With rapid development of the power electronic technology, the power electronic device has more wide application. Wherein, the inverter is used widely in power system, such as uninterruptible power supply (UPS), active power filter (APF), static var generator (SVG), and the like. In the art, various attempts and efforts have been made aiming at improving the respective specifications, such as decreasing the volume of the inverter, improving the efficiency of the inverter, reducing the cost of the inverter, diminishing the output harmonic of the inverter, the comparability of three-phase-three-wire and three-phase-four-wire, and the like. However, various circuit topologies have their respective limitations, and the respective problems which cannot be solved excellently.
For example, the prior inverter topology is shown mainly in FIG. 1˜FIG. 2. FIG. 1 shows a conventional three-phase-four-wire (3p4w) structure of two-level inverter, in which the neutral line (N line) is connected directly with the midpoint of a direct current bus. Because of the existence of the neutral line, the three phases are decoupled completely, and can be controlled individually. However, the neutral line current flows through the direct current bus, it is required to design a large direct current bus capacitor in order to suppress the ripple current. FIG. 2 shows a 3p4w structure of a traditional three-level inverter. Similarly, the neutral line (N line) is connected directly with the midpoint of the direct current bus. The harmonic component at the input side can be decreased in the three-level inverter, however, it is also required to design a large direct current bus capacitor.
In the prior art, IGBT of higher voltage is required to be selected for the two-level inverter, and the efficiency and the switching frequency will be small. In the two topologies as shown in FIG. 1 and FIG. 2, three phases are decoupled by the neutral line and has larger ripple by comparing with a three-phase-three-wire (3p3w) structure. At the same time, the 3p4w structure needs a large direct current bus capacitor in design of suppressing the ripple produced by the neutral line current.