Measurement of residual flux density in the single-phase transformer by applying DC voltage

The inrush current causes many adverse effects on electrical equipment. One of the reasons for inrush current is residual flux. When a no-load transformer is energized and the closing angle is inappropriate, the residual flux accelerates the core saturation, resulting in high-amplitude inrush current. It is necessary to measure the amplitude and direction of residual flux to reduce its harm. This paper proposes a novel strategy for measuring the residual flux of transformers based on externally applied DC voltages. Firstly, a relationship between residual flux and response current is established based on field-circuit coupling analysis. Then, the core-type transformer and shell-type transformer are modeled by the finite element method, and the fitted model for calculating residual flux is obtained. Finally, experiments are carried out on a square core and a 10 kV, 250 kVA single-phase transformer. The feasibility and accuracy of the proposed strategy are verified by the measurement results.