| Optical Current Transformer (OCT) as a novel representative of current transformer resolves problems of saturation and low transient precision created by electromagnetic current transformer. According to principles and characteristics of OCT, this paper analyzes interface and its application in protection relaying. IEC61850 standard as the basis of seamless communication system is able to realize united transport protocol. According to IEC61850 seamless communication protocol this paper presents interface scheme with digital protection relaying by using fiber optic Ethernet. Principles and criteria of differential protection based on OCT are discussed through analyzing existing problems of differential protection of traditional current transformer. A simplified 500kV extremely high voltage (EHV) transmission system is simulated by EMTP. Criteria are proved by MATLAB with simulation data. The simulation results show that this method is accurate and reliable. Meanwhile, according to capability of OCT to transform transient components accurately, a novel principle of non-unit protection based on transient components for EHV transmission lines is put forward by use of wavelet transform to extract fault generated transient signals and criterions are present.
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