Inter-layer voltage distribution and insulation failure of a spiral pulse transformer

Several spiral pulse transformers are tested to charge a water pulse forming line to more than 1 MV in ~10 ??s time. During the experiments, recurrent breakdown failures between turns of the transformer are observed. To find the cause and cure to the problem, the inter-layer voltage distribution is calculated and analyzed. The calculation models the wire wound spiral transformer turns as separate cylinders, and the inductance and coupling matrix is obtained by finite-element method. As the charging time is in microseconds range, the displacement current can be ignored. The inductance matrix is used in a circuit to find the inter-layer voltages at various time against circuit parameters. The results presented show that inter-turn voltage varies with time, position, the primary capacitance and voltage. The results explains that when transformer circuit is under-matched, so that L₁C₁ < L₂C₂, where L₁C₁, and L₂C₂ are primary and secondary inductances and capacitances, the insulation breakdown occurs early during charging, and at the outer most layers of the secondary coil, adjacent to primary turns. When L₁C₁ > L₂C₂, then breakdown takes place near central output of the secondary turns and occurs later during charging. The calculation can well explain the cause of the insulation failure. Suggestions to improve the insulation are discussed.