A Step Placement Method for Voltage Vectors to Minimize Total Harmonic Distortion in Overmodulation Region II

This paper proposes a static overmodulation method for a two-level inverter in overmodulation region II. The method minimizes the total harmonic distortion (THD) while maintaining voltage linearity. In the proposed method, the voltage vector is placed in steps whose angles are not set by a fixed rule. In overmodulation region II, the voltage vector magnitude cannot be increased further, so only the angle is adjustable. The reference angle therefore moves in several steps rather than remaining at a single position. As the modulation index increases, the step angles coincide with the vertex of the hexagon in turn. Coincident angles are not counted as distinct step angles. The number of distinct step angles decreases automatically as a result of THD minimization. In uniform placement, however, the number of steps has to be reduced according to a predefined rule. Simulations were carried out with various numbers of steps and switching frequencies. Voltage linearity was maintained, and the THD was lower than with uniform placement at most modulation indices.