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DIFFERENCES BETWEEN LOW VOLTAGE AND HIGH VOLTAGE INSULATION SYSTEMS AND

WHY IT’S IMPORTANT TO USE THE CORRECT SYSTEM

W. Howard Moudy National Electric Coil

ABSTRACT Many owners / users are confronted with the task of specifying or evaluating a stator insulation system. While on the surface this might seem simple to some, a closer look might reveal that the task is more daunting than first thought. There are many factors to consider such as machine voltage, coil type and geometry, coil throw and drop out of the core, slot space, copper cross section and desired insulation volts per mil, just to mention a few. The brief presentation will develop an awareness of different systems available in the market today. The strengths and weaknesses of the systems will be explored to help participants evaluate and select the best insulation system options for various machine applications.

INTRODUCTION This paper focuses on form wound insulation systems from 2300 volts to 26,000 volts with low voltage being less than 5KV and high voltage being above 5 KV. There are numerous industry sources for technical standards and specification guidance. Two prominent sources are IEEE (Institute of Electrical and Electronics Engineers) and IEC (International Electrotechnical Commission) Specifications are an important tool. Other considerations relating to the advantages and disadvantages of various insulation systems is also important. Many owners / users intently focus on specifications; agonizing over specific details in hopes of making a one size fits all choice. The unfortunate technical reality of a one-size fits all approach is that one insulation system alone, does not typically satisfy all machine design and unique application needs. In an attempt to offer practical insight and understanding of different systems choices key machine design features, application nuances, and system differences will be explored.

THE COMMERCIAL REALITY Finding the best combination of reliability, performance, and cost, is perhaps, the biggest challenge in selecting insulation systems. It should come as no surprise that in specifications, as details and requirements for any purchase increase, so does the price. This is certainly the case with insulation systems. As such, it is important to consider how detailed your specification really needs to be. Too much detail can result in an insulation system that is difficult to make and only available at a price you may not be able to afford. On the other hand, too little specification detail may yield a very attractively priced insulation system, but one that is not particularly well-suited to the application, and barely able to make it through a reasonable warranty period. The key point to keep in mind is similar to a question many of us were asked as children, and perhaps ourselves have asked as parents: “Is it a need or a want?” If it provides tangible benefits in reliability and performance, it is probably needed in the spec. If it is something that may help by providing some conceptual or intangible benefit, it probably is not needed.

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VPI & Resin Rich Processes Following World War II, Westinghouse and General Electric explored options to improve upon the asphalt resin systems.1 The superior mechanical strength of Polyester and Epoxy Polymer resins was seen as a solution to the problem of making longer coils and avoiding girth cracking, which was prevalent in the asphalt mica systems. Both Westinghouse and GE developed systems that integrated well with existing production processes, utilizing the expensive capital tooling and processes from the Asphalt systems for the new Polymer systems. Westinghouse developed and utilized the Coil VPI (CVPI) process and GE developed and utilized the Resin Rich tape process. Both processes produced improved results over the Asphalt systems. These processes have evolved and improved and are still in use today. Epoxy resin has become the predominant resin of choice due to its mechanical strength and chemical resistance although it is typically a bit more costly. An important similarity between CVPI and resin rich systems is that both are dependent upon expensive specialized manufacturing tooling and regimented processes to achieve desired results. This is important to note, since some companies do not make the needed capital investment in manufacturing equipment or follow proven qualified procedures to achieve satisfactory results. This is especially true with resin rich systems. With the resin already in the tape, the tape can be applied by hand or with archaic taping devices, but the consistency in tape pressure and positioning is, at best, poor and most often inadequate in yielding high-quality results. Examples of new modern robotic taping processes can be seen below in Figures 1 and 2.

Robotic taping of a Roebel bar. Figure 1

Robotic taping of a multi-turn coil. Figure 2

Without the capital investment in modern taping equipment to accurately apply tapes in the right position and tension, it is not uncommon to find the work product as represented in Figure 3 below. Tape applied with wrinkles and inconsistencies typically perform poorly on qualification testing such as IEEE voltage endurance testing as outlined in IEEE 1043 and IEEE1553 documents.

