A Technique of Parallel Connections of Pulsewidth Modulated NPC Inverters by Using Current Sharing Reactors

Abstract

The neutral point clamped (NPC) PWM inverters has many 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. The output voltage waveforms of the proposed inverter have certain voltage levels, thus it is anticipated that it will be difficult to analyze the output harmonics. For such waveforms of output voltage and current sharing reactor, frequency analysis approach is described, whose results are verified by experiments.