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 Ef is proportional to τ
⁻¹
and that E
BD
is proportional to τ
⁻¹
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 Ef versus τ of the same dielectric thickness in one coordinate. If τ is greater than τ
c
, Ef is lower than E
BD
, and the surface flashover may cause the insulation to fail. However if τ is smaller than τ
c
, Ef 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
.