Control Technique Of Cascaded H Bridge Static Synchronous Reactive Compensator

With the constant increase of electrical network capacity and raised requirement of electric power quality, flexible AC transmission system (FACTS) as an efficient technique to improve electric power quality has been rapidly developed. Static synchronous compensator (STATCOM) has currently become a hot research topic of FACTS equipments and a development trend of reactive power compensation because of its advanced control characteristic and good compensation performance.This paper analyzes the cascaded H-bridge STATCOM which avoids the high cost, huge volume, low loss and transformer ferromagnetic nonlinear caused by multi-pulse inverters and has series advantages of simple structure, less devices, easy modular design realization and packaging compared with other multilevel inverters. H bridge STATCOM topology and its operation characteristics with star and angle access mode is analyzed in this paper based on the reactive power compensation principle of STATCOM, and this is verified by MATLAB simulation.Based on the principle analysis of carrier modulation method, three kinds of modulation methods are compared and analyzed through mathematical analysis and simulation verification in this paper. The two carrier phase shifted PWM (CPS-PWM) modulation methods are applied in the cascade H-bridge STATCOM and the capacitor voltage fluctuation characteristics are studied. It can be clearly seen that bipolar CPS-PWM modulation could achieve more complex voltage waveform, larger harmonic components and unbalance of each bridge capacitor voltage than those achieved by unipolar CPS-PWM modulation.Cascade H-bridge STATCOM has the characteristic of independent DC side capacitor on each phase bridge which brings both significant advantages and some problems. So analysis the capacitor voltage fluctuation situation and the corresponding influence factors, then proposes a novel DC side capacitor voltage balancing control method based on voltage redundant states. This new method separates the capacitor voltage balance control from STATCOM control system, and this algorithm is simple, convenient, and does not affect the output voltage waveform and increase switching frequency. Simulation analysis verified the method.An experiment prototype of STATCOM controlled by DSP is designed on the basis of simulation. Design of main circuit, drive circuit and the software and hardware realization of DSP are included. Finally, experiments of the comparison of the two CPS-PWM modulation methods on cascaded H-bridge STATCOM and its reactive power compensation ability are tested on the prototype. Experiment results proved the validity and utility of this new method.