HTS Fault Current Limiters

When short circuits occur on an electrical transmission or distribution system, electrical generators feeding power to it can create unwanted surges of current.  Unless circuit breakers open up to stop this surge (known technically as a "fault current") from coursing over the grid, it can damage expensive equipment located at electrical substations and elsewhere.  As grids around the world grow, the potential for destructive power surges grows too, increasing the need for utilities to maintain and upgrade breakers that protect their grid assets.  Worldwide, the resulting cost to electric utilities amounts to hundreds of millions of dollars each year, according to the U.S. Department of Energy (DOE).

A Fault Current Limiter (FCL, but also termed a fault current controller) is a device that uses superconductors to instantaneously limit electrical surges -- or at least reduce them to more manageable levels -- before they reach a circuit breaker.  (Fault current limiters with this capability but based on conventional electrical conductors do not exist.  Sometimes large copper coils called line reactors are used, but they are bulky and lossy.  Only superconductors posses the unique physical properties that allow them to react instantly to current changes, passing electricity losslessly along the line at normal levels while limiting the surges.)

FCLs significantly reduce the wear on circuit breakers as well as the maintenance they require.  At the same time, they allow utilities to avoid or delay upgrading existing circuit breakers and electrical substations to handle ever higher electrical surges.  The substantial bottom line savings -- by reducing maintenance, avoiding replacement of damaged equipment and products limiting the need for upgrades -- has led DOE to estimate a potential U.S. market for FCLs on the order of several billion dollars over the next 15 years. (see http://www.electricity.doe.gov/ ).

The most widely developed HTS FCL capitalizes on the superconductors’ ability to transition rapidly between very low resistance and high resistance – precisely when the fault current exceeds a power level specific to their superconductor.  This type of device is called a "resistive FCL".  When designed with Second Generation (2G) HTS wire and engineered expressly for this application (using special "non-inductive" wire windings), the FCL adds minimal reactance to the grid.  HTS FCL provides a number of distinct benefits:

Fail-safe Operation:  An HTS FCL is a passive device that requires no external sensing of the current to initiate the transition.  “Smart” HTS materials provide protection by instantaneously becoming resistive under high current conditions.

Reduced System Costs:  An HTS FCL can reduce potential fault currents from 10-20 times rated current to 3-5 times rated current.  This can significantly reduce capital expenditures on the support infrastructure in the transmission or distribution system by dramatically reducing the costs of upgrading current breakers or even reducing the ratings and size of circuit breakers, eliminating the need for line reactors, and minimizing clearance requirements in substations.

Improved System Reliability:  High fault currents are a major source of outages on transmission and distribution systems.  Controlling fault currents to 3-5 times rated current would greatly reduce these outages.  In addition, line reactors, which are sometimes used to help control fault currents, can cause voltage instability by adding reactance to the system.  This forces the utility to add other equipment to counter-balance the reactive element further increasing costs.  HTS FCLs avoid this problem.

Improved Power Quality:  Unlike conventional line reactors, there is no series impedance that can cause voltage drop.

Lower Losses:  An HTS fault current limiter has lower net electrical losses than copper based line reactors.

Development of HTS Fault Current Limiters to date has involved test installations at a number of electrical substations, using HTS rods and cylinders rather than HTS wire and coils. New designs based on AMSC’s 2G HTS wire hold significant promise for delivering more cost-effective systems. For more information on HTS Fault Current Limiters please refer to the AMSC white paper, "HTS Fault Current Limiter Concept".