Useful Applications For Differential Relays With Both Kcl and Atb 87 Elements

Differential protection elements can be classified into two main categories: Kirchhoff’s current law (KCL) and ampere-turn balance (ATB). KCL differential elements sum the currents into a power system zone where all terminals are connected by a current carrying connection, such as with a power system bus or a rotating machine stator. ATB differential elements sum the ATB around a magnetic circuit, such as with a power transformer where power transfer through the differential zone relies mainly on magnetic coupling between windings on a magnetic core.

The protection requirements are often quite different for electrical apparatus that require KCL differential elements versus ATB differential elements, but often both types of apparatus exist in the same tripping zone. There are many applications where having both a KCL and an ATB differential element in the same protective relay can optimize protection, providing an optimal balance of security and dependability and an optimal balance of sensitivity and speed. This paper identifies examples of these applications and how such a relay can be applied.