REF Protection of Transformer

REF protection is one type of electrical protection that is provided to power transformers against earth faults. Full form of REF protection is restricted earth fault protection.

This protection works for a limited zone between the installation of CTs that are installed on secondary side and neutral of transformer. That's why it is called restricted earth fault protection. It will only sense earth fault in this zone. Earth faults out of this zone will not be sensed by REF protection.

In normal condition without any earth fault in this zone,  vector sum of currents that is being induced in three CTs of secondary side and one CT of neutral is zero. 

On incident of earth fault in this zone the vector sum of currents flowing in relay from CTs of secondary side of transformer and neutral connection will not be zero.

REF protection measures this vector sum of currents and if it goes above the setting value, REF will operate and will trip the breaker of transformer.

CTs on the secondary side of transformer are generally being installed in the electrical panel so this protection covers power cable up-to incomer of this panel in addition to secondary side winding of transformer.

Now we will discuss that despite of having differential protection and simple earth fault protection in transformer why we are using REF protection in addition.

The neutral connection of transformer is a point which is common point without any potential or very low potential. Due to very low potential,  the faults near to neutral connection of transformer will have small quantities of currents. These type of low currents can not be sensed by protection like differential and simple earth fault protection. This is the main purpose of REF protection.