Many of the large liquid-cooled generators operating in power plants around the world were manufactured a quarter of a century or more ago by a variety of OEMs. This aging fleet is increasingly susceptible to bar, end winding, and core variation issues as well as stator bar liquid leaks, posing serious reliability and maintenance problems. Leaks, bar or core failures that occur on machines operating at high voltage and high bar electromagnetic fields can cause a forced outage, unplanned rewinds of the stator and field, substantial contamination of the core, and considerable downtime of several months. When this happens, losses can run into millions of dollars.

GE Energy Services offers various solutions, including epoxy injection repair for leaking clips, and bar replacements for wet bars and those that have experienced damage from core or end winding vibration. State-of-the-art monitoring systems, advanced inspection techniques, and diagnostic tools are available to head off equipment problems and reduce maintenance costs.

However, to eliminate the potential for future problems and achieve the highest level of reliability as well as extending the life of the generator, a full stator rewind is often the recommended solution. The drawback is time, since rewinds of large generators have traditionally taken more than 45 days. That is not acceptable in today’s deregulated market, where power plant owners are facing greater competitive pressures to minimize outages and keep their plants on line.

Faced with this dual challenge – an aging fleet of generators in need of reconditioning and utilities demanding shorter outage times – GE Energy Services embarked on a programme to reduce stator rewinds for liquid-cooled generators to 30 days or less, while improving quality and reliability of both parts and installation.

A combination of planning, teamwork and technology enabled GE Energy Services to not only meet, but also surpass those ambitious goals. Since the autumn of 1996, GE has completed several rewinds in less than 30 days, including a recent 21-day effort at the James A FitzPatrick Nuclear Power Plant in New York state that is believed to be an industry record.

“Our team did a great job of preparing for this project, and GE did a great job of execution,” said Ned Monaghan of the New York Power Authority, owner of the FitzPatrick plant. “Overall, it was the best job I was ever involved with.”

The key factors driving the development of what GE calls its “Quality Short-Cycle Rewind” programme for liquid-cooled generators were that our customers’ needed a high quality rewind solution, and these rewinds had to occur within a customer’s outage window, usually 30 days or less. This was compounded by the increasing competitive pressures of deregulation, making shorter outages even more critical.

So the stage was set for the technolgical challenge: to take at least 15 days out of the typical generator rewind cycle. Reaching that target required an intensive effort that included detailed and contingency planning, the development of advanced tooling and high-tech equipment, improved scheduling, and comprehensive hands-on and classroom training.

Planning ahead

The foundation of the short-cycle rewind programme is planning and teamwork. The key to achieving success in cycle time reduction is planning, planning and more planning. In addition, teaming with the customer is essential, as is a dedicated team of people with a passion to succeed.

For example, planning for the FitzPatrick project began 10 months before the actual rewind. The project team held biweekly and eventually weekly meetings that included anyone who would have an impact on the rewind, to prepare a ‘roadmap’ for the overall project.

The up-front phase includes contingency planning and advance procurement and inspection of all the parts and equipment that are needed for the project. In addition to this, complete preparations are held the first week the winders and generator specialists are at the site, ensuring that all members of the project crew know what their roles are and exactly how their assignments fit into the overall schedule.

Staging and lay-down logistics also play a critical role in the eventual success of a project. A typical rewind for a large generator involves about 150 tons of parts and equipment, and space on the turbine deck usually is limited. Positioning parts and equipment into modules and arranging them so that the right parts are available when needed, from one shift to the next, requires careful planning and coordination. For example, the arrival of the larger components has to be scheduled around the availability of the customer’s crane to avoid any lost time.

Improvements in this first phase of a rewind project – contingency planning, advanced parts procurement, and staging and laydown logistics – can be the most critical for reducing outage time.

Time-saving technology

New advances in technology were integrated with new processes to further reduce cycle time, such as development of a power distribution module. Heavy duty power supplies are rarely conveniently located on a turbine deck, where they are needed during the rewind process. To simplify the wiring and equipment powering process, GE designed a new module that requires only a single connection to the plant power supply. All power required for the rewind can be distributed directly from the module to the points on the turbine deck where it is required.

Custom-designed winding removal tooling was developed to quickly, efficiently and safely remove the old stator bars without creating any airborne asbestos issues. The tooling was arranged in modules for each stage of the rewind, further expediting the overall process. Traditionally, it would take over four days to remove the 72-144 bars from a large generator. Using new removal tooling, this process is reduced to about 24 hours, and the need for asbestos abatement has been minimized.

GE’s new Flush and Bake Skid was developed to replace the traditional stator water cooling system. In the past, the rewind team would have to use the plant’s water cooling system, which might require breaking into existing piping, bringing a boiler on site to heat the water to the appropriate resin curing temperature, and then running it through the generator. In addition, site permitting for fossil-fueled boilers in nuclear plants sometimes was difficult to obtain, complicating the project.

The new flush and bake system has its own valves to eliminate the need to disrupt the plant’s water system. It flushes the generator and then runs hot water through the bars. The flush and bake process formerly took 4-5 days, but the new skid-mounted unit reduced this to below 30 hours on the project critical path.

Another key technology advancement was specially designed wands for induction brazing, to speed brazing while while improving quality and safety. Compared to traditional torch method of brazing, induction brazing saves 1-2 days in the overall rewind cycle.

