Five Technologies To Keep Blackouts at Bay

Technology already exists to prevent or mitigate the kind of outage that crippled North America on 14 August. Some of these systems are spottily deployed, others not at all, but used in combination they should lead to a much more reliable power grid.

FACTS (Flexible AC Transmission Systems) : These depend on power control arrays that are based on thyristors--large semiconductor devices that allow second-to-second control of voltage and other ac power characteristics. This level of control makes it possible to better channel the flow of power through branches of the grid, regulate voltage levels, and safely operate existing transmission lines closer to their theoretical thermal limits. As peak demand increases, utilities can shunt as much as 50 percent more power through existing lines, allowing them to avoid the expensive and politically challenging process of adding transmission infrastructure.

HTS (High-temperature superconductor) cable : Such power-carrying lines are made from a conglomeration of mainly ceramic materials that, at room temperature, are highly resistive, but offer virtually no resistance when cooled with liquid nitrogen to much lower temperatures. When these power lines replace copper ones in transmission systems, line losses will be reduced, and much more current will be carried than in copper lines of equivalent volume. HTS cables are being developed and demonstrated in grid tests in the United States, Europe, and Japan.

HTS are called high temperature because the first superconductors lost their conductivity at extremely low temperatures, around 10 K. Practical applications for those early superconductors were limited because the liquid helium required to cool them is expensive to produce. Further research led to the development of (relatively) high-temperature superconductors that are cooled to around 100 K (–173 ° C) by liquid nitrogen, which is cheap, readily available, and easy to handle.

SMES (Superconducting magnetic energy storage) : SMES systems store energy fed from power lines in the magnetic fields of superconducting coils so that power is available to make up for momentary dips in voltage or lapses in current flow. When a control device detects a voltage sag or a brief loss of power, the superconducting coils--which can deliver several megajoules of energy at power levels approaching 1000 kilovolt amperes--begin to discharge with a delay of as little as 2 milliseconds. The coils send current through switching and conditioning equipment and onto the transmission line to prop up the power.

Outfitting the grid with SMES devices would prevent billions of dollars of losses suffered by businesses that critically need a stable power supply. Industrial plants that operate heavy machinery with variable speed drives or sensitive temperature control devices--such as paper machines in paper mills or thermal treatment facilities at metal fabrication plants--can be shut down for hours because of a quarter-second-long dip in voltage.

FCL (Fault-current limiter) : These devices dampen surges of current in transmission and distribution networks. FCLs rely on a superconductor's sensitivity to temperature. Below a certain temperature, superconductors offer virtually no resistance, but above that temperature, their resistance increases sharply.

During normal grid operation, a superconducting coil on a transmission line allows current to flow almost unimpeded. But when a fault current--which can be several times higher than the normal load--hits a superconductor designed to handle a specific range of amperage, the surge heats the superconductor and pushes it into a resistive state, limiting the current and preventing it from overtaxing the line. The setup can also be designed so that the superconductor shares the burden of limiting the current flow with a separate resistor or inductor connected to it in a series. This is referred to as a series resistive limiter.

In another type of FCL, called an inductive limiter, an ordinary inductive coil inserted in the transmission line is magnetically coupled to a secondary coil made of a high-temperature superconductor. In the presence of a fault current, the secondary coil turns from a superconductor to a resistor. The shared magnetic field allows the secondary coil to turn the primary coil into a resistor as well, thereby limiting the fault current.

As was the case in the moments before the 2003 blackout, the failure of a single transmission line can create a fault current that places a greater burden on adjacent lines already operating near capacity. As the adjacent lines near the point of overloading, automated controllers disconnect them from the grid, increasing the number of fault-current loads on other lines exponentially. FCLs could have prevented this cascade by holding the fault current to a level where controllers need not have disconnected the lines from the grid.

HVDC (High-voltage direct current) : Thyristor-based converter technology is used to change current from ac to dc and then back to ac, for highly reliable power transfer over long distances, across natural or national boundaries, and between ac systems designed for different frequencies (mainly 50 and 60 Hz) or incompatible frequency controls. HVDC lines were indispensable in the creation of underwater links--like those connecting grids in Germany and Norway and the ones creating a single grid spanning Japan's islands.

High-power dc lines offer the added benefit of carrying much more power than overhead ac transmission lines over a given right of way. And because dc lines require only one cable (or two for bipolar operation), they are cheaper to make than ac lines, which require three cables.

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