Superconductors Are the Solution

As electric transmission networks become more interconnected and more power generating plants are added, the available fault current level continues to increase. In some parts of the North American electric system, the magnitude of available fault current is at the interrupting rating limit of commercially available circuit breakers. If a circuit breaker is incapable of interrupting the fault, severe and costly damage to equipment may occur as well as substantial customer outages.

When the available fault current nears or exceeds the capability of a circuit breaker, utilities typically resort to upgrading equipment. When upgrading equipment is not possible — because of a lack of existing circuit breakers capable of interrupting the available fault current — utilities traditionally implement split substation buses or add impedances to transmission lines (that is, install current limiting reactors). Central Hudson Gas & Electric Corp. is testing a new superconducting fault current limiter (SCFCL) that may eliminate potential damage to critical infrastructure and possibly improve grid efficiency in some applications.

Traditional Solutions

Traditional approaches to limiting fault currents can be complex and expensive. The substantial additional physical space required can increase costs further, especially in urban locations where more space can be difficult to obtain. Furthermore, traditional approaches can reduce the reliability of the electric network, curtail disaster-recovery capability, impose severe operational constraints, and impact efficiency through increased real and reactive electric losses.

For example, adding current limiting reactors to a system to control fault currents can degrade voltage regulation for all customers served by the bus. The added reactance also can cause phase shifts, reduce transmission efficiency and potentially contribute to system stability issues. Furthermore, the threat of voltage dips during faults and their potential operational impact can raise concerns among some larger customers, potentially leading to installation of duplicate services to enhance reliability. However, this is a costly solution that can create other challenges.

New Superconducting Solution

The potential destructive impacts of high fault currents are why Central Hudson was interested in learning more when approached by Applied Materials Inc. with an opportunity to partner on a proposal to deploy and test a new superconductor-based fault-mitigation system. In addition to eliminating potential damage to critical infrastructure, there is the added possibility this technology could improve grid efficiency in some applications.

The SCFCL was developed and manufactured by Applied Materials. The system was installed and commissioned on June 16, 2014, at Central Hudson’s Knapps Corners substation in the Town of Poughkeepsie, New York, U.S., as a research and development project funded in part by a successful bid for support from the New York State Energy Research and Development Authority (NYSERDA).

The SCFCL was installed in parallel with a conventional fault current limiting reactor to ensure testing could be conducted with minimal risk. In theory, the SCFCL working in conjunction with the fault current limiting reactor would limit a fault’s overall current to a desired level with minimal impact to system efficiency. Central Hudson would benefit from an evaluation of a technology with potential future applications on its system, and Applied Materials would be able to evaluate the SCFCL in a real-world situation.

Superconductors today offer a robust approach to controlling fault current levels on utility distribution and transmission networks, particularly at higher voltage levels. The remarkable thing about superconductors is they operate with low impedance and are essentially invisible electrically. However, in the event of a fault, a SCFCL can rapidly (almost instantly) add impedance to the circuit, limiting the fault current and simultaneously providing the utility with a low-impedance, stiff system as well as a low fault current level.

This provides an obvious breakthrough for system designers by offering impedance that varies almost immediately with operating conditions. These characteristics are inherent to the superconducting material itself; the superconductor responds to the current and no external trigger is required.

The operation of Applied Materials’ SCFCL relies on the physical properties of the superconducting material. In normal operation, current passes through a matrix of superconducting tape, its length determined by the required current and voltage of the specific installation. The superconducting tape is immersed in liquid nitrogen, rendering a near-zero impedance to current flow. The physical nature of superconductors is such that they become highly resistive when the current flowing through them exceeds a maximum level determined by the chemical composition of the material and the length and composite width of the tape in the matrix.

If a fault current occurs and exceeds the current level at which the SCFCL is designed to superconduct (that is, the quench level), the superconductor becomes highly resistive within the first-quarter cycle of the 60-Hz signal (4 msec), forcing a majority of the current into a parallel current limiting reactor that will limit the current flow. Once the fault is isolated, the superconducting material will reacquire its near-zero impedance, thereby bypassing the current limiting reactor. The system naturally resets itself and becomes transparent, without intervention, until the next fault event occurs.