Parts improvement technology

In addition to the installation improvements, new and improved parts have been developed. According to Karl Tornroos, Generator Application Engineering Manager for GE Energy Services, “The outside of the new stator bar may look the same, but there are many improvements inside, including major clip to strand braze upgrades to eliminate the crevice corrosion clip to strand leak phenomena. These improvements also include new technology induction cleaning, improved induction braze equipment, improved braze filler, and enhanced process control.

“After a lot of clip to strand testing, GE has developed a high statistical confidence in a very long lived braze. We have produced almost a year’s worth of bars with a zero braze or leak defect rate in our clip to strand braze area, and we expect this rate to continue.”

Design engineers continue to optimise the solid and hollow strand package to increase efficiency by 0.1 per cent. Since generators are 99 per cent efficient machines, the 0.1 per cent is actually 10 per cent of the total losses. For a 1000 MWe unit, this could reduce losses by 1000 kWe.

Along with the use of new top ripple springs between the bars and the stator wedges to keep the bar tighter over longer periods of time, these new features improve overall reliability and performance of the stator bars.

Success stories

Wansley, Rootville, Georgia

GE’s first short-cycle liquid-cooled generator rewind project was performed in late 1996 at Wansley Power Station in Rootville, Georgia, USA. To meet a rapidly approaching winter peak demand, Southern Company’s Georgia Power Corporation needed a high-quality generator rewind in a very short time. GE Energy Services offered a 33 day rewind, then – with the assistance and support of Georgia Power – beat that schedule by 5 days.

The rewind was on Wansley Unit 1, an 880 MWe GE steam turbine generator. The project team used all the key short-cycle rewind techniques, including fast-winding removal tooling to quickly and efficiently remove the old bars with minimal contamination. Improved site management organization on the turbine deck expedited the job flow. New parts designs, induction brazing of the copper components, and the new flush and bake skid all played key roles in the success of the project.

During testing to verify winding integrity, the leakage testing cycle was reduced by the use of ultrasonic flow measurements and a new ‘Quick Skid’ which accelerated the leak testing of the liquid cooled windings.

Although the original scope of the Wansley project included only the stator rewind, problems with the generator field also were discovered and corrected. Emergency material was provided on a very tight schedule, and the field was completely rewound. New 18Mn-18Cr alloy retaining rings were also installed to replace the original alloy rings, which were prone to stress corrosion. Both the field and stator rewinds were performed concurrently and completed within 28 days.

Since the Wansley project, GE has used its quality short-cycle rewind capabilities to complete 18 liquid-cooled generator rewinds, and seven are being performed during the 1999 spring outage season. Several of the completed projects have resulted in milestones for the industry. The following are two examples.

Three Mile Island

Owned by General Public Utilities (GPU), the 827 MWe Three Mile Island Unit 1 plant went into commercial service in 1974. In 1995, GPU began an extensive evaluation of all options, and selected GE to do a generator rewind.

The rewind of the 72-slot, double-ended, water-cooled generator was completed in 24 days, with a breaker to breaker outage of 42 days. At the time, this was considered a record for a nuclear generator rewind.

Dick Bensel of GPU cited “unprecedented teamwork” between GE and GPU. “The willingness of the team to think and act outside the lines, combining new methods with traditional processes, and the can-do attitude of the winders all played major roles in the success of the project.”

James A FitzPatrick

As mentioned earlier, this project was completed in 21 days, which is believed to be a record for a project of this size. This 800 MWe plant, owned by the New York Power Authority (NYPA), has been in service since 1975. It includes a GE boiling water reactor, steam turbine, and liquid-cooled, 72-slot, 144-bar generator.

During previous outages, water leaks had been discovered in the generator’s clip-to-strand braze connection. While eliminating those leaks and maximising the long-term reliability of the generator were the key objectives of the rewind, it also paved the way for future uprate possibilities.

GE estimated the rewind would take 27 days – and then beat that schedule by 6 days due to improved techniques implemented from previous rewinds. The project team members were from NYPA, GE Energy Services and GE Nuclear Energy.

During the outage, the project team employed “Six Sigma” methodologies to continually measure results and better prepare for the next refuelling outage. The Six Sigma Quality programme, a rigorous, highly disciplined process to reduce defects and drive quality improvements, has been implemented across the General Electric Company.

Looking ahead

It is still necessary to continue to look for ways to refine and enhance the quality short-cycle rewind programme. At every site, experience teaches new and better techniques and processes, both from the craftspeople and specialists, and from the customers. Lessons learned at one project are incorporated into the training for the next.

As the processes are updated, they are immediately communicated to the generator specialists in the field. In the days when procedures were kept in binders, updating them was cumbersome. Moving to CDs helped, but even that wasn’t enough. New technologies and changing procedures were being implemented too quickly for this to solve the problem. The generator specialists on site might receive a CD every six months or so, but they would need to know what procedures on that CD had subsequently been revised. “Now, GE Energy Services has developed a system called GENDOCS with an index that enables an electronic message to be sent out every time a procedure is changed.

The index is the first thing that a generator specialist looks at when they go to perform any job. If the index indicates the latest procedure is not on the CD, a message is sent to the document server. Within minutes, a new procedure is transmitted.

It is all part of GE Energy Sevices’ philosophy of continuous improvement. Processes are constantly being reevaluated and refined, and ‘best practices’ from previous short-cycle rewinds are applied to new projects. The demand for short-cycle, high-quality rewinds will only increase as power producers around the world face growing competitive pressures to keep their plants operating efficiently and cost-effectively.