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Current harmonics, which are injected in the utility by nonlinear loads, cause major problems that tend to deteriorate the power quality at the mains. To reduce such harmonics, active power filters (APF) are commonly employed. Challenges in improving the performance of such systems by designing sophisticated control algorithms occupy actually a major field of interest for power electronics researchers. In this paper, a new frequency-domain small- signal model of a shunt three-phase four-wire APF is presented for linear control purpose. The model calculation is based on a state-space averaged model of the filter, computed in the (d,q,0) synchronous frame. The obtained transfer functions offer new simple, systematic and straightforward alternatives for the design of linear control schemes that would ensure suitable performance for the filters.