1 Author’s note: Greg Stone, et al, in their book, “Electrical Insulation for Rotating Machines—Design, Evaluation,

Aging, Testing, and Repair,” offers an extensive history of stator winding insulation development, however, other

works listed in the References section also contributed facts to the technical history discussed throughout this paper.

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Magnified example of poorly applied tape.

Figure 3

Another necessary piece of capital equipment is the autoclave or vacuum / pressure vessel. In both the CVPI and Resin Rich processes the autoclave, begins by removing all the air and moisture form the vessel. With the Resin Rich process, the vessel then becomes a heating and pressure molding system that forms and cures the coils. With the VPI process, the resin is introduced under vacuum and pressurized with dry nitrogen to complete the impregnation process insuring the essentially void-free characteristic for which the system is known. An example of a coil VPI system can be seen below in Figures 4 and 5.

Roebel bars contained in cart prior to enclosure in the vacuum pressure vessel.

Figure 4

Vacuum pressure vessel closed for processing.

Figure 5

Tight tolerance hot presses are used to cure coils after application of the resin during the VPI process. The hot presses insure tight geometric tolerance in the straight portion of the coil / bar as well as in the involute and leads. See Figures 6 and 7.

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Hot presses prior to loading with bars. Figure 6

Close up of bar in press. Figure 7

A common advantage of both resin rich and CVPI insulation systems is that when properly manufactured, they can be electrical tested at full value before winding the coils into the machine. This is a significant advantage over Global VPI processed systems.

Global VPI Process Introduced in the 1960’s Global VPI (GVPI) was touted as an attractive option to individual Coil VPI (CVPI) or early Resin Rich processes due to lower cost of production.[1] The coils, also referred to as “green coils,” are installed into the stator dry, not impregnated with resin. Installing green coils has the advantage of the coils being relatively pliable and easy to wind. Not much mechanical stress is applied to the coils during the installation process. That is good since the green coil tapes are rather fragile and care must be taken not to tear or puncture the un-impregnated tape. GVPI advocates also tout improved thermal transfer, and mechanical strength. After processing, the GVPI stator becomes one solid collection of copper, insulation materials, core iron, and Epoxy resin. The thermal properties and lack of voids help transfer heat from the coils. There is no doubt about the mechanical strength of having all of the components glued together. However, before counting the money saved buying a GVPI system with the strong advantages noted above, it is wise to consider the other side of the coin. The coils of GVPI systems are applied green, as previously noted. Without the resin to protect the mica tape of the ground wall insulation system and the turn insulation of the multi-turn coils, these insulations can be damaged during the winding process due to the mechanical stress of manipulating the coils into the proper position in the slot and end windings on both ends. Careful consideration must be given to the test levels; the best guidance coming from tape manufacturer spec sheets for the green materials. Ground wall insulation is tested after a certain number of coils are installed or at the end of a shift, with megger and hipot test being performed. Multi-turn coils are turn tested at the same point performing a surge comparison test. Despite test values being decreased due to the green tapes, failures have been known to happen. Test failures at this point can be corrected by removing the failed coil, retesting and continuing the winding process. Once coil winding is complete and the unit is GVPI’ed, final tests are performed. Should failures be encountered at this point, schedule will be significantly impacted, if the entire work process must begin again. Obviously, the cost to the vendor/manufacturer is substantial. Maintenance can be challenging, time consuming, and costly on GVPI units of any voltage level. The GVPI process yields a solid monolithic assembly. Should a coil or major support component be damaged from one or more possible ways, including a foreign object, excessive electromechanical forces, or arcing, excavating end winding components and a top coil is challenging. If a bottom coil is involved, it may not be practical to repair, except with a complete rewind of the unit. National Electric has made stator repairs and even replaced top coils on GVPI units. The process is more like surgery on a rock or boulder, in order to remove the damaged component, prepare the area, and install the new component. The process is often considerably more time consuming and costly than a similar repair on Resin Rich or CVPI hard coil

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insulation system. Figure 8 below shows a top coil being replaced on a GVPI machine. While it is not impossible to repair or replace a bottom coil, it is not usually a feasible option considering schedule, cost and potential damage to neighboring components.

