No abstract

The characteristic of the over-volted breakdown and the gaseous recovery in short nitrogen gaps was experimentally studied. It was found that the breakdown voltage of the gap changes from shot to shot even with the same experimental conditions and obeys Gaussian distribution. The over-volted factor is reduced with an increasing pressure. With a 2.7-mm gap the over-volted factors are 4.53 for 0.1 MPa pressure and 1.74 for 0.4 MPa. The over-volted breakdown voltage depends individually on the gap spacing d and the gas pressure p, rather than on the product of pd. An empirical formula of the breakdown voltage as a function of p and d was derived. The time-dependent recovery of the breakdown voltage, RVb, was obtained using a two-pulse technique. The second breakdown voltage also obeys Gaussian distribution, but it is normally with a smaller standard deviation especially when the interpulse spacing of Δt is relatively short. As a whole, RVb rises with the increase of Δt. However, an intermediate plateau is always observed and it starts when the second breakdown voltage is a little bit higher than the static breakdown voltage of the gap. The first rising edge of the RVb curve corresponds to the recovery of the electro-neutrality and the density. The intermediate plateau and the following rising edge take the spark gap much longer time to recover. The processes governing these two latter phases are as yet not fully clear. It is attributed to the delayed recombination of the residual nitrogen atoms on the cathode to produce the initial electrons for the second breakdown. An increase in pressure has resulted in an upward shift of the intermediate plateau and a shortening in the recovery time of the gaps. The second spark generally does not follow the path of the first spark.

Space charge dynamics under AC stress is of importance as a majority of high voltage cables are under AC stress, and space charge is the driving mechanism of dielectric degradation under these conditions. Bipolar charge transport model is used to simulate space charge dynamics in polyethylene under AC stress. The current density, electroluminescence intensity and space charge density under sinusoidal, triangular and square voltages are compared. It is found that there is no phase shift between current density, electroluminescence intensity and applied voltages, which disagrees the results of Baudoin et al. The thickness of space charge layer under AC stress is much thinner that under DC stress.

The recovery of the gas density and the hold-off voltage of a spark gap after breakdown were investigated with Mach–Zehnder interferometry and two-pulse method, respectively. It was shown that the gas density in a 2.7 mm gap filled with atmospheric nitrogen almost fully recovers at t=50 ms but the breakdown voltage of the gap only recovers to its static hold-off voltage, about 21.4% of its original overvolted breakdown voltage. The mechanisms for the delayed recovery of the ability to be overvolted were discussed.