Optimization and Comparative Evaluation of Multiloop Control Schemes for Controllable AC Sources With Two-Stage $LC$ Output Filters

This study investigates the control system design and its small-signal properties for the output stage of a high-bandwidth four-quadrant three-phase switch-mode controllable AC voltage source (CVS) with an output power of 10 kW, a switching frequency of 48 kHz, and a two-stage LC output filter. Each output phase of the CVS is operated individually, i.e., the phase voltages are generated with reference to the DC input-voltage midpoint, to allow maximum flexibility in the generation of the output-voltage waveforms to supply a wide range of different load types, such as DC, single-phase, and general three-phase loads including constant-power loads leading to negative small-signal load-resistance values. Three suitable multiloop control structures with inner-currentand outer-voltage-control loops are motivated, modeled, and are optimized with respect to different control performance indicators, e.g., small-signal control bandwidth, and for common boundary conditions, e.g., maximum overshoot of the output voltage in case of a reference voltage step. All structures employ conventional P and PI controllers, due to their simplicity and widespread use. Among the three structures, the capacitor-current feedback-control structure, which controls the two filter capacitor currents and the output voltage, is identified to be most competitive. The small-signal bandwidth determined for this structure is between 7.1 kHz and 15.5 kHz, depending on the value of the load resistance. This result, in combination with an excellent matching of calculated and measured step responses of the output voltage of a 10 kW hardware prototype, point out the effectiveness of the selected control structure and the usability of control structures that are composed of conventional P and PI controllers.