Multilevel NPC inverter for low-voltage applications

Multilevel inverter topologies are an alternative for a wide range of low voltage electrical drives. In the low voltage range (<;1000 V) highly optimised switches with lower conduction and switching losses are available. For 550 V DC link voltage the power semiconductor losses of a two-level VSI with 1200 V-IGBT and a neutral-point-clamped three-level VSI in the power range between 7 to 35 kW are compared. The comparison is carried out by calculation with a linear approximation for conduction and switching losses. The calculated results are proved by simulation with the real loss curves from the manufacturers. The simulated losses are higher than the calculated especially for operation points below the rated power. Therefore, the calculation of losses which is commonly used by the power semiconductor manufacturers is not sufficient for variable loads. At 20 kHz switching frequency and an electrical power of 35 kW the semiconductor losses of the three-level inverter are 38% lower compared to the two-level inverter. At 7 kW electrical power the reduction of losses is even 66%. The further increase of the inverter levels (four- and five-level) only leads to a small improvement of the motor current quality. Moreover, suitable power semiconductors (IGBT or MOSFET) with 300 V blocking voltage are not available for five-level inverters. Therefore, four- and five level inverters cannot be included in the losses calculation within this paper. Perhaps in the near future the five-level inverter leads to a further improvement in the inverter efficiency.