Different Methods Affect Winding Cost & Reliability
When comparing manufacturing methods, not all stator windings offer comparable winding longevity or long-term value.
Generator owners contemplating a rewind face multiple choices as they prepare their budgets and write specifications. The choices and variables are not always clear, and the typical marketplace dialogues may only serve to confuse potential buyers. Information coming from a company that only manufactures one type of coil/bar and insulation system may minimize the real differences in performance, life cycle and price between its product and another. And it may promote the value and technical benefits of its product over others but in actuality, but add no useful information for an owner seeking to discern the best product for his budget and technical expectations. As a manufacturer of more than one type of coil/bar and insulation system, NEC understands that there can be real differences between the windings found in the marketplace today. Each of these systems has advantages and disadvantages, and with some types of windings, a wide variation of quality among manufacturers can be found. A short discussion of each type is offered below.
Although smaller Globally Vacuum Pressure Impregnated or GVPI machines have been made for a number of years, the technology has been more re-utilized in the manufacture of larger machines as a way to lower capital cost for the purchase of a new machine. The winding of a GVPI machine is distinct from NEC’s VPI system in that the entire generator stator – core laminations, windings and all – are VPI’d. For some manufacturers, this process presents a unit size limitation, since the entire stator must be fit into a large VPI chamber full of resin. Consequently many generator and motor units of GVPI manufacture are still limited in size by that of the manufacturer's VPI chamber. Proponents of this manufacturing method claim that GVPI units offer a stable winding at a lower manufacturing cost and that it provides superior heat transfer. But this process has some major drawbacks when it comes to repairs and maintenance. First, the unit is wound with green, uncured coils, so there is always the possibility that coil deformation may occur which will compromise the integrity of the strand bundle, as these softer, uncured coils are installed in the stator, leaving coils with undetected voids and irregular strand positioning. Over time, these may contribute to conditions that reduce winding life. (See photo below at the right.) Additionally, if a coil fails in a GVPI unit during the final acceptance Hipot or final commissioning test, removal and replacement of a single coil or bar can be extremely difficult. Dealing with an in-service failure also can present similar challenges for a timely rewind or repair, or even result in an unplanned replacement of the entire unit. The GVPI process has become more common with large air-cooled turbo- and hydro-generators, with some approaching ratings as high as 400 MVA and up to 21 kV. But overall, as a machine class, these larger air-cooled machines have not had a good reliability record. Some have suffered from high levels of partial discharge, requiring resin injection to dampen the effects of PD deterioration within the first 4 or 5 years of operation. One major OEM has exited this type of manufacture, likely due to the poor reliability record of its GVPI machines. The greatest advantage of this winding system is a lower initial capital cost, but machine life and long-term reliability cannot be expected to equal that of a conventionally-wound machine using either VPI or resin rich coils.
The term, Vacuum Pressure Impregnation (VPI), is traditionally applied to individual coils and bars that are injected with resin and fully cured before they are installed in a machine. This is NEC's premier insulation system, NECCOBond-E, and NEC is the only major manufacturer other than one or two OEMs that utilizes this system. But it is a system with a long track record in the industry for performance and reliability. Some companies that could not achieve or retain the skills and expertise in manufacturing or make the significant investment in up-to-date equipment and tooling now only offer resin rich products. NEC is different. When other manufacturers abandoned VPI, NEC invested in both research and equipment that would keep this high-performance product cost competitive. NEC has the capacity of two autoclaves, which it utilizes in its VPI processes. In recent years, NEC has made significant investments in auto-taping machines that can simultaneously apply multiple layers of groundwall insulation to a coil or bar. It also increased the number and size of its cure presses and streamlined its manufacturing flows, so that larger inner-cooled Roebel bars could be more efficiently accommodated, as well as the larger hydros with windings comprised of more than 500 individual multi-turn coils. You can see one of our auto-taping machines in action here. NEC devotes significant time and resources to continuing research related to its VPI product in an effort to maintain both its quality and affordability. NEC also has invested efforts in developing and improving its quality assurance processes in maintaining its ISO9001 certification. However, one QA task predating its ISO certification has not changed: NEC still individually tests all its coils and bars before they leave its factory. In meeting any owner's performance specifications, a VPI winding most often offers the most options in winding design and the most flexibility in utilizing materials and manufacturing processes. Consequently, VPI coils remain the preferred the rewind option for many major utilities, and typically, the Hot Press Resin-Rich type of coils and bars (described below) will be included as an alternate in their specifications, but only if the manufacturer can meet and guarantee the same long-term performance demanded of a VPI -type coil by the spec's designated life of the winding. Coils and bars made with the VPI process will be equal in quality and performance to those made with the Resin-Rich Autoclave Coils and Bars.
Hot Press-Type Resin-Rich Coils and Bars are the most universally manufactured, and the manufacturers, themselves, are more varied in size and ability, which directly affects issues such as manufacturing capacity, winding design engineering and quality assurance. National Electric Coil manufactures a resin-rich coil line that offers a number of variations, dependent on machine application and customer budget. Like the VPI coils and bars, they are individually cured, but the similarity ends with some significant differences. First, groundwall insulation systems are made up of insulating tapes that are pre-loaded with resins that must be heated to flow as designed. These type of tapes have a shelf life, which if expired, can affect the quality and performance of the final product, and some manufacturers will be more conscious of this issue than others. How well and carefully the tapes are applied can also be a significant issue, since voids between tape layers are not so easily prevented or removed as with the VPI process. Photo below shows "tape upset in the coil groundwall insulation. While the simpler groundwall build-up process during manufacturing can lead to a lower process cost, the range of performance for this product is more limited. In order for this product to achieve its most optimal performance in a machine, the highest quality standards for materials and manufacturing must be utilized. As a manufacturer, who also makes VPI coils and bars, NEC utilizes its design expertise and its experience with the more complex VPI manufacturing process to achieve the best possible performance of its resin-rich insulation systems. For example, unlike some manufacturers, NEC uses the same type of custom-built, pressure-controlled, heated dies, encompassing both the cell and end-arms of the coils/bars, which it uses in its VPI manufacture, for the curing process in manufacturing its resin-rich product lines. NEC also individually tests all of its resin-rich coils and bars before they leave its factory. NEC has a clear understanding of the costs and benefits and the best machine applications for this product. Many Resin-Rich coil manufacturers cannot offer this expertise or experience, so it is important to thoroughly vet their capabilities. NEC generally does not recommend this product for high-voltage applications or for inner-cooled types of windings. See photo below showing voids and delaminations in the cross section of a a water-cooled bar manufactured with the resin-rich process.
Autoclave-Type Resin Rich Coils and Bars are a bit of a hybrid in terms of materials and methodologies, since the process utilizes both resin-rich groundwall tapes and an autoclave process. But unlike the VPI process, this autoclave is not filled with epoxy resin but with hot asphalt. A key part of the Autoclave Resin Rich process is full tooling, fully encompassing all surfaces of the coils and bars, which will maintain their dimensional integrity during the process. The asphalt, applied under pressure, is forced into these dies and forms a conformable outer surface on the coil or bar. This process still depends on insulating tapes that are pre-loaded with resin to fill the voids between the tape layers. In this case, the heat of the asphalt, along with the autoclave pressurization greatly improves the resin flow, especially when compared to the methods of curing used by some of the lower-end resin-rich manufacturing processes. The Resin-Rich Autoclave type of process requires the significant investment in equipment and quality management systems. Coils and bars made with this process can be equal in quality and performance to those made with the VPI process.