Study On The Modulation Strategy Based On Hybrid Cascaded H-bridge STATCOM

The harmonic and reactive power can be compensated by using Static Synchronous Compensator(STATCOM), which can improve the grid quality and power factor. In the STATCOM system, the compensation effect could be influenced by the modulation strategy, this thesis took the hybrid cascaded H bridge STATCOM as the study objective, focused on the modulation strategy for its main circuit, while made the simulation analysis for current loop tracking control and DC voltage control.In hybrid cascaded structure, the hybrid cascaded H bridge converter attracts extensive attention because of its simple structure and it can be easy to expand, it also can export more level, compared with the traditional cascaded H bridge converter. This thesis first analyzed the main circuit topology based on hybrid cascaded H bridge converter, while analyzed the switch state of units combined with the main circuit topology. When the H bridge units were taken different DC voltage ratio, analysis was made for the output waveform, the maximum extension principle is introduced as the theory reference of appropriate DC voltage ratio. The simulation and experiment for the analysis have been done, and the results of simulation and experiment verified the effectiveness of theory analysis.This thesis took the hybrid cascaded seven level converter as example, studied on hybrid modulation, unipolar disposition carrier modulation and hybrid frequency carrier-based modulation. The theory reference of threshold voltage was introduced for hybrid modulation strategy, while the energy flowing backwards problem was also analyzed in this thesis. In order to solve this problem, a new hybrid modulation strategy was presented in this thesis, in which the threshold voltage become two, the new hybrid modulation avoids the case of two H bridge units output opposite polarity level, which avoids the energy flowing backwards problem. The unipolar disposition carrier modulation makes two H bridge units modulate in the same carrier frequency, also avoids the case of two H bridge units output opposite polarity level, but considering the high voltage occasion, the high voltage unit adopts IGCT or GTO, restricted by its switching frequency, in order to solve the problem, this thesis studied on hybrid frequency carrier-based modulation strategy. In this strategy, the two H bridges units adopt different frequency carrier to modulate, with the introduction of low frequency carrier, this thesis also studied on the harmonic content of output waveform caused by low frequency carrier. The simulation and experiment for the analysis have been done, and the results of simulation and experiment verified the effectiveness of theory analysis.This thesis also studied on the characteristic based on the hybrid cascaded H bridge STATCOM, including the reactive power current detection method, compensating current tracking control method and DC voltage balance control method. For unipolar disposition carrier modulation strategy, selecting the corresponding DC voltage balance control method within phase, in which the control principle was studied. At last, the compensation performance of STATCOM was analyzed when it adopted different modulation strategy through MATLAB platform.