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Insulation degradation is an irreversible phenomenon that can potentially lead to failure of power cable systems. This paper describes the results of an experimental investigation into the influence of direct current (DC) superimposed with harmonic voltages on both partial discharge (PD) activity and electrical tree (ET) phenomena within polymeric insulations. The test samples were prepared from a high voltage direct current (HVDC) cross linked polyethylene (XLPE) power cable. A double electrode arrangement was employed to produce divergent electric fields within the test samples that could possibly result in formation of electrical trees. The developed ETs were observed via an optical method and, at the same time, the emanating PD pulses were measured using conventional techniques. The results show a tenable relation between ETs, PD activities, and the level of harmonic voltages. An increase in harmonic levels has a marked effect on development of electrical trees as the firing angle increases, which also leads to higher activity of partial discharges. This study of the influencing operational parameters of HVDC converters on power cable insulation is predicted to contribute to enhancements in cable design and progressive advancement in condition monitoring and insulation diagnostic techniques that can lead to more effective asset management in HVDC systems.

A double-ended forward converter with an input inductor–capacitor filter has inherent power factor correction\ud (PFC) properties and continuous input current when operating in discontinuous-capacitor-voltage mode (DCVM). In this\ud study, the minimum separable switching configuration modelling is applied to the proposed converter operating in\ud DCVM to obtain a small-signal model of the converter. The impact of the circuit parameters on the small-signal\ud characteristic are discussed in details. The difference between the small-signal behaviour of the converter when\ud operating as a DC/DC converter and a PFC converter is investigated. The modelling methodology is intended for the\ud design of controllers in closed-loop operation. A 100 W prototype is implemented to demonstrate the feature of the\ud small-signal behaviour