High Performance Control of AC Drives with Matlab / Simulink Models

3.6 Multi-level Inverters

The voltage source inverters discussed so far produces only two levels of output, either or 0 and V_dc, hence called the two-level inverter. Two-level inverters pose several problems in high power medium voltage applications, due to limited voltage blocking capability of the power semiconductor switches. Moreover, dv/dt is considerably higher, causing high stress on the switching devices. The solution to this problem lies in using a series connection of several power switching devices or alternatively using multi-level inverters [23–30]. The former solution may lead to unequal voltage sharing among the devices. Hence, the later solution is preferred. The output voltage waveform of the multi-level inverter is significantly improved when compared to a two-level output. The THD is low and the output quality is acceptable, even at low switching frequency. In high power drives, switching losses constitute a considerable portion of the total losses and hence the reduction of this is a major goal. Nevertheless, the number of power switching devices used in a multi-level inverter is high and additional diodes and capacitors are needed. Another important shortcoming of the multi-level inverter is imbalance in the capacitor voltages at the DC link. This issue is addressed in [31–34].

Multi-level inverters have many attractive features, such as high voltage capability, ...