A novel scheme of drawout coils which are utilized in the power supply of the transmissionline state parameters online monitoring system is proposed

aiming at the ubiquity of problems in existing literatures that the drawout coils are prone to deep saturation and high heat consumption

and thus the circuits are complex. These problems are resolved through introducing an airgap magnetic resistance to the drawout coils

as well as properly matching parameters such as the configuration of the coil

the material of the magnetic core

turn numbers and air gap length. Then a power supply equipment is developed based on the drawout coil aforementioned．Integral test results illustrate that the power supply provides stable outputs with no saturation and low heat consumption throughout the alternating range of conducting currents. Besides

this paper analyzes the influence of abnormal large currents through the conductor on reliability of the power supply. Detailed measures to enhance the reliability are presented thereon. Short circuit and impulse current tests show that the measures proposed are capable of restraining the demolishment from electrodynamic force and overvoltage caused by the abnormal large current.