Time and time again, Mechanical Dynamics & Analysis (MD&A) shows the global utility industry its strength, technical capability, and resource depth to serve as a premier alternative to the original equipment manufacturer for both services and parts supply.
In fact, MD&A established itself in the beginning as the preferred non-OEM supplier, regardless of original supply or design.
Founded in 1982, MD&A takes great pride in the extensive OEM experience, knowledge, and skills of both its technical experts and management personnel.
A recent case in point: MD&A performed a full stator rewind of a GE® 7A7 generator that suffered an in-service ground fault failure. In order to meet the owner/operator return-to-service schedule for the stator rewind, the owner operator called in MD&A. The rewind was completed on schedule with no safety incidents or concerns, and the unit passed all detailed testing for return to operation.
Specialists from MD&A’s Generator Division arrived on site and completed all site-specific training, walkdowns, and tool staging.
Initial baseline testing, physical measurements, and winding configurations were confirmed. Stator end windings and core were scanned and modeled to aid in the manufacture of new stator bars. To meet the return-to-service schedule, MD&A experts were able to get the new bars quickly.
A baseline electromagnetic core imperfection detection (ELCID) analysis was performed on the stator core iron to determine the integrity of as-found laminations.
The existing wedge system was then removed according to MD&A procedure. Series loop connections were removed and collector-end circuit ring connections were cut.
After bar tie removal, top bars were removed, stacked, and placed in recycling containers. Inner radial ring and bottom-bar ties were then removed, followed by all bottom bars.
Significant levels of partial discharge were noted on several bars with high line-to-neutral voltage potential.
T4 connection ring inspection found material missing at the extension, likely a source of discharge during the generator failure. The T2 ring showed damage at the phase-strap location.
All connection ring pieces were removed and inspected to be repaired and reinstalled. Damaged pieces were replaced with newly-manufactured material. A DC HiPot test was then completed on the circuit rings. A post-disassembly ELCID was also performed to ensure no damage to the core iron, and a loop test confirmed the integrity of the stator core.
For reassembly, binding bands were received and aligned on both ends and made concentric to the core – a vital step for proper bar alignment and consistent felt buildup.
Binding bands were installed and bar installation began. A thin felt layer in each slot served as a bottom breaker between the bar and core iron.
After close-up bar installation, bottom bars were installed and permanently tied with blocking, saturated felt, and saturated roving ties. Side-ripple springs were installed to secure proper seating and avoid any vibrational movement. A bottom-bar Hi-Pot verified the integrity of bar armor insulation.
At this point, a total of 12 new Resistance Temperature Detectors, more accurate than traditional thermocouples, were installed, terminated, and verified. Inner radial rings were installed and top-bar installation began, followed by comprehensive Hi-Pot testing.
For wedging, a tapered low-shrink black canvas (LSBC) wedge and slide design with a top ripple spring was selected for sufficient radial pressing force. Filler was adjusted at each wedge for proper compressive force. Center and body wedges were installed throughout the core, followed by end wedges cut specifically for each of the 72 slots.
Brazing was completed by traditional method and MD&A procedure, and a winding resistance test ensured that the three phases produced similar resistance values. Series, circuit ring, and jumper connections were then installed. The insulating tape used by MD&A ensures no voids or cracks form along the phase breaks during operation, where the voltage potential is the greatest. Series blocking was installed and tied to reduce vibrational movement. Frequency response testing followed.
After the generator stator rewind, comprehensive electrical testing included copper and insulation resistance, final DC HiPot, and DC leakage testing of each phase. All testing results were verified and accepted.
A final crawl-through inspection was completed with the owner/operator. Field installation could begin, and the generator would soon be returned on schedule for dependable, long-term operation.
Once again, MD&A has shown the global electric utility industry its depth of knowledge, finely-tuned technical skills, and repeatedly-proven owner/operator commitment.
MD&A is truly a premier non-OEM service provider to the global gas and steam turbine-generator market. Call MD&A about your next outage today at (518) 399-3616.
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