Air-Gap Flux Probe Data
The Generatortech Shorted Turn Detection System measures flux-density changes in the stator air gap. The flux density reversals at each rotor slot are recorded at multiple loading conditions for later analysis. The flux density at each slot, being a function of the number of active turns, is compared with its opposing pole slot to determine the number of inactive, or "shorted", turns.
Due to magnetic saturation of rotor iron and modulation effects (peak height variations not due to turn shorts), proper testing requires the recording of waveforms over as wide a range of loads as possible. Both real power (MWS) and reactive power (MVARS) affect the waveform’s flux density zero-crossing (FDZC) position among the leading coil slots on the rotor.
When the FDZC position is aligned with a given coil slot:
1. True turn short indications are maximized for that coil
2. False turn short indications are minimized for that coil
An optimum data set would include waveforms whose FDZC positions were aligned with each rotor coil slot. This insures that no true shorts were missed and that no false turn short indications were reported.
Recording data using a narrow range of loads and not considering the FDZC position will produce both false negative and false positive indications. We have seen many examples of competitors’ reports that had errors of both types.
The two case studies shown below illustrate both types of errors. These were two identical 120 MVA generators at the same power plant. The rotors have 8 coils/pole with 9 turns in Coil 1 and 13 turns in Coils 2-8.
Case 1 - False negative: One true turn short existed in Coil 8-Pole A (Coil 8A). Testing at above 25% of full load could not detect the Coil 8A short, while low load testing clearly detected the short. Testing at only full load missed the short in Coil 8A.
Case 2 - False positives: At both low and high loads, false turn short indications were seen in Coil 4 and Coil 5. At medium loads, where the FDZC position was close to alignment with those coils, the false turn short indications disappeared. Testing at only full load falsely reported shorts in Coil 4B and Coil 5A.
Case 1: Rotor cross-section showing location of true short in Coil 8A.
Case 1: Single short in Coil 8A was not detectable at loads higher than about 25% of full load. However, the short was obvious in low load waveforms.
Case 2: Rotor cross-section showing no true shorts
Case 2: False shorts in Coil 4 and Coil 5 were seen in both low and high loads. At medium loads, where the FDZC positions were close to alignment with those coils, the false indications disappeared.
