PRESENT STATUS OF SUPERCONDUCTING AC GENERATORS

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PRESENT STATUS OF SUPERCONDUCTING AC GENERATORS

P. Rios

To cite this version:

P. Rios. PRESENT STATUS OF SUPERCONDUCTING AC GENERATORS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-703-C1-708. �10.1051/jphyscol:19841143�. �jpa-00223615�

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JOURNAL DE _PHYSIQUE

Colloque C l , suppl6ment au no I , Tome 45, janvier 1984 page CI-703

PRESENT S T A T U S OF SUPERCONDUCTING AC GENERATORS

P.A. Rios

Genera2 EZectric Co. Schenectady, NY 12345, U.S.A.

RdsumC - Le p o t e n t i e l apportd par l a supraconductivitd pour a t t e i n d r e d e s den- s i t d s de courant e t des champs magnstiques 6levds B s u s c i t 6 un e f f o r t mondial dans l e dGveloppement de g6ndratenrs a courant a l t e r n a t i f supraconducteurs. La supraconductivit6 repr6sente l e s e u l moyen connu d'augmenter notablement l a den- s i t 6 de puissance t o u t en r6duisant l e s p e r t e s 6 l e c t r i q u e s dans l e s machines tournant e s .

AprOs un e f f o r t concentrd i n i t i a l e m e n t s u r des p e t i t s modOles, 1 1 a c t i v i t 6 s e p o r t e maintenant v e r s des i n s t a l l a t i o n s 2 grande d c h e l l e d e s t i n d e s 2 d6montrer l a f a i s a b i l i t d technique.

Cet a r t i c l e pr6sente une revue de progrss r 6 c e n t s dans l e d6veloppement des g6- n d r a t e u r s B courant a l t e r n a t i f supraconducteurs a i n s i que l e programme en cours dans l a compagnie de l ' a u t e u r .

Abstract - The potential of superconductivity for high current density and high magnetic fields has given rise to efforts to develop superconducting ac generators throughout the world. Superconductivity affords the only known means of substantially increasing power density while simultaneously reducing electrical losses in rotating electrical machines.

Initially, development efforts were concentrated on small-scale component tests; the focus has now shifted towards larger-scale demonstrations of technical feasibility.

This paper surveys recent changes in the status of the development of superconducting ac generators. The ongoing development program in the author's company is reviewed.

The growth of the electrical power generation industry has been characterized by a growth of turbine- generator ratings from the neighborhood of 100 MVA to over 1000 MVA. This growth in rating has been accompanied by the development of new technologies that have increased power density, e.g., direct water cooling of stator and rotor windings and hydrogen cooling of rotor windings. In the late sixties and early seventies, it was anticipated that ratings would continue to grow, and alternatives were sought to increase power density to make it possible to ship these very large units assembled, since field assembly would have a negative impact on the reliability of generators. At the same time, it was necessary to maintain the high efficiency levels that were already attained by generators.

The availability of superconducting wire that can be used as technical material in large devices turned attention to the possibility of a superconducting ac generator. This was the only technology known to offer the potential for both a substantial step upward in power density and an increase in generator efficiency.

Numerous programs were started to study this alternative, and the first experiment that demonstrated the concept of a rotating superconducting field winding in a rotating dewar was carried out at Mas- sachusetts Institute of Technology in 1969 [I].

Since that time, the growth in ratings has subsided, but the evaluated cost of losses has increased dramatically. The emphasis of the development programs has turned to exploit other opportunities:

Reduction of generator losses Lower construction costs

Improved stability when connected to a power system Generation at higher voltage or transmission voltage

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841143

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C1-704 JOURNAL DE _PHYSIQUE

It has been firmly established that reduction in both size and generator losses can be achieved by superconducting generator technology. The other advantages listed above are more speculative, and there is ongoing debate as to their value and practicality. On the other hand, the mature reliability of the technology in a power plant will have a substantial impact on the economic viability of the concept, and any evaluation of the economic advantages of superconductivity in central power plant generators must await long-term tests.

