The Research On Topology And Control Of Cascaded Half-bridge STATCOM

Reactive power compensation technology,which can stabilize voltage level,optimize waveform quality and improve the efficiency of electricity,is one of the important means of building resources-conservation society.In recent years,the static synchronous compensator(STATCOM)has developed rapidly,and is widely used in the field of industrial energy-saving motor,new energy power generation,power transmission and other industry.H bridge cascaded multilevel STATCOM has become the best solution in high-voltage and high-capacity field,and has gradually replaced the traditional STATCOM topology with multiple structure.If we can further develop the advantages which reducing switching stress of devices,optimizing output waveform and and improving conversion efficiency of cascaded multilevel topology,the research value of multilevel STATCOM technology will be more prominent.At first,this paper summarizes the current multilevel converter topology,compares the advantages and disadvantages of different structure,and puts forward a new half-bridge cascade STATCOM structure.From two aspects of the overall system and the sub module internal structure,the paper analyzes the basic working principle of this new topology.According to the above analyzes,this paper deduced the transfer function model of half-bridge cascade STATCOM topology,laying the theoretical foundation for subsequent analysis of control strategy.Secondly,this paper studies the power control strategy and capacitor voltage balancing strategy of the half-bridge structure.By decoupling the active and reactive components of power in the dq coordinate system of synchronous rotation,a low frequency time domain model on which PI controller and repetitive controller are added respectively is derived by ignoring the switching characteristics.The simulation research of the control strategy is carried out at last.Simulation results show that,the PID control strategy behaves fast transient response because the proportion link guarantees the rapidity of the system,while the repetitive controller controller can realize no static error and guarantee the steady state of the system without static error.Nextly,this paper analyses the principle and advantages of several high frequency modulation methods.In terms of half-bridge cascade structure,a cascade of half bridge universal PWM(HBU-PWM)is proposed by using a single carrier to achieve multilevel modulation.Compared with the traditional way of modulation,HBU-PWM eliminates the phase synchronization,the realizes the application of SPWM in three-phase three-wire half-bridge cascade STATCOM structure in and retains the advantages of less harmonic content in traditional cascade modulation.The realization of the principle is analyzed,which provides the theoretical basis for the following experiments.The inherent high-order harmonic of PWM control is a problem faced in all kinds of high-voltage high-power inverter.So the paper simulates the output of H-bridge cascaded inverter under different modulation methods,by observing.the waveform of simulation results,the influence of different modulation methods on the output waveform is summarized.Finally,this paper describes the 7 level 40kVA a half bridge cascade STATCOM experimental platform.by counting related experimental data,the control strategy of the STATCOM is further verified.In the point of hardwire system,the topological structure and composition of the experimental platform are simply introduced,and the hardware circuit and software program of the system are analyzed.While in the experimental part,the corresponding test are performed from both static and dynamic.The results show that STATCOM can rapidly compensate reactive power shortage,verify the validity of the control strategy.By observing the waveform of STATCOM three-phase output voltage which properly tracks the line voltage of the grid under different switching states,the correctness of the modulation strategy is verified.