A six-level asymmetrical Hybrid Photovoltaic Inverter with inner MPPT capability

This paper presents a three-phase six-level Asymmetrical Hybrid Multi-Level Photovoltaic Inverter (AHMLPI) for medium voltage grid connected applications. A Low to Medium Voltage Transformer is operated in an open-end winding configuration. Thus the low voltage winding is supplied on one end by a three-phase three-level Neutral Point Clamped Inverter (NPC) and, on the other side, by an auxiliary Two-Level Inverter (TLI). A key feature of the proposed approach is that the NPC controls the main active power flow extracted by PV strings, while also accomplishing the MPPT (Maximum Power Point Tracking), thus avoiding the need of additional DC/DC power converters. The NPC operates at a low switching frequency, according to a step modulation, and can be equipped with very low on-state voltage drop IGBT devices. The auxiliary TLI instead operates at low voltage, according to a conventional high frequency two-level PWM technique, in order to accomplish a twofold task. First it controls the inverter output current. Second it acts as an active power filter in order to compensate low order harmonic components caused by the NPC step modulation. As the DC bus voltage of the TLI is remarkably lower that of the main NPC, the auxiliary inverter can be equipped with low switching losses IGBTs, or even Power MOS devices. Simulations results confirm that, compared to conventional systems, the proposed approach, features a remarkably reduced phase current harmonic content is and an increased efficiency.