# Differential Relays – Differential Relay Power System Protection

A differential relay is a power system protection relay that operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined value.

In this post you’ll learn the fundamentals of Differential relay protection systems.

Also learn basics of Electrical Relays

## Basics of Differential Protection

One of the fundamental laws of electric circuits is Kirchhoff’s Current Law, which states the algebraic sum of all currents at a circuit node (junction) must be zero. A simpler way of stating this is to say “what goes in must come out.” We may exploit this principle to provide another form of protection against certain faults in electric circuits, by measuring the amount of current entering and exiting a circuit component, then tripping a circuit breaker if those two currents ever fail to match.

An important advantage of differential protection as compared to either instantaneous- or time over-current protection is that it is far more sensitive and faster-acting. Unlike either form of overcurrent protection, which picks up only if current exceeds the maximum rating of the conductors, differential protection is able to pick up at far lower levels of current because Kirchhoff’s Current Law predicts that any amount of current imbalance, for any length of time, is abnormal. Lower pick-up thresholds along with no time delay means that differential protection is able to take action sooner than any form of overcurrent protection can, thereby limiting equipment damage by clearing the fault in a shorter amount of time.

## Working of Differential Relays

Article text from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License