The case for rewinding rotors onsite

A generator may require one or more rotor rewinds during its service lifetime, work that’s typically done in a shop for all but the largest rotors. However, onsite overhauls can make economic sense for small, medium-size, and larger rotors when there is adequate workspace on or near the turbine deck and a crane is available.

Onsite major maintenance of generators is not unusual. Paul Heikkinen, director of generator services for Turbine Generator Maintenance Inc (TGM®), a participant in the CTOTF™ vendor fair, reminded that “Generator stators generally are rewound in the plant because it is too difficult and risky to send them out. Once owner/operators consider all the factors, they typically decide to bring the tooling and supplies onsite and perform the rewind right on the turbine deck. These are the same factors to consider when a rotor must be rewound,” he said.

Very large rotors usually are rewound onsite because of the difficulty in moving such a heavy component, and the associated liabilities. Heikkinen illustrated the point this way: If you have a 1000-MW generator down for a major and the field rolls off the truck on the way to a rewind facility, it can take three or more years to get a replacement rotor.

Planning an onsite rewind. A rotor rewind is a major undertaking and detailed planning is critical when it is done onsite because standard shop practices may not apply. But there’s plenty of time for planning, according to TGM’s Pat Welch, because you usually can anticipate the need for a rewind well in advance of when the work must be done. With proper monitoring and testing of your generator online and offline, as recommended by the OEM and suggested by industry best practices, you can track performance loss over time and plan accordingly.

Ample laydown space is another key requirement for success. The turbine deck typically is used for this purpose, but because rewinds generally are performed during a major outage in conjunction with a lot of other work, it might not be available. A warehouse, or suitable temporary building, is a viable alternative to the turbine deck. But keep in mind that wherever the work is done a crane is required. Plus the workplace should have temperature and humidity controls.

Personnel and tooling. Once a suitable work area is identified and prepped, personnel, parts, and tooling are brought onsite. Because tasks generally are more mechanical in nature than electrical, machine tools (lathes, mills, etc) and specialty equipment often are required for work on retaining rings, blocks, and wedges.

Adequate floor space is important when the individual coil segments are extracted. They must be staged somewhere accessible to personnel responsible for moving them to the various rehabilitation stations. Depending on the rotor, the crew may build specialty jigs and racks to hold the windings (Fig 1).

1. Technicians recondition the copper. It is then insulated and reinstalled in the generator rotor

1. Technicians recondition the copper. It is then insulated and reinstalled in the generator rotor

Removal of debris is another important consideration—more so for rotors than for stators. Heikkinen offered the following comparison: “For a stator rewind, the heavily insulated copper is scrapped and new copper is formed and insulated. The story is different for rotors. As you are unwinding and removing rotor coils, the insulation can very quickly be stripped away and discarded. The copper is usually in good shape and just needs some grit blasting or hand polishing to remove residue.”

Possible advantages of an onsite rotor rewind include the following:

      • No transportation. Because the rotor is not being transported offsite, the risks associated with shipping—such as late delivery, damage caused by an accident, etc—do not apply. Another point to remember: Rotors are hygroscopic—they absorb moisture—and must be sealed in waterproof packaging before return shipment. Otherwise, an as-received Megger test may indicate the rotor is grounded if the trip included driving through a rainstorm or snowstorm. Also, transportation takes time. Depending on how far the plant is from the workshop, it can take more than a week to transport the rotor one way by truck within the Lower 48.

      • Engagement. Rewinding onsite allows participation of plant personnel in the process without having to travel to a repair facility. Issues can be addressed in real-time.

      • Cost. It is difficult to directly compare the cost of an onsite rewind to work done at a shop but, typically, they are about the same. While equipment must be brought to the plant and the technicians have to stay locally during the project, the cost of rotor transportation is eliminated from the spreadsheet. So is the expense of sending plant personnel or a representative to the repair facility to monitor work.

      • Shorter schedule. Onsite rotor rewinds can take less overall time than shop rewinds which may translate into revenue opportunities.

One possible disadvantage to rewinding a rotor at the plant: There is no way to perform high-speed balance test without transporting the rotor offsite. This may or may not be an issue. Welch offered this advice: “If the rotor has a history of instability, not related to the winding, then a high-speed balance should be performed.”

But if the balance was fine until rotor shorts developed, there is no compelling reason to incur the cost and risk of a high-speed balance. The argument that a high-speed run is necessary to determine if any shorts were created during the rewind has little merit according to TGM personnel: If the work is performed correctly, they said, there should be no shorts to worry about.

Onsite rotor rewind at Nucla. A lightning strike at Nucla Generating Station early in October 2014 caused a power surge that over-excited the plant’s 73-MW generator. Vibration amplitudes recorded on the machine’s two bearings, which had been about 2 mils since commissioning, suddenly doubled. Load was halved and hydrogen pressure increased to maintain pre-strike vibration amplitudes. All indications pointed towards a thermally sensitive rotor.

The unit was taken out of service while operations personnel decided how to proceed. Tim Jones, TGM technical director/generator specialist, and his team, including a vibration expert, were dispatched to the plant to conduct diagnostic tests. “We concluded that the unit was thermally unstable and that the increase in vibration was related to thermal regulation, megawatt loading, as well as reactive power,” Jones said. Also, electrical testing indicated that there were shorted turns within the main field in coils 1 and 2.

TGM told plant owner Tri-State Generation & Transmission Assn it possibly was looking at a condition referred to as “slip-sticking.” This can occur when the rotor heats up and the copper expands faster than the steel forging. The hot copper slides out of the slot exits and when it cools, for one reason or another, it is unable to slide back into position. In short, the copper stretches and then it binds.

Diagnostics complete, it was clear the 31.5-ton, 30-ft-long rotor had to be pulled (Fig 2). Greg Keller, Nucla’s maintenance superintendent, explained, “We knew that we had shorted turns, but we wouldn’t know the extent of the damage until we pulled the rotor and removed the retaining rings. When that was done, we confirmed the slip-stick condition.”

2. Pulling the rotor—all 31.5 tons of it

2. Pulling the rotor—all 31.5 tons of it

Damage identified, the decision was made to perform the rewind onsite. Keller said, “For me, I prefer work be done onsite when possible, particularly when it involves critical components. I don’t like to ship those components over the public highways because of the risks. If anything happened to that rotor, it would have been absolutely catastrophic.”

Final steps. “We knew we could not do a high-speed balance test onsite, and that was a concern,” Keller added. “So we carefully brought up the machine for the initial spin-up. The plan was to spin the rotor, identify balance problems, allow the turbine time to cool down, remove the upper end bells, and make the balance adjustments.”

None of that was necessary, the maintenance superintendent continued. “We rolled the generator rotor up initially on just air and had really good balance numbers. It was running smoothly, so we put it on hydrogen, and ran it up for an overspeed check and then back down to its operating speed of 3600 rpm. The generator continued to run smoothly. For the two bearings that historically had vibration issues, we detected approximately one mil of vibration. So, the rotor is now running with less vibration than before it was taken apart.”  

Work was declared complete just before Christmas; the plant had been down for 77 days.

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