A single-stage multi-string quasi-resonant inverter for grid-tied photovoltaic systems

A grid-tied multi-string PV inverter with a high-frequency ac (HFAC) link and soft switching operation is introduced. This single-stage topology can handle an arbitrary number of PV strings with different electrical parameters, locations, and orientations. Using a dedicated maximum power point tracker for each PV string, this inverter can harvest the highest possible power from each string independently, even though they might be at dissimilar irradiance levels and operating temperatures. The HFAC link is formed by an ac inductor and small ac capacitor without the use of bulky short-life electrolytic capacitors. The link is responsible for transferring the PV strings' power to the grid along with creating zero-voltage switching for the power devices. Therefore, the converter is expected to have high power density, high reliability, as well as high efficiency. If galvanic isolation is desired, a small-sized HF transformer can be easily employed in the HF link. After describing the control scheme and operating algorithm of the proposed multi-string inverter, a detailed analysis is carried out. Experimental results are also given to prove the effectiveness of the proposed PV topology.