Using SMES to support large-scale PV power generation

So/at Ener~ Vol. 45, No. t, PP. 35--42, 1990 0038-092X/90 $3.00 + .00 Printed in the U.S.A. Copyright © 1990 Pergamon Press plc

USING SMES TO SUPPORT LARGE-SCALE PV POWER GENERATION

KWA-SUR TAM, PREM KUMAR, and MARK FOREMAN Department of Electrical Engineering, Virginia Polytechnic Institute

and State University, Blacksburg, VA 24061, U.S.A.

AlmractmDue to environmental concerns and advance in solar technology, the potential for large-scale electric power generation by photovoltaies (PV) is promising, However, when the PV power generation level is high, the fluctuations and uncertainty of PV power would make the planning, operation, and control of a power system more difficult. This paper proposes and demonstrates that a superconductive magnetic energy storage (SMES) system can be used to overcome these difficulties and support large-scale use of PV power generation. Two system configurations for a PV/SMES hybrid system were described in this paper. In both eases, the SMES charging/discharging power is changed in response to PV fluctuations so that the combined PV/SMES power is smooth and controllable. With SMES support, large-scale use of PV power generation can be used in a power system without causing adverse effects. A coordinated PV/SMES scheme that is applicable for various weather conditions is presented in this paper. The combined PV/SMES power is dispatchable and is shown to have advantages over PV or SMES alone. PV arrays can be installed on the surface of the land occupied by an underground SMES system. With SMES support, all of the PV power can be utilized and no oversizing is needed. These benefits improve the economics of PV power generation.

1. I N T R O D U C T I O N

Two factors have increased the potential of using solar photovoltaic (PV) technology for large scale electric power generation in the near future. One factor is the growing concern of the environmental effects (acid rain, greenhouse effects, etc.) of the conventional energy sources. Environmental laws have made the building of coal-fired plants and in some cases even oil-fired plants increasingly difficult. Issues related to safety and nuclear waste disposal, among other considerations, have made nuclear power an unfavorable option for utilities. Photovoltaic power generation distinguishes itself as an environmentally acceptable way for power generation.

Another factor is the progress in solar cell technology. An experimental photovoltaic cell developed by Sandia National Laboratories is reported to be capable of converting 31% of incident sunlight into electricity[l]. This energy conversion efficiency is comparable to those of coal- or oil-fired generation. Although more work is needed, PV power generation is becoming an economically competitive option.

PV-based power generation has been used for small- scale applications and a few medium scale applications. However, some technical issues need to be resolved before PV can be considered by utilities for large scale power generation (such as central station PV power generating plants). From the power systems' point of view, two major technical issues are the fluctuations of PV power and the dispatchability of PV power.

Electric power produced by solar cells is highly dependent on the amount of sunlight the solar cells re- ceive. Since the insolation level changes as a result of variations in atmospheric conditions, the PV output would fluctuate. Because of its intermittent nature, PV power generation systems are usually oversized and

need some forms of energy storage back-up. For large scale use of PV generation such as PV central generation stations, the utility grid will serve as the storage medium. If the capacity of the PV system relative to the size of the power system is low (less than 5%), the fluctuations in PV power are relatively insignificant compared with the fluctuations of the power system's electricity demand, and do not present much problem to the operation or control of the power system [ 2,3]. As PV generation increases beyond certain level, the fluctuations of PV power would adversely affect the operation of the power system.

The impact of PV power fluctuations on power system operation is illustrated in Fig. I. Figure l shows the daily load curve for a power system in the southeastern region of the United States in spring. Data ob- tained from a 4-kW PV facility in this region is scaled up to simulate the output power from a central station PV power plant that is designed to supply up to 15% of the peak power demand of the power system. Due to PV generation (treated as negative load), the load curve seen by the rest of the generators in the system is lowered when sunlight is available but the new load curve also contains many sharp spikes of considerable magnitudes. These spikes have very high ramp rates and would impose an increase in spinning-reserve requirement, load-following requirement and control difficulty [ 2,3 ]. The problem is even more severe during sunrise hours when the system generation mainly con- sists of baseload units which have limited response ca- pability.

