Characteristic analysis of three phase transformer inrush and the discussion of second harmonic restrained method using phase angle and amplitude

The adaptive second harmonic restrained method used in T60 of GE transformer protection device was discussed. This algorithm was deduced from single transformer but it is not suitable for three winding transformer. When energizing an unload Y₀/Δ-U connection transformer on Y0 side, the circulating current in delta side may eliminate or distort inrush current waveforms on Y0 side. This kind of elimination effect phenomenon in inrush current is called the affection of add-assist current (AA) in inrush. It will cause the property of inrush different from single transformer inrush and different compensation method for differential protection also has effect on the amplitude and angle of second harmonic in inrush current. The different connection type for transformer and different compensation method should be considered in differential protection. The further research results show that the characteristic of current in magnetic branch is same as single transformer. The adaptive second harmonic restrained method was suggested if the winding differential protection scheme was used while the CT was mounted on the inner side of Δ winding. The sensitivity of protection will also be improved by using this scheme.