AIST Announces Large-capacity Superconducting Current Limiter

May 14, 2009 16:30 Yousuke Ogasawara, Nikkei Electronics

Japan's National Institute of Advanced Industrial Science and Technology (AIST) developed a 500V/200A-class fault current limiter that uses an oxide superconducting thin film.

An oxide superconducting thin film transits from the superconducting state to the normal conducting state when an overcurrent occurs. By utilizing this phenomenon, the new limiter instantly reduce fault current when short-circuiting or other accidents occur in the power distribution grid.

When a large number of medium- to small-scale power generation facilities such as wind power generators are connected to the existing power grid, the use of a fault current limiter can prevent damages of power-related equipment caused by fault current. One phase of a 6.6kV/200A-class three-phase fault current limiter can be produced by connecting eight units of the new device.

"We now see our way clear to developing a utility-scale (several kilovolt/several hundred ampere-class) superconducting fault current limiter," AIST said.

The current limiter was fabricated by attaching a highly resistive gold-silver alloy layer to a 160nm-thick YBa₂Cu₃O₇ (YBCO) thin film formed by the dipping-pyrolysis process (MOD method). The new limiter can be applied with a voltage four or more times as high as that can be applied to the existing limiter with the same length and a pure gold layer. Thus, the required amount of expensive superconducting thin film can be reduced to 1/4 or less, according to AIST.

This time, AIST fabricated an internal serial connection type limiter by dividing a 2.7cm wide, 20cm long thin film into three portions with the use of two intermediate electrodes. Then, a module was produced by connecting two units of the limiter in parallel.

A non-inductive wire-wound shunting resistor was connected in parallel with the thin film to prevent a phenomenon called "hot spot." The hot spot causes the temperature to surge locally in the portion that is first quenched (ie, entering a non-superconducting state) and burns the thin film upon short-circuiting.

The shunting resistor has a resistance value that is 1/5 or lower than that of the thin film. Thus, when the thin film is quenched, most of the applied current is branched to the external resistor. In addition, a commercially-available capacitor with a capacitance of 120μF is connected to the thin film in each of the three portions in parallel.

AIST confirmed that a 237A (effective value) current was supplied with no electric resistance when the module was cooled with liquid nitrogen. Also, it conducted an experiment of applying a high voltage and discovered that a fault current, which could have reached 2.26kA without the limiter, was reduced to 1/3 or lower.

If a 6.6kV/200A-class three-phase fault current limiter is made based on the latest development, it can be stored in a cryogenic container with a diameter of 70cm and a height of about 2m. The target price of the current limiter is ¥2,000,000 (approx US$20,896) or lower per megawatt, according to AIST.

Aiming at the practical use, AIST will continue to develop the limiter to increase its capacity to 1kA or higher. And it plans to focus on the issues such as the adjustment of recovery time and conditions for current limiting operation as well as the optimization of the device structure.

The details of the development will be presented at a conference on cryogenics and superconductivity, which runs from May 13 to 15, 2009, at Waseda University and a conference on superconducting applied electrical devices, which will be hosted by the Institute of Electrical Engineers of Japan, at Kyoto University from June 11 to 12, 2009.

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