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This paper presents a novel power supply of online condition monitoring systems for power transmission lines. The proposed power supply obtains energy from the magnetic field induced by transmission-line currents using a specially designed Rogowski coil. The design details of the power supply circuit including the unit for overvoltage and impulse line current protection are given in this paper. The impact to the power supply's reliability caused by abnormal high transmission-line currents is also addressed. Experimental tests, including the short-circuit and impulse-current tests, have been conducted on the power supply to verify its performance under different operating conditions. Test results show that the power supply is capable of providing stable outputs with no saturation and low heat generation for the entire range of line conducting currents and can sustain severe conditions such as abnormal impulse currents.