Study On Control Strategy Under Unbalanced Fault For VSC-HVDC

Different from traditional HVDC,VSC-HVDC is a new type of flexible DCtransmission technology on the basis of Voltage Source Converter (VSC),selfcommutated switches and PWM technology.VSC-HVDC can control active andreactive power flexibly and independently.VSC-HVDC system safe and stableoperation of the rectifier and inverter converter station has good control performanceguarantee, and therefore VSC-HVDC inverter control becomes the core issue ofconcern.Firstly, In theory the operating principle of VSC-HVDC system is introducedand the dynamic mathematical model for VSC-HVDC system under αβ synchronousrotating reference is developed.The power exchange control principle betweenconverters and two side ac systems are analyzed, the problem of power exchangecontrol is converted to the problem of tracking of active and reactive currentreference values under satisfying operantional constraint.Secondly,a dual closed loop controller based on direct current control strategy isdesigned for active power and reactive power between the converter and two side acsystems,active power control and reactive power control are decoupled.A terminalsliding mode is added into inner loop current controller, on this basis,the outer-loopcurrent reference siganals are deduced which take both the system side and VSC sidepower into account. In order to trace the reference signals.terminal sliding modecontroller is adopted and the discrete mode is derived in αβ reference frame, reachindependently control of active power and reaetive power, improve the systemdynamic stability under disturbance, the stability of the closed-loop system undercontroller is proved by Lyapunov theory, ensure its stability under transient voltageunbalance conditions.And VOCOL not only reducess the phenomenon of the DCovervoltage and undervoltage but also improves the system of anti-disturbanceability. Finally,VSC-HVDC system simulation model based PSCAD/EMTDC is setup.Power exchange control between VSC-HVDC system and two side ac system issimulated, and its various short-circuit faults are analyzed, the validity of theproposed controller is demonstrated by the simulation results.