Detection of Shorted Turns in the Field Winding of Turbo Generators During Turning Gear Mode

A new off-line test concept for detecting shorted field winding turns in the rotor of synchronous generators, when operated under turning gear mode, is proposed in this paper. Turbo generators are operated in turning gear mode after shutdown and/or before startup, where the rotor is turned at low speed (<50 rpm), to prevent thermal bowing of the turbine and generator rotor. The main idea of the new test method is to detect turn insulation failure in the rotor field winding by observing the pattern of change in the equivalent impedance measured from the stator side, while the rotor is rotated slowly with the turning gear. An equivalent circuit analysis is given to show that the decrease in the equivalent d-axis impedance provides a sensitive indicator of turn faults in the rotor winding. It is shown that the proposed method that has not been attempted elsewhere, is a simple and convenient test for detecting shorted field turns with high sensitivity and reliability compared to the test methods widely applied in the field. The validity of the proposed method is verified with finite element simulation and experimental testing on 4 pole, 20 kVA and 2 pole, 5.5 kVA synchronous machines.