What is the difference between differential relay and percentage differential relay?

the back ground abstract  the 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.

however the potential different  it;s sources vs current path Vs impedance and power control within the transformer  systems for more info Visit l here below. 

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Percentage differential relay is an improved form of differential relay. The main difference between them is, presence of BIAS windings that are connected to pilot wires.