Top “leg” replacement in a multi-turn GVPI machine.

Figure 8

Partial discharge is not an uncommon occurrence in air-cooled GVPI units at or above the 6,900 volts. However, if vigilant maintenance is performed at reasonable outage periods, the condition is manageable for reliable operation. The initial cost of GVPI systems is usually quite attractive. Unfortunately, at the other end of the lifecycle end of life rewind, GVPI systems are very costly in both time and money. Stripping a GVPI is more labor intensive and challenging mechanically, when compared to most other insulation systems on the market. As compared to hard coil resin rich or VPI insulation system, stripping a GVPI stator generally takes a minimum of 20% more time and money, depending upon machine design and method used. Methods used to strip the GVPI units can vary considerably but water jet, and hydraulic mechanical methods are common. Regardless of method, care must be taken not to damage the core laminations or their insulation. A large number of GVPI machines are epoxy injected to make up for the shrinkage and looseness that develops early in their life cycle. This too, adds to the challenges of stripping the stator and to an even greater extent, the need to requalify the core with an ElCid or full flux test. It is common for indications or hot spots to require aggressive acid etching and even light sanding or grinding with the grain to achieve acceptable results and to requalify the core for continued service. Load cycling has become an increasingly prevalent operational scenario in the past decade or so. Generally thought by many to have minimal effect on key generator components, load cycling has been linked to shear stresses that act aggressively on the GVPI insulation system. The shear stresses have been found to cause delamination of the ground wall insulation from the conductor stack. The insulation can be sheared from the conductor due to the temperature differential between the hotter copper on one side of the insulation and the cooler stator core on the other side. A method of evaluating sheer stresses forces and effect on various systems was presented in a paper at the 2000 IEEE International Symposium on Electrical Insulation held in Anaheim, CA. An example of the shear stress phenomena is shown in Figure 9 below.

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Separation of ground wall insulation from conductor stack due to sheer stresses. Figure 9

GVPI has served the motor market well for quite a few decades. There are smaller generators in the lower voltage levels at or below 6,600 volts that are good candidates for GVPI primarily due to the low cost and fast turnaround. Maintenance challenges should be weighed against the low cost and fast turnaround. There are other quality insulation system options that can be considered at this voltage level.

Varnish – Varnish insulation systems have been around for decades. They are readily available and are still a viable option. Key advantages include low cost, ease of maintenance, and flexible to handle the mechanical stress of the winding process, and the ability to test at full test values.

Fully-cured tape – Although fully-cured tape systems have not been around as long as Varnish systems, they are a mature technology. They have evolved significantly with advancements in the fully cured mica tape and the fusing outer layer tape that seals and protects the coil. Key advantages are reasonable price, flexibility, durability to withstand the mechanical stress of winding, and the ability to test at full test values.

Hybrids – Hybrid insulation systems combine more than one material / process on the coil. A common combination is a fully cured resin rich slot section, with a fully cured tape and sealing outer layer on the involute. Another common involute option is uncured resin rich tape. The fully cured slot section has very good dielectric strength, solid construction, and consistent geometry that allow a tight fit to be established in the core. The involutes allow good flexibility to adapt to the mechanical stress of winding. The fully cured involute also has the ability to be tested at full test values and when the sealing layer is cured has good contaminant/ chemical resistance. The uncured resin rich tape involute option can yield good results but cannot be tested at full test values until cured and until cured the tapes can be fragile and may need special storage until installed.

