A New Simple Method to Avoid Maloperation of Transformer Restricted Earth Fault Relay during External Fault Events

The restricted earth fault (REF) protection is provided for electrical power transformer in order to sense internal earth faults, mainly because it is more sensitive than the main differential protection. However, The REF relay may maloperate when current transformer (CT) saturation happens following a severe external fault. In this paper, a new simple algorithm is proposed for REF protection scheme which is realized by considering four fundamental conditions. These conditions are defined based on the differential and neutral currents as well as sum of phase currents. When these conditions are simultaneously satisfied, the relay detects the internal ground fault. This algorithm is implemented and evaluated by MATLAB program based on obtained result data from simulation of a real power system using PSCAD/EMTDC software package. The well-known Jiles-Atherton (JA) model is used to simulate the transient behavior of CTs. The satisfactory results obtained from exhaustive investigation justify the high security of the proposed protection scheme.