STATUS OF DEVELOPMENT PROGRAMS

There have been some excellent reviews of worldwide activity in development of superconducting generators [2,3]. In addition, there have been very recent publications on the status of the programs in Germany [5,6], France [61, Japan 171, and the USSR [81, where major development efforts are underway.

In the United States, three development programs have been underway. At MIT, a 10 MW generator is being developed 191, and generator tests under load are planned for 1984/1985. An

EPRIIWestinghouse development program was underway to develop and test a 300 MVA superconducting generator [lo]; plans called for testing the generator in a power plant, under load, in 1986.

This program has recently been halted. At General Electric, a 20 MVA generator has been developed [ll-131 and a 20 MW lightweight, generator is under development under Air Force sponsorship.

Since the publication of the status reports mentioned above, two major test milestones have been reached:

In Japan, Fuji Electric and Mitsubishi have completed development of a 30 MVA superconducting synchronous condenser. This represents an important milestone in that it is the highest rated superconducting machine that has been successfully developed to date.

In the United States, the development of a 20 MVA generator has been completed at General Elec- tric. The generator has been tested under full load and has been subjected to sudden short circuit from partial voltage. This is the first time a superconducting generator has demonstrated the capa- bility to operate under the torques resulting from power generation and has survived the forces caused by sudden short circuit.

THE DEVELOPMENT PROGRAM AT GENERAL ELECTRIC

The development program at General Electric has been underway for over ten years. As an initial step in this program, conceptual designs were developed for large generators to identify critical components that required development. Then a component development effort was undertaken, and those components were tested independently [14]. Having demonstrated the components' performance, a 20 MVA generator was developed. This generator was designed to model features believed to be necessary in large machines. During the generator development program, the designs were reviewed to as- sess their ability to be extrapolated to large ratings. A summary of a study where the concepts were extrapolated to 300 MVA and 1200 MVA ratings was presented by Minnich et al. [151.

Generator Design and Fabrication

The principal dimensions and computed parameters of the 20 MVA generator are given in Table 1; a cross section is shown in Figure 1. The generator overall concept is the generally accepted one of a rotating superconducting field winding and an airgap winding design for the armature.

The salient features of the General Electric rotor design are:

The superconducting rotor winding sections are racetrack shaped (Figure 2). Each racetrack section is manufactured from a filamentary niobium-titanium superconductor, reinforced with glass, and impregnated with epoxy to provide a strong, rigid winding structure. This approach provides stable windings with high overall current density that exhibit no "training." The rotor winding is support- ed in a laminated aluminum structure (Figure 3).

The vapor-cooled current leads, which carry the excitation current to the rotor winding have been designed so that burnout will n ~ t occur at the rated current, even in the event of coolant flow failure. In addition, the leads are designed to provide low losses, and they are self-regulating to the extent that the vapor flow is automatically controlled by the lead losses.

Vapor cooling of the torque tube extensions and control of the helium level in the rotor body are accomplished by a helium flow circuit design that offsets the temperature rise resulting from

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compression of the helium in the rotor body, and can cool the rotor winding to temperatures of 4.2 K or lower. In addition, the circuit provides for automatic control of the coolant flow rate and the liquid level in the rotor.

A helium transfer coupling that provides for separate control of three return helium vapor streams and uses noncontacting seals supplies the liquid helium from a stationary supply.

The rotor was assembled in a stationary aluminum electromagn,etic shield and tested at 3600 rpm and design excitation to establish the feasibility of the rotor concept prior to final stator design and fabrication.

