Improved Three-Level Third-Harmonic Current Injection Active Filter Circuit for the Three-Phase Diode Rectifier in Aircraft

Three-phase uncontrolled rectifiers have been widely adopted due to the characteristics of robustness and high efficiency, but have the disadvantage of harmonic pollution. Third-harmonic current injection (3rd-HCI) active filter circuit could remove the harmonics. However, new problems occur and lead to current distortions again when the ac line frequency increases, especially in the more electric aircraft applications where the ac line frequency is 360–800 Hz. In this article, the mechanism of current distortions at higher ac line frequency is discussed first. The inductor in the 3rd-HCI active filter circuit is volt-second imbalance. As the ac line frequency increases, the imbalance degree is more severe and then causes current distortions. Therefore, the inductance must be reduced, resulting in larger current ripple and more high-frequency components in the ac line current. Further, based on flying capacitor three-level topology, this article proposed an improved 3rd-HCI active filter circuit and the corresponding control strategy, which not only reduces the current ripple significantly but also mitigates the ac line current distortions. Besides, the input filter capacitive current compensation is also considered to improve the power factor at light load. Finally, all the analysis are verified by experimental results on a 115 Vrms/360–800 Hz/5 kW prototype.