Research On The Control Strategy Of VSC-HVDC System

VSC-HVDC is a new DC transmission technology which is based on voltage source converter and pulse width modulation.It has the advantages of supplying passive network with power,controlling the active power and reactive power independently and building a multi terminal DC transmission network easily,so the VSC-HVDC system was applied in the field of transmission and distribution widely,which attracts wide attention from industry and academia.The running of VSC-HVDC is concerned with the control strategy;therefore,it is very important to study the control strategy of VSC-HVDC system.The research of control strategy for VSC-HVDC system will be divided into the following several points in this paper:First of all,the both ends of the VSC-HVDC system's topological structure were given and the basic operation principle of the system was analyzed.At the base of a deep analysis of the converter structure,the mathematical model of the converter in three-phase stationary coordinate,two-phase rotating coordinate and the two-phase stationary coordinate system was established.On the basis of the proportional integral controller could achieve zero steady-state error control the DC component,the control strategy of VSC-HVDC system based on grid voltage orientation in two-phase rotating coordinate was achieved.Secondly,the traditional PI control method needs the complex coordinate transformation,the grid disturbance feed forward compensation voltage and state feedback to achieve independent current control,which it will reduce the robustness of the system,so the proportional resonant controller was introduced in the current inner loop of VSC-HVDC control system in this paper,which reduces the design difficulty of control system.Through analyzing the working process of converter station,we know that DC component was existed in the output current of inverter,which will affect the performance of the system.On the basis of moving average filtering method to separate DC component from the output current of inverter and the proportional integral controller which can restrain the DC component without error,the current inner loop control strategy with the DC component suppression which based on PR and PI joint controller was proposed in this paper.Then the VSC-HVDC system adopts the power of traditional PI control algorithm and the proposed algorithm in the paper variable step simulation in the balance of power grid with the MATLAB/Simulink simulation software.Through the comparative analysis of the simulation results,the control algorithm proposed in this paper could suppress the DC component and the system has good dynamic performance at the moment of power transition.Finally,based on the study of imbalance power control strategy of VSC-HVDC system,the mathematical model of VSC-HVDC system under imbalance power grid was established.The double sequence inner loop current control strategy based on proportional resonant controller was presented.Through the analysis of proportional resonant controller,the resonant harmonic filter and the resonant integrator were achieved,which be introduced into the outer loop control.The unified vector control of outer loop voltage and power was completed.The simulation analysis in the situation of unbalanced voltage which caused by voltage drop was accomplished with the simulation software MATLAB/Simulink.