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.