Design And Stability Analysis Of Polar Coordinate Vector Control Strategy For VSC In Weak Power Grid

In recent years,with the development and utilization of clean energy in China,a high proportion of new energy is connected to the power grid through long-distance lines,which actually leads to the weaken of the connection between the distributed power source and the power grid.The new energy is generally connected to AC power grid by voltage source converter(VSC).However,the interaction between dynamic characteristics of VSC system and control system in weak power grid environment may lead to system oscillation,which will threaten the safe and stable operation of new energy grid-connected and AC-DC transmission system.According to the current energy development trend,the number of ports and capacity of VSC connected to the power grid will continue to increase in the future,so the current commonly used VSC control strategy can not meet the stability needs of new energy into the grid in the future.Based on the above background,this paper designs a new strategy for VSC control in weak power grid,and analyzes the stability of the control strategy,which provides new solutions and thoughts for further improving the safe and stable operation level of new energy grid-connected and AC-DC transmission system.The main work completed is summarized as follows:Firstly,based on the narrow dynamic theory,this paper models the capacitance energy storage,inductor energy storage and resistance energy consumption in three-phase AC weak power grid system according to the energy conservation principle of Lagrangian system,and then combined with modern control theory,the circuit state equation of VSC connected to weak power grid system in polar coordinate system is obtained.After that,the control system design theory is proposed to clarify the specific steps and reasons of the control strategy designed in this paper.Secondly,based on the factors that cause the oscillation of the weak power grid and the established circuit state equation,this paper designs the converter control strategy which is suitable for VSC to connect to the weak power grid.The frequency domain stability analysis shows that the current control has good tracking ability in the controllable frequency band.The calculation of power solution interval explains the influence of weak power grid connection degree on stability from the point of view of power flow analysis.The designed frequency observer avoids the divergence of weak power grid system caused by the participation of phase-locked loop.Finally,the stability of the polar coordinate vector control strategy designed in this paper is analyzed by using Lyapunov’s first method.In this paper,the discrete controller designed in this paper is replaced by the continuous system approximation,the eigenvalue analysis of the explicit solution of the small disturbance equation of the system state variable is carried out,and the electromagnetic transient simulation is compared with the traditional control strategy.The results show that the designed control strategy has good stability and anti-interference ability.