The affected units consisted of two GE^® Lynn generators that were experiencing both aging and flooding with water due to leaking hydrogen coolers. Prior to this, others had tried unsuccessfully to dry out the windings and attain suitable test results. MD&A’s Generator Specialists determined that the proper solution was to rewind both stators, rewind one field with new copper, and while remaining focused on returning the units to service as quickly as possible. Concurrently, our Gas Turbine Services specialists performed a modified hot gas path inspection on the GE^® 7B turbine.

Stator disassembly

MD&A mobilized to disassemble and clean the two generator units. After initial tool staging, a baseline Electromagnetic Core Imperfection Detection (ELCID) test was performed on each stator core.

The existing wedge systems were first removed. Measurements were taken to reverse engineer and manufacture new flat low-shrink black-canvas wedges, and existing filler was removed and measured as an accurate baseline for re-wedge operations.

Series loop and circuit ring connections, bar ties, and bars were then removed, followed by the binding bands to access the existing axial supports. A post-removal ELCID confirmed that no damage had been inflicted on the units during the disassembly process.

The terminal plates were removed allowing replacement of the deteriorated gaskets along with new high voltage bushings and current transformers (CTs).  The original HVBs were sent to MD&A’s St. Louis Service Facility where they were disassembled so that the copper studs could be assembled with all new components.

Turbine and collector ends, as well as the slot sections of both stators, were prepared by means of CO2 cleaning then painted in preparation for installation of the new windings.

MD&A returned to the site for reassembly after the manufacture of new stator bars.

Field rewind

The Unit 3 coil was removed and transported to our one-stop-shop repair facility in St. Louis, MO. The forging was then blast cleaned, and main leads were refurbished with new copper.

NDE was then performed on the forging, retaining rings, and snap rings. The forging was then prepared and staged in the clean room for winding with new copper.

With temporary support wood in place, copper installation (0 and 180 deg.) with sequenced testing began. Top coil jumpers were then installed.

After permanent blocking installation, the field was moved to the assembly area for turbine-end retaining ring installation.

Following this and with final electrical testing complete, the field was moved to the MD&A High-Speed balance facility and then prepared for the return shipment.

Stator rewind and assembly            

To expedite the schedule, an initial wave of bottom stator bars was sent to the site while the remaining bars were in manufacturing. Outside binding bands were installed by re-mobilized MD&A personnel, then three bottom bars were fit into 120-degree-apart slots to ensure proper alignment to the circuit rings.

Two top bars were also aligned with the pitch of the bars to ensure the top and bottom bar series plates were aligned properly, and to avoid any alignment issues during the rewinds.

A thin layer of glass laminate was installed on each slot as a uniform surface for the bottom bars.

Bars were fit onto a shoe on the collector end and carefully transferred through the bore to the turbine end and properly set into the slots. Nine close-up bars were left temporarily tied to assist with final installation.

Conductive side ripple springs were installed axially throughout the bore to ensure all bars were properly seated and to eliminate bar vibration during operation. Following placement of new slot RTDs, top bars were then installed, blocked, and tied, including installation of side ripple springs.

Installation of center and body wedges followed. End wedges were cut with an axial pinhole, and adjustment wedges ensured precise alignment with ventilation holes. Brazing of series connections between the bottom and top bars was then performed along with brazing of phase connections.

Series connections were insulated to ensure no air voids; circuit ring and jumper connections were hand-taped to eliminate voids or cracks during operation.

Blocking was installed and tied between each series loop to reduce vibration. Flooding varnish was also applied to the end windings and all accessible ties and blocks.

Frequency response and modal testing were performed with good results. Terminal plates and bushings were installed with new locking tabs and hardware.

Eight new dual-element RTDs were installed on each unit, and the two RTD glands were replaced to prevent any future hydrogen leakage.

Final testing confirmed the units were fit for continued operation. MD&A’s proactive recommendation to the owner/operator was routine maintenance every 30 months of operation for minor inspections and every 60 months for major inspections.

Hot Gas Path

During this outage, MD&A’s Gas Turbine Services specialists performed a modified hot gas path inspection on one of the Frame 7B units, clearly illustrating both technical diversity and customer confidence in MD&A’s full-scope capabilities.