Because PV generation has low operating and maintenance cost and zero fuel cost, it is economical to dispatch PV power whenever it is available. How- ever, due to the weather dependency of PV generation, the amount of PV power available for dispatch is un- certain. Furthermore, due to the inclusion of PV gen-

35

36 K.-S. TAM, P. KUMAR, and M. FOREMAN

X OR I O I NAL LOAD CURVE

8 O MODIFIED BY PV

tNl I 1 I

i N I I --0', O0 8 . 0 0 1 6 . 0 0 2 4 . 0 0

HOURS Fig. 1. A power system daily load curve with and without PV

power generation.

eration, the response rates of thermal units need to be considered before they are committed. This may lead to system operations other than economic dispatch. Again, the problem gets worse as the PV generation increases beyond certain level.

Various approaches have been proposed to solve or alleviate these problems. These include reducing the PV power output, changing the generation mix of the system to provide sufficient load-following capacity, forcing the economic dispatch to always provide the higher spinning reserve, improving the design of power plants to provide faster response rate and wider regu- lating range, dispersing the solar array fields and reducing their energy densities, and forecasting ap- proaching cloud fronts via National Weather Service type data or from sensors located in the proximity of the array field.

The objective of this paper is to show the feasibility and benefits of using a superconductive magnetic energy storage (SMES) system to support large scale PV power generation. The two technical problems mentioned in the previous paragraphs can be solved by using a SMES system. The SMES system can compensate for the fluctuations of PV power so that the combined PV/SMES power output is smooth, controllable, and does not present any problem to the power system. The combined PV/SMES power can be dispatched by utilities. With SMES support, power generated from PV arrays can be fully utilized and no oversizing is necessary. PV generation can be increased to significant levels witl-_out causing adverse effects to the power system.

The proposed SMES approach differs from other proposed solutions mentioned above in one major aspect. SMES systems will be considered by utilities pri-

marily for functions (such as diurnal load leveling, ex. plained in Section 3) other than providing storage for PV systems. However, the use of SMES can be ex- tended to perform an additional function of enhancing the utilization of PV power generation. Consequently, the presence of a SMES system would provide additional incentive for the employment of more PV power, and vice versa.

The structure of a combined PV/SMES system is described in Section 2. A coordinated PV/SMES scheme that is applicable under various weather conditions is discussed in Section 3. Conclusion and final remarks are presented in Section 4.

2. PV/SMES HYBRID SYSTEM

Figure 2 shows the simplified schematic diagram of a SMES system connected to a power system. Energy is stored in the magnetic field of a superconducting magnet which, in many cases, assumes the configuration of a solenoid. At present, using metallic alloy superconductor (NbTi or Nb3Sn), the storage magnet is kept at liquid helium temperature. The refrigeration requirement will be reduced if high-temperature superconducting magnet can be developed. Electric energy from the ac power system is charged to and discharged from the dc magnet through an ac /dc converter. In addition to ac/dc power conversion, the converter also provides fast and flexible power control. SMES systems have been considered for applications in power systems and programs have been initiated in the U.S. as well as in Japan for SMES development and commercialization [ 4,5,6,7 ].

Compared with other energy storage schemes, a SMES system offers the unique advantages of high efficiency (>92%) and fast dynamic response. Unlike batteries which only have efficiencies in the range of 70% to 75%, the highly efficient SMES system can be used to store large amounts of energy without incurring much losses. The high roundtrip efficiency of the SMES system allows frequent charging/discharging operations. The fast dynamic response of a SMES system, along with its ability to absorb or release power quickly, make it an excellent choice to regulate power fluctuations caused by intermittent sources such as PV.

Figure 3 shows the functional structure of a PV/ SMES combined system. The PV generation system and the SMES system are joined by a common ac bus which is connected to the utility grid. From the utility grid's point of view, the PV system and the SMES

AC~OC CONVERTER ]

I

SUPERCONDUCTING MAGNET

Fig. 2. Schematic diagram ofa SMES system.

Using SMES to support large-scale PV power generation 37

AC BUS

Pv DC, C .t----- ARRAYS CONVERTER [ - -

COORDINATED CONTROL

DC / AC SMES CONVERTER

Fig. 3. Structure ofa PV/SMES hybrid system (Configuration I).

system are connected in parallel. Each system may operate independent of each other or they may operate in a coordinated fashion. In the latter case, the PV/ SMES combined operation is controlled by a PV/ SMES coordinated control system. The coordinated control system monitors a vector of measured signals (the PV power output, the SMES power output, the power demand signal issued from the power system control center, the current that flows to/from the superconducting magnet, and the AC bus voltage) and controls the SMES power output through the issuance of gate firing pulses to the ac/dc converter in the SMES system.

