Superconducting Fault Current Limiter

Australians quietly develop something completely different.

A "fault" in a transmission or distribution circuit is nasty business. Circuit breakers open up, and that not only interrupts service to a lot of customers, it can also put a surge on the system. Worse, most fault clear themselves almost immediately, and then a decision has to be made, either by a person or by the equipment, whether and when to reclose the breaker. This is rough on the system, and the breakers themselves are expensive and hard to maintain.

A Fault Current Limiter (FCL) is a subtler way of dealing with momentary faults. It recognizes a sudden high current that’s not supposed to happen; it "inserts" a high impedance in the line momentarily to block that current, and returns to normal once the situation corrects itself. This is not an easy task, however. Currently (no pun), FCLs are far from ideal. Air core reactors using metallic copper conductors incur high operational losses, have limited response time, and wear out easily. What’s more, the breakers usually trip anyhow.

It’s long been recognized that FCLs are a great application for high temperature superconductors (HTSC). In fact, it’s seen as the first and best application of HTSCs on the power system. The basic idea is to put a superconducting element in the circuit in such a way that if too high a current comes along, the element goes "normal" or momentarily stops being a superconductor. This supplies the temporary high impedance to limit the current, and once the current drops, the superconductor goes back to being a superconductor and lets the current can flow again. This happens almost instantaneously, faster than a mechanical switch, and with "softer" transitions.

A SC FCL could thus detect abnormally high current transients in the grid, e.g. from lightning strikes, in a fraction of a cycle, and control the fault current so that system equipment can absorb it safely, protecting valuable downstream infrastructure.

Superconductors go "normal" if the temperature gets too high, or if the magnetic field gets too high. A SC FCL relies on the latter type of "quenching". The base current passing through the device produces a magnetic field below the level that would turn off the SC — a fault current will increase the magnetic field enough to do the trick.

SC FCLs are the subject of intense R&D efforts worldwide. ABB installed a prototype at a substation in Switzerland in 1997. The DOE is funding a new $12M program (, and EPRI is offering a major study (

A conference earlier this month presented the very latest on SC, including power applications. Note the three FCL sessions. Applied Superconductivity Conf, ASC 2004, Jacksonville, FL, October 3-8, 2004

Essentially all these efforts to date are using the bulk property of SC, and involve putting the entire load current through the SC itself, as described above. This leads to designs that are highly complex and which require a lot of SC material (i.e. very expensive wire or tape – which is proving difficult to make in large quantities). Moreover, none have progressed beyond the R&D stage and or early field beta trials. (Note – in most designs, a shunt actually supplies the impedance, not the quenched SC element, — even more complicated.)

Meanwhile, Down Under!

Meanwhile, a quiet development program in Australia has come up with a novel approach which has already been successfully demonstrated, and which is coming to North America. They developed their own SC tape and SC coils (and manufacturing method), and they invented and patented a 3-phase FCL that works in an entirely different way. It is actually more of a "controller" than a limiter of fault current.

It is a HTSC-enabled saturated magnetic core inductor. The load current passes through a copper coil on one side of a laminated-steel core. A DC coil on the other side maintains the core in a fully saturated state of magnetization. The number of copper turns are set so that a fault current in the AC coil will drive the iron core out of saturation (on the negative swing of the waveform). The coil then presents a large current controlled reactance, clipping the fault current at the design value.

All of this is explained in detail in a white paper presented in 2003, and which is available on request. Download 3.5 MB — (password required)
The design uses only a small amount of superconductor, simply to maintain the core magnetization (the only reason you need SC for this is that ordinary coils would be too big and lossy). More important, it works; it’s simple, robust, and versatile; and it will be available in a year at a reasonable price point. Key advantages include:

Superior Fault Condition Performance
– Very fast response time – protection functions activate in a fraction of a cycle.
– Large dynamic range – accommodates overloads without degradation and recovers instantly.
– Superior dynamic performance – suppresses initial transients more fully with much shorter decay times.
– Self-triggering/self-governing – operates instantly because of fundamental physical laws, no external sensing or controls required.

Low Cost
– Low operational cost – very little electrical losses in standby mode.
– High durability – very low cycle fatigue – operates through multiple operating cycles or fault events with little or no degradation.

– Expandable architecture – can be field or shop reconfigured to meet future requirements or changing grid characteristics.
– Small footprint and flexible form factor – compact to fit within space constraints and can be configured differently for local requirements.

Positive Grid Impact
– Improved grid reliability – clips fault currents completely without de-energizing the downstream grid.
– Transparency to the grid – no discernable impact during standby.

The technology has undergone substantial simulation, prototyping, and testing. The company sees no significant technical barriers and is on target to begin low-volume manufacturing and field installations of three-phase commercial units within 12 months.
The Australian company was recently acquired as a subsidiary of SC Power Systems, a US company, and operations have been moved to the US. They’ve already engaged in substantive dialogue with potential early customers and have validated the demand for its first three-phase units (15KV, nominally 10KAmps/phase).

They’ve contracted with NEETRAC (see UFTO Note 17Jan02) to prepare test procedures compatible with IEEE standards. NEETRAC member utilities are lining up to be the hosts for utility field tests scheduled for Q4, 2005. The company welcomes the opportunity to explore application needs, and will be taking orders as early as 2005.


Woody Gibson, 415-277-0179
SC Power Systems, Inc.
San Francisco, CA

The company is also raising equity funding. They presented at the NREL Industry Growth Forum, Oct. 18-20 in Orlando A business plan is available from the company.

