Rapid overhaul of generator rotor needed due to lack of maintenance
Carrying out routine maintenance on a large turbine generator calls for expertise as well as the experience to achieve an efficient and cost-effective project.
We all take electrical power for granted; knowing we have lights, heating or music at the touch of a button. It all relies on power stations meeting the demand as it fluctuates throughout the day, which depends on maintaining reliable generation equipment.
For an Irish power station, the plan was to carry out routine maintenance on a 42 MW generator, which provides 50% of the generating capacity of the site. As such, time is always a crucial factor in these projects, so careful planning is essential to minimize downtime. However, when the initial findings discovered issues beyond routine maintenance, a rapid repair was needed. The generator was originally built in 1997 and initially spent two years in the Caribbean before being installed in the Irish power station. As part of the move the machine was converted from 60 Hz to 50 Hz output frequency.
The repair project was coordinated with assistance being provided by nearby facilities in Daventry, Aberdeen and Southampton. The allocation of work across different service centers ensures each designated task is carried out by the service center that specializes in each required discipline. This also allows several tasks to be carried out at the same time, which is necessary to achieve the short turn-around expected by the customer.
A team of engineers from Aberdeen built the platform, stripped the unit, installed the custom rotor removal gear then unthreaded the rotor which was sent back to the service center. The stator was cleaned before a full life-assessment investigation was carried out to determine the integrity of the component. The tests carried out included offline partial discharge, tangent-delta, wedge tightness, core lamination imperfection and end-winding resonance.
Unfortunately, the recommended maintenance schedule for the generator had not been followed and this contributed to a number of more serious issues that were discovered during the routine maintenance.
Uncovering hidden issues
In the service center, the initial visual condition of the generator had already highlighted some detrimental issues. The complete rotor body was covered in rust and the single-length aluminum wedge was corroded and flaking away across the entire length. As-received electrical tests were carried out before the retaining rings were removed.
A cross connection coil was found to have migrated with bare copper only millimeters away from the earthed shaft. A catastrophic failure would have occurred if there had been contact while in service. The overall insulation system on the rotor was also found to be very poor. The retaining rings were corroded and both failed ultraviolet dye penetrant testing. Ultrasonic testing on the single sleeve bearing also highlighted extensive de-bonding of white metal from its parent backing material.
The repair engineers recommended that the rotor be completely refurbished. This was to include a full strip down to the main rotor forging, removing all rust, cleaning all copper coils, machining new retaining rings, machining a set of new rotor wedges, re-metaling the sleeve bearing and rewinding the unit using existing copper. The original work scope was to be turned around in 25 days, with this variation in the order, the complete refurbishment and installation would be completed in 50 days.
At the onset, the challenge was to remove the individual slot wedge which was approximately 2.5 m long. The corrosion and bonding of dis-similar metals between the wedge and rotor forging did not allow easy removal. All wedges had to be split in half using a custom-made milling head.
Once the rotor was fully stripped and cleaned, the individual parts were detailed for reverse engineering. Due to the destructive removal of the rotor wedges, the new wedges were detailed off the rotor forging wedge slot.
Creating exact replica parts, fast
As with any reverse engineered part replacement, it is essential to use the same or superior material while maintaining the same tolerance, as per the original part, to ensure that the rotor performs and meets the original equipment manufacturer’s (OEM’s) design criteria. Material analysis on the wedges matched an old French standard for aluminum alloy which took some considerable time to source the required quantities and lengths. All machining works were carried out and quality controlled in-house in the Birmingham Service Center.
The re-metaling of the bearing was tasked to the Southampton Service Center. Bearing clearances for this specific application were calculated, specified by the engineers and supplied to Southampton to include into the quality work pack.
The biggest challenge was to source the specialized high tensile, stainless-steel forgings in the correct size, which are not readily available. Only a handful of forge masters and manufactures have the experience and expertise to produce these items. A typical lead-time for procuring retaining ring forgings and machining is around six months; this could be longer when procured from the OEM.
The projects team was able to procure a couple of forgings which met the engineers’ specification from the U.S., which arrived within five days. In order to minimize the time required to complete the machining, the task was split between the Birmingham and Avonmouth Service Centers.
The retaining rings were finish-machined and inspected within a 15-day period, with the time from point of ordering included. This extraordinary effort went a long way to keeping the overall project schedule below the 50-day target.
Ensuring perfect balance
The complete rotor was rewound with new insulation and the cross-over connection was redesigned to eliminate any future migration. Once completely rebuilt, the rotor was electrically tested and dynamically balanced in a high-speed balance pit.
As this unit was of a single bearing design, a stub-shaft with a journal was designed and manufactured to complete the balancing and over-speed exercise. This substantial stub shaft was fabricated and machined at Daventry to the engineers’ drawings and specifications.
As the unit was practically a brand-new rotor, a 120% overspeed test was carried out for two minutes on this rotor and witnessed by the customer. This specification is in line with international standards for brand new turbo-generator rotors.
With the project almost complete, engineers returned to the site to help with reassembly of the generator and to conduct the onsite load testing and recommissioning before the project was successfully concluded.