Research On CHB SVG Control Strategy Under Power Grid Voltage Imbalance Condition

In modern society,people’s demand for power quality is getting higher and higher,but the instability of new energy generation,long-distance transmission of power,aging of power grid and other problems will affect the power quality.Reactive power compensation equipment can solve this problem.Static Var Generator(SVG)is one of the widely used reactive power compensation equipment,which has fast response speed,continuous regulation,less harmonic content and low loss.Cascaded H Bridge(CHB)has the advantages of modularity,fewer harmonics,and more electrical levels.Therefore,this paper uses SVG of this topology as the object of research.Firstly,the research background and significance of reactive power compensation technology are introduced.The development of reactive power compensation devices and the advantages and disadvantages of different devices are collected,so as to determine the adoption of CHB SVG and analyze its research status.The topology structure and working principle of CHB SVG are analyzed.The mathematical model of SVG is established under the two conditions of power grid voltage balance and imbalance respectively,so as to facilitate the subsequent research.Then the topology and working principle of CHB are analyzed.For the traditional CHB inverter,the working principle,advantages and disadvantages of two common modulation strategies,carrier stacking and carrier phase shifting,are analyzed,and their simulation analysis and experimental verification are carried out.For hybrid CHB inverter,the working principle of hybrid carrier cascade modulation and the power imbalance mechanism of low-voltage unit are analyzed,and then the improved hybrid modulation strategy is proposed.Then the low-voltage unit is optimized for power balance,and its reliability is verified through simulation and experiment.Then the basic structure of the PLL is analyzed.The working principle of single synchronous PLL,double synchronous PLL and PLL based on double second-order generalized integrator are expounded,and the corresponding system structure block diagram is drawn.The simulation models of three kinds of PLL technologies were built respectively,and they were run under the same condition,and the simulation results were recorded.The performance of three kinds of PLL is analyzed according to the simulation results.Finally,the working principle of reactive current detection,voltage and current double closed-loop control and three-layer voltage equalization control are analyzed,and the corresponding structural block diagram is drawn.Simulation is used to verify the performance of SVG control strategy.The results show that the performance of SVG control strategy can reach the expectation when the grid voltage is balanced,but the performance of SVG control strategy is poor when the grid voltage is unbalanced.To solve this problem,the current loop control with positive and negative sequence separation and the phase-to-phase voltage equalized control with zero sequence voltage injection are analyzed,and the simulation is used to verify the effect of the control strategy when the grid voltage is not balanced.Figure[78] Table [4] Reference[77]...