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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.

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