| The proposal of "carbon peaking and carbon neutrality goals" shows that China will vigorously develop clean energy during the 14 th Five Year Plan period and even for a long time in the future.Line commutated converter based high voltage direct current(LCC-HVDC)is the main channel for clean energy transmission,which can help build a green and low-carbon energy system.However,since the LCC-HVDC inverter consists of thyristors which has no self closing ability,the system has the risk of commutation failure.Commutation failure will threaten the safe and stable operation of AC/DC power grid with increasing coupling degree.By connecting a third-party device in the inverter station of LCC-HVDC system,it can provide emergency commutation voltage can be provided once AC faults occur,which can effectively reduce the probabilities of commutation failures.This paper studies the commutation failure suppression method of the third-party device represented by series voltage commutation converter(SVCC)and cascade H-bridge energy storage converter.The main research contents are as follows:(1)The development of LCC-HVDC technology and the research status of commutation failure suppression methods are briefly introduced.The mechanism of commutation failure is analyzed from two aspects of thyristor turn-off characteristics and commutation process of LCC-HVDC system,and the influencing factors of first commutation failure and subsequent commutation failure are studied respectively,which provid theoretical basis for the proposed commutation failure suppression method based on SVCC and cascade H-bridge energy storage converter.(2)The principle of suppressing commutation failure of SVCC is studied.Aiming at the problems of large impulse current borne by insulated gate bipolar transistor(IGBT)in sub module and lack of suppression ability of subsequent commutation failure after traditional SVCC commutation failure in case of serious fault,an improved series voltage commutation converter(I-SVCC)and its control strategy are proposed.After commutation failure,I-SVCC can quickly cut off the sub module capacitance from the commutation circuit,avoiding the fault overvoltage problem of the sub module capacitance,reducing the impact of fault current on each IGBT in the sub module after commutation failure,and improving its engineering practicability.In addition,the re input strategy after the first commutation failure is designed for I-SVCC,which makes I-SVCC have the ability to suppress the subsequent commutation failure and speeds up the fault recovery process of the system.The simulation results of PSCAD show that I-SVCC has good ability to suppress the first commutation failure and is effective in reducing the peak current of IGBT and restraining the subsequent commutation failure.(3)Aiming at the economic problem of adding additional reactive power compensation device,method of suppressing commutation failure by using the transient reactive power output capability of energy storage converter is proposed.The working principle of triangle connected cascade H-bridge energy storage converter is studied,the mathematical model of the main circuit of cascade H-bridge energy storage converter is deduced,the device level control strategy of cascade H-bridge energy storage converter is designed,and the system level control strategy of cascade H-bridge energy storage converter is designed according to the characteristics of large reactive power demand to suppress commutation failure.The simulation model built by PSCAD verifies the effectiveness of device level and system level control strategies of cascaded H-bridge energy storage converter.In order to further verify the suppression effect of cascaded H-bridge energy storage converter on commutation failure,a prototype of 380V/50 k VA cascaded H-bridge energy storage converter is developed and verified on 1600 V HVDC physical simulation platform.The experimental results show that the prototype has the ability to suppress the commutation failure. |
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