DC bias identification method for transformer differential protection

The DC bias will lead to maloperation of the second harmonic restraint when entering the power system, which causes the failure of transformer differential protection. Therefore, this paper explores the impact of DC bias on differential protection and proposes a DC bias detection algorithm based on harmonic components. Firstly, the theoretical mechanism by which DC bias causes transformer differential protection failure is analyzed, exploring the influence of different magnitudes of DC bias and different fault types on differential protection. Subsequently, under the conditions of DC bias or inrush current, this study simulated the root mean square (RMS) value and phase angle characteristics of the second harmonic of transformer differential current, and established a DC bias detection algorithm using two criteria: RMS value identification and phase angle identification. When the algorithm detects DC bias, it will block the second harmonic restraint to ensure the normal operation of differential protection during internal transformer faults. The proposed method is validated through simulations using typical protection failure scenarios, demonstrating its effectiveness in resolving the refusal issue of transformer differential protection under DC bias conditions.