Complex State Variables as Analytical Tool for Control System Design of Medium-Voltage Drives

Complex state variables are used to analyze the dynamics of medium-voltage drives. These operate at low switching frequency to reduce the dynamic losses of the power semiconductor devices. It results in a low sampling rate of the digital signal processing system, causing signal delays that deteriorate the dynamic behavior. Undesired cross-coupling results between the current components id and iq. The effect is shown to be even more adverse than described by the conventional control theory. The solution is modeling the system by single-complex state variables. These permit designing a current controller of high dynamic performance, which exhibits zero cross-coupling. Improved performance at very low switching frequency is demonstrated by experimental results.