Shinobu Sekito

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.

Turbo-generator life management and service technology

To reduce electricity generation cost, utilities, IPP and industrial users are strongly asking to minimize life cycle time cost and maximize utilization and effective operation of existing power units. To meet these requirements, we replaced the conventional interval-based life management of units with the new risk and condition-based life management to extend the inspection intervals and to shorten inspection periods, and realized the significant improvement in availability and the reduction in maintenance cost. We also clarified the mechanism of aged deterioration of stator and rotor coil insulation, and established the life estimation technology. We furthermore established and put into practical use the wet-beat deterioration life estimation technology for water-cooled stator bars. Regarding the enhancement use of existing units, we improved cooling technology and newly developed HTC (high thermal conductivity) stator insulation system resulting in substantial output increase and efficiency improvement. We have established the technologies for the 2-shift operation. In the early 80s, these technologies were applied to a large-capacity water-cooled unit of DSS operation. This unit is currently running satisfactorily after 20-year operation. It is important, should a failure occur, to immediately reach the root cause of the failure and take the appropriate corrective actions considering economy. It is possible to propose optimal corrections to meet the user needs by using a new failure analysis technique and a new method to evaluate the different approaches to correction. This paper presents a new life management concept and service technologies for turbo-generators.