Mechanical Dynamics & Analysis (MD&A) has completed another successful generator rewind as part of life extensions programs on a gas turbine fleet in the northwest US. This project featured a unique focus on the totally enclosed water-to-air cooled (TEWAC) assembly including complete GE® 7A6 generator isomode pad replacement.
MD&A provided all tooling, supervision, and labor to perform the planned generator major inspection outage scope.
This generator life extension project exemplifies MD&A’s position as a preferred non-OEM one-stop organization throughout the turbine-generator industry.
Generator enclosure
After removal of all attachments, MD&A experts held the critical lift meeting and removed the 24,000 lb generator enclosure and installed all FME covers, removed upper half T4 and T5 bearings, and prepared the base for generator field removal.
The field would be prepared and transported to MD&A’s Turbine-Generator Repair facility in St. Louis, MO. The T4 and T5 bearings and inner and outer oil deflectors would be transported to MD&A’s Bearings, Seals and Hydraulics division in Euclid, OH for inspection and repair. Â
Scaffolding was then installed for the generator rewind activities.
Coolers were removed from the TEWAC assembly and cooler heads were disassembled. New isomode pads were prepared for the reinstall.
Stator
This was a planned stator rewind, scheduled as preventive maintenance due to significant observed partial discharge. Over time, this could lead to a ground fault condition and forced outage.
A baseline electromagnetic core imperfection detection (ELCID) was performed on the stator core iron to determine the integrity of the as-found laminations.
The existing flat wedge system was removed by traditional methodology and MD&A procedure.
Top bars were then removed without issue, using come-alongs to remove the bars out of individual slots. Bottom bar removal was completed in the same manner. Significant levels of partial discharge were found on several bars.
Following full visual inspection, all connection ring blocking and ties were removed and replaced with new conforming material. A post-disassembly ELCID was performed to ensure no damage to the core iron. Each of the 72 core slots were cleaned of all contaminants and thoroughly inspected.
A core knife test determined the need for complete stator core tightening, completed by MD&A procedure. At the completion of torque activities, an additional ELCID was completed with satisfactory results.
Once received, binding bands were aligned axially and made concentric to the stator core. Three bottom bars were then fit into slots 120 degrees apart to ensure proper alignment. Two top bars were also aligned with the pitch of the bars to ensure the top and bottom bar series plates were properly aligned.
A thin layer of insulation was installed in each slot, and bottom bars were then installed and bottom-bar testing was performed.
A total of six dual-element RTDs were removed during disassembly and were replaced with twelve new dual-element RTDs and operation was verified.
All top bars were then installed, blocked and tied, with no issues.
A tapered wedge and slider design with a top ripple spring was selected to provide sufficient radial pressing forces. Following all winding activities, a final ELCID was performed to ensure no damage to the core iron.
After brazing, a winding resistance test was performed to ensure each of the three phases produced similar resistance values, and all other electrical testing was completed with good results.Â
Field
Following removal of the generator field, the customer opted for a generator field exchange. We sent a refurbished field to site that had been completely rewound with the latest in rewind technology. The field forging had undergone NDE and inspections to ensure long term viability, along with it being high speed balanced with a full heat run to ensure no shorted turns. The exchange of the generator field produced a shorter outage duration which allowed for the customer to be back online sooner.
The original generator field was transported for complete inspection and rewind. On arrival in St. Louis, the field received in-depth visual and mechanical inspection. Three-dimensional scanning and hand mechanical dimensions were obtained.
During disassembly, indications were found on a few field body wedges, and some material damage was found on both retaining rings. All field coils were removed and sent for cleaning.
The cleaned coils were determined fit for service and reinstallation began.
Each installed coil was HI-POT and turn-to-turn tested. A baker surge test was also completed on each coil. During the rewind process, the MD&A standard strap and design blocking was installed. Turn insulation was coated, requiring a rotor bake cycle. After the bake cycle, a full set of electrical tests was completed.
The field was then moved from the clean room to the shop floor to install retaining rings and complete the electrical testing. It was then moved to the high-speed balance facility. After balance, it was staked and painted and prepared for exchange sale.
Isomode PadÂ
During this project, the TEWAC assembly was removed. Mating surfaces with the generator enclosure were cleaned and weatherstrips were installed. Generator cooler heads and all piping were cleaned of corrosion and buildups, and new gaskets were installed.
The generator stator was raised to remove the old isomode pads and shims. The MD&A team cleaned the area and installed new isomode pads with pre-existing shims. Â
The TEWAC assembly was then reinstalled.
Summary
All MD&A inter-divisional work was performed on schedule without any injuries or accidents.
There were no issues with turbine-generator synchronization during startup.
Once again, the MD&A organization drew on its experience and multi-divisional expertise to complete a major inspection and repair project and ensure owner/operation confidence in continued unit operation.
MD&A is truly a one-stop shop! Call MD&A about your next outage today at (518) 399-3616.
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