"Supercool Protector," Feature Article, October 2004

supercool protector

A runaway-current limiter passes a test.

by Harry Hutchinson, Executive Editor

A grid-protection device that takes advantage of a property of superconductors has passed its proof-of-concept test, the developers say. SuperPower Inc. of Schenectady, N.Y., is leading the development of a device called a matrix fault current limiter, designed to protect transmission lines from power surges.

A fault current, a spike on the line caused by a short circuit, can burn out cables, connectors, and transformers. It can black out a region. As new generating capacity comes online and demand for power increases, the means in place to protect against power surges can be overworked.

According to the developers, the superconductive device, although it will be expensive, will provide an alternative to other expensive protections for transmission lines and will cause no power losses when there is no emergency.

According to Len Kovalsky, program manager for
SuperPower's switchgear technologies, the basic device is a superconductive circuit in parallel with a coil. Under normal operating conditions, current will seek the path of least resistance—in this case, the zero resistance of the superconductor. Because there is no resistance across the superconductive circuit, it will introduce no power losses during normal operations, Kovalsky said.

If current spikes beyond a safe limit, the superconductor shuts down. The ability to go from no resistance to high resistance almost instantly is unique to superconductors and underlies the strategy for the device. When the superconductor becomes a resistor, power is shunted to the coil, where inductance will impede the flow of current and so will protect components of the line downstream.

Tests on a small-scale prototype were run at KEMA Powertest Inc., a high-power electrical testing laboratory in Chalfont, Pa.

Prototype of the matrix fault current limiter, which combines superconductor with a coil, began to redirect current within 4 milliseconds of a surge.

According to SuperPower's president, Philip Pellegrino, tests showed the fault current limiter starting to react to a power surge within four milliseconds, and limiting as much as 50 percent of the current within 50 milliseconds. Test currents ran as high as 27,000 amperes, he said.

The superconductive material for the matrix fault current limiter is a ceramic compound of bismuth, strontium, calcium, and copper supplied by Nexans Superconductors GmbH of Hürth, Germany.

The conductors are hollow tubes about 20 cm long, Kovalsky said, and that is where the "matrix" comes in. The limiters are small but modular, so several can be combined to increase the scale of the current they can handle.

The limit to expansion is the size of the cryogenic container, which must hold liquid nitrogen. Although the material is called a "high-temperature" superconductor, it must be chilled to about 77 kelvin, or -196°C, if it is to carry current. This is relatively high when compared to the 4 kelvin to which low-temperature superconductors must be cooled. SuperPower is the high-temperature superconductor subsidiary of Intermagnetics General Corp. of Latham, N.Y. The parent company makes the superconductive magnetic cores for magnetic resonance machines. MRI superconductors are of the low-temperature variety, and are cooled by helium.

The estimated cost for the program to develop a full-scale matrix fault current limiter is $12.2 million. Superconductivity Partnerships with Industry, a program of the U.S. Department of Energy, has put up $6.1 million. The Electric Power Research Institute, the nonprofit energy research organization based in Palo Alto, Calif., has contributed $600,000.

The program is scheduled to field-test a commercial-scale prototype in 2006. SuperPower said the goal is to place the test unit on a 138 kV transmission grid operated by a host utility. A commercial installation will probably take up more space than conventional passive circuit breakers, but will remain comparable in scale to other utility equipment, Kovalsky said. He said the footprint will be similar to that of a 138 kV transformer.

According to Kovalsky, at an estimated cost of $1 million to $1.5 million per unit, the device is expected to be competitive when utilities compare it with the cost of adding substations and replacing large groups of circuit breakers with more powerful ones.