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A variable-structure observer for solar-array current estimation in a photovoltaic power-generation system is presented. The solar-array-current information is obtained from the variable-structure observer and fed into the maximum-power-point tracker to update the reference voltage. The parameter values such as inverter DC-link capacitances can be changed by up to 50% from their nominal values and the linear observer cannot estimate the correct state values under parameter variations and noisy environments. The configuration of the variable-structure observer is simple, but it shows robust tracking performance against parameter variations and modelling uncertainties. The method for constructing the variable-structure observer is presented and the convergence of the proposed observer is verified using the Lyapunov method. The mathematical modelling and the experimental results verify the validity of the proposed method.