Research On The Influence Of Defects In HVDC Cable Accessories On The Electric Field Distribution

With the maturing of voltage source converter based direct current technology and the cable manufacturing technology,the cross linked polyethylene insulated high voltage direct current(HVDC)cables have become the preferred DC plastic cables due to the outstanding performance in crosschannel transmission,transmission through a tunnel and renewable power generation and other areas.Cable accessory is an important part of the cable system and plays the crucial roles in converge,transition and other aspects.At the same time,cable accessory is also the weakest part and malfunction prone areas of the transmission line.Selecting insulation materials reasonably and studying the electric field distribution comprehensively in the cable accessory with typical defects can contribute to monitor the cable line running state to detect and repair the faults,and improve the reliability of cable systems,which is significant to the long-term stable operation of HVDC cable system.In this paper,first of all,the simulation model of HVDC cable accessory was established using multiphysics simulation software(Comsol Multiphycics)based on the basic properties of the materials for HVDC cable accessories.And an excellent scheme of the combination of reinforced insulation and cable insulation was selected according to the electric field distribution simulation results.Secondly,the electric field distribution rules of cable accessories with typical defects under different voltages,different temperature gradients and the effect of overvoltages were calculated based on the selected HVDC cable accessory models.At last,the relationship between the defect location and the maximum electric field location were researched,and the threat of the type of defect on the cable accessory was evaluated.The results indicated that nonlinear silicone rubber had strong capacity to homogenize the electric field distribution under DC voltage,making the maximum electric field in DC cable accessory without defects concentrate in the main insulation,and the higher the voltage was,the more obvious the homogenization effect was;when the stress cone installation was dislocation,the electric field distortion became worse as the distance of cable screen layer exceeding the stress cone root increased and the large exceeding distance would extinguish the electric field homogenization effect of the stress control body;the electric field mainly concentrated near the air-gap when there were gas holes in the enhanced insulation,and the maximum electric field strength was beyond the breakdown strength of air;the tiny bulge on the stress cone surface would obviously enhance the local electric field around the bulge;when the temperature gradient was large,the electric field overturn phenomenon would occur in the main insulation of cable joint,and the maximum electric field location would transfer from the end of high voltage shield tube to the stress cone root;when conductive particles existed on the cable main insulation surface,the maximum electric field in the whole cable joint located around the conductive particles;when the impulse voltage was overlaid on DC voltage,the maximum electric field strength in the cable accessory always located in crosslinked polyethylene(XLPE)insulation near the core regardless of the polarity of the impulse voltage,and the maximum electric field strength under the homopolarity impulse voltage was higher than that under the reversed polarity impulse voltage.