Applied to transformers in the transmission and distribution networks.
An overall differential protection is connected to current transformers located on each transformer HV and LV winding and provides a unit protection to operate circuit breaking devices connected to the HV and LV windings in the event of a fault being detected. Transformer differential relays incorporate a number of features to compensate for factors such as CT location, transformer vector group, non-optimised CT ratios, transformation variations introduced by the presence of an on load tap changer and inrush current during switch on.