Eighteen major anti-accident measures taken by State Grid Corporation of China make the corresponding requirements on relay operating power and action voltage from the point of view of design. But if direct current(DC)grounding fault or alternating current(AC)intruding happens

when distributed capacitance of control cables exceeds the critical value

the export relay may still malfunction. The on-line monitoring system of distributed capacitance of control cables is built to realize early warning of the risk: firstly

the low-frequency alternating current signal is injected into the DC system; then

the AC current value is measured and the distributed capacitance is calculated; finally

the measured value is compared with the critical value. In order to measure the distributed capacitance accurately and steadily

a small AC and DC measurement current transformer is designed using the magnetic saturation principle and flux induction principle. Based on constant resistance mode of DC electronic load

a voltage equalization module is designed to balance DC bus voltage.