Optimized Space Vector Modulation for DC-Link Balancing in Three-Level Neutral-Point-Clamped Inverters for Electric Drives

Multilevel inverters can replace conventional two level inverters in many applications, as they offer less harmonic distortion on the output voltage and reduce losses in certain cases. Especially in electric vehicle traction applications, multilevel inverters can improve the system efficiency significantly and increase the cruising range. A popular topology is the three level neutral-point-clamped (NPC) inverter. However, in contrast to conventional two-level converters, a low frequency ripple on the midpoint of the dc-link capacitors can occur. In this paper, an optimized space vector modulation (SVM) scheme for dc-link balancing based on a combination of nearest three vector SVM and radial state SVM is introduced. The presented modulation scheme ensures a low voltage ripple on the midpoint of the dc-link over a wide operating range. Additionally, static voltage deviations can be eliminated to obtain a fully balanced dc-link. Simulation results of the proposed modulation scheme are presented for the entire operating range of a 160 kW induction machine. The performance is evaluated by a comparison with other modulation schemes.