Robust evaluation procedure for lightning impulses

The existing recommendation for high voltage tests, IEC Publication 60060, is ambiguous in the evaluation of the impulse test voltage, when oscillations or overshoot are superimposed on the peak of the smooth, double exponential waveshape. Particularly impulse tests on transformers may be critical due to the high stress on the insulation during the impulse tests. The contribution describes a proposal for the evaluation with a frequency dependent test voltage factor which is derived from experimental investigations with some basic insulation materials. The test voltage factor method allows robust evaluation procedures which can be used for manual as well as for automatic evaluation. Presenting some examples the contribution shows that the proposed methods are suitable to increase the reproducibility and reliability of lightning impulse tests.