Coordination of Surge Arresters

[hidalgoe]

Folks, please answer the following:

Is it good design practice from a coordination point of view to have station class arresters connected to a 69kV Bus in close proximity to surge arresters connected directly to the primary inputs of a 20MVA 69kV/12.47kV transformer connected to the same bus?

Thanks,

hidalgoe

 

[jghrist]

Having the bus arresters close to the transformer arresters is probably not the most effective location. Surge voltage on the bus increases as the distance from the arrester increases because of reflected surges at the arrester and at open points on the bus. If the bus arrester is close to the transformer arrester, it adds little to the bus protection because the transformer arrester protects the bus close to the transformer. Arresters at incoming lines would be a more logical location. Lighting surges get onto the bus from the incoming lines, so this is the best place for the arresters. If the bus is short and the transformers are always connected, the transformer arresters alone may be sufficient.

 

[RRaghunath]

I have seen 132kV substations with surge arrestors connected to bus alone and 132kV incoming lines /transformer feeders having no surge arrestors.

This is economical and may be OK for small substations but may compromise on protection to the equipment to some extent (depending on the margins in BIL of equipment vis-a-vis the expected residual voltage across the surge arrestor at the time of lightning strike).

 

[jghrist]

I would put arresters on the transformers in any case because the consequences of a flashover in the transformer are a lot more expensive than those of a flashover of the bus insulation. The surge voltage at the transformer would be higher than at the bus arrester because of reflections, and it may exceed the transformer BIL. You could do an extensive study to determine if the bus arresters alone would protect the transformers, but it would be better to arresters on the transformer instead. Lightning is a probabilistic phenomenon and you could get higher surges than whatever you assume in the study.