Abstract: On December 26,2021,the upper section of the bushing at the high voltage side of phase C of main transformer in 750 kV substation was displaced and fractured resulting in the phase C open-phase of the transformer.Under low load conditions, there was no protection alarm for a long time.If the open-phase occurs in the bushing of the main transformer, there was a risk that the bushing would fall and may cause permanent damage such as internal deflagration, insulation breakdown, winding deformation of the main transformer.In order to deal with the threat of open-phase failure, an open-phase protection scheme of the transformer based on the measurement level current transformer was proposed.By detecting the current and voltage at the high voltage side of the transformer and the bushing current at the low voltage side of the transformer, which can effectively identify the open-phase failure of the transformer under the light load condition, and the problems were solved that the transformer protection was not sensitive to the open-phase detection and the tripping was not timely, It is conducive to the safe and stable operation of the power grid.