Distributed SMES: a new technology supporting active grid management22 January 2001
Distributed superconducting magnetic energy storage offers a way of responding to grid demand changes on a fractional second timescale
As power industry competition unfolds, an emerging sticking point is the difficulty of managing power reliability on free-flowing AC networks. Traditionally, power networks have been operated as passive systems, with power flows following paths of least resistance. While system operators have long had the resources to respond to changing conditions in a five- to ten-second time frame, they have historically lacked resources capable of handling short transient problems extending to one or two seconds. As a result, they have been required to overbuild systems to ensure that adequate capacity is available, on an instantaneous basis, to handle transient power flows caused by plant trips, line outages, and lightning strikes. In recent years, however, opposition by residents has made it very difficult for utilities to site major new transmission facilities.
In the US heartland, two utilities have addressed reliability issues by deploying a new type of grid stabilisation technology known as D-SMES, or distributed superconducting magnetic energy storage. These deployments by Wisconsin Public Service Co (Green Bay, WI) and Alliant Energy (Milwaukee, WI) are the first time worldwide that superconductor-based technology has been incorporated as an integral part of a live utility system. They also suggest a new philosophy of active management of the power grid, one that enables utilities to extract higher performance and revenues from their existing infrastructure.
D-SMES offers a promising new approach to enhancing the security and stability of power systems. Offered by American Superconductor and GE Industrial Systems under the terms of a strategic sales alliance formed in April 2000, it makes use of power storage and sophisticated electronics (Figure 1) to overcome the limitations of passively-operated grids. Magnetic coils, operated at cryogenic temperatures, effectively “cache” megawatt-level amounts of real power close to customer loads. These magnets are connected to the power grid through an advanced, proprietary inverter technology. The system is capable of large, instantaneous bursts of real power, providing smooth carryovers in the face of power system disturbances. The inverter banks are also able to inject, on a continuous basis and simultaneously with real power injections, large amounts of reactive power (VARs) to ensure local voltage support.
The system is packaged in a trailer configuration (Figure 2) that is tied into the power system at distribution voltage and sited within substations. D-SMES units monitor local line conditions on a continuous basis. When system disturbances occur, each unit responds instantaneously and independently. While the units boost local voltage, the transmission grid sees their operation as the functional equivalent of load shedding, and gains a precious interval of time to recover to nominal voltage.
The D-SMES projects now operating at WPS and Alliant represent an adaptation of a power quality technology that had been proven in the field in a number of installations since the early 1990s. Traditionally, customers for the technology included large, high-speed continuous process industrial customers in such industries as paper, plastics, chemicals, aluminum and semiconductor fabrication. In these power quality applications, systems are calibrated to provide the highest level of power quality, at a cyclical level, to individual customers with the most demanding tolerances. In these power quality applications, American Superconductor's micro-SMES technology has accumulated a track record of nearly 50 unit-years of reliable operation.
In the D-SMES application, this same technology is configured to provide a wide-area reliability benefit by easing the burden on high-voltage transmission lines during system disturbances (caused by lightning, line trips and forced plant outages) as well as periods of extreme system stress. The WPS system provides an example of this application. Consisting of six units, it supports a portion of the company's 115 kV system north of Wausau, Wisconsin, serving a load of approximately 200 MW (see Figure 3).
Several criteria not dependent on price or performance were pivotal in WPS's decision to deploy D-SMES. These included the rapidity and ease of siting the mobile, trailer carried SMES units, the modularity and inherent reliability of a distributed solution, and the units' portability. As system needs change, the company will be able to move these units to other parts of its network, avoiding the threat of a stranded capital asset. In addition, WPS has determined that, by allowing it to tap unused thermal capacity in its existing lines, it will be able to handle a 15 per cent growth in load in the area supported by the D-SMES system without constructing additional lines, improving revenues and profitability.
WPS operational experience
During the summer of 2000, the D-SMES units were called into service on several occasions to provide carryovers through a variety of system disturbances. These included both two- and three-phase events, and voltage drops that resulted in magnet discharges as well as depressions that were handled by the inverters alone. One of the more severe events handled smoothly by the system is shown in Figures 4 and 5. A sharp voltage drop occurred during a late-summer heat wave on September 1, a day when WPS experienced an all-time system peak. Figure 4 illustrates the rapid response achieved by a discharge of the magnet, which brought local voltage up from approx. 53 per cent p.u. to 90 per cent p.u. within 200 ms. This response, forestalling customer problems, was substantially better than the 500 ms criterion originally established by WPS.
The new “active” approach to grid management made possible by D-SMES is timely, to meet the challenges faced by power grid operators across North America and around the world. Power demand and siting problems are forcing grid operators to move larger amounts of power over longer distances, despite the physical constraints of AC networks. By easing these constraints, D-SMES supports a number of policy objectives such as enhanced competition and improved air quality.
A principal application of D-SMES will be to improve local area stability in the face of load growth. This was the case with the WPS installation. In September, Entergy purchased a D-SMES unit from American Superconductor and GE for installation in spring 2001. This project will provide a critical short-term solution to the problem of ensuring transmission system reliability in a rapidly-growing suburban area. The Woodlands area of metropolitan Houston has seen rapid growth in recent years, requiring additional transmission support. However, local opposition to new overhead facilities has hindered Entergy's efforts to support service in the area. By installing two D-SMES units on its system, Entergy will be able to maintain reliable service for several years while it pursues a long-term solution.
A second group of applications currently under study would facilitate the siting of new generators. In a competitive environment, it is proving highly problematic to forecast the exact sequence and location of new independent power producers. This uncertainty impedes planning for interconnection facilities. D-SMES can be used to solve short-term grid problems, filling in as generators are added and retired in a fast-changing competitive environment. Dispatching D-SMES units from a “fleet” to key locations gives tomorrow’s RTOs and transcos the ability to plan for expansion of permanent grid facilities in an orderly, cost-effective fashion.
A third type of D-SMES application would enhance transfer capacity by easing the "bottlenecks" that impair efficient regional trading in electricity. In many instances, limitations on the rating of power lines arise from stability concerns rather than thermal limits -- that is, additional flows could be accommodated but for concerns that system contingencies could trigger instability. D-SMES can be used to provide the real and reactive power resources needed to handle a wide range of such contingencies. Facilitating the increase of power flows while assuring system security, D-SMES improves the functioning of the broader geographic markets being sought by regulators and policymakers.
A fourth group of studies focus on the opportunity to improve overall system efficiency and improve air quality by reducing reliance on so-called “reliability must-run” generating units, which are typically located in the heart of urban areas known as “load pockets.” While these units are often both dirtier and less efficient than today’s state-of-the-art generating facilities, system operators are often required to operate them to assure local reliability. By addressing local stability concerns, D-SMES can allow markets to adhere more closely to merit order dispatch, resulting in both efficiency and air quality benefits.
New technology solutions
The trend towards competitive electric power markets is compounding the challenges faced by today's power system operators. Civil opposition to siting new energy facilities conflicts with the policy objective of geographically broad markets. In this challenging environment, new technology solutions are needed to ensure that customers receive reliable service. D-SMES is proving to have a broad range of applications to address many of these challenges. It is an example of new technology offering not merely single solutions, but a new vision of active grid management benefiting customers and system operators alike.