Study On The Suppression Measures Of Commutation Failure In Multi-infeed DC Transmission Systems

Compared with the single-infeed HVDC transmission system,MIDC transmission systems has larger capacity and more flexible operation mode.However,due to the recent electrical distance of inverter stations,a disturbance may leads to commutation failure in MIDC systems,once commutation failure of one or more loop can not effectively controlled,unipolar or bipolar power outage occurs,the DC transmission power will be greatly reduced,meanwhile frequency stability of power grid will be affected.MIDC transmission system brings new problems to the safe and stable operation of power grid.Therefore,the appropriate control strategy for MIDC systems to inhibit subsequent commutation failure has become an urgent need to study the current problems in the field of DC transmission.Based on the domestic and foreign research of commutation failure in MIDC systems,firstly a more profound study and analysis in the mechanism of commutation failure has been conducted,through set faults in different times in the low and high voltage side to study the fault characteristics of commutation failure.Then analysis influence factors in MIDC systems comprehensively,the mathematical model about the effect of MISCR,harmonics and AC system fault on commutation failure is deduced and verified by Matlab.Meanwhile,the inverter directly connecting with the faults occurs commutation failure,divided the influences on other DC systems into voltage drop fault and commutation failure component,then study the interaction between the DC systems under fault conditions.Finally puts forward the blocking voltage components measures based on VDCOL,through the PSCAD/EMTDC simulation verify the transient characteristics of MIDC systems has been improved,the probability of commutation failure is also reduced.Then put forward the control strategy optimization about the blocking voltage coefficient K switch,which means the K value will cut-in when occurs serious fault,while in normal and slight fault condition the K value is 0.Built the simulation model of Shanghai power grid includes 4 loop HVDC systems in PSCAD/EMTDC and through simulation research,the conclusions can be getted that the control strategy not only can reduce the probability of commutation failure in MIDC systems,but also can improve the recovery performance to prevent the commutation failure the unbalance power angle leads to,which caused by the slow recovery of MIDC systems.