54 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 1, MARCH 2006

The New Generation of Superconductor Equipmentfor the Electric Power Grid

Alex P. Malozemoff, Senior Member, IEEE

Abstract—High-temperature superconductor (HTS) powerequipment, such as power cables, synchronous condensers, faultcurrent limiters (FCLs), and transformers, are poised to addresskey technical issues in the U.S. Power grid. This equipment isenabled by the successful development of robust long-length HTSwire, available commercially from a variety of manufacturersworldwide. Compared to conventional cables, HTS power cablesoffer higher capacity with minimal disturbance to neighboringunderground infrastructure, and their low series impedanceopens the opportunity for cost-effective power flow control withphase angle regulators. HTS cables are being actively prototypedworldwide. Based on technology developed for industrial and shippropulsion motors, an HTS synchronous condenser has been de-veloped and successfully tested in the Tennessee Valley Authority(TVA) grid; this is the first HTS power equipment to be offeredcommercially. It addresses the growing need for dynamic reactivepower compensation for grid stability and power quality. HTSFCLs and transformers are also in active development.

Index Terms—Fault current limiter (FCL), power cable, super-conductor, synchronous condenser.

I. INTRODUCTION

THE U.S. electric power grid faces serious technical chal-lenges arising in large part from three decades of falling

investment during the same period when electric power con-sumption was doubling.

High-temperature superconductor (HTS) power equipmentoffers new and timely solutions to these challenges. Oneproblem is bringing adequate power into dense urban andsuburban areas. Overhead lines have become almost impossibleto permit, while conventional underground cables are becomingincreasingly difficult to install because the heat and electro-magnetic fields (EMFs) they emit interfere with the existingdense underground infrastructure. HTS superconductor cables,which emit no heat or EMF, offer an easily installed, nonin-terfering cable solution. HTS synchronous condensers enablehigher throughput on existing lines and cables by cost-effec-tive dynamic power factor correction. These solutions will bedescribed further below.

An even more serious problem is the need for increasedstability against power disturbances—faults, sags, surges, andfinally, blackouts—occurring on increasingly overloaded grids.This issue has been highlighted by a crescendo of power out-ages, with the latest and most serious being the Aug. 14, 2003Northeast Blackout in the U.S. and Canada. A key contributorto these outages has been identified as inadequate reactive

Manuscript received November 7, 2005; revised January 5, 2006. This paperwas recommended by Editor-in-Chief J. Schwartz.

The author is with American Superconductor Corporation, Westborough, MA01581 USA (e-mail: amalozemoff@amsuper.com).

Digital Object Identifier 10.1109/TASC.2006.870837

power support, an issue to be described further below. Again,the HTS synchronous condenser can provide such supportcost-effectively. Furthermore, overloaded grids are the mostcommon precondition for blackouts, and it would be highly de-sirable to controllably reallocate power in such grids. However,the ac currents in these grids follow Kirchoff’s laws and are noteasily controlled, while dc links where power can be controlledby ac/dc power electronics remain expensive and in limiteduse. HTS power cables, coupled with phase angle regulators(PARs), offer a new cost-effective method of achieving thiskind of controllable reallocation of power.

High fault currents pose yet another challenge in overloadedgrids as new power sources are added. Future HTS fault currentlimiters (FCLs) offer a solution to this problem, providinginstantaneous current limitation without increase of systemimpedance.

Behind these issues lies the broader requirement for environ-mentally clean and safe power equipment, with improved effi-ciency driven by cost considerations and now by the Kyoto pro-tocol. Power losses in the present U.S. grid, approaching 10%from generation to final load, generate heat which is typicallydissipated through oil cooling, yet oil is a fire hazard and poten-tial contaminant. By contrast, the zero dc electrical resistance ofsuperconductors offers significant improvements in efficiency,while cooling by liquid nitrogen or closed cycle mechanical re-frigerators is environmentally clean and safe.

Viewed in this context, superconductors are ideally posi-tioned to address some of the most urgent and significant issuesin the electric power grids of the 21st century. This brief reviewdescribes the rapid progress being made in commercializingsuch electrical equipment using HTSs.

