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A new algorithm for three-phase active power filters is proposed, which is expanded current synchronous detection (ECSD) theory. It can detect the active or the fundamental reactive currents in each phase symmetrically and equally, based on the decomposition of the fundamental reactive component and the harmonics under unbalanced power condition. Nonlinear load is composed of a 2-hp three-phase squirrel-cage-type induction motor and motor drives (inverter). To prove the validity of the proposed ECSD algorithm, some experiments were performed in steady states and transient states under 15% unbalanced power system. A stand-alone-type TMS320C31 digital signal processor (60 MHz) board is employed to calculate and to decompose the power and the current components of nonlinear load. The experimental results show that the active and the fundamental reactive components detected by the proposed theory were balanced and equal in each phase despite an unbalanced power source.