A Space Vector PWM Algorithm for a Three-Level Seven-Phase Voltage Source Inverter

The paper develops for the first time a space vector PWM technique for a seven-phase, three-level voltage source inverter (VSI), based on vector space decomposition (VSD). The algorithm represents an extension of a recently introduced VSD-based space vector algorithm for a three-level five-phase VSI. However, due to the increase of the phase number from five to seven, which simultaneously increases the number of planes that have to be considered from two to three and the number of switching states from 3(5) = 243 to 3(7) = 2187, the development of the space vector PWM is here much more involved. Using the ordering-per-sector law in time domain, the number of switching states that have to be considered is reduced to an acceptable number of 297. Choice of switching sequences and division of sectors into sub-sectors are further done in such a manner that space vector projection cancelation in the second and the third plane is achieved, where the references are zero. Each of the fourteen sectors in the first plane (with non-zero reference) is divided into 18 sub-sectors, meaning that each sector is characterised with 18 specific switching sequences. Developed space vector PWM method requires a lot of preliminary calculations for selection of sequences and for sub-division of sectors, but the algorithm is rather straightforward. Once when the appropriate constants have been established, development of the software for real-time implementation is simple. Validity and implementation simplicity of the proposed algorithm are proved at this stage by simulations.