| Summary: | This paper introduces the basic principle of fibre optical current transformer (FOCT), and explains the advantages of FOCT compared with electromagnetic current transformer (CT). FOCT can be wound around the primary conductor in any shape, has high harmonic accuracy and doesn't suffer from saturation, thus a good solution for generator relay protection. As a common electrical fault within large generators, the inter-turn short circuit in field windings (ISCFW) is likely to cause earth faults between the field winding and the rotor body and magnetization of the main shaft without timely intervention. Steady-state unbalanced currents of even orders and fractional orders related with pole pairs inside phase windings are required to monitor the ISCFW. The inter-turn fault of stator windings may rapidly develop into a phase to phase fault, which seriously threatens the safety of the unit. Split-phase transverse differential current reflects the steady-state current imbalance inside the stator winding. Nevertheless, for most steam turbine generators and some hydro turbine generators, it is unable to obtain phase-segregated transverse differential current due to impossibility of installing branch current transformer in the narrow space inside the generator. However, FOCT which uses fibre optic cable as a sensor is installable within limited space to measure the current of each group of winding branches of each phase. FOCT furthers phase-segregated transverse differential protection, partial differential protection and online monitoring of the ISCFW, thus optimizing the generators protection scheme. In addition, operating speed and sensitivity of generator differential protection are improved based on the reliable measurement from FOCT. The proposed scheme is verified by an application on a 300MW generator at a pump-storage power plant. This is the first attempt of applying FOCT to the relay protection of generator set, which provides reference for further development and application of FOCT in power plant.
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