A transfer function is a reliable tool for comparison of full- and chopped lightning impulse tests

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Abstract
A new diagnostic tool available with digitally recorded waveforms for lightning impulse tests on a transformer is the so called transfer function. For the calculation of the transfer function, two waveforms are recorded. The first waveform is the impulse voltage and the second waveform is the impulse current. Both waveforms recorded in the time-domain are transformed to the frequency-domain and the transfer function is calculated. This paper proves that the transfer functions of a sequence of impulse tests with the same test circuit can be compared with each other. In the author's opinion, the differences found between transfer functions are mostly due to hardware and software which is not suitable for the determination of the transfer function. A statement about reliability is only possible if the technology used for determination of the transfer function is beyond any doubt. This paper compares the transfer functions of the full impulse waveform and the chopped impulse waveform on the same terminal of the transformer and test circuit, which is the worst case condition for the comparison, to prove that comparison of transfer functions is possible.

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