Flow-Through Inrush and its Effect on Transformer Differential Protection

This paper analyzes the effects of flow-through inrush on CTs, especially on transformer and its differential protection. The flow-through inrush can cause the transformer to produce series-type sympathetic inrush, when the series-type sympathetic inrush current is followed by heavy CT saturation in the secondary side of transformer, false operation may be introduced in the existing 2nd harmonic restraint relays. This paper discusses the reason of false operation and presents a restraint criterion based on 2nd and 3rd harmonic according to the characteristic of this kind of phenomena. The presented criterion is an extension of the traditional 2nd harmonic restraint criterion and is proved to be feasible by lots of simulation tests.