MD&A gas turbine experts followed all critical work scope and sequencing details for gas turbine HGP inspections including casing and component removal, opening clearances and setback readings, concentricity measurements, and rotor clearances.

Bucket, nozzle, and shroud inspections then began.

In general, minor foreign object damage was noted along with thermal barrier coating (TBC) loss and several non-critical cracks. First stage nozzles, however, required additional action. Due to excessive TBC loss and critical cracks in various locations, these nozzles were replaced by MD&A with a refurbished set.

First stage shroud inspection showed that one seal had migrated aft. This seal was re-inserted and re-staked.

More work was required in the combustion section. Liners and transition pieces, once removed, showed severe TBC loss and were replaced with refurbished sets. Inner crossfire tubes and tube retainers showed significant wear and were replaced with new equipment. Primary fuel nozzles, although not available for detailed inspection due to outage duration, were replaced with a refurbished set.

In the compressor section, inlet guide vanes were hand-cleaned from heavy fouling, and migrated inner bushings were repositioned and re-staked. Rotating blades were hand-cleaned, blended, and polished due to foreign object damage and fouling. Selected compressor stationary vanes were hand-cleaned as needed to remove any heavy fouling or minor FOD. The compressor rotor was also hand-cleaned.

Beyond-traditional-scope work was also performed. MD&A installed lube oil coolers, make-up cooling water piping, and vent and drain valves, and performed a complete borescope inspection of the compressor section, which resulted in the removal of the upper half fuel gas manifold and upper half compressor casing.

For the generator, T-4 and T-5 bearings were replaced with refurbished parts.

Also, hydrogen seals and housings, and inner and outer oil deflectors, were replaced on both the turbine and collector end bearings.

A visual inspection showed no indications of any issues with the accessory gearbox.

Final recommendations included regularly-scheduled borescope inspection of combustion, compressor and turbine sections, inlet debris monitoring, periodic inlet guide vane inspections, inlet expansion joint integrity checks, crack detection of the exhaust frame, unit monitoring for hot gasses/air leaks, and replacement of 1^st stage shroud horizontal seals at the next opportunity.

Summary

MD&A is a full-service, OEM-alternative resource globally recognized for our response, craftmanship, and long-term owner/operator confidence. Projects like this stator & field rewind and hot gas path inspection, are excellent examples of our fast response, in-depth analysis, emphasis on precision, meticulous attention to detail, creativity, and inter-divisional commitment to owner/operator online trust.

Call MD&A today for any overhaul at +1 (518) 399-3616 or use our Contact form.

The post Aging Generator Stators and Field Rewinds Plus Modified Hot Gas Path Inspection appeared first on MDA Turbines.

Incoming visual and mechanical inspections were first completed, as well as a runout inspection in the lathe which resulted in machining the turbine end journal to fix high runout in this area.

As found electrical data was then collected on the generator field by completing a 500V winding insulation “Megger” test, an AC impedance test and testing for shorts within the field winding.

After testing, the retaining rings were removed, and a full blocking inspection was performed. Significant blocking damage was found which resulted in replacing the entire blocking scheme with the MD&A recommended blocking.

Additional damage was found during wedge removal, which revealed 320 wedges with significant arc damage of about 2-3 arc marks per wedge. The wedges were successfully repaired by removing a thin amount of material from the effected wedge face. After which, they were cleaned and underwent non-destructive evaluation (NDE.)

With the wedges and copper completely removed, the field was glass bead blasted and underwent NDE. HiPot testing was then performed on the inboard and outboard collector rings as well as the respective bore copper with satisfactory results.

Prior to reassembly, the copper coils were sent out for blast cleaning. The rotor body and the field body wedges were glass bead blasted. The field body was then cleaned, followed by additional NDE. The non-destructive examination revealed multiple arc marks throughout the field forging body. A formal recommendation was made to remove these indications which was accepted by the Customer.

Repairs involved machining a relief to both sides of the dovetail where the wedges meet, as well as changing the #1 slot wedges from steel to aluminum, which incorporate larger end radii. The field then underwent a second NDE which resulted in no indications.

