Equivalent instantaneous inductance based inrush current identification for three-phase V/V traction transformer

The magnetizing inrush and the sympathetic inrush may cause the malfunction or operating delays of differential protection for traction transformers, and affect their normal operation. The algorithm based on equivalent instantaneous inductance can reflect the essential characteristic of the traction transformer operating conditions and correctly distinguish magnetizing inrush and sympathetic inrush from the internal fault current. For three-phase V/V traction transformer, the connection is different and the operating circumstance is much worse than the power transformers, and the second harmonic restraint scheme can not effectively restrain the false action. At present, the equivalent instantaneous inductance based algorithm is still not applied to three-phase V/V traction transformer to recognize magnetizing inrush and sympathetic inrush, even though it is popular in application to the power transformers. This paper improves the applied conditions and the arithmetic elements to calculate the average equivalent instantaneous inductance of none-saturation zone of the differential current to distinguish magnetizing inrush from the fault current. The dynamic experimental and the simulated results show that the algorithm can distinguish the magnetizing inrush and sympathetic inrush from the fault current, with high reliability. Also, the algorithm is well immune to the saturation of current transformers.