Optic Fiber Inside Transm Cable Measures Temperature

Here is most of the text of a summary prepared by the developers, Com Ed and Southwire. The complete Word document with graphics can be downloaded at: (password needed)

**ComEd – Southwire Alliance Develops Novel Fiber Optic Transmission Conductor (FOTC)

In 1999, ComEd began work with Southwire to investigate a new concept to accurately determine the thermal behavior of overhead transmission lines during operation. It is the conductor temperature that dictates the thermal rating and available clearance under a line. However, as of yet no satisfactory method has been developed that measures conductor temperature axially throughout its length as well as radially.

A novel overhead transmission conductor system that uses optical fibers as an integral part of the phase conductor has been developed by ComEd and Southwire (Patent Pending) and placed in service on the ComEd system.

Operational since February 21, 2002, the 138 kV FOTC system uses distributed temperature sensing (DTS) to measure the temperature of the optical fibers that are embedded in the conductor. DTS allows accurate temperature measurement along the entire length of the FOTC line at different locations within the conductor.

Prior to the field demo, the FOTC system was tested and characterized by the NEETRAC {see UFTO Note, 17Jan02} and Oak Ridge National Lab (ORNL). Significant discoveries on the temperature behavior of the transmission conductor under various test conditions were found. For example, the impact of wind on radial temperature drop across a conductor and the impact of solar radiation on a conductor varied significantly from IEEE Std 738 during extreme weather conditions.

Field Trial Installation: The Fiber Optic Transmission Conductor (FOTC) was installed using a special dead-end assembly and an optical insulator. The installation method was the same as a conventional one, except that special care was taken to separate and protect the optical fibers from the conductor at the dead-end location.

The graph shows an example of the temperature data that is available in real-time from the FOTC system. With the FOTC system it is a simple matter to show the temperature of any desired interval lengths of the FOTC line. [graphic: Temperature versus Time Profile of 138 kV FOTC Line]

Utilities have a need to maximize the use of their assets. FOTC provides the medium for utilities to determine the real-time thermal operating limit of a transmission conductor in the most accurate way possible. It also provides the means to transmit data or voice communications. As the utility industry continues to evolve through transmission open access, new innovations such as FOTC will help pave the way to competitive advantage.


Southwire will pursue the development and commercialization of FOTC under a license from ComEd. A market study is underway, and in particular the partners want to learn more about how much FOTC can increase transmission capacity, and how utilities will judge the merits and value for use on their own systems.

Contacts for Additional Information
Jim Crane, ComEd, 630-576-7034,
Gene Sanders, Southwire, 770-832-4988,

NEETRAC R&D Focused on Power Delivery

While at the IEEE T&D Expo last November, I had the opportunity to meet folks from the National Electric Energy Testing, Research and Applications Center, a not-for-profit at Georgia Tech which focuses exclusively on power delivery technology, including (the integration of) storage and distributed generation. (See below* for a list of services provided–clearly a very practical “nuts and bolts” approach.)

NEETRAC grew out of the R&D Center that Georgia Power transferred to Georgia Tech in 1996, with all its staff and large facilities. Since then, the program scope and constituency have been broadened. The fulltime staff now exceeds 30, in addition to faculty and students who work with them. NEETRAC has access to all kinds of expertise and facilities across the entire school. They are ISO-9001 certified.

Sometimes it’s easier to start with an explanation of what something is not. NEETRAC is not a research management organization. Work is done in-house, and almost nothing is subcontracted out. They are not a funding agency. There is no technology watch function, except as part of scoping studies at the front end of projects. As the name says, they do testing, research, and applications.

Membership includes 23 major utilities (including 3 current UFTO participants, TXU, Xcel, and Exelon) and manufacturers. This number is expected to rise to 25 this year; they will stop at 30. Most pay $105K/year (much larger companies pay more).

Half of this money goes into “baseline projects”, which are selected and overseen by the Management Board. Forty-seven such projects have already been done, for a total cost of $4.2 million — on or below budget. Each project has a technical advisory committee, which usually meets by teleconference. There is a total commitment to the idea that members are to determine project content and program direction.

The other half of the money is placed directly into individual proprietary projects for the individual members (IP is very carefully protected). There is also about $1 million/year in contract research performed for other (nonmember) clients.

A new program of “Focused Initiatives” will offer non-members the chance to participate, though at 2.5 times the member cost. The proposal for the first such Initiative will appear in July, for Cable Diagnostics. NEETRAC already is doing a lot of work (for its members only) on Cables, including a test facility with cables with known defects. Vendors are invited to test and demonstrate their equipment. There are similar programs for other components.

Hans (Teddy) Püttgen, Director*

(*Dr. Püttgen is also the new President-Elect of the IEEE Power Engineering Society.)


*Services provided by NEETRAC:

-Transmission and Distribution Component Failure Investigation
-Incoming Material Inspection for Quality Control Program
-High Voltager Testing of Transmission and Distribution Components
-Connector Evaluation
-Insulator Testing
-Testing of Aerial Personnel Devices
-Soil Thermal Property Measurements
-Lighting Fixture Evaluation
-Testing of Routine Utility Devices
-Frequency Characterization
-Transfer Functions
-Line Hardware Evaluation
-Thermal Evaluation
-Electric/Hybrid vehicle testing & research
-Mechanical Testing
-Underground Cable Pulling
-Fault Current Testing
-Performance Evaluation of Overhead Conductor and Accessories
-Vibration-damper Testing HV 60Hz Watts Loss
-Measurements Power Cable and Accessory Evaluation
-Weathering and Corrosion Evaluation