Detection of inter-coil short circuits has a significant importance in nowadays diagnostics of rotating electrical machines. In this paper two modern, reliable and economically viable methods for such detection in excitation windings of turbo generator have been described. One can be applied in operation and the other in standstill condition.
First method for detection of excitation winding insulation state at standstill condition is based on impulse voltage test. Test impulse passes through the winding turns and in the case of weak or faulty point in insulation, directly continues through this weak point to the next winding coil turn, coil, group, phase, or if possible to the ground. Inter-coil short circuit between turns reduces inductance of the faulty winding coil, which results with higher oscillating frequency. In that case voltage stress response curves have oscillations that are not identical i.e. fault is present in winding. By combining winding connections and analysis of response characteristics, inter-coil short circuit extent and its approximate location (± 0, 5 m) can be determined.
Second presented method for detection of inter-coil short circuits in excitation winding during generator operation is based on magnetic field measurement in the air gap of the machine. Magnetic field can be measured using Hall sensors or measuring coils fitted on the stator tooth. If an inter-coil short circuit appears in excitation winding, it will cause disturbance of magnetic field in the machine. This change will be seen as localized disturbance in measured magnetic field or flux density waveform. Signal from measuring sensors are collected via analog input modules of adequate monitoring system and process measuring data using algorithm specifically developed for this purpose, based on mathematical model using FEM (finite element method). Monitoring system can provide information weather inter-coil short circuit is present in excitation winding coils and number of shorted turns per coil.