II. HTS: AN ENABLING TECHNOLOGY

Superconductors are materials which show zero elec-trical resistance below a critical temperature. Until 1986,the highest known critical temperature was a frigid 23 K,which corresponds to 250 C. But in 1986, a new class ofcopper-oxide-based compounds, called HTS, was discovered,with much higher critical temperatures which have now reached135 K, or 138 C. While still cold, this temperature is wellabove 77 K or 196 C, the temperature of boiling liquidnitrogen, enabling practical and economic cooling.

A key development during the years since the initial HTSdiscovery has been the development and commercialization ofpractical and robust wires based on these materials, capable ofcarrying a current density and, therefore, a power density wellover 100 times that of copper in typical power equipment. Aswill become apparent below, this huge power density advantagehas actually become the main driver in the initial use of HTS

1051-8223/$20.00 © 2006 IEEE

MALOZEMOFF: NEW GENERATION OF SUPERCONDUCTOR EQUIPMENT FOR THE ELECTRIC POWER GRID 55

Fig. 1. Timeline of HTS cable projects.

power equipment, although improved efficiency is also impor-tant [1].

A first generation (1G) of wire is based on a multifilamentarycomposite of the HTS material Bi Pb Sr Ca Cu O withsilver alloy and is manufactured using a powder-in-tube and de-formation processing approach. Electrical and mechanical per-formance is fully adequate for commercial power equipment,with wire produced by a number of companies around the world,in greater than kilometer lengths with an engineering currentdensity at 77 K of over 14 000 A/cm . While adequate for pro-totypes and some limited commercial applications, the presentprice in the range of $150–200/kAm is still almost an order ofmagnitude higher than the $15–25/kAm range for copper in con-ventional power equipment. While the 1G HTS wire price con-tinues to drop, a second-generation (2G) HTS wire promises amore significant reduction of manufacturing cost, with price-performance matching or bettering that of copper. This wire,called a coated conductor, is based on a thin film of the HTSmaterial YBa Cu O on a textured substrate, and it is being ac-tively developed and scaled up around the world, with commer-cial level production expected to commence in several years [1].

The technology to use this HTS wire to fabricate coils forrotating machinery and multistrand helically wound conduc-tors for power cables is already well established, and the cryo-genic technology is also available from multiple manufacturersaround the world. Thus, the enabling technologies are in place tosupport the development and commercialization of HTS powerequipment.

III. HTS POWER CABLES

As shown in Fig. 1, many HTS power cable projects ofincreasing sophistication have been and are being conducted

around the world, moving the technology rapidly to commer-cialization [2]. Of particular significance is the Long IslandPower Authority (LIPA) project sponsored by the U.S. Depart-ment of Energy, bringing together American Superconductoras project manager and wire manufacturer, Nexans as cablemanufacturer, Air Liquide as cryogenics supplier, and LIPA assystem installer and operator [3]. It will be the first permanentinstallation in the power grid, providing a first step in whatis envisioned as a high-capacity power spine along the lengthof Long Island. The cable will operate at 138 kV, carry up to600 MVA of power and stretch 600 m.

A major driver for HTS power cable stems from the currentdensity advantage of HTS wire, enabling a cable carrying 3–5the power of a conventional cable in the same cross section.This makes it possible for an HTS cable to carry almost as muchpower at, say, 69 kV as a conventional XLPE cable at 345 kV.Keeping the voltage at or below 69 kV has a major impact onsimplifying permitting and offers further cost advantage by re-ducing the number of transformation steps in bringing powerfrom source to load. Another major cost driver is the simplifica-tion of installation: Conventional cables require a meter or moreof space between phases and other underground infrastructure,with special thermally conducting sands to aid in the dissipationof heat to the surface. HTS cables avoid these restrictions, emit-ting neither heat nor EMF into their surroundings, enabling theLIPA cable, for example, to be installed through a duct boreddeep under the surface infrastructure.

