American Electric Power (AEP; Columbus, Ohio, U.S.) and a team consisting of SuperPower Inc. (Schenectady, New York, U.S.), Nexans SuperConductors GmbH (Hürth, Germany) and Oak Ridge National Laboratory (ORNL; Oak Ridge, Tennessee, U.S.) have made it a priority to address fault current over-duty problems at the transmission voltage level of 138-kV and higher. Underway since June 2002, they’ve embarked on a project to develop three prototype fault current limiters (FCLs) using high-temperature superconductor (HTS) technology.

As new sources of generation are added, the matrix fault current limiter (MFCL) is expected to help cost effectively meet the threat of higher levels of fault current without the adverse side effects imposed by existing solutions. The MFCL will reduce the available fault current to a lower, safer level, so existing switchgear can still protect the grid. Half of the US$12.2-million cost to develop the MFCL is being provided by the U.S. Department of Energy’s Superconducting Partnerships with Industry Program and the Electric Power Research Institute (EPRI) is contributing $600,000.

Leading the project, SuperPower has developed the proprietary, patent-protected MFCL technology. Nexans is providing the HTS materials fabricated with its proprietary melt cast processing (MCP) technique. ORNL is assisting with the development of the technology needed to operate equipment in a cryogenic environment at high voltage. AEP will be the host utility for the MFCL Beta prototype to be installed and evaluated at its Philip Sporn Substation in New Haven, West Virginia, U.S., starting in late 2006. The first of three proof-of-concept prototypes, as shown in the photo on page 63, with a nominal rating of 8.6-kV line to ground at 800 A steady-state current was successfully tested at KEMA Power Test in Chalfont, Pennsylvania, U.S., in June 2004.

When tested with a prospective asymmetric first peak fault current of 25.6 kA, the device achieved the ratio of limited to prospective current of 84% at the first peak and a ratio of 56% by the third cycle of the fault. Reduction of fault current by the first peak helps to reduce mechanical and electrical stress on utility system components, and limiting at the third cycle reduces the fault current that the breaker must interrupt.

The MFCL achieves this type of response by using a matrix of multiple modules—each consisting of an HTS element in parallel with a conventional coil for current limiting. Under normal operation, the HTS elements are in a superconducting state and the device is invisible, presenting no impedance to the power system. Under fault conditions, the HTS will rapidly “quench” and go to a very high resistive state, while the fault current is then limited by the parallel connected coils. After the fault, the HTS quickly returns to a superconducting state, once again bypassing the coils. Following the successful completion of the proof-of-concept device in July 2004, a single-phase Alpha prototype is currently being designed. After fabrication, it will be tested at KEMA laboratories late in 2005 at the full 138-kV rating requirements required for the AEP application. During 2006 a full three-phase Beta device will be fabricated, qualification tested and installed at AEP’s Sporn Substation late in 2006 for in-grid evaluation. l