T a b l e I

P R I N C I P A L D I M E N S I O N S A N D C O M P U T E D P A R A M E T E R S O F T H E 20 M V A G E N E R A T O R Rotor winding region current density

Stator winding region current density Inside radius of rotor winding region Outside radius of rotor winding region Inside radius of stator winding region Outside radius of stator winding region Outside radius of electromagnetic shield Electromagnetic shield thickness Resistivity of electromagnetic shield Inside radius of the iron yoke Iron yoke thickness

1 Active length

Half-angle at pole of field winding region

'

Number of pole pairs Harmonics considered in design Rotor speed

Operating power factor

10,000 ~ l c m ~ 130 ~ l c m ~ 2 in.

S in.

9.5 in.

14.0 in.

8.6 in.

2.0 in.

0.0435 x m

14.9 in.

9.1 in.

52 in.

30"

1

fundamental only 3600 rpm 0.90 (lag)

Field winding inductance Armature inductancelphase Armature resistancelphase EM shield inductance EM shield resistance

Fieldlarmature mutual inductance Shieldlarmature mutual inductance Field/shield mutual inductance Synchronous reactance Transient reactance Subtransient reactance

Generator rating at operating power factor Rated voltage line-to-iine

Rated current per phase Rated field current Armature turnslphase Field turns

Figure I. Cross Section of 20 MVA Generator

5.2 H

4.1 H

5.5 1 0 4 0 1.0 X 1 V 6 H 6.0 x R 1.9 x H 1.5 X lo-' H

1.2 H

0.389 p.u.

0.324 p.u.

0.164 p.u.

20.6 MVA 3600 rms 3300 rms 500 A 14 2790

Figure 2. Superconducting Field Winding Assembly

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/

Figure 3. Superconducting Rotor Assembly

The stator winding contains no iron teeth and is a self-contained structure fabricated from fiber- reinforced composites into which the stator bars are wedged. The stator winding is surrounded by an iron yoke to shield the external environment from the stray alternating magnetic fields produced by the rotor and stator. The iron yoke also enhances the flux density in the stator winding region.

Air-, water-, or oil-cooling of the stator can be considered, depending on the duty and the rating of the machine. In the 20 MVA generator, the stator winding is water-cooled.

Since the stator bars are now exposed to a rotating magnetic field rather than a pulsating magnetic field of relatively high intensity, it is necessary to utilize stator bars comprised of small insulated and transposed strands to reduce the eddy current and circulating current losses to acceptable levels.

The salient features of the stator design are:

The internally cooled, transposed stator bars (Figure 4) are built up from 0.051 inch diameter copper conductor and 0.332 x 0.0166 inch copper-nickel cooling tube. The transposition is of the 540" type.

The armature winding support system employs spring-loaded wedges to maintain mechanical secu- rity, despite the dimensional instability of the materials employed. Plastic teeth are keyed to the laminated iron stator core. There are half as many teeth as there are bars in a layer. One bar from each layer is placed adjacent to each side of each tooth, as shown in Figure 5. Fixed wedges are placed adjacent to each bar, and a movable wedge is placed between each pair of fixed wedges.

Each movable wedge is drawn securely into place by a nonmagnetic stainless steel cable. Each cable passes from the core back radially inward through a ventilation duct in the core.

Figure 6 shows the rotor being assembled into the completed stator.

Figure 4. Stator Bar

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Figure 5. Stator Winding Support S t ~ c m r e

Figure 6. Generator Assembly

Generator Test

The 20 MVA generator has been tested in a pumpback arrangement (Figure 7 ) . Completed tests are:

Steady-state short-circuit test at 120% of design current Steady-state open-circuit test at 120% of design voltage

.

Load test at power factors up to 0.9.

Application of sudden three-phase short circuit from 50°/o of design voltage.

CONCLUSION

The generator test shows the technical feasibility of the design concepts used in this machine, but the short-term operation of the generator gives no indication of what may be expected under long-term continuous operation in a power plant environment. It is anticipated that the development effort required to attain reliable operation of the generator over a long lifetime is considerable. At the present time, evaluation of the commercial potential for this technology is being carried out to determine the appropriate level of effort for further development.

