Effective Current Limitation for Multifrequency Current Control With Distortion-Free Voltage Saturation and Antiwindup

Abstract

In three-phase converters, it is frequently important to control current components other than the positive-sequence fundamental. Adequate voltage saturation and antiwindup (VSA) should also be included to prevent malfunction under output-voltage saturation due to, e.g., large voltage disturbances or dc-link-voltage reduction. A multifrequency VSA technique was recently proposed, based on modifying the current references so that the control output voltage is brought back to within the hexagonal boundary. Thus, it avoids the additional distortion that would arise if the control output surpassed (even partially) the hexagon. However, no current limitation was considered. If the current references demanded by the VSA are saturated, windup of the integrator contained in said VSA scheme may occur. Furthermore, current saturation also causes steady-state overmodulation; then, the actual current differs from that fed back to the control (due to the internal-model-control structure included in the VSA), and hence it is not effectively limited. This article adds current-saturation functionality to the multifrequency VSA by several relevant modifications. Windup of the integrator of the VSA when current saturation occurs is prevented. The actual current is effectively limited to the desired value also when there is steady-state overmodulation. Experimental results with a three-phase permanent-magnet machine are provided.