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

A new test method for detecting shorted turns in the field winding of synchronous generators during turning gear mode operation is proposed in this paper. Turbo generators are operated in turning gear mode, where the rotor is turned at low speed (0-50 rpm), after shutdown or before startup to prevent thermal bowing of the turbine and generator rotor. The main idea of the new test method is to test the rotor field winding condition from the stator winding side by observing the change in the equivalent impedance as a function of rotor position while the rotor is rotated slowly. The decrease in the d-axis impedance due to shorted turns in the field winding is observed for fault detection with high sensitivity. It is shown that the proposed method provides a simple and convenient means of testing shorted field turns with high sensitivity and reliability compared to existing off-line and on-line test methods. The validity of the proposed method that has not been attempted elsewhere, is verified on 4 pole, 20 kVA and 2 pole, 5.5 kVA synchronous machines.