Research On The Hybrid Cascade Multilevel Topology And Its Application In STATCOM

With the development of the Power Electrical technology, the applications of high power PE devices in the high voltage high power field such as FACTS,HVDC,VFVS are more important than before. In the several solutions of high voltage high power transformation, the Multilevel Converter received wide attentions for its capability of increasing the output voltage magnitude and reducing the output voltage and current harmonic content, decreasing the switch frequency and the voltage of each power semiconductor. As an innovative technology, the hybrid multilevel converter brings a new way to the research of the high power converter. STATCOM is the basic technology of the FACTS technology; therefore based on the hybrid cascade multilevel converter's topology research, a noval STATCOM of a hybrid cascade multilevel converter is particularly discussed. The order of the disquisition is from the topology's structure to modulation, spectral analysis, control circuit and the application of the static var compensator. The followings are the main work:Analyzing of the numerous hybrid multilevel topologies, a universal generative topology is obtained by the parallel and series ways based on the generalized multilevel half bridge. By this method, more hybrid multilevel topologies can be derived from it. From the hybrid multilevel converter's characters and the commands of the actual application, a valuable improved hybrid cascade multilevel converter is presented. This proposed converter is the series-connection of a Neutral-Point-Clamped (NPC) converter and several common H-bridge converters with equal dc bus voltage, and by a hybrid modulation method which incorporates square-wave synthesis in conjunction with unipolar triangle carried-based PWM method the output voltage wave is obtained. Based on the anatomy of hybrid modulation process in nature, 3-D mathematic models are built about three kinds of carrier dispositions frequently. Further, by double Fourier series expansion and intricate derivation, strict output analytic solution is acquired. The output analytical solution contains all original informations of the output waveform spectra.The problem of the power flow change in the cascade modules in the traditional hybrid cascade topology is considered, and the square waves' modulation ranges which ensure the converter don't have the power flow change in the improved hybrid cascade multilevel converter's hybrid modulation strategy is obtained. For the actual application of the topology, a based on FPGA-DSP control circuit is devised, and a hybrid cascade multilevel inverter is carried out by the repetitive control.After the reactive power compensation principle of the STATCOM is analyzed, a novel STATCOM structure based on the improved hybrid cascade multilevel converter is presented. In virtue of the separated phase character of the cascade topology, a reactive power compensation system is designed by the non-zero-order current mode to control. By this design, the simulations which included the conditons of the symmetry or asymmetry grid voltage and balance or unbalance reactive load current obtained good results, and is improved to be a valuable control method. Following the analyses of the digital current loop control system, a comparison research between the PI controller, the repetitive controller and their composite control method is presented, and the composite control based on the digital PI controller and the digital repetitive controller is proposed to improve the composition performance of the digital PI controller, when the repetitive controller enhancs the steady state performance, the PI controller ensuring dynamic performance.Through theoretically investigating into the convergence process of the two weighting factors in the adaptive harmonic filter, two novel algorithms are presented to modify the least mean square (LMS) algorithm. The first LMS algorithm update the step-size from the time-average of error signal and the second algorithm based on avoiding the repetive harmonic disturb to provide a fast convergence at early stages and ensure a small final maladjustment. Both of them can greatly shorten the transient period while keeping steady-state accuracy.