Abstract
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.
Highlights
Stator winding inter-turn short circuit protection is essential for large generators
Fibre optical current transformer (FOCT) which uses the fibre optic cable as a sensor can be mounted on any shape of conductor in a limited space to measure the current of each group of winding branches of each phase, to optimize the protection scheme for a generator
With the help of FOCT installed on stator winding branch, incomplete longitudinal differential protection was added to the existing differential protection to form a complete stator and rotor winding inter-turn short circuit fault protection scheme
Summary
Stator winding inter-turn short circuit protection is essential for large generators. In some hydro turbine generators, only one dedicated CT for unit transverse differential protection can be installed at the connecting point between the neutral points of two groups of three phase winding branches. In some inter-turn fault cases, the sensitivity and operating time of unit transverse differential protection and longitudinal zero sequence voltage protection may not be satisfactory for clearing the fault quickly and preventing the generator from damage. A generator of a hydropower plant in Zhejiang Province, China has been damaged after twice experiencing long-lasting stator winding inter-turn short circuit fault, the unit transverse differential protection operated correctly. Fibre optical current transformer (FOCT) which uses the fibre optic cable as a sensor can be mounted on any shape of conductor in a limited space to measure the current of each group of winding branches of each phase, to optimize the protection scheme for a generator. L where V is the Verdet constant of optical medium, l is the propagation distance of light in the optical medium, H is the magnetic induction intensity, NL is the number of optical fibre loops and I is the electric current of the conductor
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