EVALUATION OF THE RESTRICTED EARTH FAULT PROTECTION FUNCTION IN NUMERICAL PROTECTION RELAY

Abstract

For earth fault near-neutral point of a grounded wye-connected transformer winding, the transformer different protection function (F87T) may not sensitive enough to detect this condition due to small fault current value. Therefore, many relay manufactories have diligently searched for different kinds of a restricted earth fault function (REF) which is requested to solve misoperation. So that it can be reliably operated. The paper explains and compares the sensitive detection between ground faults of F87T and REF, detects a typical miss connect secondary current transformer wiring in a 110kV Dak Nong substation. Then the paper presents the tripping characteristic of five relay manufactories such as Abb RET670, Schneider P633, Ge T60, Siemens 7UT6, and Sel 387. These characteristics are also simulated to evaluate the performance of relay when a single phase to earth internal fault from 10% to 100% winding in a transformer 25MVA 115/24kV Yy0, or external fault occurs and normal condition by using Matlab/Simulink software. REF with the required test point calculations and explanations is included for clarity of evaluation.