A Novel Leakage-Current-Based Online Insulation Monitoring Strategy for Converter Transformers Using Common-Mode and Differential-Mode Harmonics in VSC System

摘要

Online insulation monitoring techniques for power transformers have been widely studied. However, challenges rise when they are applied to the converter-connected transformers which are operated under high electrical stress conditions. This article proposes a nonintrusive leakage-current-based insulation monitoring strategy for converter transformers, using inherent characteristic harmonics in voltage source converter system. Analytic relation between common-mode and differential-mode harmonics and insulation capacitances is first presented. Then, a combined strategy is proposed based on the differential measurement of leakage current, which enables online prognosis of not only overall ageing degree but also ageing localization of the phase-to-ground insulation. Experiments are conducted on a 15-kVA transformer driven by an industrial inverter. The proposed combined strategy shows robust performance over effective characteristic frequency spectrums. Several considerations for field implementation are further discussed. The major features of the proposed strategy are as follows: 1) easy implementation and nonintrusive measurement; 2) little impact on system operation and immunity to system interferences; and 3) lower requirements of data processing.