Reduction of Spurious Switching in Direct Vector Quantized Space Vector Pulse Density Modulation Scheme for Multilevel Inverters

A generalized online switching scheme for multilevel inverter based on direct vector quantized space vector pulse density modulation (DVQSVPDM) suitable for adjustable speed drive is proposed in this article. The existing DVQSVPDM schemes have spurious switching as the voronoi region is determined from integrated error vector of sigma delta modulator (SDM). This results in high total harmonic distortion (THD) of common mode voltage (CMV) at lower modulation index. In the proposed DVQSVPDM, the nearest three space vectors to the reference vector forms the triangular voronoi region. The integrated error vector of SDM is quantized to its nearest space vector in the voronoi region to generate the switching vectors. This reduces spurious switching associated with DVQSVPDM schemes and improves the performance at lower modulation index by reducing THD of CMV. The proposed scheme works for all n-level inverter topologies and the computations required remain the same irrespective of the level of inverter. Redundant states of switching vectors are not considered for switching which results in the discontinuous nature of the proposed scheme. The performance of the proposed DVQSVPDM is compared with space vector-based modulation schemes and the third harmonic injected sine PWM scheme and the results are presented. The proposed scheme is experimentally validated for a 3 hp open-end winding induction motor fed with three-level inverters on both sides.