The invention relates to a high reliable
inrush current brake method for a
transformer, which comprises the steps of sampling, calculating the three-phase differential current fundamental wave content and the second
harmonic content of a current sampling point, setting and calculating a floating threshold, and carrying out differential starting judgment, the judgment of the change characteristics of the fundamental wave content and the judgment of the change characteristics of the ratio between second
harmonic content and the fundamental wave content by a
discriminant so as to determine reliable locking or immediate differential opening. On the basis of a traditional second
harmonic braking principle, the method sets the floating threshold, tracks and masters the change characteristics of the fundamental wave and the second harmonic in a real-time manner, analyzes and judges the operation state of the
transformer in time by utilizing the change characteristics of the fundamental wave and the second harmonic, solves the contradiction of the right and
fast motion between the
inrush current brake and
differential protection in the current method and ensures the reliable
brake of the
transformer under the condition of the only existence of the
inrush current and the right and
fast motion of the transformer under the situation of any fault in the
sample space. Meanwhile, the method reduces the value scope of the brake coefficient of the second harmonic and improves the sensitivity of inrush current brake.