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This paper presents a novel modulation strategy for a seven-phase three-level inverter, aimed at variable-speed drive applications. Developed space vector pulsewidth modulation (SVPWM) strategy is based on vector space decomposition (VSD) approach and an analysis of multidimensional space vector projections in mutually orthogonal planes. The algorithm represents an extension of a recently developed five-phase three-level SVPWM algorithm and is considerably more complex, due to the existence of three mutually orthogonal planes in a seven-phase system. The difficulties of generalization of space vector strategies, which stem from the nature of the multiphase systems, are highlighted. Increasing the number of phases from five to seven increases the number of switching states from 3 5 = 243 to 3 7 = 2187 and the number of orthogonal planes from two to three. This considerably affects the complexity of the offline calculations. However, the final implementation of the algorithm is simple. Developed SVPWM algorithm is compared to an equivalent carrier-based strategy and it is shown that they yield identical performance. The complexity of the algorithms for real-time implementation is compared. Simulation and experimental results, obtained using neutral-point clamped inverter, verify the theoretical developments.Index Terms-Carrier-based pulsewidth modulation (PWM), seven-phase, space vector pulsewidth modulation (SVPWM), three-level, vector space decomposition (VSD).

Abstract-Three-phase multilevel inverters are used in many medium-and high-power applications such as motor drives and grid-connected systems. Despite numerous PWM techniques for multilevel inverters have been developed, the impact of these modulation schemes on the peak-to-peak output current ripple amplitude has not been addressed yet. In this paper the analysis and the comparison of current ripple for two-and three-level voltage source inverters are given. Reference is made to optimal and popular modulation, so-called centered PWM, easily obtained by both carrier-based modulation (phase disposition, with proper common-mode voltage injection) and space vector modulation (nearest three vectors). It is shown that the peak-to-peak current ripple amplitude in three-level inverters can be determined on the basis of the ripple in two-level inverters, obtaining the same results as by directly analyzing the output voltage waveforms of the three-level inverters. This procedure can be readily extended to higher level numbers. The proposed analytical developments are verified by both numerical simulations and experimental tests.Index Terms-Multilevel inverters; three-level inverter; output current ripple; carrier-based PWM; space vector PWM.

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The version presented here may differ from the published version or from the version of the record. Please see the repository URL above for details on accessing the published version and note that access may require a subscription.