Mechanical Dynamics & Analysis has analyzed and resolved generator stator bar abrasion events that have become common on some newer GE® Frame 7F and 324, 330, or 390 generators. The damage can cause ground faults leading to forced outages and complete stator rewinds.

A principal cause of the abrasion is traceable to side ripple spring-induced damage.

All high-output, high-voltage machines experience elevated stator bar forces during operation. These forces can also be intensified by today’s start/stop operations.

OEM documents on these machines include warnings about ripple spring migration resulting in possible bar abrasion and leading eventually to stator failure. Although not all units are susceptible to these bar abrasion issues, MD&A recommends investigation and, if required, correction before the problems become critical.

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The details

Stator bars lay within the stator core iron, and bar sparking can occur when the energized bar loses contact with the stator core. The loss of contact occurs when there are air gaps, or voids, between the bars and the stator core.

Unwanted movement or bar fluttering must be eliminated. Usually, the bars are supported radially by top filler, top ripple springs, and stator wedges. Support in the tangential direction, to avoid fluttering, is normally accomplished by semi-conductive side ripple springs or side packing.

The side ripple springs provide continual pressure and contact, compressing the bar against the stator core iron. The ripple has a 45-degree orientation to the bar as opposed to the 90-degree ripple orientation for top springs.

Proper installation of side ripple springs must maintain 80 to 90 percent contact between the face of the spring and the side of the insulated bar. This gives optimal compression to eliminate any voids between the bar and core.

Example/case study

MD&A experts found evidence of bar abrasion in an air-cooled machine containing side ripple springs and experiencing partial discharge.

Bar removal and investigation revealed clear evidence of partial discharge that could be attributed directly to side ripple spring damage.

Forces applied to the bar and spring during operation or a lapse during the manufacturing process had flared the spring’s corner, causing a sharp edge to be present resulting the damage to the wall insulation. Penetration into the side wall insulation was approximately 3 mm (0.11 in.).

MD&A found similar damage events in 2015 and 2017, leading to full rewinds. The first unit had operated just over 19,000 hours, with 700 starts.

Solution and guide

MD&A experts can inspect and analyze these side ripple spring installations to detect any potential for stator bar damage.

Replacement with smooth-corner side ripple springs should eliminate the potential for cutting into the bar insulation. It remains critical to specify the proper thickness of the ripple spring, and contact between the spring and bar should remain at 80 to 90 percent.

Stator bars need to be manufactured for proper space between the stator core and insulated bar and must account for the chosen dimensions of side ripple springs or side packing design. The bars must also be uniform down the slots.

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

Additionally, our experts have Stator Rewind Kits available for sale. There are 2 options: GE® 7FH2 Stator Rewind Kit with 6” of Step Iron Stator Bars and GE® 7A6 Stator Rewind Kit.

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 (314) 880-3000 or use our Contact Form.

The post Generator Stator Bar Abrasion Risk and Prevention appeared first on MDA Turbines.

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.

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.

GE^®  7FH2 and 324 type generators have a history of end-winding dusting as a result of materials used during manufacturing. A major contributor to this issue is the connection ring blocking.

On some affected generators, the utilization of consolidated dry ties for the connection ring and connection ring to axial support blocking causes the Textolite™ blocking wrap felt and the connection ring mica insulation to lose their bond. When this occurs the connection ring blocking material can abrade the mica insulation to the point where the copper is exposed.

If not addressed the dusting can lead to serious damage to the generator up to, and including, a ground fault.

MD&A has trained personnel who are qualified to inspect the stator end-winding and identify if the unit has consolidated dry ties being affected. Our experts can solve this problem by replacing both the connection ring and connection ring to axial support blocking and ties with new blocking and wet ties.

Have MD&A look at your stator today. Call our Turbine-Generator Repair Facility at (314) 880-3000 or use our Contact form.

Also, see MD&A’s Solution 7FH2 Spring Migration Repair

The post MD&A Solution for 7FH2 & 324 Stator End Winding Dusting Issues appeared first on MDA Turbines.