Why Neutral Earthing Resistors are ineffective for autotransformers with the delta tertiary winding

Neutral Earthing Resistors (NERs) are effective in limiting single-phase fault currents at the Low Voltage (LV) side of a two winding transformer. This paper aims to demonstrate that NERs are not an effective solution to limit the earth fault current on the LV side of autotransformers with delta tertiary windings. In a ground fault condition, the delta tertiary winding used in the autotransformers forms a shared path between the NER and the delta tertiary. This shared path of the zero-sequence model of an autotransformer substantiates why earth fault currents on the LV side of autotransformers cannot be effectively limited using the NER in the way they can be on conventional Delta-Star (Dyn) transformers. The findings demonstrate that significantly larger NERs are needed in autotransformer applications to limit the fault current compared to the conventional transformer.