Investigation of tertiary fault in 500/150/66kV 500MVA inter bus transformer

Tertiary winding connection arrangement might give various effects in conjunction with modern Inter Bus Transformer (IBT) failure. An investigation was conducted upon the failure of an IBT which consist of three one-phase-banks and had one side of delta tertiary winding connection grounded through the tertiary bushings. Apart from the oil test, all routine maintenance historical data showed no early failure indication while visual inspection showed arc marking on tertiary bushing rod. Phase to ground fault simulation on tertiary winding was performed with actual field data and arrangement reference. Current waveform of the simulation result matched with current waveform of Digital Fault Recorder (DFR) data; hence it was suspected that the failure had happened on tertiary winding connection side. According to the simulation, tertiary bushing which connected tertiary winding of each IBT banks to form delta connection and grounded at one side suffered voltage stress above its phase to earth rating. This continuous voltage overstress decreased the insulation strength of tertiary bushing which lead to breakdown.