Research On Model Simulation And Application Of DC Cable On Internal Insulation Fault In A VSC-HVDC Transmission System

In recent years,the rapid application of VSC-HVDC technology has played a vital role in the field of high voltage direct current transmission,while constantly improved voltage levels put forward the realibility of the high-voltage dc cable.Operating experience shows that internal insulation is the most improtant part of an high-voltage transmission cable,simulation and analysis on fault characteristics of a transmission system when internal insulation faulted have an important significance to the hige-voltage dc cable’s design and the transmission system’s protection in the future.Aimed at the over-voltage induced in metal sheath of a transmisson cable when internal insulation faulted in the VSC-HVDC transmission system,theoritical model and simulative platform have been built for researching in this paper,the main contents and results are as follows:(1)A high-voltage direct current cable model with distributed parameters is approximately modeled by the method of cascading admittance matrix of each basic unit(BU).Since the distributed parameter characteristic has been the most difficult problem for cable’s theoritical modeling,the approximation of a long distance transmission cable in theoritical modeling process of several BUs with concentrated parameters has been applied in this paper,as long as the length of each BU does not excess 0.3km,this approximate method is valid.In addition,as long as the length of the BU is suitable,a transmisson cable with arbitrary length can be simulated by applying the method of cascade iteration on admittance matrixes mathematically.(2)In the Matlab platform,the ac source discharging model of internal insulation fault system has been realized,fault current and induced voltage on metal sheath have been simulated and calculated by applying to actual project.After internal insulation faulted in the transmisson system,the induced over-voltage on metal sheath may damage the transmisson lines,due to the fault current flowing through the grounding sheath which has been produced by the distributed capacitors’ discharging effect.Calculation of the induced over-voltage on sheath can provide basis for the design of high voltage dc cable and fault protection of the transmisson lines in the future.(3)The R-L-C fault system model is built in the Simulink platform when internal insulation faulted,the fault current and induced voltage on metal sheath are simulated and calculated by applying to the practical project.The platform is helpful for the comparision with the theoretical model,meanwhile,the simulation results have been verified and corresponding defense measures have been proposed as well.