A central station PV generation system requires a considerable amount of land to install the solar arrays. With a solar cell conversion efficiency of about 5%, about ! square km of PV arrays is needed to produce 20 MW. Oversizing may be needed to account for the intermittent nature of solar power. If the new cell with conversion efficiency of 30% is used, I square km of PV arrays can produce about 120 MW.

Depending on the storage capacity, a SMES system with a solenoidal magnet design also occupies a sub- stantial area of land. The SMES system is usually installed underground so that the bedrock can be used for mechanical support to sustain the large forces usually associated with bulk magnetic energy storage. The land requirement is mainly caused by the environmental and safety concerns due to the stray magnetic field surrounding the superconducting magnet. While the natural background magnetic field is around 0.5 gauss ( 1 gauss = 0.0001 Tesla), a magnetic field in- tensity of less than 10 gauss is considered to be safe even for heart pacemakers. Thus, the property limit for a SMES site is set at the 10-gauss line. According to a design of an SMES system (5000 MWH storage capacity, 1000 MW power rating) for utility applications, the fence line for 10 gauss is the circumference of a circle with a radius of 2.06 km. Magnetic field of less than 100 gauss has very tittle effect on the electronic equipment and so the converters are located just out- side the 100-gauss line. A plan view of this SMES system is shown in Fig. 4.

Although a central station PV system and a SMES system both require a lot of land, the land requirement

for the combined PV/SMES operation is not neces- sarily the sum of the land requirement for each indi- vidual system. While the SMES is installed underground, the earth surface surrounding the magnet can be used for the installation of PV arrays. Since magnetic field below 100 gauss has very little impact on electronic equipment, the impact of magnetic field of such low magnitudes on the performance of the PV cells is ex- pected to be very small. Whole body exposure of 100 gauss is acceptable for short-term exposure. Therefore, maintenance and repair of PV arrays can still be performed, although PV arrays need little maintenance.

For the SMES design shown in Fig. 4, PV arrays can be installed between the 100-gauss line and the 10-gauss line. This 10.46 square km of land, which is already owned by the utilities, can be used for the generation of about 209 MW of electric power if PV cells of 5% conversion efficiency are used, or about 1255 MW if PV cells of 30% conversion efficiency are used. The presence of a SMES system practically provides free land for the installation of PV arrays. This factor would have a positive impact on the large scale use of PV generation.

An essential prerequisite for PV/SMES operation is that the SMES can react fast enough to any PV power fluctuation such that the combined PV/SMES power output is smooth and dispatchable. In order to study this aspect, coordinated operation o fa PV/SMES system with a configuration as shown in Fig. 3 was sim- ulated through computer modeling. The output power from a 40 MW (peak power) PV generation system was modeled using information provided by Ref. [2]. Equations describing the operation of a SMES system of storage capacity of 200 MWH and power rating of 40 MW were developed to build the computer model

\ \ N .~T \ \

SMES MAGNET (UNDERGROUND)

[ ] LOCATIONS FOR PV ARRAYS

[ ] CONVERTERS

[ ] SWITCHYARD

T TRANSMISSION UNES

Fig. 4. Plan view ofa PV/SMF..S system.


of a SMES system. The validity of these models has been verified.

Figure 5 shows the simulation results of a case in which the PV power increases (with fluctuation) during the sunrise hours. The load demand is usually low at this time and so only part of the PV generated power is used by the power system. The rest is used to charge up the SMES system. The convention being used in this paper is that the SMES power output is positive when it is being discharged (releasing energy) and is negative when it is being charged (absorbing energy). When the PV power rapidly drops, the SMES system changes its operation from charging to discharging very quickly so that the PV/SMES combined power remains at the desired level. When the PV power rapidly increases to a higher level, the SMES quickly changes from discharging back to charging. The total PV/SMES

power supplied to the power system remains at the desired level at all times and is not affected by PV tluctuations at all. The SMES system can readily handle the changes, and the transition between the charging mode and the discharging mode is smooth.

With SMES support, the PV/SMES combined output is free of fluctuations because the SMES power is adjusted to compensate for the changes in PV power. However, with ac/dc converters used for high power applications, any change in real power is accompanied by a change in reactive power. As shown in Fig. 5, the reactive power consumption for the SMES system fluctuates as a result of PV/SMES combined operation. Fluctuations in reactive power would introduce dis- turbance to the power system and are undesirable. This constitutes a major drawback for the system configuration (configuration I) shown in Fig. 3,

8

b- I-- "~m

i i (A)

61.6 , .33 MINUTES

~ . 0 0

8 LD

I ! (B)

8 OOU~"

8

'-o.oo sl.s I 13.~ 20.00 MINUTES

8 LO,

hQIILD- =E

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M I NUTES

8 u) --. I i

(o)

I , ~ ~ . 1 : ~ 61. ff7 1 3 . 3 3 2 0 . 0 0

M I NUTES Fig. 5. Coordinated PV/SMES operation using configuration I. (A) PV power output. (B) SMES power

output. (C) PV/SMES combined power output. (D) Reactive power consumed by the SMES system.


