Enhancing FDS Low-Frequency Micro-Current Measurement: A Lock-in Amplification Approach with High Precision

To address the challenges of weak current signals in low-frequency band (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1 \text{mHz}-1 ~\text{Hz}$</tex>) during Frequency Domain Spectroscopy (FDS) testing, where on-site measurements are susceptible to electromagnetic interference and effective data acquisition is difficult, this paper proposes a micro-current detection technology based on phase-sensitive detection (PSD), and demodulation algorithm. By integrating digital signal processing techniques such as narrowband filtering and timedomain sinusoidal fitting, the method achieves ultra-lowfrequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$n A$</tex>-level current measurements under complex background noise. First, the characteristics of low-frequency current signals of oil-paper insulated bushing are analyzed. Experimental validation on a 40 kV oil-paper insulated bushing sample demonstrates that the proposed approach enables rapid measurement of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$n A$</tex>-level currents in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1 \text{mHz}-1 ~\text{Hz}$</tex> range while maintaining a phase resolution on the order of 0.005°, with excellent reproducibility and stability in repeated measurements.