Problem Solver

New shorted-turn detection technology has been developed which can identify the coils with shorted turns and the number of shorted turns in the effected coils. The test data can be obtained while the generator is at speed and normal operating load conditions. With this information, rational corrective action maintenance decisions can easily be made. No special off-line test set-up is required. (Impedance tests or stator short-circuit tests originally described in D.R. Albright's 1970 IEEE paper.(2)) Obtaining a baseline set of test data before a critical shorted-turn condition develops enhances the sensitivity of the detection system.

The new technology uses a simple permanently mounted air-gap flux probe positioned on a stator wedge. The probe is sensitive to changes in radial flux density magnitudes as the rotor surface passes by. This waveform is the time rate of change of the radial air-gap flux density waveform. Thus, it greatly amplifies minute variations in the slopes. Analyses of the flux probe waveforms can detect critical shorted turn conditions.

Installation

Generatortech, Inc., mounts a permanent flux probe at a selected location on a stator wedge. Signal cabling is routed to the outside of the generator wrapper plate through a hydrogen gas tight seal to a rugged test cable connection fixture. The Generatortech, Inc. flux probe, cabling and installation materials are inert to the harsh environment within a generator. Probes are designed for each installation to assure that it does not interfere with rotor installation and removal. Probe installation requires advanced planning and maintenance personnel should contact Generatortech, Inc. during the planning stages of a major overhaul.

Two different air-gap probe designs are pictured above. The probe design used is dependent on the generator's air-gap length. The probe must be assembled during a major inspection when the rotor has been removed. Partial rewedging is not required. The probe is doweled and bonded to the assembled wedge.

Note the probe tip location will be two inches or less from the rotor surface. The dimension will be established by providing a 0.5 inch radius clearance to the retaining ring. Thus, there will be no interference with the retaining ring during rotor assembly and disassembly. The probe is wired to the instrument terminal gland which is mounted on the O.D. of the generator casing. The casing gland seals hydrogen pressure and has a BNC terminal for an instrument connection. The cable routing, probe to casing gland, is similar to the stator bars RTD wiring.

Generatortech, Inc., 31 Sutherland Drive, Scotia, NY 12302. Tel: 518-399-4646; Fax: 518-399-4646.