Harmonic analysis of PWM multi-level converters

The method of modulation based on a triangular carrier signal and a sinusoidal modulating signal has been widely used in Graetz bridges. This method is applied to multilevel converters where, with the chain-circuit converter as an example, there are two carriers per chain link. Each carrier is responsible for one output voltage pulse per half power-frequency cycle. The fundamental and harmonic components of the converter output voltage are calculated by a three-dimensional modulation model and the double Fourier series, without the need for iterative numerical techniques. The harmonics results are compared with those of a single-phase two-level converter with a high-frequency carrier. The harmonic performance of the chain-circuit converter is generally better. The extension of the method to more than one output voltage pulse per half power-frequency cycle per chain link is also discussed.