Insulation failure significantly contributes to the unpredictable shutdown of power equipment. Compared to the partial discharge and high-frequency (HF) injection methods, the HF common-mode (CM) leakage current method offers a non-intrusive and highly sensitive alternative. However, the detection of HF CM currents is susceptible to interference from differential-mode (DM) currents, which exhibit high-amplitude and multifrequency components during normal operation. To address this challenge, this paper proposes a double-ring current sensor based on the principle of magnetic shielding for inverter-fed machine winding insulation monitoring. The inner ring harnesses the magnetic aggregation effect to isolate the DM current magnetic field, whereas the outer ring serves as the magnetic core of the Rogowski current sensor, enabling HF CM current monitoring. First, the magnetic field distributions of the CM and DM currents were analyzed. Then, a correlation between the sensor parameters and signal-to-noise ratio of the target HF CM current was established. Finally, an experimental study was conducted on a 3-kW PMSM for verification. The results indicate that the proposed double-ring HF CM sensor can effectively mitigate DM current interference. Compared to a single-ring sensor, a reduction of approximately 40% in the DM component was achieved, which significantly enhanced the precision of online insulation monitoring.
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