Analysis on transformer protection potential fault trip resulting from CT's wrong connection

Aiming at decrease of main transformer CT(current transformer) secondary current,wrong connection leading to additional circuit is concluded according to practical current data. Prime cause of secondary current’ decrease is analysed in detail applying electromagnetic theory. Protection's differential circuit affected by additional circuit is calculated according to transformer's load status. In conclusion,potential fault trip exists in RCS-978 stable ratio differential protection. Measures are brought forward to avoid fault trip,which should pay attention to corresponding between transformer radio and outlet impacted bolt when replacing CT,cooperation between control string and raising up differential protection's startup threshold,so that reliability of transformer protection is improved.