Research On Control Strategy Of Cascaded H-Bridge SVG In Coal Mine Power Grid

With the gradual increase of coal mining intensity and depth,a large number of non-linear and unbalanced loads are connected to the power grid,resulting in a decrease in power factor and affecting power quality.Installing reactive power compensation equipment is an effective way to improve power quality.This paper proposes to apply cascaded H-bridge Static Var Generator(SVG)to improve the power quality of coal mine power supply system,and carries out research on its current tracking control and DC-link capacitor voltage balance control strategies.Combined with the power supply system of Longhu Coal Mine,the feasibility and effectiveness of the proposed method are verified.The mathematical model of cascaded H-bridge SVG under balanced and unbalanced conditions is established,which provides a theoretical basis for the study of current tracking control strategy.The modulation principle of unipolar frequency doubling carrier phase shifting SPWM(CPS-SPWM)is studied,and the simulation model is established to verify its effectiveness.Aiming at the problem that the synchronous rotating coordinate system phase-locked loop cannot quickly and accurately extract the grid voltage phase information under unbalanced grid voltage conditions,a double second-order generalized integrator phase-locked loop is introduced,and a simulation comparison analysis of the two phase-locked loops is carried out.The simulation results show that under unbalanced grid voltage conditions,the double second-order generalized integrator phase-locked loop can quickly realize the positive and negative sequence separation of grid voltage,extract the grid voltage phase information more accurately,and reduce frequency fluctuation.Aiming at the problem that the feedforward decoupling control using PI controller alone cannot track the command current signal containing harmonics without steady-state error,this paper proposes an improved feedforward decoupling control based on quasi-PIR controller,and compares and analyzes the two control methods by simulation.The simulation results show that the improved feedforward decoupling control can better track the instruction current signal containing harmonics without static errors.In order to effectively solve the problem of unbalance capacitor voltage at the DC-link,this paper adopts the layered control strategy of capacitor voltage at the DC-link,that is,global voltage equalization,voltage equalization between phases and voltage equalization within phases,and builds control models at each layer for simulation verification.The simulation results show that through three-layer voltage control,the DC-link capacitor voltage of each H-bridge in the cascaded H-bridge SVG is balanced,verifying the effectiveness of each layer control strategy.Combined with the current situation of power supply in Longhu Coal Mine,various working conditions that may occur in the coal mine power grid are simulated by MATLAB/Simulink,and the simulation results are analyzed.The simulation results show that the cascaded H-bridge SVG under the control strategy has good dynamic compensation effect for various working conditions that may occur in the coal mine power grid.It can quickly increase the power factor from 0.8before compensation to more than 0.98,and improve the power quality of the coal mine power grid.