On to assembly
Alstom's initial inspection of the stator winding found it to be in relatively good condition. Visual inspection of the winding heads detected some areas of "greasing" and partial discharge on both the DE and NDE. There also were some broken ties on spacer blocks. Electrical tests similarly showed the stator windings and insulation to be in generally good condition. A Transposition Voltage Comparison Test detected a broken strand in the bottom coil of slot #35, but PGE determined that it was a manufacturing defect. Finally, a Vent Tubes—Vent Tubes Resistance Test found two "floating" tubes in the bottom bars of slots #2 and #31. These were repaired by installing a new resistor on slot #2 and by re-soldering the resistor connection on slot #31. All other initial testing found no problems with the stator winding that required attention.
Alstom prepared the winding heads for the installation of the new endwinding support system by removing the old support system's coil support brackets, coil braces, top coil support ring, and (on the NDE) the phase rings and phase ring supports. Inspection of the winding head surfaces found places where the diamond spacers between the bars protruded significantly above the surface. To ensure good contact at the interface between the support system and the winding heads, Alstom ground flush with the surface of the winding head any protruding spacer blocks that might contact the inner support ring or pressing plates.
Assembly of the new endwinding support system began with installation of the inner support rings. Layers of epoxy-impregnated glass and polyester felt were applied to the rings' outside surface to serve as a conformal layer and provide a good mechanical bond between each ring and the surface of the windings. The outer support ring structure then was assembled in place and centered on the winding head. With temporary tooling holding the ring structure in place, the tightening devices were assembled between the support plates and the winding heads. Layers of glass and polyester also were applied between the pressing plate and outside of the winding head.
The final step of the assembly process was to install the phase rings at the NDE of the stator by securing them to the brackets (Figure 6). Connections then were made to the existing phase bars by means of parallel copper plates brazed to either side of the phase ring ends and soldered to the phase bar strands.
6. Holding on. Brackets for mounting the phase rings were installed on the outside of the outer support ring structure. Courtesy: Alstom Power
With the assembly complete, the next step was to heat-cure all of the epoxy in the support system. As part of this process, it was critical to maintain constant pressure on the winding head. This was accomplished by means of special tooling installed against the inner support rings and on the retightening devices.
Bump testing was performed after the heat curing was complete. The NDE results were very good, indicating no excitable vibration modes within the 110-Hz to 140-Hz range (Figure 7). Several 6-node modes did show peaks in this range, but they did not cause concern because they are not easily excited by the 120-Hz forcing frequency of the machine.
7. Final non-drive end bump test results for the new support system. Shown is the basket average response at the outer ring. Courtesy: Alstom Power
Passing final exams
After the successful completion of final electrical tests on the support system, the Boardman plant's generator was reassembled. The plant then was brought back on-line and closely monitored for any operational problems. No significant problems were detected.
More importantly, data provided by the endwinding vibration monitoring system indicated dramatic reductions in endwinding vibration levels. Data taken before Boardman was shut down indicated a peak vibration level on the end windings of the DE of approximately 10 mils, with the generator loaded at 541 MW. Post-overhaul readings indicated that with the generator loaded at 628 MW, the peak vibration levels at its non-drive and drive ends were 2.4 mils and 1.4 mils, respectively.
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