Aiming at these two severe problems OCT faced

temperature excursion and unstable operation that baffle its utility in power system

the adaptive optical transducing principle and the solenoid collecting magnetic field optical path are adopted to solve the problems after analyzing the open loop mechanism of OCT.The performance of an OCT is examined by applying the standard testing system.The results indicate that under the steady state the measurement precision of the AOCT reaches class 0.2

and under the transient state the maximal peak instantaneous alternating current component error of the AOCT reaches class 1.The proposed method can measure the non-periodic component and various AC harmonic components exactly

without saturation phenomenon.Bergeron model of transmission lines is used to set up the entire current differential protection of EHV transmission lines based on the electronic transducers.This algorithm considers capacitive current automatically

and can pass over the compensation of capacitive current.Simulation results demonstrate that the analysis is correct and the algorithm is reliable.This protection method is suitable for digital substations.