| Multilevel conveter topology has great potential in high voltage and large power applications, considering its significant features such as lower switching stress, higher efficeintcy, and better output waveforms. Dynamic reactive power compensation based on Static Synchronous Compensator (STATCOM) plays an active role in ensuring voltage stability of power load centers and improving power quality, thus has important realistic meaning and engineering value. Modular Multilevel Converter (MMC) is lastest result of Multilevel Converter which provides new ideas for R&D work of high power converter. In this dissertation, the control strategy of STATCOM based on MMC topology is investigated by means of theoretical analysis and simulation validation. The main contests and related conclusions are as follows:(1) Based on the analysis on the structure and operation principle of MMC, Three main important issues are taken into discussion which includes modulation strategy, capacitor voltage balancing and circulating current suppression. A comparision of existing modulation strategies sutiable for multilevel converter is made, a novel capacitor voltage balancing algorithm based on hierarchical sorting and related optimization have been presented. The proposed algorithm ensures voltage balancing among sub-modules while reduces swiching frequency and improves efficieny of MMC. As to circulating current suppression, based on the analysis on its generation mechanism, feature of circulating current under both positive and negative networks is investigated. According to the concepet of decoupled double synchronized transformation, the method of ciruculating current suppression controller is proposed, which efficiently suprreses the fluctuation without increasing arm inductance.(2) Based on the operation principle of reactive compensation, the mathematical model and equivalent circuit are created for STATCOM in both three phase coordinates and dq two phase synchronous coordinates. As to the coupling of dq component, a kind of statefeedback decoupled PI controller is presented. This control strategy can eliminate coupling between dq components by introducing dq components feed-forward, simulation results verify fast dynamic and non-steady error reactive power trace.(3) Considering quite large amount of sub-modules (SMs) used in MMC topology, attention is drawn on the reliability of operation. Based on the analysis of existed fault-tolerant design for H-bridge converters, a novel fault-tolerant control strategy suitable for MMC-based STATCOM is proposed. According to proposed strategy, all redundant SMs are set in hot stand-by state, once fault occurs, redundant SMs will rapidly take place of the faulted SMs, thus ensures the STATCOM under normal condition. Furthermore, according to different number of faulted SMs, control strategy adopts different objectives which can greatly improve operating range of STATCOM.(4) Based on the concept of instanous symmetrical component and synchronized transformation, power balance relation between the AC and DC side of MMC are analyzed under both balanced and unbalanced ac voltage conditions, corresponding low frequency dynamic mathematical models of MMC are developed in dq synchronous reference frame. In order to increase detection accuracy of synchronous phase and instantaneous symmetrical components, a novel detection method based on the concept of decoupled double synchronized transformation is presented. The proposed methed can not only eliminate the adverse effects of negative sequence componets, but realize sequence component extraction as well. Control strategy for MMC under unbalanced condition is proposed which can eliminate negative sequence components or ensure DC voltage stable without fluctuation, thus enlarge the operating region of MMC.(5) A novel control strategy for suppressing circulating current of modular multilevel converter (MMC) under unbalanced conditions is proposed. Based on decoupled synchronous transformation, dynamic behavior of converter can be expressed in both positive- positive and negative-negative networks. Mechanism and dynamic performance of circulating current in both networks will be further discussed. Based on the concept of decoupled double synchronous transformation, circulating current suppressing controller suitable for both networks is presented which can eliminate coupling between two netwroks by introducing dq components feed-forward. The proposed control strategy can efficiently suprress the fluctuation without increasing arm inductance under unbalanced condition. |
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