Evaluation of the low-frequency neutral-point voltage oscillations in the three-level inverter

The nearest vectors to the reference vector are commonly used in space-vector modulation (SVM) strategies. The main advantages of these modulation strategies are the low switching frequencies of the devices, the good output voltage spectra, and the low electromagnetic interference. However, when these techniques are applied to the three-level neutral-point (NP)-clamped inverter, low-frequency oscillations appear in the NP voltage for some operating conditions. As a result, the value of the dc-link capacitors must be increased in order to attenuate such oscillations. In this paper, these amplitudes are quantified for two modulation strategies that use nearest vectors to the reference vector. Owing to the nondimensional variables used in the analysis, the information provided will help for the calculation of the dc-link capacitors in a given specific application. 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. In some applications, the neutral-point (NP) voltage is imposed by a solid neutral terminal at the dc side provided by two independent rectifiers [2]. This solution requires a transformer with two isolated secondary windings that increases the overall cost of the system. This special transformer can be avoided with a back-to-back connection of NPC converters [3], [4]; nevertheless, the cost of this connection is also high, because it requires an active NPC rectifier with the same topology than the inverter. This solution is especially interesting for regenerative motor drives applications. Fig. 1. NPC inverter.