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 (http://www.engineeringtalk.com/news/nex/nex111.html), and EPRI is offering a major study (http://www.epri.com/destinations/product.aspx?id=439&area=10&type=2).
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 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 firstname.lastname@example.org
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 http://www.cleanenergyforum.com/. A business plan is available from the company.
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