After each coil was checked and installed, they were AC Hi-Pot tested and turn-to-turn tested. A baker surge test was also completed on each coil to ensure that there were no shorts present in the winding. The turn insulation was then coated on one side with heat-cured glue, which required a bake cycle of the rotor after winding.

After the bake, a full set of electrical tests were completed including a 500V Megger test, a winding resistance test, an AC impedance test, a pole balance test, an RSO and a complete DC turn to turn test. A full set of electrical tests were also completed after the retaining ring insulation was installed and repeated after the retaining rings were installed. All results were satisfactory.

After a heat run and electrical testing were completed in our high-speed balance facility, the field was staked and painted and prepped for shipment. Before the field was shipped, a final outgoing Megger test was completed.

The generator field rewind was then complete with a Rotor Slot Dovetail Repair.

MD&A is a full-service OEM alternative for generator service, parts, and repairs with an over 30-year track record of successful generator stator and field repairs. We focus on delivering consistent quality and value with fast response, superior communications, and innovative solutions that meet or exceed OEM alternatives.

Call MD&A today for all your generator needs at +1 (314) 880-3000 or use our Contact form.

The post Field Rewind with a Rotor Slot Dovetail Repair appeared first on MDA Turbines.

Stator Rewind

MD&A performed a complete rewind of the generator stator during this outage. The rewind kit supplied by MD&A included new stator bars, connection rings, and end winding support components.

The rewind process began with the removal of the stator wedges and stator bars. Following bar removal, the stator slot walls and core vent ducts were cleaned and prepared for bar installation. The core was both visually inspected and electrically tested using an Electromagnetic Core Imperfection (ELCID) test to identify any shorted laminations that could lead to future overheating and damage. ELCID tests were performed three times throughout the rewind process with no relevant indications found.

The winding installation included the replacement of the original end winding supports and connection rings. Additionally, Glaskyd sausage (conforming material) was installed in place of the original radial rings between the top and bottom bars in the end winding. Conductive side ripple springs were installed to ensure proper electrical contact between the slot wall and the bar. Following bar installation, a contact measurement test was performed to ensure the bars were properly grounded to the core.

New connection rings were then installed, and all series and phase connections were brazed using induction heating. The bars were appropriately blocked, and the series loops were insulated with two-piece molded caps filled with insulating epoxy filler. Following stator slot wedge installation, all final electrical tests were completed with excellent results.

Field Rewind

Once removed from the stator, the generator field was identified with damaged end-winding blocking. The field was then sent to our Turbine-Generator Repair Facility in St. Louis, MO.

Incoming electrical testing was completed once the field arrived in St. Louis, confirming multiple shorts within the field winding. After the retaining rings were removed and the unit was inspected, MD&A recommended a full rewind to remove the shorted turns and properly adjust the end-winding blocking. After the rewind was authorized by the customer, the unit was stripped and cleaned. During this time, all removed components underwent a complete visual and NDE inspection. Indications were noted on one of the snap rings. Ultimately, the customer elected to have both snap rings replaced at the same time.

After the copper windings were removed, electrical testing was completed on the bore copper and collector rings with acceptable results. The coils were blast cleaned and inspected. Following the inspection, the coils were deemed suitable for continued operation and returned to service. The wedges were blast cleaned and sent for NDE inspection. The field body was blast cleaned and underwent NDE inspections. All NDE results were satisfactory, and the field body was repainted. The field forging and field body wedges were returned to service.

After each coil was re-installed, an AC Hi-Pot test was performed, and an AC turn-to-turn test was completed. A baker surge test was then completed on each coil to ensure that there were no shorts present in the winding.

The turn insulation was coated on one side with heat-cured glue. This glue required a bake cycle of the rotor after it was wound. After the bake, a full set of electrical tests were completed and the unit underwent a high-speed balance, two heat runs (with a flux probe test before each heat run). All test results were acceptable.

MD&A is a full-service, OEM-alternative for generator service, parts, and repairs with a 35+ year track record of successful generator stator and field repairs. All inspections and necessary repairs are completed by our experts in a timely manner that allows units to be placed back online on an expedited basis.

Call MD&A today for any generator repair at +1 (314) 880-3000 or use our Contact form.

View the video to see how you can minimize costly outages by having our expert technicians perform your next full stator rewind.

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