A tuning procedure for the current regulator loops in multiple three-phase permanent magnet machines with low switching to fundamental frequency ratio

With an increase in the wind turbine ratings, power converter ratings increase as well, thus raising the interest in medium voltage power converters. Switching frequency in said converters has to be kept below 1 kHz; hence the sampling frequency, which normally nowadays equals twice the switching frequency, is low as well. Sampling at low rates introduces additional non-idealities in the control of the converters that do not appear in low voltage applications with high switching (and sampling) frequencies. To overcome this drawback, a decoupling procedure for the machine's model and a detailed model of the current loops are developed first. This is followed by presentation of a tuning algorithm for the current regulators, which can select the controller parameters that give the desired response in terms of overshoot, settling time, minimisation of the squared error, etc. Theoretical developments are verified by simulation and experiments.