Research On Cascaded H-bridge STATCOM Based On Three-phase Balance Regulation

Nowadays,with the development of medium-high voltage in power industry,the requirement of power quality of power grid and voltage tolerance level of power electronic device is higher and higher.It is difficult to meet the demand of medium-high voltage situations for the existing limit of the voltage tolerance level of the power switch and the actual power system often operates under unbalanced conditions,which brings adverse effects to the safe and stable operation of the device.As a new type of reactive power compensation device,cascade H-bridge STATCOM can not only improve the capacity and voltage tolerance level of the device,but also quickly and accurately compensate the system for reactive power.Therefore,this paper studies the control strategy of cascaded H-bridge STATCOM under unbalanced conditions of three-phase four-wire system.Firstly,the working principle of h-bridge STATCOM,modulation strategy and reactive compensation principle are analyzed.The mathematical model of the device under balanced condition is established and its control strategy is analyzed.On this basis,the mathematical model of the device under the unbalanced condition of three-phase four-wire system is established,and the control strategy of positive and negative zero sequence decoupling is proposed.A new method of dc side voltage phase-to-phase balancing control based on the nearest level modulation is proposed to address the phase-to-phase imbalance of cascade H-bridge dc side voltage.Secondly,a new hardware test device of cascade H-bridge STATCOM based on DSP+STM32 distributed control is proposed.The master control DSP is responsible for the upper control algorithm of positive,negative and zero sequence.The trigger signal of the power switch of H-bridge unit is controlled the slave controller STM32,and overcome the limitation of the number of H-bridge cascades due to the limited pin of single controller.The hardware part designs the chip power supply,H-bridge power switch multi-path isolation drive power supply,cascade H-bridge basic circuit,H-bridge voltage acquisition circuit,STM32 chip hardware address setting circuit,DSP master control unit and AD7606 data acquisition circuit,etc.The software part designs the main program of the device,the control algorithm program,the master-slave I~2C communication program and the voltage AD acquisition program of H-bridge arm.At last,the multi-path isolated power supply simulation model and the 12H bridges cascade STATCOM simulation model were built respectively in the Multisim and EMTP simulation software,and the test equipment was debugged.The simulation and test results show that the multi-path isolated drive power supply can safely and stably drive the H-bridge power switch,the control strategy of cascade H-bridge STATCOM under the unbalanced condition of three-phase four-wire system can accurately and efficiently compensate the system for reactive power and the cascade H-bridge phase to phase balance control strategy based on the nearest level modulation can effectively balance the imbalance between the three-phase dc side voltage.