7FH2 Partial Re-Stack of the Core Step Iron & Full Stator Rewind

Mechanical Dynamics & Analysis performed a 7FH2 Partial Re-Stack of the core step iron & Full Stator Rewind on a GE® 7FH2 generator.

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.