On-line Method for High-sensitivity Leakage Current Measurement of Converter-connected Transformers in Microgrids

Leakage current is an effective indicator for groundwall (GW) insulation condition of power transformers. The existing sensing technology based on magnetic shielding design and differential measurement principle enables on-line leakage current measurement from strong magnetic noises caused by load currents. The technology has been verified in the application of the traditional power system. In this paper, an improved method is proposed for harmonic leakage current measurement, with the goal of on-line GW insulation monitoring of converter-connected transformers in microgrids. First, the simulation model of modular multilevel converter system is built in MATLAB/ Simulink. The amplitude-frequency characteristics of the load current and the leakage current of the converter-connected transformer are analyzed. Second, the sensing performance of the improved method is investigated in converter harmonic frequency band, by considering the effect of the frequency-dependent core permeability. Third, the methodology is clarified to select the appropriate monitoring frequency with the highest sensitivity. Finally, a sensor unit with the dual-core structure is manufactured and its performance is verified by off-line and on-line experiments. On-line leakage current measurement is enabled with 1-mA sensitivity and 100-kHz bandwidth from strong noises caused by load currents up to 400-A.