Impact of Transformer Inrush Currents on Sensitive Protection Functions

Transformer inrush currents are known to cause problems for sensitive protection functions applied to a power transformer itself, or in a near vicinity of a transformer. Magnetizing inrush currents contain significant and slowly decaying dc components. This makes Ct saturation plausible and causes more problems for associated protection. Sensitive protection functions that respond to currents are affected most. This paper reviews impact of transformer inrush currents on five protection function: (i) main differential function of the transformer, (ii) restricted earth fault protection, (iii) stator differential protection for generators, (iv) sensitive ground directional overcurrent functions used for line and feeder protection, and (v) distance protection. For each of the covered protection functions the problem is reviewed and quantified, and practical solutions presented