Operation and control of cascaded H-bridge converter for STATCOM application
Abstract: In the last decade, particular attention has been paid to the use of Modular Multilevel Converters (MMC) for grid applications. In particular, for STATCOM applications the phase leg of the converter is constituted by a number of single-phase full-bridge converters connected in cascade (here named Cascaded H-Bridge, CHB, converter). This multilevel converter topology is today considered the industrial standard for STATCOM applications and has replaced other converter topologies, mainly due to its small footprint, high achievable voltage levels (allowing transformer-less operation), modularity and reduced losses. However, there are still areas of research that need to be investigated in order to improve the performance and the operational range of this converter topology for grid-applications. The aim of this thesis is to explore control and modulation schemes for the CHB-STATCOM, both under balanced and unbalanced conditions of the grid, highlighting the advantages but also the challenges and possible pitfalls that this kind of topology presents for this specific application. The first part of the thesis is dedicated to the two main modulation techniques for the CHBSTATCOM: the Phase-Shifted PulseWidthModulation (PS-PWM) and the Level-Shifted PWM (LS-PWM) with cells sorting. In particular, the focus is on the impact of the adopted modulation on the active power distribution on the individual cells of the converter. When using PS-PWM, it is shown that non-ideal cancellation of the switching harmonics leads to a non-uniform active power distribution among the cells and thereby to the need for an additional control loop for individual DC-link voltage balancing. Theoretical analysis proves that a proper selection of the frequency modulation ratio leads to a more even power distribution over time, which in turns alleviates the role of the individual balancing control. Both PS-PWM and cells sorting schemes fail in cell voltage balancing when the converter is not exchanging reactive power with the grid (converter in zero-current mode). To overcome this problem, two methods for individual DC-link voltage balancing at zero-current mode are proposed and verified. Then, the thesis focuses on the operation of the CHB-STATCOM under unbalanced conditions. It is shown analytically that regardless of the configuration utilized for the CHB-STATCOM (star or in delta configuration), a singularity exists when trying to guarantee balancing in the DC-link capacitor voltages. In particular, it is shown that the star configuration is sensitive to the level of unbalance in the current exchanged with the grid, with a singularity in the solution when positive- and negative-sequence currents have the same magnitude. Similar results are found for the delta configuration where, in a pure duality with the star configuration, the system is found to be sensitive to the level of unbalance in the applied voltage. The presence of these singularities represents an important limit of this topology for STATCOM applications.
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