Hybrid Active Power Filters (HAPFs) can effectively suppress harmonic current in power-lines, the compensation precision of which rely heavily on the design of the current loop and dc voltage controllers. In this paper, the operation principle of a transformerless HAPF is introduced and analyzed, and then a double closed loop control strategy with power-line current feedback and the fifth harmonic current feedback is proposed aiming at enhancing filtering characteristics at the fifth harmonic frequency. Finally, a dc voltage regulator is designed based on the system model of dc link voltage control. Experimental results obtained from a 380 V, 20 kVA experimental prototype verify the validity and effectiveness of the proposed control strategies for the transformerless hybrid filter, which has high compensation precision as well as fast dynamic response and also reduces the cost of the equipment since only two phases of power-line current are required to be detected.
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