Optimization Of Control Parameters Of AC/DC Hybrid System Based On Optimal Power Flow

Multi-terminal flexible DC transmission has played an important role in high-voltage and large-capacity applications,such as offshore wind power grid-connected,distributed energy disposal and so on.It will also play an important role in smart grid in the future.It is of great significance to study the AC-DC hybrid system composed of multi-terminal flexible HVDC transmission.In order to obtain the power flow distribution of AC/DC hybrid system with multi-terminal flexible HVDC transmission system,it is necessary to analyze the influence of different DC system control modes on its power flow calculation.In order to improve the stability and economy of AC/DC hybrid system with multi-terminal flexible HVDC transmission,it is very important to optimize the control parameters of the system.In this paper,the following aspects of power flow calculation and power flow optimization for AC/DC hybrid system with multi-terminal flexible HVDC transmission are studied.The main contents include:(1)The working principle of voltage source converter,which is the key component in multi-terminal flexible HVDC transmission system,is introduced,and the characteristics of controlling active and reactive power independently are analyzed.The three-level control of VSC-MTDC system,including upper control of system-level,control of converter station-level and control of converter valve-level,is briefly described.Three common DC voltage coordination control strategies are also introduced:master-slave control.The principles,characteristics and application occasions of system,margin control and droop control provide theoretical reference for determining the control mode of DC power grid before power flow calculation and optimization.Three common DC voltage coordinated control strategies are introduced:master-slave control,margin control and droop control.Their principles,characteristics and application occasions are also introduced,which can provide theoretical reference for defining the DC grid control mode before power flow calculation and optimization.(2)Aiming at the problem that the simplified model of converter station and the single-point DC voltage control are mostly used in the existing literature,which results in insufficient accuracy and limited application occasion of the current AC/DC hybrid power flow calculation,an improved alternating iteration method for VSC-MTDC power flow calculation is proposed.FiRTSly,an accurate steady state model of converter station is established,and the mathematical relationship between internal variables of converter station including loss of converter station is deduced.Then,the influence of control modes of converter station and voltage control modes of DC power grid on power flow calculation is deduced Taking the test system of IEEE_RTS1996 as an example,the effectiveness and applicability of the proposed method is verified by setting different scenarios.(3)According to the power reference value,which is the key factor affecting the power flow distribution in master-slave control in the existing literature,the power flow optimization is not accurate enough because of selecting the value based on experience.With the help of multi-objective particle swarm optimization(MPSO),the power reference value of converter station in AC/DC hybrid system is optimized with the objective of minimizing network loss and AC voltage deviation.Finally,taking the setting of master-slave control mode in the dual-area test system of IEEE_RTS1996 as an example,the effectiveness of the proposed method is verified by comparing with the value of the objective function before optimization.(4)In view of the drawbacks of the existing multi-point DC voltage control mode of AC/DC hybrid system,voltage-power droop control,which is mostly based on the capacity of converter station to determine droop coefficient and keep it fixed and unchanged,makes the DC power grid unable to participate in the regulation of power flow distribution flexibly.With the help of particle swarm optimization(PSO),the optimal droop coefficient of each converter station is obtained with the objective of minimizing network loss and AC voltage deviation.Taking the droop control method in the dual-area test system of IEEE_RTS1996 as an example,the effectiveness of the proposed method is verified by comparing with the value of the objective function before optimization.