Echo of Transformer Tertiary Winding Fault in Test Results – A Case Study

Tertiary winding provided in Star-Star transformer to stabilize neutral point voltage and to provide a path for circulation of zero sequence currents, is the innermost (hidden)winding. Tertiary winding, if not used for any auxiliary supply or compensation purpose, is generally less prone to fault as far as system fault levels aren't on higher side, as it remains almost idle i.e. no significant current flows in normal balanced network conditions. In one variant of such cases, tertiary winding delta is formed inside the tank and only one end is brought out to earth it and therefore, individual testing of tertiary winding is limited. This paper presents a case study showing how a fault in tertiary winding, with such configuration, echoed (reflected) in the field test results.