On control of cascaded H-bridge converters for STATCOM applications

Doctoral thesis, 2017

Cascaded H-bridge (CHB) converters are today considered the industrial standard for STATCOM applications, mainly due to their small footprint, high achievable voltage levels, 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, 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 technique 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 nonuniform 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. Two different strategies for DC-link capacitor voltage balancing, based on zero-sequence voltage/ current and negative-sequence current control, are investigated and compared. The comparison shows that none of the investigated control strategies allow for the full utilization of the converter capacity. Aiming at enhancing the converter utilization, two alternative control strategies, based on the proper combination of the zero-sequence and negative-sequence current control, are proposed and investigated. Finally, the operation of the CHB-STATCOM when controlling the voltage at the connection point in case of unbalanced system conditions is considered. It is shown that even if the device is intended for compensation of the positive-sequence voltage only, control of the negativesequence voltage might be necessary in order to avoid undesired overvoltages. Three structures for the negative-sequence voltage control are investigated and compared.