Ripple Analysis and Capacitor Voltage Balancing of Five-level Hybrid Clamped Inverter (5L-HC) for Medium-Voltage Applications

The five-level hybrid clamped inverter (5L-HC) is a promising new topology for medium-voltage applications. However, to minimize the capacitors and ensure good performance of the converter, capacitor voltage ripples, as well as voltage balancing control, need further study. Based on carrier-phase-shift PWM (CPS-PWM), this paper gives a rigorous analysis of the ripple characteristics of all the capacitors (which consist of three groups: the flying capacitors, the central dc capacitor, and the upper and lower dc capacitors) under both normal and unbalanced operating conditions. The results can serve as a guide for the optimal design of the capacitors. First, as for the balancing issue, a self-balancing mechanism of the central dc capacitor, which is similar to the better-known self-balancing feature of the flying capacitors, is revealed. Then, a new voltage balancing strategy is proposed for these two groups of capacitors to cope with disturbances arising from various nonideal factors. For the upper and lower dc capacitors, which suffer from intrinsic imbalance, this paper proposes a better method to calculate the optimal zero-sequence voltage (ZSV), which is to be injected to achieve the balancing objective. In this paper, the ripple analysis and balancing strategy proposed are verified with simulations and experiments.