Multilevel Converter

Since the development of the neutral-point clamped three-level converter (Nabae et al., 1981), several alternative multilevel converter topologies have been reported in the literature. The general idea behind the multilevel converter technology is to create a sinusoidal voltage from several levels of voltages, typically obtained from capacitor voltage sources. The different proposed multilevel converter topologies can be classified in the following five categories (Lai & Peng, 1996); (Lai & Peng, 1995); (Manjrekar & Venkataramanan, 1996); (Marchesoni & Mazzucchelli, 1993); (Suh et al., 1998):

• Multilevel configurations with diode clamps (Nam et al., 1991); (Sun-Kyoung Lim et al., 1999); (Nabae et al., 1981); (Lixiang & Fahai, 1999).

• Multilevel configurations with bi-directional switch interconnection (Brumsickle et al., 1998); (Nabae et al., 1981).

• Multilevel configurations with flying capacitors (Xiaoming et al., 1999).

• Multilevel configurations with multiple three-phase inverters (Cengelci et al., 1998).

• Multilevel configurations with cascaded single phase H-bridge inverters (Peng et al., 1997).

A common feature of the five different multilevel converter concepts is, that in theory, all the topologies may be constructed to have an arbitrary number of voltage levels, although in practice some topologies are easier to realize than others. The principle of the five topologies is illustrated in Figure 45.

Figure 45. Multilevel topologies. a) One inverter leg of a three-level diode clamped multilevel converter. b) One inverter leg of a three-level multilevel converter with bidirectional switch interconnection. c) One inverter leg of a three level flying capacitor multilevel converter. d) Schematic presentation of a three-level converter consisting of three three-phase inverters. e) One inverter leg of a three-level converter consisting of H-bridge inverters.

Below is a brief comment on the different multi-level converter topologies:

a) Despite the more complex structure, the diode clamped multilevel converter is very similar to the well known back-to-back PWM-VSI. Unlike the multilevel topologies shown in Figure 45d and Figure 45e, the diode clamped multilevel converter may be coupled directly to the grid without transformers. For a converter based on the diode clamped multilevel converter a voltage-balancing problem occurs for levels higher than three, but for a three level converter this is only a minor problem.

b) For a three level structure, the topology of the multilevel converter with bidirectional switch interconnection requires the same number of switches as the diode clamped three-level converter and the three-level flying capacitor converter. However, half of the switches have to block the full DC-link voltage.

c) The topology of the flying capacitor multilevel converter is very similar to that of the diode clamped multilevel converter shown in Figure 45a. In the literature it is stated, that the voltage-balancing problem is relatively easy to solve, compared to the diode clamped converter. The difference in component count between the diode clamped multilevel converter and the flying capacitor multilevel converter is, that two diodes per phase may be substituted by one capacitor.

d) The number of switches (and other components) required realizing a three level converter is very high compared to the concepts a-c, but the converter can be realized from the well-proven VSI technology.

e) The multilevel converter based on multiple H-bridge inverters is heavy, bulky and complex (Nam et al., 1991), and maybe of most importance, -connecting separated DC-sources between two converters in a back-to-back fashion is very difficult because a short circuit will occur when two back-to-back converters are not switching synchronously (Lai & Peng, 1996), (Suh et al., 1998).

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Renewable Energy 101

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