In view of the malfunction of the ratio differential protection of the main transformer after a single-phase short circuit of the line

the analysis is carried out from the aspects of the differential protection setting

operation logic and alarm signal of the main transformer.It is considered that the damage of the internal circuit of the AD chip of the main transformer CPU1 is the cause of portection misoperation.We propose the main transformer protection device adds functions such as zero drift over-limit alarm to improve the sensitivity of the current sampling full deviation and ensure the reliable action of the main transformer ratio differential protection.