A high performance three-phase active power filter is proposed in this paper. A novel circuit for detecting the amplitude of the active part of the fundamental load current is also proposed. The detection time of the current is not more than 1/4 cycle. Its transient response is superior to the other conventional techniques. Moreover, the proposed algorithm avoids the use of mains voltage signal in the calculation of reference compensation current. Therefore, the mains current after compensation still has a purely sinusoidal waveform even when the mains voltage is distorted. In addition, a simple control scheme, based on the energy balance concept, is proposed to control the voltage of an energy-storage capacitor. Because the energy change in the energy-storage capacitor can be fast compensated, a small energy-storage capacitor is required. The advantages of the proposed active power filter are low cost, small size and fast transient response. Finally, some simulation results are presented for verification.