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The authors have studied fitting methods to extract a more reasonable base curve from the lightning impulse voltage test waveforms. In this paper, by assuming a test circuit of actual large-sized UHV class equipment, simulated recorded waveforms were created, to which various fitting methods were applied in order to study their merits and demerits. Consequently, it was found that a new base curve extraction method could calculate an overshoot rate close to the correct value for waveforms actually generated. In particular, while the existing method resulted in significant dissociation for waveforms superimposed by oscillation waves of several hundred kHz, which was actually at issue, the new method significantly improved the evaluation results of the overshoot rate. Meanwhile, it should be noted that the k-factor filtering scheme was implemented after the application of various fitting methods, and then the shape parameters of the test voltage waveforms, such as the crest value, wavefront duration, and wavetail duration, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform.