Percentage differential relays remain the most
sensitive protection tool applied as backup protection on power
transformers, busbar, and generators. Relays sometimes do
mis-operate with the current transformer being affected by
external fault leading to saturation, and the subsidence current
present after clearing external faults. The cause of misoperation of percentage differential relays cannot be ignored
that it entirely depends on magnitudes more than directionality
for tripping decisions. This paper covers evaluating differential
element performance, analysis of transformer inrush current,
internal faults, external faults, and overexcitation conditions.
The accurate computing of current transformers is also
included. This protection only applies to 10MVA and above on
transformers; however, it is not limited to transformers, but
also transmission lines, busbars, and generators. The balance
of the paper is on reliability assessment based on the HVDC
grid protection scheme operation
