Generator upgrades produceoperational dividends
By R.A. Halpern,GE Power Systems
While generator rewinds continue to be used primarily to increaseunit reliability and for life extension, additional economic benefitsof stator improvement can be significant and should not be ignored
Many factors must be considered when evaluating generator upgrade alternatives. Particular emphasis should be placed on the features of stator and field rewinds and aspects of reliability, uprating and efficiency gain that typically accompany them. Many advanced components and processes developed for new units are available for existing generators during refurbishment.
The first step in considering a turbine-generator rebuild is to define the intended use of the equipment. Included in this analysis should be a review of the unit`s service life, baseload or start-stop cyclic duty, MW and megavar load requirements, and reliability issues. Establishing these parameters is critical in determining the extent of the rebuild required. For example, GE Power Systems` approach is to perform a comprehensive review of the total power train, including the generator design. Then, these elements can be compared against the existing machine design; included in this review should be an examination of the generator coolers, excitation system, auxiliaries and monitoring systems, as well as the stator and field windings.
While the unit`s intended use can provide general guidelines for the rebuild, the equipment`s existing condition is usually an equally important factor in determining the extent of rebuild. Proper maintenance on a regularly scheduled basis, as recommended, will help maintain reliability. On the other hand, lack of regular maintenance produces accelerated wear and more extensive rebuilds.
By comparing inspection results of a particular generator with a database of information from similar units, it is possible to identify suspect components that are likely to impact the generator`s future reliability. If a unit has not been inspected for several years or there has been a recent incident that potentially affects the condition of the generator, it is prudent to perform an inspection before making any final decisions on rebuild workscope. Recent advances in inspection procedures have improved the manufacturer`s ability to make timely recommendations regarding the need to repair or rebuild portions of the generator. Inspection time and cost can be minimized by completing this work without removing the rotor, utilizing advanced robotic inspection systems.
Advances in the area of nonmetallic materials, insulation systems and composites allow significant reliability improvements and life extension to be achieved through replacement of old materials. One example of this type of enhancement is replacement of an asphalt-insulated stator winding with a modern epoxy-based insulation system. However, beyond simple material replacement, significant reliability improvements can be obtained through upgrading the design of winding support systems and by improving the rotor`s cooling arrangement. Stator slot wedging systems can be upgraded to use modern wedge designs, thus reducing the damage potential created by winding movement in the slot. Conversion of an indirectly cooled rotor winding to a direct-cooled one can reduce rotor and other temperature differentials and reduce winding stresses caused by cyclic duty.
Direct rotor cooling may be required to permit generator uprating. GE has had significant success in replacing older radial-axial, radial-cooled field windings with modern diagonal-flow cooling patterns, thereby obtaining improved reliability and reduced thermal operational sensitivity. Rewinds may also be considered as a way to eliminate known problems that could lead to reduced future reliability. Top-turn fatigue failures have been a problem with longer fields equipped with spindle-mounted retaining rings. Rewinds specifically tailored to isolate top-turns and to replace the copper with more fatigue-resistant materials can be designed to overcome this problem. Similarly, machines with multilayer field windings may be rewound to eliminate copper dusting generated by galling between the layers.
Enhancing output, efficiency
Rewinds always present an opportunity to enhance machine performance. The economics of such design upgrades can often help to justify the cost of the rewind activity. The type of opportunity varies with the vintage and type of the original machine design. In terms of stator windings, modern epoxy-mica insulated components, which replace the original asphalt-mica insulated windings, can be designed to take advantage of improved dielectric capability of the new insulation system. This permits the use of thinner groundwall insulation and allows the inclusion of more copper in the winding at a given rating. This design is more efficient than a same-rated, original winding and will operate at lower temperatures.
Alternatively, improved temperature margins allow the winding to carry more stator current and to generate more output. Early designs of liquid-cooled windings can also be redesigned for improved efficiency or uprate, even though they may incorporate an epoxy-based insulation system. Improvements in strand insulating materials, including the introduction of glass, permit thinner-strand insulations to be used. These changes allow more copper to be incorporated into the stator bars. Additionally, as liquid cooling technology has advanced, bar design has evolved from designs with identical top and bottom bar cross sections having all-hollow copper strands to designs with unequal top and bottom bars having mixtures of solid and hollow strands. These changes reduce stator bar a.c. losses; furthermore, this advanced bar cross-section design may be retrofitted into a full stator rewind. Depending upon the particular design involved, substantial efficiency gains or uprate capability can be obtained with these types of changes.
