Research On Coordination Control And Optimization Of Multi-terminal DC Transmission System Based Voltage Source Converter

At present, in the situation of a large number of renewable energy accessing into the grid, the grid must timely transport and consume its active power, which requires the grid with higher reliability and greater flexibility. As the extension of flexible direct current(VSC-HVDC) electric power transmission technology, the multi-terminal flexible direct current (VSC-MTDC) electric power transmission technique not only has the advantages of VSC-HVDC transmission technology, for example, transmission capacity, good controllability, with dynamic reactive power compensation and the ability to improve power quality, environmental friendliness, etc., but also, as an important part of smart grid, can be used to solve the stability and power quality problems caused by the renewable energy accessing into grid and to solve the transmission problem of multiple power supply or multiple power consumption. With so much flexible and convenient features, VSC-MTDC transmission technology has very broad application prospects and currently more than one VSC-MTDC project has been put into actual operation in the world. However, compared to the VSC-HVDC transmission technology, more questions need to be compared for VSC-MTDC transmission technology and its control strategy is more complex, and therefore it is necessary to study it in depth. On one hand, in order to ensure DC voltage stabilization and active power balance under all operation conditions, coordination control strategy of VSC-MTDC system needs to be researched in depth and reasonable controllers need to be designed; on the other hand, in order to ensure the economy of DC grid transmission systems, the optimization control based upper controller also needs to be researched.In this background, based on the power system simulation software PSCAD/EMTDC, this paper researches and discusses the mathematical model and controller design of the voltage source converter, coordination control strategy and optimization control of VSC-MTDC transmission system.Firstly, this paper analyzes existing topology of voltage source converter, including three-phase four-wire VSC topology and three-phase three-wire VSC topology; then the mathematical model in the three-phase stationary coordinate system and transient models in d-q synchronous rotating coordinate system are respectively established, the model can be decoupled, and it is conducive to the design of the controller; on the basis of the model, local control of the flexible HVDC system is designed, and based on vector control, also called direct current control, its inner loop current controller and outer loop voltage controller are analyzed and designed respectively. Moreover, taking two-terminal and three-terminal flexible DC transmission system for example, this chapter sets their parameters, analyzes the simulation results and verifies the effectiveness of vector control strategy in PSCAD.Then, based on the traditional DC voltage-active power control characteristics, this paper proposes improved DC voltage-active power drop characteristics control mode, and the coordinated control strategy is designed. The improved control strategy for assistant and APC converter station in the VSC-MTDC transmission system are proposed respectively based on dc voltage-active power control characteristic and the control schematic diagram for the above control modes are presented; then taking five-terminal flexible DC transmission system for example, this paper presents the process of parameter selection and calculation and the DC power flow calculation; finally simulation and analysis in the simulation software PSCAD/EMTDC are conducted to validate the coordination dc voltage control method under normal, master converter station fault and assistant converter station fault conditions respectively.Finally, the basic principle of the internal point optimization algorithm is analyzed, and the flow diagram of the algorithm is presented. The objective function, equality constraints and inequality constraints in the optimization control strategy of five-terminal VSC-MTDC transmission system are presented respectively, and its existence of solutions is also analyzed; then variable setting for interfaces is carried out in the joint operation between PSCAD and MATLAB; and the simulation for optimization voltage algorithm is conducted in the five-terminal VSC-MTDC system, the simulation graphics in which voltage of all the nodes in the DC grid changing with active power in the wind farms are presented and loss reduction for DC grid is analyzed.