Research On The Control Strategy Of Cascaded H-bridge STATCOM Under Unbalanced Grid Conditions

As a reactive compensation device with superior performance,cascaded H-Bridge STATCOM which owns broad research value and application prospect has the advantages of fast response speed,dynamic reactive power compensation,high degree of modularization,easy maintenance.However,the imbalance of grid connection point will bring a sharp drop of the output current quality and a serious imbalance of the dc voltage to the cascade H-bridge STATCOM,threatening the safe operation of the device.In this paper,the cascaded H-Bridge STATCOM under unbalanced grid condition is taken as the research object and the corresponding control strategy is studied to enhance the capacity to withstand the impact of three-phase unbalanced power grid and maintain stable operation of STATCOM.The main work is as follows:Starting from the basic H-Bridge module model,the mathematical model of STATCOM under unbalanced grid condition is established step by step.At the same time,the principle of unipolar frequency doubling CPS-PWM modulation is studied and a simulation model is established to verify its effectiveness.One of the key technologies of cascaded H-Bridge STATCOM grid-connected control is to obtain the phase angle of the positive sequence voltage of the grid.The defects of the traditional phase-locked loop used by STATCOM is firstly analyzed in this paper.Four second-order generalized integrators are compared.And a structure with no attenuation of output and easy realization is selected.On this basis,the phase-locked loop based on the DSOGI is studied.Aiming at the problem of slow frequency response,the dynamic response speed of frequency output is improved by adding frequency lock loop.The design method of key parameters of frequency lock loop is studied,and its effectiveness and reliability are studied by simulation.Based on STATCOM mathematical model of the positive and negative sequence,current loop control strategy of double order synchronous rotating coordinates system is built.On the basis of analyzing its characteristics,the current control strategy based on reference signal and the error signal decoupling network of double synchronous reference coordinate system is studied.Its application effectively eliminates the double frequency tracking error of the current tracking link.This paper theoretically analyzes the influence of active and reactive power of unbalanced grid to STATCOM and designs a layered control strategy for dc voltage.According to the factors affecting the voltage balance between phases,negative sequence current injection method and zero sequence voltage injection method are studied respectively and this two control strategies are combined to adjust the proportion of two above in the control more flexibly.For the dc voltage balance control,the method of superposition active voltage is analyzed and the dc voltage balance is realized.The simulation model is built to verify the cluster and individual control strategy of dc voltage.The results show the effectiveness and practicability of the proposed strategy.Based on the existing grid simulator platform and the cascaded H-Bridge STATCOM platform in the laboratory,the phase locked loop technology,the modulation strategy of STATCOM and the voltage balance strategy are studied experimentally.The results show that the strategy can effectively achieve the control goal.