EMF is eliminated through the use of a shielded cable archi-tecture [4] shown in Fig. 2. The tape-shaped HTS wires consti-tuting the phase conductor are surrounded by a dielectric layersuch as polypropylene laminated paper infiltrated with liquidnitrogen, and then by a second layer of HTS wires constitutingthe shield layer. When the shield is grounded, the property of thesuperconductor is to flow currents equal and opposite to the cur-

56 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 1, MARCH 2006

Fig. 2. Cross section of a cold dielectric (VLI: very low impedance) HTS cable(courtesy of Sumitomo Electric Industries) [4].

rents in the phase conductor, resulting in a complete shieldingof the magnetic fields outside the shield.

This architecture has some other remarkable consequences[5]. Since inductance is determined by the magnetic field energyduring each cycle of current, and the field in this architecture isconfined to the cylinder between the phase and the shield, theinductance is very low, up to six times lower than in conven-tional copper cable and less than in overhead lines[6],hence the terminology VLI or very low impedance cable. Sincethe inductance is the dominant series cable impedance, cur-rent in a meshed network will redistribute to this high capacitylowest-impedance link. In this way, the HTS cable can unloadoverloaded networks.

What is more, if a PAR is added in series with the HTScable, power flow through the link can be controlled accordingto the equation , where is the phase angleand the link impedance. Given the relatively high impedanceof conventional power cables, the phase angle changes requiredfor significant power flow control are large—many tens of de-grees—and such PARs, which are specially wired tap-changingtransformers, are very expensive, preventing widespread use.All power-electronic solutions like Universal Power Flow Con-trollers are also extremely expensive and used so far in only twoU.S. installations. However, the low impedance of HTS cablesenables small phase angle changes, lowering significantly thePAR cost. This new cost-effective means of controlling ac powerflow application has been simulated but not yet demonstrated.

IV. ROTATING MACHINERY

As with power cables, many projects of increasing sophisti-cation and power have demonstrated HTS rotating machinery[7]. As summarized in Fig. 3, these include industrial motors,generators, and high-torque ship propulsion motors. Shippropulsion motors are particularly advanced. A 5-MW 230-rpmmotor manufactured by American Superconductor and Alstomunder contract from the U.S. Office of Naval Research (ONR),has been tested at the Center for Advanced Power Systems ofFlorida State University, Tallahassee, FL. The motor has nowoperated successfully at full load for periods of up to 14 h, andalso under conditions simulating load variations expected atsea [8]. Its size and weight is about 1/2 that of the two 2.5-MWdynamometers used for its load, demonstrating the remarkableadvantage in size and weight stemming from the power density

advantage of HTS wire. The success of this program has led to afollow-on program from ONR for a 36.5-MW motor capable ofdriving a destroyer. This motor has successfully passed a Navydetailed design review and is presently under construction.

However, perhaps the most remarkable development in thearea of rotating machinery has been the manufacture of an 8MVAR HTS synchronous condenser [7]. The importance of thismachine stems from the growing importance of dynamic reac-tive power in the power grid [2], [5]. Reactive power representsthe out-of-phase component of electric power and is defined asthe product of the root-mean-square (rms) ac voltage and the90 out-of-phase rms current. Often called VARs or imaginarypower, reactive power appears in conjunction with inductive orcapacitive components in the power grid, such as the coils oftransformers or induction motors, or the capacitance of cables. Itcomes in two flavors, with current lagging the voltage in induc-tive VARs and current leading the voltage in capacitive VARs.

The effect of reactive power in a grid can be understoodthrough the simple electrical circuit of Fig. 4(a), in which avoltage source representing the generators of a grid is coupledto a resistive load through an inductor representing thetransformers and other coils in the grid. As power use grows,more resistive loads are added in parallel, which can be repre-sented in this simple circuit as a decreased net resistance. It isan elementary problem of electrical engineering to solve thiscircuit for the real and reactive powers. As shown in Fig. 4(b),the reactive power rises as the square of the real power; inother words, reactive power becomes increasingly significantin overloaded grids. The out-of-phase currents of the reactivepower occupy much needed capacity of transmission lines andcables. Furthermore, as the real power demand grows, voltageacross the load drops, as shown in Fig. 4(c), until a critical pointis reached, beyond which the system becomes unstable. Thisvoltage collapse is the primary cause of blackouts. The curvein Fig. 4(c) is often called the “nose curve.”

