Comparative Evaluation of Control Dynamics of 3-Φ AC–AC Current DC-Link and Voltage DC-Link Converters Based on DC–DC Equivalent Models

Variable speed drives (VSDs) operating from the mains can be realized either with a voltage dc-link (voltage-source converter, VSC) or, alternatively, with a current dc-link (current-source converter, CSC), which has received renewed interest recently due to the availability of monolithic bidirectional GaN power transistors (M-BDS). Considering equally rated ac-ac VSC and CSC systems with sinusoidal output voltages (additional LC filter for the VSC, inherently present filter capacitors for the CSC), this paper first derives equivalent dc-dc converter models that accurately capture the dynamic behavior of the ac-ac converters and thus advantageously allow for a straightforward design and analysis of the converter control systems. Detailed experimental results obtained with realized ac-ac VSC and CSC prototypes (1.4 kW, 200V line-to-line rms, 600V GaN monolithic bidirectional power transistors in the CSC) show very close matching of the open-loop and the closed-loop behavior with that predicted by the dc-dc equivalent circuits. Therefore, these circuits are then utilized to compare the small-signal and large-signal output voltage control performance of the VSC and the CSC, indicating certain advantages for the CSC regarding small-signal bandwidth (5 kHz vs. 1.8 kHz for 72 kHz switching frequency, identical for both systems) and, in case of the CSC, highlighting the trade-off between control dynamics (dc-link current kept at the nominal value) and efficiency (dc-link current adapted to the load). Finally, a case study considering the time until nominal current is reached in an exemplary motor at standstill finds mixed comparative large-signal performance of VSC and CSC; a CSC variant with the output capacitor selected for equal high-frequency ripple as the VSC is slower than the VSC (by around 45% for the considered converter’s specifications) whereas a CSC variant with the output capacitor selected for equal fundamental-frequency reactive power consumption as the VSC is faster (by around 40% for the considered converter’s specifications).