When any change in the rating of the generator is contemplated, a coordinated examination of all the generator components is necessary (see figure). Area 1 indicates the increased capability potentially available if the stator is rewound. Area 2 is the additional capability potentially available if the field is also rewound. Notice that if the stator alone is rewound, the power factor in the lagging region at the new maximum rating is increased, unless the rotor has margin that was unused at the previous rating.
Unless there is unused temperature capability in the core-end region, leading power factor capability becomes more restrictive. Some cases require changing from magnetic to nonmagnetic retaining rings to control core-end heating. These changes also require a design study and possibly upgrading or replacing the coolers and excitation system. For this reason, rewinds can best be addressed by the original equipment manufacturer having the knowledge of the unit`s electromagnetic design.
The performance improvement that can be achieved varies widely from machine to machine. In a conventional hydrogen-cooled generator, an uprate of up to 40 percent, as well as improved efficiency with a combination of a new direct-cooled field, new exciter and new armature winding and coolers is possible.
Stator winding considerations
Generator winding methods have evolved significantly over time. The tendency in the past has been to rewind the generator as it was originally built. However, many of the old materials have become obsolete, and new materials must be substituted. This forces changes in the winding design. One approach to simplify this process is GE Power Systems` efforts to standardize rewind methods and materials. The method used depends on the type of cooling and insulation for a particular unit.
In small machines, where flexible insulation is used to replace either coil or bar asphalt, windings are pushed down in the slot by a simple flat wedge rather than a tapered wedge. Tapered wedges and springs tend to deform the insulation due to high mechanical loads. However, in conventionally cooled machines with rigid insulation, side-ripple springs, top-ripple springs and tapered wedging arrangements are all effective; but only solid-rigid fillers are used between bars. Electromagnetic force levels increase when liquid cooling is used and, in such machines, conforming materials are used between bars and under bars and a hose-mold bake process is required. This introduces a complexity into the rewind process, but is essential to the zero-clearance assembly required to suppress bar vibration. Regardless of machine size, great attention must be paid to the wedging tightness. Careful selection and control of wedge material is essential to avoid shrinkage and creep.
There is great divergence of designs for end-winding support systems from one manufacturer to another. Despite divergent approaches, basic objectives are the same. The winding must withstand running and fault electromagnetic forces, accept expansion and contraction of the bars, and be free of mechanical resonances at running frequency and double frequency. For conventional machine rewinds, GE Power Systems uses a resin-impregnated felt and resin-impregnated glass fiber for bar support, with an underlying structure of fiberglass rings that move freely and axially in relation to the winding. These materials are suitable for operation at Class F temperatures.
In large machines, the Tetraloc end-winding support system is used. The system is a combination of blocks, ties and conforming material, which provides excellent stator end-winding restraint. Conventional and Tetraloc systems both require baking of conforming materials. In special cases where added capability or strength of the bake system is not required, a nonbake rewind system for smaller, Class B applications has been used successfully. In some cases, this method represents the best rewind solution for a given application.
Old asphalt windings, which are converted to a rigid insulation system for uprating reasons, have special requirements. The tape of the rigid system is applied by machine, which requires slightly different geometry of the bar than the original design. This leads to longer end-winding lengths, requiring careful review of electrical clearances and ventilation. Some coil windings must be converted to bar designs. These conversions sometimes require changes in air-shield location, and again, there is a need to obtain knowledge and experience of the original equipment manufacturer.
