Improvements to transformer differential protection — Design and test experience

Transformer differential protection schemes normally use multi slope characteristics to provide stability during CT saturation with 2nd harmonic restraint or blocking for inrush conditions. This paper presents new algorithms that have been developed to improve the performance of the differential protection. Extensive testing has been used to demonstrate that the transient bias algorithm can reduce the CT requirements for transformer differential protection. Testing has also shown that the new CT saturation and no gap detection algorithms improve the tripping time for internal faults with CT saturation.