Lightning withstand of medium voltage cut-out fuses stressed by nonstandard impulse shapes experimental results

This paper aims at evaluating the lightning withstand voltage of cut-out fuses designed for overhead distribution lines of 15 kV and 25 kV. For this purpose, it is used the critical flashover voltage (CFO) and the V-t curves parameters. These two parameters are usually derived for 1.2 × 50 μs impulse shape, which represents overvoltages originated by direct lightning strikes on phase conductors or shield wires of transmission and distribution lines. In order to assess the lightning withstand levels with respect to induced overvoltages, it has been chosen five nonstandard impulse shapes with front and tail times of 0.5 × 5 μs, 0.5 × 20 μs, 1.2 × 10 μs, 3 × 16 μs and 5 × 50 μs. The standard impulse shape 1.2 × 50 μs results are also provided for the purpose of comparison with nonstandard ones. The tests for determining the CFO and V-t curves were performed on dry condition, for positive and negative polarities. The CFO and V-t curves results show an important issue considering lightning withstand of cut-out fuses. For both test methods and impulse shapes, the lowest withstand values are observed on negative polarity. Considering porcelain insulators and manual disconnectors for distribution systems, it means a change on the critical polarity with impacts on insulation coordination procedures.