I ARRAYS CONVERTER

I PV / SMES COORDINATED

CONTROL

I__1 °c,°° I _ SMES CONVERTER

DC BUS

OC/AC CONVERTER

Fig. 6. Structure ofa PV/SMES hybrid system (Configuration II).

An alternative system configuration (configuration II) is shown in Fig. 6. The PV system and the SMES system are joined by a common d¢ bus which is connected to the power system through a t i c /a t converter. The de/tic converter in the PV system is used for maximum power tracking. The dc /dc converter in the SMES system is used to control the charging and discharging of the SMES system. Both dc /dc converters perform their respective control functions by changing their duty ratios. Operational details of these converters can be found in Ref. [ 8].

The PV/SMES coordinated control system monitors a set of measured signals (the PV power output, the SMES power output, the power demand signal issued from the power system control center, and the dc bus voltage) and regulates the SMES power output through the control of the duty ratio of the dc /dc converter in the SMES system. The a t / d e converter then transforms the dc power from the PV and the SMES system into ac power. Through the action of the coordinated control, the SMES output would vary to compensate for PV power fluctuations so that the combined PV/SMES dc power can be without fluctuations. There is no need to change the firing angle for the a t / d e converter. Consequently, there is no fluctuations of reactive power. Furthermore, the firing angle for the a t / d e converter can be set at values that minimize the consumption of reactive power (about 5 ° for rectifier operation, and 165 ° for inverter operation). The coordinated operation o fa PV/SMES hybrid system using Configuration II is very similar to that of Configuration I (as shown in Fig. 5 ) except that the reactive power consumption is kept constant at a very. low level.

For both Configuration I and Configuration II, the PV system is connected in parallel to the SMES system. The two systems can operate independently or in a coordinated fashion. For both configurations, the SMES system can operate by itself without the PV system. When a PV system is installed later, only minor modifications in the SMES control are needed to change it to a coordinated PV/SMES control. The changes basically involve incorporating one more in- put, the PV power output, to the control circuitry. Thus, the cost to modify a SMES to perform an additional function of supporting PV power generation is quite modest.

Compared to Configuration I, Configuration II has the advantage that no reactive power fluctuation is introduced to the power system. However, Configuration II has the disadvantage that power is processed two times before it reaches the power system. This results in higher losses, although the additional losses are relatively small due to the high efficiencies of the converters.

Like Configuration I, Configuration II can also maximize the use of land by installing the PV system on the land that the underground SMES system occupies. With power compensation provided by the SMES system, power generated by PV arrays can be fully utilized and there is no need to oversize the PV system, resulting in further reduction in the cost of PV power.

3. COORDINATED PV/SMES OPERATION

With SMES support, large scale use of PV power generation can be introduced to the power system without causing adverse effects mentioned in Section 1. A system level study has been conducted on a new scheme involving the coordinated use of PV and SMES and its impact on the power system. This section pre- sents the major results of this study.

A typical daily load curve is represented by the curve A-B-C-D-E-F-G-H-I-J-K in Fig. 7. Although the use of SMES for function such as system stabilization has been demonstrated [ 5 ], SMES systems have been considered by utilities mainly for diurnal load leveling. When a SMES system is installed to perform diurnal load leveling, electric power generated from low-cost baseload units is used to charge up the SMES during light-load hours (late evening and early morning hours) between to and t2 and the stored energy is then released during heavy-load hours (atternoon to early evening hours) between t4 and h to replace the power generated from higher cost peak/intermediate units. The major benefits for diurnal load leveling is the savings from the energy cost difference between periods ofpeak and minimum demand. With diurnal load leveling, the load curve is modified to A-D-E-F-I-J-K.

