Hideyuki Shimada

Technology for detecting wet bars in water-cooled stator windings of turbine generators

Stator bars of large-capacity direct water-cooled turbine generators may suffer from cooling water leak into the ground wall insulation by galvanic corrosion in clip to strands brazed area. Once the cooling water leaks, the water accelerates insulation deterioration of bars by hydrolysis which is caused by wet and heat. Consequently early detection of the water leak is necessary for safety operation and proper maintenance. While common insulation of stator bars are made of only mica tapes lapped, there are bars with another design in which some conducting layers (internal electrode layers) are inserted in intermediate layer of ground wall insulation at coil end. The internal electrode layers work as a voltage divider so that the electric field at the surface of end portion of bars are suppressed. While a capacitance measurement method is commonly used for detecting wet bars with the former design of bars, the method can not be applied to the latter design of bars. For detecting wet bars of the latter design we have developed a new and sensitive method. After our so many experiences of capacitance method is simply described, the detail of newly developed method for the bars with internal electrodes is described in this paper.

Development of a capacitance measuring robot for the water-cooled stator winding of turbine generators

When water permeates the winding insulation in large-scale direct water-cooled turbine generators, dielectric strength drops steeply due to wet heat deterioration. To ensure operational reliability, it is essential to know whether or not water has permeated the insulation, and if so, to determine accurately the degree of deterioration. We have developed a capacitance measuring robot which enables measurement near the stator core end without removing the rotor, and confirmed good results in the field. The robot travelling along the retaining ring can be accurately positioned, and measured capacitances are stable and reproducible. The robot minimizes the extent of disassembly and reassembly, thus greatly reducing the out-of-operation period. Further, because the insulation condition can be accurately determined through its periodical use, we believe that it will contribute to both preventing accidents and assuring generators with high reliability.