One of the challenges on the increasing reliance on isolated renewable generation sources is the transmission of power from these sources to centers of power demand. One possible approach is the use of high voltage direct current (HVDC) transmission. The power electronic converters are key components in HVDC transmission system. The converters produce the intended DC voltage for transmission but there may also be AC harmonics superimposed. The superimposed harmonics on the HVDC may have synergistic effects and may lead to further degradation in the cable insulation. Previous research has shown that partial discharge was the main cause of degradation in polymeric insulation under AC stress. However, few publications have demonstrated the effect of combined stress on cable insulation degradation. Additionally, the most popular cable insulation material, cross-linked polyethylene (XLPE), cannot be recycled. Alternative materials which can be recycled have been proposed and one such solution could be thermoplastic materials. In this study, HDPE was investigated as a reference material for thermoplastics and their potential use as insulation in HVDC cables. In this paper the effect of frequency on HDPE degradation under superimposed stresses was studied using the following approaches; equivalent phase resolved partial discharge (PRPD) plots, fourier transform infrared spectroscopy -attenuated total reflection (FTIR-ATR) and dielectric spectroscopy (DS) measurements were carried out. The results show that during aging and with a frequency increase, the voltage of PD events increased which in turn created more polar molecule groups on the surface. The amount of polar molecule groups was found to affect ε' and tanδ, with both increasing when more polar molecules were created. The results show that applying a higher AC frequency enhances polymer degradation.

This paper details testing conducted under DC conditions on a dielectric sample containing internal voids. The DC testing was conducted using a ramp method to vary the voltage applied to the dielectric sample. The dielectric sample was de-energised for a week prior to two separate identical ramp tests and the results are presented showing the variability of PD activity. After the second ramp test an additional ramp test was performed in quick succession and PD activity was reduced, emphasizing the importance of de-energising the sample between tests. A major challenge associated with void type dielectric samples is ensuring that repeatable results are generated and possible approaches are discussed.

The most popular cable insulation material used is XLPE due to its excellent electrical and thermal properties. However, it does not lend itself to ease of recycling. As a result of an increase in concern worldwide regarding environmental protection, it is the objective of this work to investigate whether a thermoplastic material could be used to replace XLPE for cable insulation. Among thermoplastic materials, HDPE is regarded as one with the most similar properties as XLPE. Although it is clear that the performance of polymeric material changes with different stresses, especially polymer nanocomposites aging process under AC electric field stresses, there are also not many publications on how a superimposed AC voltage would affect the insulation's performance in HVDC power systems. This paper reports the dielectric properties of HDPE under thermo-electrical stresses. DC stress with and without a superimposed AC stress were applied in the experiments undertaken. The degradation of materials with change in frequencies are summarized and discussed.

The main focus of the paper is to develop a better understanding of partial discharges under DC excitation. Partial discharges studied will initially be limited to discharges from well-defined discharge sites. These include corona, surface discharges and internal voids. The samples are first tested under AC excitation as a sense check to ensure the samples yield the expected PD events. The samples were then subjected to DC excitations where the PD events were recorded and subsequently analysed. A number of analysis techniques will be applied to potentially enable the identification and classification of the type of PD event occurring in the DC system under investigation

To support, plug-in electric vehicle (PEV) growth, there is a need to design and operate charging stations without increasing peak system demand. In this chapter, first, an overview of on-going demonstration and testing studies are presented to show the complexity of the actual charging infrastructures and uncertainties related to customer demand. Then, an analytical model for a large-scale charging station with an on-site energy storage unit is introduced. The charging system is modelled by a Markov-modulated Poisson Processes with a twodimensional Markov chain. A Matrix geometric based algorithm is used to solve steady state probability distribution to compute optimal energy storage size. Case studies are presented to show (i) the relationships between energy storage size, grid power and PEV demand and (ii) how on-site storage can reduce peak electricity consumption and the station's monthly electricity bill.