Single-phase double-buck three-level inverter design

1 Introduction

As a non-polluting energy source, solar energy research has been a hot topic of research. In order to improve the energy conversion efficiency of solar energy, the research of photovoltaic grid-connected inverter is the focus of photovoltaic utilization. For photovoltaic grid-connected inverters, the topology can be divided into:

High-frequency transformer type, power frequency transformer type and transformerless type. The high-frequency transformer is small in size, light in weight and high in efficiency, but the control is complicated; the power frequency transformer is bulky, heavy, and simple in structure; in order to improve the efficiency and reduce the cost of the photovoltaic grid-connected system, it can be used when there is no special requirement. Transformerless topology. However, since there is no transformer, the input and output are not electrically isolated, and the parasitic capacitance of the photovoltaic array formed by the series and parallel connection of the photovoltaic modules becomes large, and the capacitance is greatly affected by the external environment, and the resulting common mode current will be large. For the study of leakage current, there are various solutions: when the full-bridge inverter adopts the unipolar modulation mode, there is a common mode voltage of the switching frequency pulsation, and when the bipolar modulation mode is adopted, the common mode The voltage is constant, and its amplitude is equal to half of the bus voltage; in the half-bridge inverter, the parasitic capacitance voltage to the ground is also clamped to the half of the bus voltage by the input voltage divider, which remains basically unchanged. These are methods based on bridge circuits to solve leakage current. In recent years, a double-buck inverter structure has emerged. This type of inverter has the characteristics of no bridge arm straight through, body diode not working, and bipolar operation. Therefore, it is widely used. This paper proposes a new three-level dual-buck inverter scheme, and sets the corresponding control strategy to achieve maximum power point tracking and grid-connected control.

2. The overall solution of the three-level dual-buck inverter

As shown in Figure 1, the circuit topology of the dual-buck inverter, the dual-buck inverter uses a half-cycle mode. When the output current is in the positive half cycle, the power transistor S1, the freewheeling diode D1, and the filter inductor L1 and the filter capacitor Cf together form the Buck1 circuit. When the output current is negative half cycle, the power tube S2, the freewheeling diode D2, the filter inductor L2 and the filter capacitor Cf together form a Buck2 circuit, and the two Buck circuits do not work at the same time. Compared with the traditional bridge inverter circuit, the circuit has no bridge arm through, and the body diode does not need to participate in the working process. However, in this case, the voltages that the power tubes S1 and S2 are subjected to during the half cycle of operation are twice the DC bus voltage Ud. Since the voltage waveform output by the bridge arm itself is still bipolar, its harmonic content is still very large.