Research On Control System Modeling And Commutation Failure Of ±800kV DC Project With Split Connection To 500kV/1000kV AC Grid

The ±800 kV DC project with split connection to 500 kV/1000 kV AC system is first created by china in the world,and there is no existing experience for this kind of DC control and protection system.High voltage converter and low voltage converter of inverter station need to equip with voltage,current and trigger angle controllers separately under split connection and it is necessary to study the problem of coordinated control between high and low converter.In order to accurately study the dynamic behavior of ±800 kV DC transmission,analyze fault characteristics and study the interaction between AC and DC systems under split connection,it is necessary to establish an electromagnetic transient simulation model which consistent with the actual DC engineering characteristics.Under the fault condition of AC system on the inverter side,the traditional predictive control method of commutation failure fails to commutate the converters connected by the faulty AC network.At the same time,the converters connected by the non-faulty AC network fail to commutate.When DC line faults occur in unipolar DC system under split connection,the induced current during DC recovery causes commutation failure of normal operating poles,which leads to DC power oscillation.High voltage converter and low voltage converter of inverter station need to equip with voltage,current and trigger angle controllers separately under split connection,which is proposed in this paper.In addition,500 kV and 1000 kV independent reactive power control functions need to be added to the bipolar control layer.Based on the traditional control principle of ±800 kV DC converter tap-changer,the control principle of high-voltage and low-voltage converter transformers on the inverter side under split connection is simulated.Based on the background of ±800 kV Xitai DC transmission project,the static equivalent parameters of AC power network are obtained according to the AC power supply access on both sides of the project.In PSCAD/EMTDC software,the electromagnetic transient simulation model of ±800 kV DC with split connection is established.The results of electromagnetic transient simulation model are compared with the field recording of Xitai project and the dynamic response of the simulation waveform is basically consistent with the field results,which verifies the accuracy of the simulation model.Paper analysed the problem of commutation failure in traditional predictive control of commutation failure when AC system faults occur on inverter side under ±800 kV split connection.It is concluded that the rapid increase of DC current in the inverter side is the direct cause of commutation failure of the fault-free AC network connection valves;the failure of the predictive controller for commutation failure in the valve group to produce the correct trigger angle increment is the fundamental cause of commutation failure of the fault-free AC network connection valves.It is concluded that the rapid increase of DC current in the inverter side is the direct cause of commutation failure of the normal AC network connection converters;the failure of the predictive controller for commutation failure in the valve group to produce the correct trigger angle increment is the fundamental cause of commutation failure of the normal AC network connection converters.On this basis,an improved predictive control method of commutation failure under split connection is proposed.The simulation results show that the continuous commutation failure of the converters connected to the normal AC network can be effectively suppressed and the DC power oscillation can be avoided when the AC system fault occurs on the inverter side.When DC line faults occur in unipolar DC system under ±800 kV split connection,the induced potential during DC recovery causes commutation failure of normal operating poles,which leads to DC power oscillation.To solve this problem,the dynamic characteristics of DC line faults are studied.It is concluded that the induced current is the direct cause of commutation failure of normal operation poles during DC recovery process,and the reduction of actual commutation margin is the fundamental cause of commutation failure of normal operation poles.On this basis,this paper proposes to increase commutation margin by modifying rated extinction angle of normal operating pole during DC line fault.The simulation results show that the method is effective and avoids DC power oscillation.