A novel type of utility-interactive inverter for photovoltaic system

A novel utility-interactive inverter for photovoltaic system is proposed. This inverter has no transformer to reduce the size but has a unique power processing system consisting of a dc-dc converter and a single-phase full-bridge inverter. The dc-dc converter is a two-quadrant type and performs boost/buck-mode operations to feed-forward/feed-back the instantaneous power to/from the utility. The feed-backing function is rarely utilized for such as controlling the power-factor in utility side to regulate the utility voltage. Although both the dc-dc converter and the inverter operate with high-frequency switching at all the time in the conventional scheme, only the dc-dc converter or the inverter operate with high-frequency switching in the proposed one. Thus, the switching losses are reduced significantly in the new system. Additionally, the dc-dc converter controls the input current so that the dc-inductor traps the ripple-power fed back from the utility. As a result, a large capacitor is not necessary to connect in the input of inverter in this new system. In the products, the efficiency marked very high value (i.e., 95.5 [%] and 95.1 [%] in the half load and the rated load condition of 4.5 kW, respectively) where the utility voltage is 240 [V_RMS] and the solar-cells voltage is 200 [V]. On the other hand, total harmonic distortion of the utility current of the product marked very low value (i.e., 1.6 [%]) in the rated condition. The size of the power unit including the heat-sink is reduced to 2/3 of the conventional one.