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JOURNAL DE PHYSIQUE

Figure 7. Generator Test REFERENCES

[ l l THULLEN, P., SMITH, J.L., DUDLEY, J.C., WORDEN, H.H. and GREENE, D.L., "An Experimental Alternator with a Superconducting Rotating Field Winding," IEEE Transactions on Power Apparatus and Sys- tems, Vol PAS-90, No. 2 (1971) 611.

121 GLEBOV, I.A., CHUBRAEVA, L.I., EDMONDS, J.S., MCCOWN, W.R., RUELLE, G. and SABRIE, J.L., "Superconducting Generators: Current Situation and Prospects," CIGRE International Conference on Large High Voltage Machines, Paper 11-14, (September, 1982).

t31 SMITH, J.L., Jr., "Overview of the Developement of Superconducting Synchronous Generators," IEEE Transacions on Magnetics, Vol. Mag 19, No. 3 (May 1983) 522.

141 LAMBRECHT, D., "Status of Superconducting AC Generators," IEEE Transactions on Magnetics, Vol.

Mag 17, No. 5 (September 1981) 1551.

[5] INTICHAR, L. and LAMBRECHT, D., "Technical Overview of the German Program to Develop Super- conducting AC Generators," IEEE Transactions on Magnetics, Vol Mag 19, No. 3 (May 1983) 536.

[61 SABRIE, J.L. and GOYER, J., "Technical Overview of the French Program," IEEE Transactions on Mag- netics, Vol Mag 19, No. 3 (May 1983) 529.

[71 FUJINO, H., "Technical Overview of the Japanese Superconducting Generator Development Program,"

IEEE Transactions on Magnetics, Vol Mag 19, No. 3 (May 1983) 533.

[81 GLEBOV, I.A. and SHAKTARIN, V.N., "High Efficiency and Low Consumption Material Electrical Gen- erators," IEEE Transactions on Magnetics, Vol Mag 19, No. 3 (May 1983) 541.

191 SMITH, J.L., Jr., WILSON, G.L., and KIRTLEY, J.L., "M.1.T.- D.O.E. Program to Demonstrate an Ad- vanced Superconducting Generator," IEEE Transactions on Magnetics, Vol Mag 15, No. 1, (January 1979) 727.

[lo] MCCOWN W.R. and EDMONDS, J.S., "300 MVA Superconducting Generator; Plan for Design, Testing and Long-Term Operation," CIGRE, Paper 11-08 (September 1980).

1111 LASKARIS, T.E. and SCHOCH, K.F., "Superconducting Rotor Development for a 20 MVA Generator,"

IEEE Transactions, Vol. PAS-99, No.6, (Nov.-Dec. 1980) 2031.

[I21 LASKARIS, T.E., KEIM, T.A., SCHOCH, K.F. and RIOS, P.A., "Development of the Stator for a 20- MVA Superconducting Generator," Advances in Cryogenic Engmeering, 27 (Plenum Publishing Corp. 1982) 57.

[I31 GAMBLE, B.B. and KEIM, T.A., "A Superconducting Generator Design for Airborne Applications," Ad- vances in Cryogenic Engineering, 25, (Plenum Publishing Corp., 1980) 127.

[I41 RIOS, P.A., GAMBLE, B.B., JEFFERIES, M.J., JONES, D.W. and LASKARIS, E.T., "Component Development for a 20-MVA Superconducting Generator," Proceedings of the World Electrotechnical Confer- ence, Moscow, (June 1977)

I151 MINNICH, S.H., KEIM, T.A., CHARI, M.V.K., GAMBLE, B.B., JEFFERIES, M.J., JONES, D.W., LASKARIS, E.T. and RIOS, P.A. "Design Studies of Superconducting Generators," IEEE Transactions on Magnetics, Vol Mag 15, No. 1, (January 1979) 703.