The fact that reactive power can be either inductive orcapacitive provides the basic method for preventing voltagedrop and collapse, namely by reactive compensation of oneby the other. In other words, capacitive reactive currents cancancel out inductive ones and vice versa. Utilities accomplishcompensation by adding capacitors to inductive grids andinductors to capacitive grids. However, as inductive loadschange dynamically over time, the reactive power on the gridchanges. To follow these changes, utilities often introducethyristor-switched capacitors or inductors, called Static VARCompensators or SVCs, or fully power-electronic systemscalled STATCOMs (Static Compensators) or D-VARs (Dy-namic VAR Compensators) [5]. Rotating machines can in somecases provide a more cost-effective solution for these problems.Such machines, called dynamic synchronous condensers, areessentially synchronous generators without a prime mover(energy source). [2], [5]. When the voltage inducedin the stator coils by their rotor coils differs from the statorterminal voltage , the synchronous condenser injects anout-of-phase or reactive current of magnitude

(1)

where is a proportionality constant called the synchronousreactance. is capactive when , inductive oth-erwise and is proportional to the reactive power or VARs. Syn-

MALOZEMOFF: NEW GENERATION OF SUPERCONDUCTOR EQUIPMENT FOR THE ELECTRIC POWER GRID 57

Fig. 3. Timeline for HTS rotating machinery projects.

Fig. 4. Simple model (a) to illustrate the effect of reactive power, which grows quadratically. (b) With increasing real power, and which causes voltage collapsebeyond a critical level of real power (c).

chronous condensers can provide almost instantaneous injec-tion of inductive current in response to sudden drops inand, thus, help stabilize the grid against voltage collapse. Theycan also be used to provide dynamic reactive compensation overtime by adjusting with an exciter which controls the rotorcoil current.

Conventional synchronous condensers have been widely usedin grids around the world, but they suffer from reliability prob-lems because of the heating from large rotor currents which mustbe applied to reach rated VAR levels. They are also bulky andexpensive. HTS synchronous condensers solve these problems.For reasons which stem from the high current density of HTS

58 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 1, MARCH 2006

wire, compact rotating machines can be designed which pro-duce high but without magnetic teeth in the rotor and statorcoils, and this leads to very low values of synchronous reactance

. According to (1), this means high VAR output in a compactand, therefore, low-cost machine. Furthermore, low meansthe rotor coil current change required to achieve rated output islow, eliminating the reliability problem.

American Superconductor, in collaboration with Ideal Elec-tric and supported by Tennessee Valley Authority (TVA), hasmanufactured a first 8 MVAR dynamic synchronous con-denser, which has been undergoing successful testing at a TVAsubstation during the last half year. TVA has also ordered twocommercial units. This order is a major benchmark for the fieldof HTS power equipment: Although many government-sup-ported prototypes have been and are being demonstrated aroundthe world, this is the world’s first truly commercial order forHTS power equipment. With the growing demand for dynamicreactive power, the potential market for dynamic synchronouscondensers can be substantial.

V. FCLs

As power demands in large urban areas grow, new sourcesof generation, often from independent power producers, are in-creasingly being added to the grid and their output impedancelowers the net impedance of the grid, thus increasing the faultcurrent. Fault currents have reached levels ( 60 kA), for ex-ample in the New York City and Long Island grids, approachingthe limits of existing breaker technology. While fault currentscan be controlled by adding reactors (coils) to the network, thesealso increase operating losses and endanger voltage collapsewith larger voltage drops.

Superconducting FCLs offer a solution to this problem[9]. Since superconductors turn normal when currents exceeda certain critical current level, and the normal resistance ofHTS materials is quite high, they can act like a gate to limitcurrent. Numerous FCLs have been demonstrated around theworld, including the ABB demonstration at the Kraftwerk amLoetsch hydropower plant in Switzerland or the more recentAccel/Nexans/RWE demonstration at the Netphen substationin Germany.