With a significant level of PV power generation, the load curve would be changed to A-B-C-L-M-N-H-I-J- K plagued with spikes like those shown in Fig. 1. As shown in Fig. 5, these spikes can be readily smoothed


PL

G I

E N ,

, ,

A ~ II

, ' I', i I I I I1! I I It

o t: t:2 t3t4 t:5 c6 t:7t:,

Fig. 7. A power system daily load curve modified by coordinated PV/SMES operation.

out by appropriately controlling the SMES power output, making the PV/SMES combined power dispatchable. With PV generation shouldering some of the peak load, the SMES discharge can begin at ts and continue until ts when the stored energy level is decreased to the designated level.

When sunrise begins at h , there is enough generation (consisting mainly of baseload units) to supply the load as well as to charge up the SMES. Between t~ and t2, the PV power can be used to provide extra charging power for the SMES. Any intermittent change in PV power would only cause a temporary change in the charging power for the SMES. The SMES system can readily handle these fluctuations so that the power system would not be affected.

Starting at t2, the baseload units' power output eL is no longer enough to supply the system load. The baseload units' power output can be maintained at the same level PL until t3, but between t2 and t3, PV/SMES power would be employed for dispatch. When PV power is available during this period, part of it would be used to supply part of the load enclosed by D-M- E-D and the rest (enclosed by D-L-M-D) would be used to charge up the SMES. If PV power fluctuates, the SMES would switch rapidly from the charging mode to the discharging mode, or do whatever is necessary, so that the combined PV/SMES output power is dispatchable and that the fluctuations would not affect the power system.

Beyond t3, all of the available PV power is used to supply the system load. The load curve would follow the curve M-N until ts at which the SMES discharge begins and would end at l,. From ta to is, the SMES output would still be used to smooth out PV fluctuations.

With PV/SMES combined operation, the system load curve is modified to A-D-M-N-J-K. This is better than the load curve modified by PV or SMES alone.

The SMES is being charged from to to t2 by the baseload units with total energy indicated by the area enclosed by A-B-C-D-A, and by PV generation from t, to t3 with total energy indicated by the area enclosed by C-L-M- D-C. The latter is relatively small when compared to the former. Thus, the SMES charging would not be affected significantly by the weather dependent PV power. On cloudy days, the PV power output is reduced. However, the pattern shown in Fig. 7 still remains the same, except that the curve C-L-M-N-H would be shifted closer to the curve C-D-E-F-G-H. On rainy days, the PV output is very small and the curves C-L-M-N-H and C-D-E-F-G-H almost coincide (point N coincides with point F, point M coincides with point D). In this case, the SMES operates by itself to perform its primary function, diurnal load leveling. In any event, the combined PV/SMES output power is dispatchable and is fluctuation free. This operation scheme is applicable for various weather conditions.

A simulation study has been performed to inves- tigate the effect of the PV/SMES coordinated operation scheme under different weather conditions. The power system and PV data used in this study are the same as those used to generate Fig. l and are typical data for the southeastern region of the United States. The installed PV system on a sunny day can deliver power up to 40 MW, about 15% of the power system's maximum load. Added to the power system is a SMES system with a power rating of 40 MW, a storage capacity of 200 MWH and a roundtrip efficiency of 97%. The operation scheme described in this section was implemented.

Figure 8 shows the PV/SMES operation on a rainy day. In this case, the PV power is about 5% of its peak power. The PV contribution is very small and so the

8 X ORIGINAL LOAD CURVE

O MODIFIED BY PV-SMES N I !

.

H O U R S 1 6 . 0 0 2 4 . 0 0

Fig. 8. A power system daily load curve with and without coordinated PV/SMES operation on a rainy day.


PV/SMES combined operation is almost the same as the SMES operation alone. The SMES is charged up from 12 a.m. to about 7 a.m. Discharge of SMES begins at around 12 p.m. and ends around 10 p.m. The system's peak load is reduced from 259 MW to 227 MW.

Figure 9 shows the PV/SMES combined operation on a sunny day. In this case, the PV peak power is 40 MW, which accounts for about 15% of the system's maximum load. Comparing Figs. 1, 8, and 9, the benefits of PV/SMES operation can be identified. Firstly, with SMES support, all the PV power is fully utilized and there is no power fluctuation introduced to the power system. Secondly, because of PV generation, SMES discharge can begin after 3 p.m., resulting in a larger reduction in the system's peak load to 217 MW.

In the southeastern part of the United States, the PV generation and the load demand do not peak at the same time. PV generation peaks around noon, while the system load peaks in the late afternoon. For this region, if PV generation is employed alone, not only will the PV fluctuations adversely affect the operation of the power system, the system peak load would still remain at around 259 MW. On the other hand, if the SMES system operates alone, as illustrated by Fig. 8, the system peak can only be reduced to 227 MW. Hence, the combined use of PV/SMES also en- hances the primary purpose of the SMES system-- diurnal load leveling.

