Grid-connected PV system using a three-phase modified dual-stage inverter

This paper presents a PV grid-connected system in centralized configuration and constructed with a three-phase dual-stage inverter. In this topology, usually, a DC-DC converter performs the maximum power point tracking (MPPT) and an inverter is responsible for controlling the grid-current. For the DC-DC stage the three-phase series resonant converter (SRC3) is chosen thanks to the advantages that it exhibits. However, it is inadequate for the accomplishment of MPPT, due to its efficiency strongly depending on the implemented deadtime and switching frequency. Then, this paper proposes a conceptual modification, i.e., a dual-stage inverter in which the inverter stage is responsible for the MPPT and the grid-current control. In addition, the DC-DC converter operates with constant duty cycle and frequency. Such configuration requires a new concept, introduced as Behavior Matching. It serves as a fundamental feature for the DC-DC converter to reproduce the PV array I-V characteristic when they are connected, without control action. The maximum power operating point (MPOP) is found by maximizing the direct axis current, obtained by Park's transformation from the inverter, through the perturbation and observation algorithm (P&O). Any specific measurement to realize MPPT is not needed. The galvanic isolation is achieved by using a high-frequency transformer. The structure is appropriate for high power applications, above 10kW.