Research On Key Technologies For Modular Multilevel Converters And Application In STATCOM

With the rapid development of power electronic technology, modular multilevel converters attracted more and more attentions on industry and power systems applications due to the excellent output waveform qualify. Modular multilevel converters, as a new topolgy of multilevel conveters, features many advantages:switch with the low switching frequency and low voltage rating realizing high switching frequency and withstanding high voltage, modularity, easy expansion of power and voltage rating, simple realization of redundancy, thus, suitable for the medium and high voltage applications. Such as, high voltage dc transmission (HVDC), power quality, renewable energy integration, traction system and so on. So the study on mathematical model, harmonics analysis and suppression, control of capacitors voltage balancing are conducted in depth in this dissertation. The main contribution of this dissertation is summarized as follows:To simplify the controller design and describe the power transmission clearly, the simplified equivalent model is established. From the power transmission of view, the MMC is simplifed with the combination of an equivalent DC circuit and AC circuit, and respective control systems are developed, which describes the power transmission cource where DC source charges the amounts of floating capacitors and then energy of capacitors is transmitted to the load in the form of the alternate; Based on the equivalent circuit, the mature control structure is directly employed to control the MMC; Structue focused on the power transmission faciliates control of harmonics according to the requirement of system. Moreover, to alleviate the load of the communication of contoller, the open-loop control of equivalent DC circuit also is studied, and its stability is proved, which paves way for the harmonics analysis in Chapter three.The relationship between voltage and current of the equivalent arm capacitor is establised, the harmonic characteristic of MMC is analyzed. Firstly, the component of harmonics is derived, and the reason of generating circulating current is studied, from which a non-closed-loop suppression of 2nd circulating current is proposed, furthermore, a compund method is developed to suppress the 2nd order circulating current; Different from the existing methods, the analytical expression of 2nd circulating current derived, which faciliates the design of the main circuit parameters; Secondly, the distribution of 2nd order power between DC source, capacitors and the load is analyzed, then, a conclusion, from the capacitors of view the 2nd power of capacitors is balanced with that from the load and the DC source caused by the 2th circulating current, phase relationship between them decieds the amplitude of 2nd pulse of the capacitors, is drawn, which can provide the basic guidance for the relationship between the suppression of 2nd circulating current and capacitor ripple; Additionally, the expression of alternate output voltage which is reflected by the circulating current and alternate current is derived, and components of output voltage harmonics is analyzed.The capacitor voltage-balancing mehods based on the popular CPSPWM and PDPWM is studied in depth. An intra-arm voltage-balancing method is proposed, which combines the advantages of CPSPWM and PDPWM, featuring one balance controller, no voltage sorting algorithm, even module switching frequency, no effect on the synthesized arm votlage; Due to the drawback of existing inter-arm voltage-balancing methods, energy channel based method is proposed to balance the energy difference between up and low arms. As a result, the control of arm voltage and current is decoupled with that of the inter-arm voltage balancing, which is suitalbe for the application with DC source or bus capacitor, as well as the positive and negative bus suspension, Furthermore, the coordination between the intra-arm and inter-arm voltage balancing control also is analyzed.Mehtod of dealing with the phase unbalancing of STATCOM based on star-type and delta-type cascaded multilelvel converters is anlayzed in depth, the drawback of which is demonstrated. Thus, the energy based channel mehod is applied to the MMC based STATCOM, which can avoid the drawback of the common-model injection mehtod. Based on the established simplified equivalent model in this dissertation, the self-balancing priciple of STATCOM based on MMC is analyzed, Meanwhile, to impove the dynamic performance of phase balance, the modifed controller is employed. The capacitor voltage reference is calculated according to the requirement of outputting maximum phase voltage. Finally, the experimental prototype is designed and constructed, the simulation and experimental results show that the MMC-based STATCOM can compensate the reacitve power, negative current and suppress the harmonics under the unblance grid and load.