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The quality of the power is impaired by several reasons. This degradation of the quality of the power is seen e.g. as supply interruptions, transient overvoltages, voltage dips, flicker, harmonics and voltage unbalance. The quality of the power may be improved by using power conditioning equipment, which may be either passive or active. This paper discusses one of the most modern power conditioning devices, i.e. the unified power quality conditioner (UPQC), which consists of the parallel active power filter (PAPF) and the series active power filter (SAPF). The UPQC is a power conditioning device able to compensate all kinds of power quality faults. In this paper the UPQC is used to filter both current and voltage harmonics and to compensate the voltage dip. The paper deals with a way to improve the voltage compensation performance of the SAPF at fundamental frequency in steady-state operation. This improvement is achieved by improving the conventional SAPF control system. The control system of the SAPF is improved by using closed-loop control instead of open-loop control. Finally, the results of the experimental tests verifying the functioning of the proposed control system are presented.