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As the core component of a converter transformer, the valve-side bushing is used to provide insulation and mechanical support for high-voltage current-carrying conductors passing through the transformer's metal casing at different potentials. During long-term operation, the valve-side bushing is subject to various transient over voltages and complex higher-order harmonic voltages, and the insulation condition faces serious challenges, requiring monitoring of its insulation status. DC leakage current is an important indicator reflecting the insulation status, while current leakage current measurement methods exist for measuring accuracy up to the μA level, making it more difficult to achieve accurate measurement of mixed signals at the mA-level AC μA-level DC. When a fault occurs in the system, the leakage current undergoes a large sudden change, and the existing sensors are unable to determine whether the sudden change comes from the deterioration of the casing itself or from a system fault. In this paper, based on the study of the leakage characteristics of the valve-side casing of HVDC transmission systems, and based on the harmonic characteristics of leakage in normal and fault conditions, a new method of abnormal data rejection is proposed to design a wide-frequency characteristic electromagnetic current sensor with μA-level high resolution and an external AC wide-frequency sampling structure to collect the AC voltage in the presence of weak DC signals in the presence of large AC characteristics. At the same time, the current criterion is set to reject the abnormal data to ensure the accuracy of the sensor measurement, and thus the accuracy of the leakage current measurement.
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