Suitable Grounding Transformer Placement and Vector Group Compensation Matrices for Enhanced Autotransformer Differential Protection

Autotransformers, in which the primary and secondary windings share a part of the same winding, are essential for transmitting large amounts of electrical power and thus require dependable fault protection to ensure network stability. Typically, autotransformers use a star-star-delta configuration, with the delta winding grounded through a grounding transformer to provide a fault path that can be detected by a differential relay. This paper aims to enhance autotransformer differential protection by determining the suitable location for the grounding transformer, whether inside or outside the differential protection boundary, and by selecting the appropriate vector group compensation matrix for each winding to detect faults in the delta winding effectively. MATLAB/SIMULINK is used to simulate a practical power system with an autotransformer and differential relays from two manufacturers to identify these selections. The results demonstrate that placing the grounding transformer outside the differential protection boundary, combined with selecting the delta winding as a reference, provides the best fault detection performance.