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The use of air-gap magnetic flux probes has proven effective in the detection of generator rotor winding shorted turns and has helped to improve the quality of predictive maintenance decisions concerning rotor rework. Analysis of air-gap flux probe data can pinpoint the number and location (pole and coil) of shorted turns without having to take the generator off-line.
Shorted turns in the rotor winding can have significant effects on a generator and its performance. If the percentage of total turns shorted out is small, the generator may be able to run at rated load for years without further problems. Larger shorted turn percentages, however, can cause operating conditions that may limit unit loads. If the problems become severe, forced outages may occur. Conditions that may result in running a rotor with shorted turns include:
* Rotor unbalance that varies with field current changes (thermal sensitivity),
* Rotor/stator vibration due to unbalanced magnetic force,
* Higher field current than previously experienced at a specific load, and
* Higher operating temperatures due to higher field currents.
On-line monitoring using an air-gap flux probe can detect turn shorts that are established only when the rotor is at normal operating speed and temperature. The flux probe is sensitive to the time rate-of-change of the radial flux in the air gap. As each rotor slot passes the flux probe, the slot leakage flux from that slot is detected. Rotor slot leakage flux refers to flux that does not cross the air gap to reach the stator windings. Since the leakage flux does not induce stator current, the leakage flux does not contribute to power generation. However, it is local to each rotor slot and its magnitude is proportional to the current flowing through the turns found in the slot; it is diagnostic, therefore, of active turns in each slot.
The flux probe waveform displays a peak for each rotor slot. The magnitude of that peak is related to the amp-turns in the slot. Since amp-turns are directly related to the number of active turns in the slot, a coil with shorted turns will display a smaller peak than a coil without shorted turns. By comparing slot peak magnitudes between poles, the number of shorted turns can be calculated for each coil in the rotor. To fully characterize a generator rotor, a series of load points is needed whose flux density curve zero-crossings (FDZC) align with each of the leading coil slot peaks in the flux probe waveform. With this data set, each coil can be analysed at the maximum possible sensitivity.
