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Voltage and 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. Where as shunt APFs are used for current-type load, series APFs are applied to voltage-type load and allow both voltage and current harmonic compensation. A new constant-frequency control scheme for a series APF is designed. The elaboration of the control law is based on a small-signal averaged model of the converter, computed in the (d,q) synchronous frame. The control scheme consists of two successive loops: the inner loop that ensure voltage harmonic compensation at the AC side is designed on the basis of the linear decoupling principle, whereas the outer loop that uses a simple PI controller ensures voltage regulation at the DC side of the filter. The control system is implemented numerically using Matlab/Simulink tool. The performance of the proposed control approach is finally discussed through the obtained simulation results.