Effects of imbalances and nonlinear loads on the voltage balance of a neutral-point-clamped inverter

The effects of linear imbalances and nonlinear loads on the voltage balance of the neutral-point-clamped converter are described in this paper. The study reveals that a negative sequence of output currents (linear imbalance) may produce additional low-frequency oscillations to the neutral-point voltage. Similar consequences are produced by odd-order current harmonics from a nonlinear load, while even-order harmonics can cause the neutral-point voltage to shift. Furthermore, the second, fourth, and eighth output current harmonics might produce instability to the neutral-point voltage. The second and fourth harmonics are the worst components. The maximum amplitudes of these harmonics superposed to the current fundamentals that the system can tolerate are described. Simulated and experimental examples are presented.

I. Introduction

Since its introduction in 1981 [1], the neutral-point-clamped (NPC) voltage-source inverter (Fig. 1) has demonstrated some advantages over the conventional two-level inverter for high-power applications. To maximize the performance of this converter, the voltages of the two series-connected dc-link capacitors must be confined to one half the level of the dc-link voltage. Fig. 1.

NPC converter.

Fig. 2.

Functional diagram of the NPC converter.

Several publications discuss ways to solve this balance problem [2]–[5]; however, this objective may be unattainable when operating with high modulation indices [6]. In such conditions, a low-frequency ripple appears in the neutral-point (NP) potential. As a result of this oscillation, the output line-to-line voltages will also contain low-frequency harmonics, and additionally, the devices of the bridge and the capacitors themselves must withstand higher voltages than when balance is achieved.