Space-Vector Pulsewidth Modulation for Three-Level NPC Converter With the Neutral Point Voltage Control

This paper proposes a new space-vector pulsewidth modulation (SV-PWM) strategy for a three-level neutral point clamped inverter. The presented SV-PWM strategy makes it possible to control the neutral point voltage by optimum choice of switch sequence for any position and length of output voltage vector. The proposed solution takes into consideration the unbalance of the dc-link voltages. It also analyzes the influence of vector sequences on the predicted unbalance of the dc-link voltage. The solution allows selecting optimum vectors and their on-time durations in order to reduce quickly the dc-link voltage unbalance. The calculation of the space vector area proposed in this paper takes into consideration voltage unbalance and its influence on the length and position of vectors. The proposed approach assures properly generating the voltage output vectors, even in the case of the existing large voltage unbalance in the dc-link. The results of the experimental investigation of the proposed modulation strategy are presented in this paper.

I. Introduction

Multilevel inverter topologies are experiencing increased application in the industrial environment, particularly in high-power drive systems [1]–[4]. The main advantages of these inverters are improving quality of voltage waveforms and an increase in the dc-link voltage for a given blocking voltage capacity of the semiconductors. Generation of voltage vectors in multilevel inverters requires taking into consideration all of the phenomena which have an influence on the accuracy of the output vectors. While the neutral point clamped (NPC) multilevel inverter (Fig. 1) converts higher voltages using devices with lower rating, the dc-link voltage balancing problem seriously limits the applicability of these inverters if not solved. The dc-link voltage unbalance causes an increase of the voltage stress on switching devices and additional harmonic distortion in the inverter output voltage. To maximize the performance of the three-level inverter, the voltages of the series connected dc-link capacitors should be equal. Fig. 1.

Three-phase NPC inverter.