The systems above may not seem very impressive when compared to the hard coil VPI and Resin Rich options but at lower voltage levels can be a much better option due to the “Egg Shell Effect”. The Egg Shell Effect is a term used to describe the fragile nature of hard coil ground wall insulation when applied to lower voltage applications. At lower voltages, the ground wall insulation can be very thin and takes little effort to twist or bend the coil to crack or break the ground wall insulation. Especially in multi-turn coils at lower voltage levels, the mechanical stress of winding can be too great for hard coils systems to reliably withstand. Many specs have been misguidedly written to require a hard coil insulation system, typically applied to bars and in high voltage applications, be applied to a 2,300 volt multi-turn application. Having questioned a dozen or so writers of such specifications, the first point out of their mouth justifying their choice is “if it is good for 13,800 volts, I should be safe using it at 2,300 volts”. Once they see an actual coil of his type being wound or pictures of the process, most realize that the insulation systems ability to withstand

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mechanical stress is at least as important as withstanding the voltage stress. Figure 10 below illustrates the mechanical flexibility needed to install the lap on some machine designs.

“Winding the lap” of a stator with multi-turn coils. Figure 10

Multi-turn Coil and Single Turn Roebel Bar Designs There are a number of applications where multi-turn coil designs are frequently used, including small hydrogenerators, turbogenerators up to 15 kV, and most GVPI units. Bars are typically applied to higher rated and higher voltage generators. Some key advantages and disadvantages of the two designs include:

Multi-turn Coils are faster and less costly to manufacture

Bars often allow greater technical opportunities to minimize losses and to potentially uprate units

Bars see less mechanical stress when being wound

Multi-turn coils generally wind and connect faster. The illustrations below in Figure 11 and 12 show the key component parts of a multi-turn Coil and of a single turn Roebel bar.

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Components of a multi-turn coil. Figure 11

Components of a single turn bar. Figure 12

When choosing either design, there are several important considerations to keep in mind. Multi-turn coils have served the market well for many decades. Turn insulation is critical to the performance of this design. Designers have latitude with how turn and strand insulation materials are applied, as well as how they contribute to the turn to turn insulation value and ability to withstand turn to turn voltages. Up until the 1970s, most designers took a conservative approach and utilized dedicated turn insulation, usually mica tape. Since that time, some designers have taken and aggressive approach to save space and/or money by having strand insulation do double duty, serving as both the strand insulation and turn insulation. Clearly using turn tape is a more robust design form, both an electrical and mechanical perspective. Consider that if a fault develops in the turn insulation, the current applied to the fault is much greater. Taking the normal current and multiplying by the number of turns is the current applied to the fault. With that high a current applied, temperature rapidly increases, copper melts, and a ground fault occurs. Applying dedicated turn tape to multi-turn coil designs is an option that should not be overlooked to help insure long-term reliability. Another important consideration, with either design choice, is minimizing circulating current losses. This is typically accomplished by transpositions. Transpositions can be made in coil connections, in either design, but will not be explored in this paper. Twist transpositions can be made in multi-turn designs. Usually the turn nearest the center of the turns, is twisted 180 degrees, without damaging the strand insulation and with additional turn insulation applied for protection and to fill the geometric voids. Figure 13 below illustrates the formed twist transposition and Figure 14 shows how it integrates into the multi-turn coil loop.

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A formed twist transposition. Figure 13

Twist transposition integrated into coil’s conductor loop.

Figure 14

Single turn Roebel bars have the advantage of not having turn insulation, essentially only strand and ground insulation. Transpositions in single turn bars are call Roebel transpositions. The amount of the transposition in a single turn Roebel bar is referred to by the degrees in a circle. A bar with a 360° Roebel can be understood by picturing a double stack of strands in the bar’s conductor bundle. Follow the strand at the top on one end of the bar. Along the entire coil length, you will see that in the middle of the coil length, it has traveled down one side of the conductor stack to the bottom. Here it transitions to the opposite bottom side and begins to climb up the other side of the conductor stack, reaching the top in its original position at the other end of the stack. In effect, the strand made a complete 360° circle around the coil stack. Other common single turn Roebel bar transposition designs are 540°, 720°, and 900°. At the lower values the rebelling is in the straight or slot portion of the bar. The higher values, especially the 900° have a portion of the Roebel in the involute portion of the bar.

