Current controller for low harmonic distortion and low switching frequency

In this work a new digital current controller for three phases loads is proposed. It uses a modulation between active and null VSI (voltage source inverter) states in order to minimize both switching frequency and harmonic distortion. The controller has been developed using polar coordinates instead of alpha-beta coordinates in order to obtain a circular area, defined by the hysteresis border, for the current error trajectories. The active vectors are applied to achieve the strongest opposition to the back-EMF. The total back-EMF trend is always determined under a null vector application. During an active state, a feed-forward calculation of the current error trajectory under null vectors is continuously calculated. As this calculation considers the back-EMF trend, it is possible to find the longest trajectory of the current error when the new null vector will be applied. Then, when the longest trajectory is found the system will switch from the active to the null state without waiting for the hit on the hysteresis border. This controller not only minimizes the total harmonic distortion but also the switching frequency. A qualitative analysis has been carried out, showing the robustness of the controller in front of both exogenous and endogenous perturbations. Simulations confirm the behaviour expected from the proposed current controller.