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Partial discharge activity in oil - paper insulated system under DC superimposed AC and harmonic voltage

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In the present study, the partial-discharge activity under nonstandard voltage formed by ac, harmonic, and dc voltages was studied. It is identified that the negative dc voltage superposed with ac and high frequency harmonic enhances the discharge activity in the medium and if the negative dc voltage is superposed with ac and low frequency harmonic, it can mitigate the discharge activity. The signal of the partial discharges in oil-paper insulated systems under the composite voltage formed by dc superposed with ac and harmonic voltages, has a dominant frequency content at around 20 MHz. Streszczenie. Analizowano wyładowania niezupełne przy niestandardowych napięciach złożonych z napięć AC i DC. Stwierdzono, że obecność napięcia stałego zwiększa prawdopodobieństwo wyładowań. Zbadano wyładowania niezupełne w izolacji papierowo-olejowej. (Wyładowania niezupełne w izolacji papierowo-olejowej przy napięciu DC oraz składowej AC) Keywords: Harmonic voltage ,composite voltage, oil-paper insulated system, partial discharge. Słowa kluczowe: iziolacja olejowo-papierowa, wyładowania niezupełne. Introduction HVDC power transmission networks are widespread nowadays and converter transformers form key components of these systems. The insulation structure is same as in conventional power transformer but it is stressed with dc, ac, and harmonic voltages[1]. The major insulation in transformer is a combination of pressboard material and the transformer oil, which is a multi-dielectric system.[2, 3].The manufacturers estimate the quality of the insulation of the converter transformer by performing certain routine tests before installation, which includes AC voltage withstand test, DC voltage test with simultaneous partial discharge measurement, polarity reversal test with partial discharge (PD) measurement etc [4]. Considerable research work of PDs in oil-paper insulation system under ac or dc voltages have been performed for long time [5-9]. Howeve[...]

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