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 or use our Contact Form.

The post MD&A Generator Stator Rewind with Timely New Bars appeared first on MDA Turbines.

Several 7FH2 units have experienced failures of the core step iron, caused by inadequate compression of the core laminations.  This issue was first brought to light by GE® TIL 2260, which only covered certain units; however, this recent failure was outside the scope of this TIL, indicating that other units may be at risk of a similar failure.

MD&A recommends that all 7FH2 generators be subjected to core tightening, preferably during the rewind of the stator.  However, if no rewind is in scope, then the core tightening should be performed during the next planned outage. Many of the 7FH2 units are more than 25 years old and are approaching their intended design life.  This particular unit had the damaged core iron replaced and windings replaced.

The unit was last inspected during spring 2020 utilizing a generator robot and was classified as “good” with no issues noted, indicating that the issue can develop & present over a short period of time.

The outage started as a Gas Turbine Hot Gas Path with Generator Robotic In-Situ Inspection. The Generator scope included a full battery of electrical testing on the field & stator. The Robotic In-Situ Visual Inspection included a Wedge Map, & ELCID core iron assessment.

The Robot crawler traversed axially from CE (collector end) to TE (turbine end) utilizing both a straight facing camera and 360-degree rotating camera to assess wedges, core iron condition, field body, and to note any abnormalities. The initial inspection proved highly challenging due to the amount of oil/greasing present on the stator, particularly on TE – as the robot began slipping. At the 12 o’clock position, MD&A noted high amounts of contamination at TE step iron and body wedge system.

After discussing recommendations and repair options, the Customer opted for a partial re-stack of the core step iron and full stator rewind. Three full packs of iron and full set of 7FH2 windings were readily available as part of MD&A’s inventory. The generator field was shipped to MD&A’s Turbine-Generator Repair Facility where thorough inspection and testing of the field was performed. This allowed the field to be in a controlled climate to reduce the potential for moisture ingress during the stator repair activity duration.

The repair of this unit required that the generator be stripped of all stator windings, followed by a thorough cleaning of the stator to remove all core ferrous debris from the stator prior to performing the rewind.

The damaged iron was removed from the stator core in circumferential sections moving axially inboard from the TE step iron where the fretted iron was located. Core iron packs were removed just past the damaged section to ensure reliability for future operation of the unit.

Once the core iron was removed, new core iron was replaced in the reverse process, moving axially outboard in circumferential bands. With the new core iron in place, MD&A then axially compressed the core to the required values. A Core Loop test was performed at the completion of the partial re-stack to verify successful compression of the new core and to ensure no hotspots were present on the newly installed or original remaining core iron.

After the core iron was replaced, new 7FH2 generator stator windings were installed. All quality checks were performed according to MD&A procedures to ensure reliable future operation of the unit.

Stator Experts

MD&A has a long track record of successful generator stator inspections and rewinds on-site. You can count on MD&A’s attention to quality!

Additionally MD&A has Stator Rewind Kits available for immediate use along with new step iron laminations in stock for 7FH2 units. There are several options of on the shelf rewind kits including GE® 7FH2, 7A6, 9A4 and 324 units.

For any questions or concerns about operational issues with these large generators, or to arrange service or inspection, contact MD&A’s generator experts today at +1 (314) 880-3000 or use our Contact Form.

The post 7FH2 Partial Re-Stack of the Core Step Iron & Full Stator Rewind appeared first on MDA Turbines.

MD&A uses superior materials to complete the job. You can count on MD&A’s attention to quality!

View the video to see how you can minimize costly outages by having our expert technicians perform your next full stator rewind. For additional information call MD&A’s Generator Division today at (314) 880-3000 or use our Contact form.

The post Generator Stator Rewind 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.

The post Comprehensive Generator Stator and Field Rewind appeared first on MDA Turbines.