A current transformer (CT) is one of electrical power system main equipment, whose performance has an important influence on measuring instruments, meters and protective or control devices . In an iron core CT, a remanent flux will remain after a current interruption. Once the remanent flux is established, very little of it is dissipated under service conditions, and might remain for almost infinite time in the core until it is demagnetized . Many test results and literatures indicate that, to the commonly used 0.2C and 0.5C measurement CTs, the remanent flux may cause the error to displace 0.1%∼0.2% to the negative direction, causing the measurement to be small [3, 4]. In addition, the remanent flux will also reduce the available margin for flux increase. This will naturally reduce the time to saturation when the remanent flux is in the same direction as the flux increase required to reproduce the primary fault current [5, 6].