Comparison of DC-bus voltage balancing strategies for three-phase DSTATCOM based on cascaded H-bridge multilevel converter

This paper presents a comparison of two DC-bus voltage balancing strategies for cascaded connected H-bridge converter-based STATCOMs. These converters have classically been commutated at fundamental line-frequencies, but the evolution of power semiconductors has allowed the increase switching frequencies and the power ratings of these devices, permitting the use of PWM modulation techniques. This paper mainly focuses the DC bus voltage balancing problems and proposes the use of two different balancing strategies, the individual voltage balancing strategy (IVBS), and the modulation based balancing strategy (MBBS), which solve these balancing problems using PWM modulation. Both balancing strategies maintain the delivered reactive power equally distributed among all the H-bridges of the converter. These two balancing strategies have been previously published by the authors for single-phase DSTATCOM applications. In this paper the good performance of both strategies is shown for three-phase applications, even in presence of grid perturbances. A comparison of the behavior of both balancing strategies is made based on simulation results.