In high voltage direct current (HVDC) topology, the transformer insulation system continuously experiences nonsinusoidal voltage waveforms due to its proximity with the power converters. This may cause changes in the discharge mechanisms, and as a consequence, the isolation system might undergo accelerated degradation. This study aims at investigating partial discharge (PD) measurements with different combination of solid/liquid insulation systems along with pressboard/mineral oil which is predominantly used in transformers. In this study, different barriers were tested in mineral oil and synthetic ester liquid. Also, an optical detection technique has been used beside electrical detection to acquire phase resolved partial discharge (PRPD) patterns, as it is a promising detection technique even in the presence of complex electrical waveforms. The optical detection was found to detect PD events during same phase as electrical detection and a correlation was established between two detection techniques. Moreover, the PRPD pattern and partial discharge inception voltage (PDIV) of different solid/liquid combination tested during this work have been presented.

Partial discharge (PD) measurement of an insulation system is widely used as a diagnostic tool for the quality control of high voltage apparatuses. In ac, the conventional method of PD detection is well established. However, under square voltages such as those generated across power electronic converters, the conventional method cannot discriminate between PDs and switching transients. In this context, this paper demonstrates PD detection under fast-rising square voltages by deploying a photomultiplier tube. For this, an experimental set-up generating unipolar square voltage (USV) waveforms is developed, generating a dV/dt in the range of 10-100 kV/µs. The device under test (DUT) comprises of needle-barrier plane configuration immersed in dielectric liquids. Barriers differing in permittivity and thicknesses are chosen to test with two types of dielectric liquids, to assess varying conditions. The results thus obtained show a lower partial discharge inception voltage (PDIV) in USV than in bipolar ac for the same DUT specimen. The phaseresolved (PR) PD patterns exhibit a higher PD occurrence during rising and falling front of the USV waveform. Experimental results demonstrate the relevance of the PMT as a PD detector for square voltage waveforms exhibiting high dV/dt in dielectric liquids.INDEX TERMS Partial discharge, square voltages, high dV/dt, ac, optical method, photomultiplier tube, high divergent field, dielectric liquids