However, cost remains a major challenge for most of theseprojects. Elementary considerations dictate that a certainamount of energy must be dissipated during the fault hold timeof 0.05–0.3 s until breakers can open [10]. That energy must beabsorbed in a uniform way by the heat capacity of the currentcarrying material and any other contiguous materials. Thisdictates a certain mass of material, which can be bulk HTSsuperconductor, or a thin layer of superconductor bonded to asubstrate and/or stabilizer. Many combinations have been tried,including bulk materials and relatively expensive single-crystalsubstrates. There is increasing recognition, for example in arecent American Superconductor/Siemens collaboration, thatthe most cost-effective solution should come through low-cost

2G HTS wire bonded to an inexpensive stainless steel or similarstabilizer. Such systems are under development.

VI. CONCLUSION

HTS power equipment has made huge strides and is poised forcommercialization. The HTS synchronous condenser is the firstsuch equipment for which a commercial order has been placed.HTS cables and HTS motors and generators are being demon-strated at commercial performance levels. FCLs as well as trans-formers require new HTS wire functionality—high resistancestabilizers and low ac-loss wire, and work on such systems isactively underway. Given the urgent needs in the electric powergrid, the maturing of practical HTS power equipment is timelyand is expected to make a significant impact.

REFERENCES

[1] R. M. Scanlan, A. P. Malozemoff, and D. C. Larbalestier, “Supercon-ducting materials for large scale applications,” Proc. IEEE, vol. 92, no.10, pp. 1639–1654, Oct. 2004.

[2] A. P. Malozemoff, J. Mannhart, and D. Scalapino, “High temperaturesuperconductors get to work,” Physics Today, pp. 41–47, Apr. 2005.

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[4] T. Masuda et al., “High-temperature superconducting cable technologyand development trends,” SEI Tech. Rev., vol. 59, pp. 8–13, Jan. 2005.

[5] A. P. Malozemoff, B. Kehrli, J. D. de Leon, and S. Kalsi, “Supercon-ductor technologies for a controllable and reliable high capacity grid,”in 2004 IEEE PES Power Systems Conf. Exposition, New York, Oct.10–13, 2004, CD publication.

[6] J. Jipping, A. Mansoldo, and C. Wakefield, “Impact of HTS cables onpower flow distribution,” IEEE/PES, vol. 2, pp. 736–741, 2001.

[7] S. Kalsi et al., “Development status of rotating machines employingsuperconducting field windings,” Proc. IEEE, vol. 92, no. 10, pp.1688–1704, Oct. 2004.

[8] S. Woodruff et al., “Testing a 5 MW high-temperature superconductingpropulsion motor,” in Proc. 2005 IEEE Electric Ship TechnologiesSymp., Philadelphia, PA, Jul. 25–27, 2005, CD publication; IEEECatalog no. 05EX1110C; ISBN 0-7803-9260-4.

[9] M. V. Hassenzahl et al., “Electric power applications of superconduc-tivity,” Proc. IEEE, to be published.

[10] S. Kalsi and A. P. Malozemoff, “HTS fault current limiter concept,” inProc. IEEE Power Engineering Soc. Meeting, Denver, CO, Jun. 6–10,2004, CD IEEE Catalog no. 04CH37567C; ISBN 0-7803-8466-0.

Alex P. Malozemoff (M’75–SM’80) is a graduate ofHarvard and Stanford Universities.

As Executive Vice President and Chief TechnicalOfficer of American Superconductor Corporation(AMSC), Westborough, MA, he is responsible forR&D strategy, advanced conductor development, andthe intellectual property portfolio. He has more than20 years of experience in superconducting materialsand systems, as well as in magnetic materials anddevices. He has numerous publications and patentsin magnetism and superconductivity. He has worked

at IBM Research, Yorktown Heights for 19 years before joining AMSC.Dr. Malozemoff is a Fellow of the American Physical Society, and IEEE Dis-

tinguished Lecturer for Superconductivity in 2005.