From the viewpoints of system operation as well as the reliability of power supply, including a significant level of PV generation without SMES support may not be acceptable to power system planners. Therefore, during the period when the SMES system is under

X ORIGINAL LOAD CURVE

8 ~ MODIFIED BY PV-SMES

eM I I I

HOURS Fig. 9. A power system daily load curve with and without

coordinated PV/SMES operation on a sunny day.

maintenance or repair, the PV generation level would need to be reduced to a lower level. However, like the PV system, the SMES system also requires little maintenance.

4. CONCLUSION

Due to environmental concerns and advance in solar technology, the potential for large scale electric power generation by photovoltaics is promising. How- ever, when the PV power generation level is high, the fluctuations and uncertainty of PV power would make the planning, operation, and control of a power system more difficult. This paper proposes and demonstrates that a superconductive magnetic energy storage system can be used to overcome these difficulties and support large scale use of PV power generation.

Compared with other energy storage schemes, a SMES system offers the unique advantages of high roundtrip efficiency (>92%) and fast dynamic response. These features make SMES an excellent choice to regulate PV power fluctuations. Two system configurations for a PV/SMES hybrid system were described in this paper. In both systems, the SMES charging/discharging power is changed in response to PV fluctuations so that the combined PV/SMES power is smooth and controllable.

PV arrays can be installed on the surface of the land occupied by an underground SMES system. With SMES support, all of the PV power can be utilized and no oversizing is needed. These benefits improve the economics of PV power generation.

With SMES support, large scale use of PV power generation can be used in a power system without causing adverse effects. A coordinated PV/SMES scheme that is applicable under various weather conditions is presented in this paper. The combined PV/ SMES power is dispatchable and is shown to have benefits over PV or SMES alone.

PV/SMES combined operation makes large scale PV power generation a more attractive option for utilities to consider. The large scale PV generation discussed in this paper refers mainly to central station PV power generation. From the viewpoint of operation of the entire power system, a centralized SMES system can still minimize the adverse effects caused by large scale use of smaller and distributed PV generation systems. However, although the overall system control and the power system as a whole would not be affected, the portion of the power system in the immediate neighborhood of a distributed PV generation system would experience some effects caused by PV power fluctuations. For distributed PV generation, perhaps smaller and distributed SMES systems should be considered.

Acknowledgments--This material is based upon work sup- ported by the DuPont Young Faculty Gram, the Center for Innovative Technology, and the Virginia Center for Coal and Energy Research.


REFERENCES

I. K. Wollard and G. Zorpette, Power/energy, IEEE Spec- trum 26( 1 ), 56-58 (1989).

2. M. G. Thomas, J. W. Stevens, G. J. Jones, and P. M. Anderson, The effect of photovoltaic systems on utility operations, Conf. Record of the 17th IEEE PV Specialist Conference, 1229-1233 (1984).

3. S. T. Lee and Z. A. Yamayee, Load-followiog and spinning reserve penalties for intermittent generation, IEEE Trans., PAS-100(3), 1203-1211 ( [981 ).

4. S. M. $choenung, W. V. Hassenzahl, and P. G. Filios, U.S. program to develop superconducting magnetic energy storage, 23rd Intersociety Energy Convers. Eng. Conf. Paper no. 889504 (1988).

5. J. D. Rogers, R. l. 3chermer, B. L. Miller, and J. F. Hauer, 30-MJ superconducting magnetic energy storage system

for electric utility transmission stabilization, Proc. IEEE 17(9), 1009-1107 (1983).

6. R.J. Loyal, S. M. Schoenung, T. Nakamura, D. W. Lieu- rance, M. A. Hilal, J. D. Rogers, J. R. Purcell, and W. V. Hassenzahl, A feasible utility scale superconducting magnetic energy storage plant, IEEE Trans. Energy Conver- sion, EC-I(4), 63-68 (1986).

7. K. Hayakawa, M. Masuda, T. Shintomi, and J. Hasegawa, Test plant as the first step towards commercialization of SMES for utilities, IEEE Trans. Magnetics 24(2), 887- 890 (1988).

8. M. M. Foreman, Control and operation of SMES and SMES/PV systems. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA (1989).

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Using SMES to support large-scale PV power generation