Natural balancing of three-level neutral-point-clamped PWM inverters

This paper explores the natural balancing mechanisms of the three-level neutral-point-clamped (NPC) multilevel inverter. An equivalent circuit of the three-level NPC inverter is derived that facilitates an understanding of the balancing mechanisms. This is followed by a detailed analysis of the balancing mechanisms. The theory is applied to a three-level NPC inverter under vector control. It is shown that this inverter possesses natural balancing mechanisms in which the load impedance and spectra of the switching functions play an important role. Finally, it is illustrated how the natural balancing can be enhanced by using a passive balancing circuit which provides a low impedance at the sampling frequency.

I. Introduction

Multilevel inverters are mainly applied in high-voltage and high-power applications. They offer the advantage of effectively stacking the switching devices in series, thereby providing a higher dc-bus voltage than can be obtained by conventional techniques. At the same time, the harmonic content of the output voltage waveform is significantly reduced compared to conventional two-level inverters. This comes at the expense of a number of added components like clamping diodes [1] or a number of flying capacitors [2]. Furthermore, to generate the large number of gating signals requires a more complicated controller. Another inherent problem with these inverters is possible unbalance of the capacitor voltages which may result in overvoltage of one or more of the switches.