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This paper presents an online grid impedance estimation method for single-phase grid-connected systems, such as photovoltaic systems, small wind turbines, fuel-cells power systems. The method is based on producing a small perturbation on the output of the power converter that is in the form of periodical variations of active and reactive power (PQ variations). The main idea is to make the power converter working in two operation points in order to solve the equation of the equivalent grid impedance. During the perturbation, measurements of voltage and current are performed and signal processing algorithms are used in order to estimate the value of the grid impedance. The online grid impedance estimation method can be used for compliance with the anti-islanding standard requirements (IEEE1574, IEEE929 and VDE0126) and for adaptive control of the grid-connected converters. The proposed method is embedded in the existing power converter control. The selected results validate the effectiveness of the proposed method.