Comparison of motor neutral point overvoltage oscillations in SiC-based adjustable speed drives using two-level and three-level inverters

In SiC-based adjustable speed drives, power cables have similar behaviour to transmission lines due to the steep voltage pulses (high dv/dt) which experience back and forth voltage reflections while travelling across the cables, known as the reflected wave phenomenon, resulting in serious motor overvoltage oscillations. Besides the widely known overvoltage at the motor terminals, the motor neutral point experiences overvoltage oscillations that can be more detrimental than the terminal overvoltage. This paper investigates the motor neutral point overvoltage oscillations due to the reflected wave phenomenon in SiC-based adjustable speed drives using the two-level (2L) and three-level (3L) power inverters under the conventional PWM scheme. In fact, the overvoltage at the motor neutral point arises from the propagation of the inverter common mode (CM) voltage pulses through the motor winding itself, which emulates the same effect of long cables. The analysis and experimental results show that the motor neutral point overvoltage oscillations are characterized by the first anti-resonance point of the motor drive system, while the amplitude of the neutral point overvoltage is affected by the inverter CM voltage waveform. The results show that the motor neutral point experiences a higher overvoltage when the inverter is operated at high switching frequency and low modulation index. Moreover, when the switching frequency coincides with the first anti-resonance frequency of the motor drive system, the motor neutral point experiences the highest overvoltage. Specifically, the maximum motor neutral point voltage is 3 times the dc-link voltage for the 2L inverter. In contrast, the maximum motor neutral point voltage is 1.5 times the dc-link voltage when the 3L inverter is used.