Internal fault fast identification criterion based on superimposed component comparison for power transformer

• Defects of existing differential protection for power transformer are introduced.
• There is proved to be a time interval if internal fault occurs.
• A preset filter is designed to extract the superimposed component.
• The proposed criterion is evaluated under various fault conditions.

Due to the interference of false differential current caused by the saturation of the iron core, the differential protection for the power transformer may suffer mal-operation and long response time. According to the investigation for the characteristic of transformer core, there should be a time interval between the sudden change of phase voltage and the emergence of differential current. And it can be utilized to distinguish between the external and the internal faults. A new time-interval based criterion for transformer differential protections is proposed, aiming at identifying transformer fault type by virtue of highly accurate time interval. Furthermore, a novel filter algorithm is proposed to determine the time interval for its excellent singularity detecting and effective noise-repressing ability as well as high-speed response. The EMTDC-based simulation, which covers various fault situations, is analyzed, of which the results confirm that the criterion can effectively prevent the differential protection from mal-operations and meanwhile guarantee the high-speed response to the internal fault.