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Basic study of fitting method for base curve extraction in lightning impulse test techniques

The IEC, which prescribes lightning impulse test techniques for electric power equipment, is about to introduce the k-factor method for evaluating the overshoot of impulse waveforms. Its procedure specifies that waveforms are to be fitted with a double exponential function to derive a base curve. Some derived waveforms, however, deviate upwards from the central lines of the recorded waveforms in the wavefront area. This paper examines a more rational method of extracting a base curve by considering the relationships between equivalent electric circuits and the solutions of their governing equations. A trial calculation with the new methods gives results with lower crest values and slower rises than the existing method.