Fault Analysis of the Misoperation of 220kv Transformer Differential Protection Caused by Restorative Inrush Current

A single-phase transient fault occurred at the forward outlet of the power incoming line in a 220kV user substation. After the line protection acted then the line switch on this side tripped, the B-set ratio differential protection of the 1# power transformer acted, while the A-set protection did not. The analysis shows that due to the voltage rise after the external fault is removed, the transformer produces restorative inrush current. In addition, when calculating the second harmonic content in the differential current, the B-set differential protection does not use the real differential current, but constructs a "virtual differential current" with zero sequence current. Theoretical analysis and data verification show that using the "virtual differential current" to calculate the second harmonic content can not more effectively identify the magnetizing inrush current, but may cause the braking failure of the protection and then misoperation.