Research on Residual Flux Density Measurement for Single-Phase Transformer Core Based on Energy Changes

When a power transformer is reconnected to the grid, if there is a large residual flux in the iron core, significant inrush current may occur and result in reclosing failure. Thus, accurate measurement of residual flux is necessary to avoid the generating of inrush current. This paper proposes a residual flux density measurement method for single-phase transformer core based on energy changes. After the transformer is de-energized, the core is connected to the external dc measurement circuit. First, when positive and negative DC voltages (referred as the direction along or opposite to the initial residual flux density) are applied, the positive energy of power source islarge than the negative energy and thus the direction of residual flux density can be determined. Then, the residual flux density value is calculated by the empirical formula between positive energy and residual flux density. This paper takes the square iron core as the research object, establishes the corresponding empirical formula by finite element method, and verifies the accuracy of proposed formula on the experimental platform. The experimental results show that the proposed method has higher accuracy.