Graphic depiction of movement of strands in a 360° Roebel transposition.

Figure 15

With inner-gas cooled Roebel bars, the strands cross over the hollow

conductor to complete the transposition.

Figure 16

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Neither multi-turn nor single turn Roebel bars can be applied to all machines, small to large and low voltage to high voltage. However, there is a significant overlap in the middle range where either design is a possible option. Where both are an option, the multi-turn coils have a distinct advantage in price, while single turn Roebel bars have a technical advantage in reducing circulating current losses, which aids in increasing output, decreasing temperature or a combination of both. Below in Figures 17 and 18, completed hard coil VPI multi-turn coils and single turn Roebel bars are shown packaged for shipment.

Multi-turn coils. Figure 17

Roebel bars. Figure 18

CONCLUSION

Specifications are an important tool that should have the necessary level of detail to insure important characteristics are met and the needed level of quality is ensured. Help in preparing an adequate specification can be found from many industry sources including the IEEE, and IEC and several good books. There are many different machine models, ratings, sizes, and applications. If consideration is given to the insulation characteristics, coil design and their advantages and disadvantages discussed herein, a choice can best be made to fit the specific unit design and application characteristics that will be reasonably affordable while at the same time provide long-term reliable operation.

ACKNOWLEDGEMENTS

The author wishes to thank Steve Jeney, Gary Slovisky, Jane Hutt, and others of the National Electric Coil Company team for their contributions, encouragement, and patience.

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REFERENCES

[1] Stone, G.C.; Boulter, E.A.; Culbert, I.; Dhirani, H. Electrical Insulation for Rotating Machines—Design, Evaluation, Aging, Testing, and Repair. Hoboken, NJ: Wiley-Interscience, 2004.

[2] Andritz Hydro. “Generator Technology Expanded.” Hydro News, no. 16 (October 2016): p. 5.

Accessed March 20, 2018. https://www.andritz.com/resource/blob/31516/ c065e2781c933191f0b935df21089489/hy-hn16-en-data.pdf .

[3] Plesa, Ilona, Petru V. Notingher, Sandra Schlogl, Christof Sumereder, and Michael Muhr. "Properties

of Polymer Composites Used in High-Voltage Applications." Polymers 8, no. 173 (April 28, 2016): 1-63. Accessed February 17, 2018. doi:10.3390/polym8050173.

[4] Robinson, Scott. "Vendor Presentations Provide GUG Attendees Short Courses on Important Topics -

Part II." Combine Cycle Journal, January 23, 2017. Accessed January 27, 2018. http://www.ccj-online/vendor-presentations-provide-gug-attendees-short-courses-on-important-topics-part-I/.

[5] Maughan, Clyde V. "Generators: Problems Experienced with Modern Generators." Combined Cycle

Journal, October 1, 2013. Accessed January 27, 2018. https://www.ccj-online.com/4q-2013/generators-problems-experienced-with-modern-generators/.

[6] Klempner, Geoff, and Isidor Kerszenbaum. Operation and Maintenance of Large Turbo- Generators.

Hoboken, NJ: Wiley-Interscience, 2004. [7] Batista Do, Giovan, Luigi Cattaneo, Daniele Liberia Condotto, and Francesco Tufaro. "EPRI Workshop

- Rome 17 April 2013." Proceedings of Lifetime Extension Solutions for Turbo Generators in Fully Inpregnated Stator Winding Technology.

BIOGRAPHY

Howard Moudy, Director of Operations, has been employed by National Electric Coil for 16 years. He graduated from Western Kentucky University and began work at the Apparatus Repair Division of Westinghouse Electric. Throughout his 37-year career in the power industry, he has been engaged in the rotating electrical machine service business and coil manufacturing operations.