抄録
The neutral point clamped (NPC)-PWM inverter has significant advantages such as its ability to operate the motor with nearly sinusoidal current waveforms. For this reason, in larger capacity inverter systems, NPC-PWM inverters using the GTOs and the like have been also put into practical use, because of great advantages to large capacity ac motor drives, such as lower ripple currents and higher output voltages. With the spread of applications, still larger capacity inverters are also expected. However, since the capacities of such switching devices are insufficient, a certain technique and controlling method are proposed for the parallel connections of NPC inverters. In this paper, the conventional modulation techniques are extended to improve the inverter characteristics, in which the asymmetrical modulation and over-modulation techniques are introduced for the parallel connected NPC inverter.
In the proposed modulation strategies, the waveforms become more complicated, so it is difficult to analyze the output waveforms. For those waveforms, the analytical waveform analysis method using the switching function is proposed. By means of such method, a physical meaning of the harmonic generation can he realized to some extent.
