T&D World Magazine

Transmission Voltage Resistive Fault Current Limiter Demonstrated

Nexans, Siemens and American Superconductor Corp. have successfully qualified a transmission voltage resistive fault current limiter (FCL) that uses high temperature superconductor (HTS) wire.

This marks the first time a resistive superconductor FCL has been developed and successfully tested for power levels suitable for application in the transmission grid (138 kV insulation class and nominal current of 900 A).

As electrical demand increases, more power generation must be added to the grid. The addition of generation capacity also tends to increase the destructive overcurrent available when a fault occurs on the power system, taxing the capabilities of installed equipment, such as circuit breakers. Faults can be caused by equipment failures, severe weather, accidents or even acts of willful destruction. Such faults can damage major, expensive components and, if not cleared quickly, can lead to lengthy, costly outages.

Used in a substation, FCL’s acts as current surge protectors for the power grid. A resistive FCL consists of low inductance superconducting coils that work in parallel with a shunt reactor. Unlike other approaches, this type of system has low impedance, meaning it is virtually transparent to the grid until it “sees” a fault. At this point, the superconductor coils transition from a conductive to a resistive state to suppress the fault current.

The system that was tested by Nexans, Siemens and AMSC proved to reduce fault current levels by more than 50 percent. This smart grid system can strengthen the grid by reducing the destructive nature of faults, extending the life of existing substation equipment and allowing utilities to defer or eliminate equipment replacements or upgrades. The resistive nature of this superconductor-based FCL can also improve the ability of the high voltage transmission power grid to remain stable, reducing the likelihood of more widespread system collapse.

The collaboration between the three industry leaders resulted in a solution that has virtually no electrical impact to a large electric utility grid under normal operation but limits currents in response to a downstream short circuit, limiting damage and the stress that other grid components experience.

The FCL development and testing was done as part of a project cost-shared by industry partners and sponsored by the U.S. Department of Energy and was aimed at accelerating the modernization of the U.S. electricity grid using superconductor technology. Nexans, Siemens and AMSC jointly designed, developed and tested the FCL. It features a proprietary Siemens-developed, low inductance coil technology that makes the FCL invisible to the grid until it switches to a resistive state. Nexans designed and built the high-voltage terminations and their connections to the FCL module in the cryostat. AMSC provided its proprietary Amperium HTS wire for the system.

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