Advanced multilevel converters for grid integration of renewable energy

Abstract

In the realm of renewable energy generation, including photovoltaic systems, wind power generation, and battery storage, inverters are pivotal components. In high-power applications, multilevel inverters offer significant advantages such as enhanced power quality, reduced switching losses, minimized electromagnetic interference (EMI), and improved system reliability. Among these, three-level inverters are widely adopted, with the three-level T-type inverter garnering particular attention due to its low conduction losses, minimal component count, and straightforward working principle. Traditional inverters generally function in hard switching mode, which often results in significant switching losses and electromagnetic interference, restricting the system's overall efficiency and power density. In contrast, soft switching technology can effectively reduce or eliminate the overlap of voltage and current during the turn-on and turn-off processes of switching devices, thus lowering switching losses. By operating the converter in boundary conduction mode (BCM), the inductor current can reverse in each switching cycle, enabling zero-voltage switching (ZVS) without requiring additional auxiliary circuits. Currently, T-type inverters operating in BCM commonly employ pulse frequency modulation (PFM), resulting in a non-constant switching frequency that varies widely between 30 kHz and 200 kHz. This variability presents several challenges: it complicates the control strategy and filtering design, and it also introduces electromagnetic interference issues. Moreover, as the load increases, the peak switching frequency and its range of variation in PFM also rise, ultimately reducing system efficiency. To tackle these issues, this dissertation thoroughly examines the operating principles of T-type three-level inverters in Boundary Conduction Mode (BCM) and the two-phase interleaving mechanism. Building on this analysis, a new BCM soft-switching technique with a constant switching frequency is introduced. The research also contrasts the switching losses of T-type inverters under hard switching and soft switching conditions. Proportional-Resonant (PR) control is utilized to implement closed-loop control for the grid-connected T-type three-level inverter employing the proposed approach of soft-switching. The efficacy of the suggested method is confirmed via a simulation model of a grid-connected two-phase interleaved T-type three-level inverter constructed in Simulink.