Optimization and Comparative Evaluation of Multiloop Control Schemes for Controllable AC Sources With Two-Stage <inline-formula> <tex-math notation="LaTeX">$LC$</tex-math> </inline-formula> Output Filters

Abstract

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 $10\;\mathrm{kW}$ , a switching frequency of $48\;\mathrm{kHz}$ , 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-current- and 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 $7.1\;\mathrm{kHz}$ and $15.5\;\mathrm{kHz}$ , 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 $10\;\mathrm{kW}$ 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.