A group of complete images on the discharge channel developed in PMMA in quasi-uniform electric field under nanosecond pulses are observed with an on-line transmission microscope. The characteristics of the cumulative breakdown process are also generalized, which include initiating from the vicinity of the cathode, developing to the anode with a branch-like shape, and taking on a wormhole appearance when final breakdown occurs. The concluded characteristics are explained by referring to the conceptions of “low density domain” and “free radical” and considering the initial discharge channel as a virtual needle. The characteristics are helpful for designers to enhance the lifetime of insulators employed on a nanosecond time scale.

A Tesla-type repetitive nanosecond pulse generator including a pair of electrode and a matched absorption resistor is established for the application of solid dielectric breakdown research. As major components, a built-in Tesla transformer and a gas-gap switch are designed to boost and shape the output pulse, respectively; the electrode is to form the anticipated electric field; the resistor is parallel to the electrode to absorb the reflected energy from the test sample. The parameters of the generator are a pulse width of 10 ns, a rise and fall time of 3 ns, and a maximum amplitude of 300 kV. By modifying the primary circuit of the Tesla transformer, the generator can produce both positive and negative pulses at a repetition rate of 1-50 Hz. In addition, a real-time measurement and control system is established based on the solid dielectric breakdown requirements for this generator. With this system, experiments on test samples made of common insulation materials in pulsed power systems are conducted. The preliminary experimental results show that the constructed generator is capable to research the solid dielectric breakdown phenomenon on a nanosecond time scale.

The critical pulse width (s c ) is a pulse width at which the surface flashover threshold (E f ) is equal to the bulk breakdown threshold (E BD ) for liquid-polymer composite insulation systems, which is discovered by Zhao et al. In this paper, the mechanism of s c is interpreted in perspective of the threshold and the time delay (t d ) of surface flashover and bulk breakdown, respectively. It is found that two changes appear as the pulse width decreases which are responsible for the existence of s c : (1) E BD is lower than E f ; (2) t d of bulk breakdown is shorter than t d of surface flashover. In addition, factors which have influences on s c are investigated, such as the dielectric type, the insulation length, the dielectric thickness, the dielectrics configuration, the pulse number, and the liquid purity. These influences of factors are generalized as three types if s c is expected to increase: (1) factors causing E BD to decrease, such as increasing the pulse number or employing a dielectric of lower E BD ; (2) factors causing E f to increase, such as complicating the insulator's configuration or increasing the liquid purity; (3) factors causing E BD and E f to increase together, but E f increases faster than E BD , such as decreasing the dielectric thickness or the insulation length. With the data in references, all the three cases are verified experimentally. In the end, a general method based on s c for solid insulation design is presented and the significance of s c on solid insulation design and on solid demolition are discussed. V C 2015 AIP Publishing LLC. [http://dx.