High-temperature superconducting cables deliver up to 10 times as much power as conventional electric power transmission cables. They are poised to help to reduce grid congestion as well as installation and operating costs.
All over the world, electric utilities are dealing with the challenge of trying to move more electricity through urban grids to meet the growing power demands of 21st-century customers. In addition, they must protect end users from increasingly larger power surges, known as fault currents.
A technological breakthrough in cable technology promises to assist utilities in overcoming both of these problems. The new cable is manufactured using hair-thin high-temperature superconductor (HTS) wires that conduct 150 times the electricity of similar-sized copper wires (Figure 1). When placed in a cable, these superconductor wires act as almost perfect conductors of electricity as long as a few conditions are met, the most notable one being that the temperature of the cable must be maintained below a certain critical temperature. This requires the cable system to be continuously cooled with liquid nitrogen, which is inexpensive and environmentally safe. This also eliminates the oil used in many conventional high-power cables in cities across the U.S.
1. Doing more with less. Hair-thin HTS wires conduct 150 times the electricity of similar-sized copper wires. Courtesy: American Superconductor Corp.
Overview of the New Technology
In November, POWER interviewed Jack McCall, director of business development of transmission and distributions systems for American Superconductor, which is the company that developed [correction: co-developed] the HTS electric power cables
"An understanding of the basics of HTS cables sets a good point of reference in discussing their application," he said. "There are four primary characteristics of HTS cables that differentiate them from traditional copper cables: higher power transfer capability, very low impedance, simplified placement considerations, and optional fault current limiting capability."
McCall explained that the power density advantage enables an HTS cable of any voltage to conduct up to 150 [correction: 10 times for the cable] times more power than traditional copper-based cables. Conversely, it is possible to carry a given amount of power at a much lower voltage level than is typically used. For example, a single 15-kV class HTS cable can carry 100 MVA, a level usually associated with 69-kV copper cables.
The very low impedance of HTS cables results in much lower power losses compared with equivalent cables, he pointed out. When placed in a network application, the cable’s lower impedance attracts current flow from parallel circuits, reducing the power losses in those lines as well, although the refrigeration system required by the cable system does offset some of the efficiency gains.
According to McCall, two characteristics of HTS cables combine to produce simplified siting requirements. The first of these is that HTS cables generate little to no magnetic field. This both dramatically reduces the required right of way and eliminates the need to derate the cables when they are placed near other cables or underground infrastructure. The environmental and public relations benefits due to the absence of magnetic fields are also apparent (there is no electric field either, but that is true of all cables). Secondly, as the cables are in a self-contained thermal envelope due to the refrigeration system, there is no need to consider derating of the cables due to cable burial method, depth, or soil type. Therefore, HTS cables are ideal for placement in constrained right-of-way locations, especially when large amounts of power transfer are desired.
McCall pointed out that another important innovation in the [American Superconductor] HTS cable’s design is its built-in fault current limiting capabilities. The cable will act as a very low impedance, high-ampacity conductor under normal operating condition, and then become highly resistive during faults, limiting high-magnitude fault currents.
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