Percentage restrained differential, percentage of what?

Percentage restrained differential protection is one of the oldest forms of adaptive protection algorithms. The slope characteristic provides high sensitivity when low levels of current are flowing in the zone of protection but has less sensitivity when high levels of current are flowing and false differential current due to current transformer (CT) saturation is more likely. The percentage restraint characteristic operates on the ratio of operate-to-restraint current in the zone of protection. The operate quantity is universally defined as the magnitude of the differential current in the zone of protection. However, several different methods have been developed to quantify the restraint quantity, which is a measure of the through current in the zone of protection. The definition of the restraint magnitude will have an impact on the effective sensitivity and security of a given percentage slope characteristic. This paper examines several common methods for defining restraint and slope characteristics and provides guidance on selection of the correct slope setting. Examples are used to examine a multirestraint transformer differential application with steady-state, proportional, and transient sources of mismatch in the zone of protection.