Space charge determination in HVDC power cable and its influence on electric field

Abstract

Considerable amount of transmission and distribution of electricity is carried out by power cable. At present, most high voltage direct current (HVDC) installation uses traditional (oil-impregnated paper insulation and oil-filled-types) cables which pose a risk of environmental pollution in case of an accident.

Thus, significant advances and usage of polymeric material, notably cross-linked polyethylene (XLPE), have been made as the insulation material for power cables due to its economical production, environmental benefits and its electrical properties. However, unwanted disadvantages to its performance are featured when operated under direct current (DC) application. Such as the accumulation of comprehensive immobile charges in the XLPE, this superimposes to the Laplacian field resulting the changes of electric stress across the dielectric material. Additionally, in most HVDC transmission systems both the presence of temperature gradient across its insulation and bi-directional power flow are also needed to be considered. Space charge existence within the insulation is particularly dangerous in the event of polarity reversal, which has been recognised as the root source of breakdown in the early extruded insulation of commercial DC cables. High electric stresses within the insulation may be created, especially in the case when rated voltage is applied on the cable and with the presence of temperature gradient. Therefore, investigations are needed on both the space charge dynamics and also the accurate determination of electric field across the insulation of a full sized cable.

In this research, space charge accumulation within the polymeric material of a XLPE power cable is measured using a modified pulsed electro-acoustic (PEA) system with a current transformer attached. The presence of these accumulated space charges along with the consideration of conductivity influences the electric field distribution across the insulation material. As it is well known that the conductivity of an insulating material is dependent of both temperature and electric field, the coupled problems impose significant difficulty to know the electric field distribution in HVDC power cables.

In this thesis, scientific contributions have been made towards the research on power cables by allowing both users and cable designers to obtain a much more accurate calculation of the total electric field. This total electric field calculation is based on the influences of both the variation of the conductivity and the space charge field across the insulation material under its specific temperature gradient. In addition, parameters pertinent to the insulation material of the power cable are obtained based on the hopping model of conduction in dielectric and are utilised in the calculation of the total electric field.

To stage the cables working under a real world HVDC transmission system scenario, experiments on the space charge dynamics of the full sized polymeric insulated cables were conducted by replicating it under the service conditions where both temperature gradient present across its insulation and bi-directional power flow are being considered. The field enhancement obtained within the cable under these scenarios allow us to estimate the lifetime of the power cable which are relatively important towards the power companies in realising the time for replacement of their aging cables.

In the polarity reversal experiments, results have shown that the total electric field is higher during polarity reversal when a 10 0C temperature gradient is applied across the insulation as compared to the application of no temperature and 20 0C temperature gradients. Therefore, this higher electric field due to the 10 0C temperature gradient will stage a higher potential risk due to the amount of heterocharge accumulation adjacent to the outer electrode.

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