Generator Rotor Slot Tooth Weld Repair
Mechanical Dynamics & Analysis (MD&A) experts recently performed a Generator Rotor Slot Tooth Weld Repair. Initially, the generator rotor suffered a double ground fault incident that tripped the unit offline. The customer contacted MD&A to perform disassembly, testing, and evaluation of the condition of the unit.
Once the generator rotor was removed, visual inspections confirmed melted copper turns, damaged insulation, and arcing damage to the rotor forging.
Inspections after disassembly (unwinding) of the generator rotor revealed arcing damage to the inboard side of the collector end retaining ring and to the slot wedge above the ground fault area. The rotor forging was then blast cleaned, clearly revealing the severity of arcing damage to the generator rotor forging slot tooth sidewall.
The unit owner/operator asked MD&A to evaluate and perform an analysis of the damage in support of providing a recommendation for running/repairing/replacing the generator rotor forging.
The slot tooth sidewall arc-damaged and heat-affected material was removed, the forging NDE inspected, material composition confirmed, and the geometry of the rotor forging as well as the excavated tooth sidewall obtained. A detailed stress analysis was performed consisting of both hand calculations and finite element analysis for the damaged geometry. The analysis calculated the damaged tooth stresses under centrifugal load for both normal operational speed as well as over-speed. The results of the analysis revealed the current condition was not acceptable for safe and reliable operation.
Solution: MD&A recommended and performed a local weld repair of the damaged tooth section utilizing a weld filler material that met the base material chemistry, strength, and magnetic permeability of the generator rotor forging.
The welding testing for this repair option not only included the typical base material and weld filler material qualifications, but also actual base material HAZ (Heat Affected Zone) testing utilizing a bore ring sample removed from the coupling end of the rotor forging. The results of these tests were utilized in a final weld repair acceptance and remaining life analysis.
Repair of the rotor included: Initial dimensional inspections, nondestructive testing, and rotor runout checks; Weld prep machining to remove a section of the slot tooth; Preheat and weld buildup of the removed slot tooth section; Post weld heat treatment; Rough machining along with both surface and subsurface nondestructive testing; Final machining, final dimensional checks, final nondestructive testing, and final rotor runout checks. The rotor was then released for winding, final assembly, and balance.
A detailed acceptance/life expectancy analysis was performed on the rotor weld repair. The analysis consisted of a 3D Finite Element stress analysis, fracture mechanics calculations, as well as low cycle fatigue calculations. The results of the analysis offered positive results and increased confidence in the repair performed.
Based on the past and expected future operational parameters of the unit, it was calculated that the original rotor material, the deposited weld material, and the HAZ material would conservatively last for 36, 41, and 16 years, respectively.
The repaired rotor was returned to service without any operational restrictions and it was recommended the rotor forging be inspected after ten years of operation or 150 stop/starts, whichever comes first.
This generator rotor tooth local weld repair allowed the unit owner/operator to return to service much quicker than a full rotor forging replacement option.
MD&A experts are there when you need us! Providing comprehensive refurbishment and repair of generator stators and fields for many different OEMs across the world.
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