A Multilevel Energy Buffer and Voltage Modulator for Grid-Interfaced Microinverters

Microinverters operating into the single-phase grid from solar photovoltaic (PV) panels or other low-voltage sources must buffer the twice-line-frequency variations between the energy sourced by the PV panel and that required for the grid. Moreover, in addition to operating over wide average power ranges, they inherently operate over a wide range of voltage conversion ratios as the line voltage traverses a cycle. These factors make the design of microinverters challenging. This paper presents a multilevel energy buffer and voltage modulator (MEB) that significantly reduces the range of voltage conversion ratios that the dc-ac converter portion of the microinverter must operate over by stepping its effective input voltage in pace with the line voltage. The MEB partially replaces the original bulk input capacitor, and functions as an active energy buffer to reduce the total size of the twice-line-frequency energy buffering capacitance. The small additional loss of the MEB can be compensated by the improved efficiency of the dc-ac converter stage, leading to a higher overall system efficiency. The MEB architecture can be implemented in a variety of manners, allowing different design tradeoffs to be made. A prototype microinverter incorporating an MEB, designed for 27 to 38 V dc input voltage, 230-V rms ac output voltage, and rated for a line cycle average power of 70W, has been built and tested in a grid-connected mode. It is shown that the MEB can successfully enhance the performance of a single-phase grid-interfaced microinverter by increasing its efficiency and reducing the total size of the twice-line-frequency energy buffering capacitance.