A New Multilevel Converter Applied to Photovoltaic Grid-connected system

In the realization of high voltage and large power, the conventional photovoltaic grid-connected converter using PWM modulation usually needs to consider the dynamic voltage equalization of the power switches in series. At the same time, it is necessary to consider the issue of achieving low switching frequency and harmonic suppression. In view of the above problems, this paper uses voltage reinjection conversion technology to design a DC voltage conversion circuit on the DC side of a conventional series 12-pulse voltage source converter, and constructs a new multilevel photovoltaic grid-connected converter. The new converter can effectively suppress harmonics so that the ac voltage into the grid can obtain an approximate sinusoidal waveform without filtering. In addition, it can ensure that the main bridge power switches achieve zero voltage switching under 50Hz switching frequency and solve the dynamic voltage equalization problem of the power switches in series, which is easy to realize high voltage and large power. Based on the analysis of the working principle of the converter, the theoretical waveforms of the ac and dc sides of the system are given. A unified coordinated control strategy for maximum power tracking, unit power factor grid-connected power generation and reactive power compensation is designed based on the mathematical model of the system. The simulation model of photovoltaic grid-connected system based on the proposed converter is constructed in PSCAD. Finally, the simulation verifies the feasibility of the converter and the effectiveness of the control strategy.