Analysis and Design of Third-Harmonic Current Injection Active Filter Circuit for the Aircraft

Third-harmonic current injection (3rd-HCI) active filter circuit could be utilized to eliminate low-frequency harmonics generated by the three-phase uncontrolled rectifier. However, some problems exist when the circuit operates in the 115 Vrms/400 Hz aircraft power system. The inductor in the 3rd-HCI active filter circuit is not strictly volt-second balance in one switching period, resulting in injection current error and ac line current distortions. The problem is more severe in the 400 Hz power system since the imbalance degree increases with a higher ac frequency. Moreover, the common method to mitigate the ac line current distortions is using a smaller inductor, which will increase the inductor current ripple and high-frequency harmonic components. This article investigates the distortion mechanisms and reveals the relationship between current distortion and volt-second imbalance, and the maximum allowed inductance is determined under every operating point. Then, an improved circuit based on ripple current cancelation technique is proposed to realize quasi-zero ripple in the inductor current and to reduce the high-frequency harmonics in the ac line currents, which extends the operating range of the circuit significantly. Finally, the correctness of analysis and validity of proposed circuit are verified by experimental results based on a 115 Vrms/400 Hz/5 kW prototype.