Emergency repair alternatives
When an emergency situation arises due to a forced outage or a maintenance high-potential failure, a variety of generator repair options are available. A complete rewind permits the owner to have the winding returned to a condition that is as close to new as possible. This may be the only option in cases of severe damage or contamination. While a complete rewind is the most costly and time-consuming solution, it ensures future unit reliability is maximized and provides the greatest flexibility to the designer for providing uprates and efficiency improvements. Full rewinds can be installed during short outage durations if new design retrofits and advanced installation techniques are used. A recent example is the 28-day rewind of Georgia Power Corp.`s Wansley Station (see sidebar). An added post-rewind benefit is the ability to extend the outage inspection interval to double the original time, thus reducing maintenance costs for testing and field removal.
If damage is not extensive and spare bars are available or can be quickly manufactured, a partial rewind is usually the fastest repair option. Owners of duplicate machines also may have spare bars, and a utility may be able to negotiate purchase. With smaller units, if damage is limited, a possible strategy is to isolate damaged bars from the rest of the winding. This should only be done under the guidance of the original manufacturer, as isolation of bars will increase the air gap flux waveform`s harmonic content and create excessive rotor surface heating if harmonic levels are too high. This technique limits generator output and should be regarded as only a temporary solution.
Leaks on water-cooled generators have occurred since the 1960s, and the diagnostic tools and testing to locate and repair them have existed for several years. The major leakage locations have been at brazed joints, such as the Teflon-hose-to-liquid-header connection, hose-to-bar clip, copper piping braze joints and cracks, bar clip window, cast clip porosity and strand-to-clip. Most of these can be repaired by brazing or TIG welding, with the exception of clip-to-strand leaks. It must be noted there are about 9,000 potential leakage locations in a typical water-cooled generator.
Because brazing repairs and clip replacements do not offer reliable long-term repair, GE turned to new technologies that had proven successful in the nonmetallic field–namely epoxies that have been used extensively on new construction. An epoxy which had been used previously for its qualities of bonding to oxide layers on copper was selected as the best overall repair that addressed the issues of effectiveness, reliability, ease, safety and speed.
Using this approach, all work is done remotely without removing the clip. This proprietary epoxy advantage of the oxide layer on the strand face by chemically bonding to it. Two layers of epoxy are applied to provide a barrier to water flow and prevent leaks. The inside surface of the clip is also sealed with epoxy at the window to prevent any potential leak sites. This repair method is a proven permanent solution to clip-to-strand water leaks and crevice corrosion and offers an alternative to stator rewind for those units that do not have wet bars. To date, this repair process has been proven to be 100 percent successful on more than 20 units and more than 900 clips during the past two years; continuous testing supports this experience.
For certain small industrial units, increased capability may be the major goal of a stator rewind. Changes in materials can be of significant benefit when rewinds are considered. If uprating is a primary goal, the entire machine should be reviewed and all potential weaknesses of unreliability addressed. z
Ron A. Halpern has been with GE for 22 years and is generator product line manager, responsible for strategic direction of GE`s generator parts and service business. Prior to his current assignment, Halpern held various management positions in contract administration and gas turbine parts, after having worked 10 years in generator engineering as technical leader and engineer on all aspects of generator operation, maintenance and rebuilds. Halpern received his bachelor of science degree in mechanical engineering from the City College of New York.
The small amount of space taken up by rotor slot insulation and mechanical design restrictions limit the options for enhancing thermal performance of an existing generator field.
Photo courtesy of GE Power Systems.
Figure: Generator uprate capability expressed in graphic form: Area 1 indicates increased capability potential if stator is rewound. Area 2 represents additional capability potential if field is also rewound. Photo courtesy of GE Power Systems.
To meet a rapidly approaching winter peak demand, The Southern Company`s Georgia Power Corp. needed a high-quality generator rewind in an extremely short period. GE Power Systems responded by offering a 33-day rewind window, then–with the assistance and support of the client–beat the set plant outage schedule by five days.
The rewind was performed on Georgia Power`s Wansley Unit No. 1, an 880 MW GE steam-turbine generator-one of the company`s highest-rated power density machines.
The unit was removed to correct damage from bar abrasion and to prevent future wear and stator bar water leaks. Although the original scope of the Wansely project included just the stator rewind, problems with the generator field also were discovered and corrected. Emergency material was provided to solve this problem on a tight schedule, and the field was completely rewound to prevent copper dust formation and potential field grounding. Both the field and stator rewinds were performed concurrently.