Space charge modelling and measurement in HVDC extruded cable insulation

Abstract

Space charge accumulation under the DC field is one of the most challenging issues in the further development of HVDC extruded cable systems. The electric field distribution over the insulation thickness is strongly affected by space charge, which further control the long‐term reliability and life expectancy of the cable system. Although extensive efforts have been made to investigate the space charge behaviour in dielectrics, the charge generation and transport mechanisms are still not yet well understood. The space charge and field distribution in cable system cannot be calculated accurately by the conductivity model, which serves as an important reference for DC cable design. Thus this dissertation devotes to study space charge by numerical modelling and experimental investigation of space charge behaviour in cable insulation. A numerical modelling approach based on bipolar charge transport theory has been developed to simulate the space charge dynamics in polymeric insulation of coaxial geometry. Based on previous experimental observations, a threshold electric field (10 kV/mm for polyethylene‐based material) at which the charge injection takes place is introduced. The build‐up of space charge in the medium size cable insulation in the presence of temperature gradient has been modelled and the field inversion phenomenon has been verified under the applied voltage of 90 kV. Compared with the traditional conductivity model, the new modelling method performs better as it well describes the charge generation and transport mechanisms. The space charge and field distribution in cable system under the voltage polarity reversal have been calculated, and the significant field enhancement near the conductor suggests that particular attentions need to be paid on the DC cable design and the operation methods of polarity reversal. Considering the practical operation of HVDC cable systems, the thermal transient effects on the space charge behaviour in cable insulation have been investigated. It is found that the field inversion can only take place with a higher load current which represents a higher general temperature and a larger temperature gradient, and this phenomenon can still be maintained even with the temperature decreasing. It is suggested that different loading conditions need to be considered in designing DC polymeric cables. To study the hetero charge accumulation in cross‐linked polyethylene, the formation and transport of the ionic charges have been considered and fed into the bipolar charge transport model. By introducing the impurity molecules serving as the ion‐pairs with a dissociation coefficient, the modified model can be employed to investigate the hetero charge formation in XLPE material. Simulations have been performed in XLPE flat specimens, and it is found that if the ionic charges distribute predominantly and offset the electronic charges, the net charge distribution will exhibit as a hetero charge accumulation. A pulsed electro‐acoustic system for measuring space charge accumulation in cable insulation has been designed and built, and considerable hetero charges have been experimentally observed in the XLPE insulated cables. The modelling approach considering both the charge injection and ionic dissociation is employed to investigate the space charge behaviour in XLPE cables, and both the space charge and field distribution are consistent with the experimental observations in the original cables. It is found that the ionic charge behaviour interplays tightly with the electronic charge behaviour, and it is also suggested that the impurity concentration gradient needs to be considered in the degassed cables.

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ORCID for Yunpeng Zhan: orcid.org/0000-0002-8171-8107

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