The critical pulse width for surface flashover and bulk breakdown in oil-immersed polymers

The general regularities of the electric surface flashover threshold (E f ) of polymers immersed in transformer oil and the polymers' bulk breakdown threshold (E BD ) versus pulse width (τ) are obtained respectively by summarizing the experimental data in literatures. Those regularities are that E f is proportional to τ -1/4 and that E BD is proportional to τ -1/6 when τ is smaller than 30 ns, when τ is larger than 30 ns, E BD is independent on τ It is discovered that there exists a critical pulse width (τ c ) by comparing the regularities of E BD and E f versus τ of the same dielectric thickness in one coordinate. If τ is greater than τ c , E f is lower than E BD , and the surface flashover may cause the insulation to fail. However if τ is smaller than τ c , E f would be larger than E BD , and the bulk breakdown would cause the insulation to fail. The existence of τ c is experimentally verified via a specific example of dozens of polymer samples immersed in transformer oil. In addition, methods on how to design short-pulse long-life insulators are proposed in this paper based on the concept of τ c .