A Sensor for the Ultraweak DC Current Covered by Strong AC Currents

In the field of insulation monitoring of high-voltage power equipment, the dc leakage current is an important indicator to evaluate the insulation state. However,
$\mu \text{A}$
-level dc leakage currents are usually mixed with mA-level power-frequency ac currents, bringing obstacles to the accuracy of the dc measurement. In this regard, a new magnetic modulator with a suppressing structure is proposed. Based on the fact that Lenz law only keeps valid for a changing magnetic flux, the ac suppressing structure is developed to enclose the original magnetic modulator. Then, the factors of the suppression structure’s performance, including weakening the ac magnetic flux and reducing the dc measurement sensitivity, have been analyzed. And the performance is found to be closely related to the magnet dimensions, the magnetic permeability, and the suppressing winding turns. Finally, the improved prototype current sensor for the converter transformer valve side bushing has been developed, and experiments are conducted to verify its dc accuracy and ac suppressing performance.