Research On Enhanced Capacitor Commutated Converter Based On Anti-parallel Thyristor Full Bridge Sub Module

Line-commutated-converter based high voltage direct current(LCC-HVDC)has been widely used in bulk-power long-distance transmission and asynchronous power grid interconnection.However,commutation failure(CF)is an unavoidable issue for LCCHVDC.The commutation failure leads to undesired risks to converter equipment,the power transmission of HVDC system as well as the stability of AC/DC system.Compared with LCC,the capacitor commutated converter(CCC)have the ability to improve the converter power factor and effectively reduce the probability of commutation failure under ac system faults.Generally,CCC can tolerate a sudden drop of 15-20% in the ac bus voltage without suffering a commutation failure.However,once the commutation failure occurs,it's difficult for CCC to restore to the initial state after the fault is cleared,or even continuous commutation failure will happen.The main reason is that series-connected capacitors are charged undesirably during fault condition.Firstly,series capacitor's charging or discharging characteristics are analyzed.With the anti-parallel thyristor full bridge sub module(APT-FBSM)embedded between the converter and the converter transformer,an enhanced capacitor commutated converter(ECCC)is proposed in this paper to improve the transient performance of capacitor commutated converters under fault conditions.Compared with CCC,ECCC exhibits more flexible control ability of capacitor voltage.The operating principle of sub-module and the coordinated control strategy are designed.Secondly,the theoretical calculation and simulation analysis are conducted to evaluate the voltage-current stress of the APT-FBSM.A parameter design method of APTFBSM is proposed.Then the ECC-HVDC and the presented control strategy are developed in PSCAD/EMTDC.Simulation results show that the parameter selection of APT-FBSM is reasonable and the voltage and current stress of the sub-module is within reasonable range.The valve voltages in the case of the ECCC are same as those for a conventional bridge at rated conditions.Finally,the transient operation characteristics under different conditions and fault recovery characteristics of ECCC and CCC are compared.The commutation failure immunity and critical voltage are adopted to evaluate of ECCC to mitigate the commutation failure.The characteristics of capacitor voltage and failure recovery rate after commutation failure in the case of ECCC and CCC are compared.The results show that,ECCC exhibits stronger immunity to CF than LCC during both three-phase and singlephase faults condition.Furthermore,with flexible control of capacitor voltage,ECCC can protect capacitors of APT-FBSM against over-voltage.ECCC has a stronger fault recovery capability and it can help system restored to the rated operating state quickly.