Abstract-Due to multilevel inverters can have more levels of output voltage at the output waveform, closer to sine waves, and with small output waveform THD, and low voltage stress, low system advantages of EMI, multilevel inverters are widely recognized, and get a certain application. Based on the analysis of the characteristics of the basic multi-level inverter topology and the scope of application, this paper explores the idea and method of multi-level inverter topology using multi-terminal switch network. By comparing the new topology and basic topology, it can be seen that this new multi-level inverter topology is easy to expand, requiring small filter inductance volume and inductance, loss and low cost, which is an important direction for the development of topological circuits. The new multi-level topology has broadened the research scope of multi-level inverter topology and created new ideas and directions for multi-level topology research, which provides theoretical guidance for further scientific research and engineering applications.

Abstract-Facing the unbalance drop of three-phase grid voltage, jumping frequency and other complex working conditions, the accuracy and dynamic response of phase-locked loop (PLL) have become the important performance indicators of the photovoltaic grid-connected inverter. Aiming at the problem that the low accuracy of SOGI unit in the traditional double-order generalized integral phase-locked loop (DSOGI-PLL), when the frequency variation of the grid is happened. Thus, the paper proposes a compensation and improved algorithm that the stability of output signal is ensured by the stationary input frequency and the extra compensation. Finally, simulation and experiment show that the proposed PLL system can filter out the harmonic part of the grid voltage, but also, lock the phase fast and accurately. And it ensures that the grid-connect inverter goes through the fault. So that the PLL has strong adaptability to the grid.Keywords-phase-locked loop; second order generalized integrator; low voltage ride through