Snubber Circuits

Hello all,

I am working on a desing of a new substation for a client and when discussing whether to use electronic vaccuum circuit breakers or fused switches the discussion of snubber cirucits come up.

From what I have gathered so far, snubbers are required when using vaccum circuit breakers and certain transformers in close proximity. All the studies I've come across so far are very technical, but most seem to utilize snubbers in step-down configurations (i.e. 13.2kV utility - 480V customter).

My application is different in that we will be stepping up the 480V utility voltage to 4.16kV distribution on the customer side. Will there still be adverse switching transient effects that require snubbers in this isntance?

I attended a presentation given by Eaton on this topic. They indicated the problem mostly exists when the distance between the source vacuum breaker and transformer is less than 200 feet (at the medium voltage level) and the transformer is dry type. The 200 feet of cable adds impedance which helps to lessen the elevated voltages and rate of rise which causes the insulation to be damaged during some switching events when the arc is being extinguished during the opening of the breaker. The oil in an oil filled transformer acts as impedance and also helps to mitigate the problem caused by inductance in the transformer and system. This is why dry type transformers are more susceptible to the problem. At 480V I would expect the arc to extinguish in fewer cycles when opening or closing compared to the same amperage at medium voltage levels but it would still generate a switching transient. It may dampen fast enough at the 480V level that it is not an issue but it would need to be calculated to verify that was true. I would expect at 480V that the 200 foot of cable distance in the sweet spot of this problem would need to be significantly less to see the problem at 480V. Eaton uses software called EMTP (EMTP.com) to verify if a snubber circuit is needed to protect the transformer from the transient destroying it by popping a hole in its winding insulation. It would be best to run the calculations or have a vendor run them to confirm if a snubber and an arrestor is needed, or if this would only need an arrestor. Using an assumption on this one could end up having high liability down the road. Good luck.