kv stepdown transformer. The T/F is star -delta ( 132 side is star) and its star neutral is solidly earthed. What will happen if the 132Kv star point of the T/F is not solidly earthed.

Hi,

From your brief description, during normal operation the loss of the earth connection will not make a lot of difference. However, in losing this reference to earth you may find some of your protection is lost, for example typically you would have a protection relay looking for flow down the earth to pick up earth faults or imbalances. I suggest that you look at your protection schematics to see what is there,

Derek

Check your protection relays and zero sequence netwok. With your limited query, I'm afraid you'll get a limited answer.

Euclid

I would also have thought that for a 132kV fault you will destroy the transformer as the insultation toward the star point is probably graded on the assumption that the star point can only ever be at the same potential as the tank. Which it won't be for a 132kV fault if the star point is not tied down.

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mikek

Fundamentally the grounding of primary side of transformer is not recommended. For more information you can refer to Protection Riddle No.62 from http://electrical-riddles.com

I would agree with mikek.

UK practice is to solid earth the 132kV Star Point to protect the transformer from transient overvoltages, lightning and the similar. The 132kV winding insulation is 'graded' as it easier to design a winding where the star point is always at or near earth potential.

All EHV Transfromers have star winding, 132,275 & 400kV, tought so are auto wound. An it is common for even 66kV to be star.

Don't apply Standby earth fault to the star connection as it will see every earthfault on the UK grid, including the 400kV system. Think about how the earth faults flow.

An certain Distribution Company in East Anglia asked for it on a 132/33kV Transformer Protection scheme I was engineering, I question it but they still wanted it. A few months later they had a 132kV fault coupled with a stuck CB which in turn tripped out both mesh corners and transformers at a grid site. The trip link was quickly and permantley pulled!

Tim