A superconductor application graduated in May from localized, industrial use and will enter a transmission network in January-marking its first connection to the grid.
Wisconsin Public Service Corp. (WPS) ordered six Distributed Superconducting Magnetic Energy Storage (D-SMES) systems from American Superconductor Corp. (ASC) for deployment within the next year at five electrical substations for grid stabilization.
Single SMES units have provided power quality backup for individual industrial customers, storing and instantaneously discharging large quantities of power for carry-through during voltage dips or momentary interruptions. Housed in portable 48-foot trailers (see Figure 1), SMES units use an energy storage electromagnet made with low-temperature superconductor (LTS) wire. The superconducting properties of the electromagnet allow it to carry large currents without resistance, and to be quickly charged and discharged. The electromagnet can be operated indefinitely without degradation. SMES units use power electronics to sense momentary electrical power disturbances and instantly provide supplemental power to eliminate the disturbance. Once incoming electric power is stabilized, the customer`s equipment is run again entirely on the utility power source.
Postponing new transmission
D-SMES systems are modified versions of the single SMES units designed for connection to substations. Also housed in portable trailers, they increase transfer capacity and protect utility grids from the destabilizing effects of short-term events such as voltage dips caused by lightning strikes and downed poles, sudden changes in customer demand levels and switching operations. In the WPS application, the D-SMES units will provide fast voltage recovery after a disturbance and continuous voltage support if key transmission lines are disrupted. D-SMES units are designed to restore 90 percent of the grid`s original voltage level within 0.5 second. According to ASC, most grid disturbances last less than 0.5 second.
WPS president and COO, Patrick Schrickel, speaking at the Edison Electric Institute`s annual conference held in Long Beach, Calif., in June, said the D-SMES units will be installed in WPS` northern transmission loop, a network approximately 200 miles in circumference. The area served is about 40 miles wide and extends 60 miles from north to south in the Wausau and Eagle River areas of Wisconsin.
Schrickel said the load has changed from winter peaking to summer peaking due to the increased demand associated with the growing tourism economy and construction of seasonal homes in this area. The reactive load (i.e., air conditioning and refrigeration) has also increased. The area`s load is about 200 MW, currently supplied mostly by 115 kV transmission lines.
WPS plans to ultimately build a 200-mile, 345 kV line connecting central Wisconsin and Duluth, Minn., to meet the growing demand. Schrickel said WPS compared D-SMES with other options, including static VAR compensators, standard transmission capacitors, series capacitors, new generation, and construction of new transmission lines. He said by using D-SMES, “WPS avoided or delayed a $4.5 million investment at this point in time.” In the interim, the northern loop gains stability with the D-SMES units.
Greg Yurek, ASC`s president and CEO, said, “The cost is between $650,000 and $800,000 per D-SMES unit. This was one-half to one-third of the cost of the alternatives.” The deal with WPS provides flexibility with a buyback provision, and ASC opted to defer recognition of revenue from this sale to a future date based on the terms of the buyback option.
The D-SMES units will be remotely monitored via phone link with systematic downloading of information on an ongoing basis to ASC`s technical center in Middleton, Wis. Preventive maintenance will be required one time each year.
Tallying HTS` costs vs. benefits
SMES and D-SMES units use LTS wire. Other applications now being developed for use in the electric industry use high temperature superconducting (HTS) wire.
To achieve superconducting properties (defined as nearly zero resistance to the flow of electrons) in the compounds used to conduct electricity, LTS requires cooling by liquid helium to near absolute zero, or zero on the Kelvin scale, which is about -273 C. The discovery in 1986 of new conducting materials (HTS) allowed superconductivity at 77 K, which is about -196 C.
Because they are superconductive at relatively higher temperatures, HTS materials require less cooling, so liquid nitrogen, which costs only about one-fifteenth as much as liquid helium, can be used for cooling. This discovery theoretically makes widespread applications more feasible.
HTS wire used in cables (see Figure 2), electric motors and transformers (see Figure 3) boosts efficiencies and allows for smaller equipment sizes and weights. For example, HTS cable can now carry three to 10 times the current of conventional cables at 1/100th the weight of the conductor.
Demonstration projects hint at the possibilities of HTS applications. Detroit Edison`s Bill Carter presented an update at the EEI conference on the Frisbie substation retrofit project in Detroit. Three 400-foot flexible HTS cables will be connected to a spare transformer at the substation and will then turn underground and snake through several 90-degree bends to connect with switchgear inside the substation.
In mid-June, Detroit Edison completed a trial pull of the cable, measuring tensions and checking pulling techniques. The targeted installation date is fall of 2000.
Carter highlights the advantage of such retrofitting when urban areas must face right-of-way constraints. After conventional 24-kV copper cables have been removed from nine conduits, the 24-kV HTS cables will be installed in three of the conduits. Because these three HTS cables can carry the same amount of current as the nine copper ones they will replace, six cable conduits will remain available for use in meeting future load growth or to be made available to other service providers such as telecommunications companies.
Carter listed another advantage of HTS cables. High-current HTS cables rated at subtransmission and distribution voltage levels offer “the opportunity to not only maximize your existing infrastructure, but to avoid building additional substations. You actually cut the need for transformation.”
Another project making steady progress is Southwire Co.`s installation of a working, commercial-level HTS cable to power its own corporate headquarters and manufacturing facility in Carrollton, Ga. Ground has been cleared and poles have been set near Southwire`s Building Wire Plant in preparation for the installation and startup by the end of 1999.
R.L. Hughey, Southwire`s project manager for the HTS cable project, said HTS cable will be used under real-world conditions, including varying loads and exposure to lightning. The load is estimated to be equivalent to that required for the city of Carrollton (about 18,000 people). Additional HTS cable applications noted by Hughey include heavy-duty current line (transmission loops) and use at crossover points of major north-south and east-west transmission lines.
The cost of HTS cable remains a barrier to widespread use today. A realistic evaluation of what costs might be for actual installations is difficult because the U.S. Department of Energy now subsidizes most of these projects. The competitive economics of the HTS market cannot evolve until mass production begins. Examples of funding include:
– $2.75 million (50 percent of the total cost) for the Detroit Edison retrofit project. Industry participants will fund the remainder.
– $5.5 million (about 50 percent of the total cost) for a four-year collaborative project between ABB, ElectricitÃ© de France and ASC to develop HTS wires for HTS transformers. Industry participants will fund the remainder.
– $7.5 million (50 percent of the total cost) for the Southwire project. Southwire and its partners will fund the remainder.
– $21 million for design, development and testing of 1,000- and 5,000-hp HTS motors for industrial applications (Rockwell International`s Reliance Electric in collaboration with ASC).
The primary cost driver identified in a paper by N. Kelley, et al., Pirelli Cables and Systems North America, is the price of the superconducting tapes required to make the cables. Challenges in HTS tape development and manufacturing are achieving the desired current capacity and producing longer lengths of tape. Although ASC and Southwire Co. won`t reveal a cost range for their HTS products, they do point out that utilities realize this technology could expand their possibilities, and that there are benefits above and beyond the initial cost outlay.
John Redmon, Public Service Electric & Gas Co.`s manager of special projects, said HTS applications “are essentially in the pilot or in the late research phase now. Some initial applications or products may come out in the next one to two years.” Initial commercial applications will probably be best suited for specialized purposes, such as transformers on electric train locomotives.
He said, “Widespread applications will come out in maybe 10 years or maybe a little bit more.” Smaller transformers (1 to 10 MVA) might be among the first general applications.
Southwire`s Hughey estimated HTS cable could be commercially available in three to five years.
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