A DPWM-Controlled Three-Level T-Type Inverter for Photovoltaic Generation Considering Unbalanced Neutral-Point Voltage

The three-level T type inverter has been extensively used in photovoltaic (PV) generation. Various PV panels in cascaded connection normally require unequal dc voltage to accomplish separate maximum power point tracking (MPPT). This paper proposes a DPWM-controlled three-level T type inverter for two strings of PV panels in cascaded connection. The proposed modulation is developed based on the so-called circuit-level decoupling principle so that the inverter is able to generate balanced line voltage even in unequal dc voltage. The different dc voltages of PV strings are controlled by injection of zero-sequence voltage while total dc voltage is regulated to control the real power of the inverter. Thus both PV strings are able to operate at their respective MPPT while there is almost no low-order harmonic distortion at the inverter output. The proposed method was experimentally evaluated.