Abstract The burning of phase-R, 166 MVA, 500 kV/150 kV inter bus Power Transformer of Tasikmalaya EHV main substation on 28th January 2009, has been disturbing the JAMALI (JAVA-MADURA-BALI) interconnection system. The investigation on root cause of the trouble is still under studied. After replacement of the inter bus power transformer, the existing IBT transformer still show some abnormalities on the neutral earthing current. Neutral earthing current found relatively higher than other IBT transformer even though the load level only minimal. Within this paper, measurement and simulation and analysis will be done to verify the cause of abnormality neutral earthing current on this Ynyd configured IBT 500/150 kV power transformer. The measurement on neutral earthing current were conducted by true rms current logging for one week on three area of similar configuration of EHV main substation including the Tasikmalaya EHV main substation to compare with other normal conditions. Measurement with true rms current meter, will investigate the effectiveness of delta tertiary winding for reducing zero sequence current on IBT transformer and also if there is dc component on neutral current due to geomagnetic field interference. Analysis and simulation was also conducted to verify the measurement result. The analysis and simulation show that if it is in the normal condition whether distorted, balanced or unbalanced the total resultant neutral earthing current should be zero when there is no insulation leakage to ground.