Energy-Balance Based Voltage Regulation Method for Multiple DC-links in Asymmetrical Cascaded Multilevel Inverters

This paper presents a dc-link voltage regulation method for the asymmetrical cascaded multilevel inverter (ACMI). Such method is based on energy-balance analysis and is here evaluated in the context of a single-phase grid-connected photovoltaic (PV) system. However, the proposed strategy can be easily extended to different kinds of applications related to distributed generation with multiple primary power sources. There are several works that deal with the dc-link voltage control for conventional PWM inverters, but such methods cannot be applied to multilevel topologies due to the presence of multiple and isolated dc-links in the ACMI topology. Regarding the proper operation of this topology, the dc-link voltages must be strictly held in a particular set of scalable values. Therefore, a method for regulating the dc-link voltages is required. Although the here proposed method could be applied to any N number of h-bridge inverter series-connected cells, this work analyses a 3 h-bridge ACMI case, which is able to provide an (up to) 19 levels output voltage. A computational model built in the MATLAB/SIMULINK® platform was considered in order to evaluate the dynamic performance of the proposed control method on continuous time-domain under two different scenarios: with proportional (P) controllers; and with a set of proportional and PI (P+PI) controllers. Simulation results show that the proposed method is suitable, offering appropriate transient and steady-state responses even during load and available power variations.