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The emission of voltage and current harmonics from power electronic devices causes extra stress of equipment in power systems. Therefore, the need of harmonic distortion measurements is increasing. Harmonic measurements in high or medium voltage power systems with magnetic voltage transformers or capacitive voltage transformers may cause large errors. A new measuring system based on a coupling capacitor is presented in this paper. However, the transfer ratio of the new measuring system is susceptible to the environment, due to the small capacity of the coupling capacitor. To improve the measurement accuracy, a parameter estimation algorithm is adopted to calibrate the transfer ratio. The laboratory test proved this new method can achieve a precise result, even for voltage harmonics with high frequencies and low amplitudes.