Comparison of Three Space Vector PWM Methods for a Three-Level Inverter with a Permanent Magnet Machine Load
Much work exists on multilevel space vector pulse width modulation (PWM) to drive induction machines, in which the rotor currents are induced by stator rotating field effects. However, there are few investigations that analyze these modulation methods applied to permanent magnet (PM) and wound-field synchronous machines, in which the rotor induces a back emf in the stator. In this thesis, three different three-level space vector PWM switching sequences are applied to a three-level neutral-point-clamped (NP) inverter driving an internal permanent magnet (IPM) machine load. The inherent qualities of each of the switching sequences when under the influence of a forcing function (the back emf) created by the permanent magnets of the machine are investigated. In particular, output voltage quality, output current quality, and dc bus neutral point balance are analyzed and compared. Two machine operating conditions are considered: rated speed, rated load and half speed, rated load. By considering these two different operating speeds, the three switching sequences may be analyzed under both two-level operation and three-level operation of the inverter. A circuit model based on the machine state space model in the abc current frame of reference is used to model the IPM machine load. First, a short introduction to two-level inverters and a theoretical development of two-level space vector PWM are presented to introduce these basic principles. Then, an overview of the three main multilevel inverter topologies including their associated advantages and disadvantages is presented. A theoretical development of three-level space vector PWM is built upon the concepts introduced in the two-level case, and the three switching sequences under investigation are explained. The system model, including the IPM machine load, the three-level NPC inverter, and the space vector PWM algorithms, is implemented using MATLAB Simulink. All simulation results are analyzed based on output voltage and current distortion and neutral point imbalance, and a comparison between the three switching sequences is presented.