Superconductor devices to tackle growing power network problems

London – Power networks are generally designed with a relatively low impedance between the generators that provide the source of electrical power and the users of the power. This helps maintain network stability by providing a fixed, stable system voltage while the current varies to meet the changing loads.

The downside is, though, that substantial fault currents - typically between five and 20 times the nominal current – can develop during network disturbances.

Over time, the maximum fault current in a network has tended to increase due to:

• Increasing demands for power and the resulting need for increased generation are pushing MV power grids to their maximum operating limits?

• Parallel distribution paths are being added to networks to support load growth and there are a greater number of interconnections within the grid?

• The development of distributed generation, such as wind power and CHP schemes, are adding to the complexity of an already complex system

The net effect is that short circuits can occur more often and are more likely to cause high, uncontrolled fault currents, leading to damage of electrical networks and consequent power failures.

Until now, operators of public and industrial electrical networks could only have limited protection against high short circuit currents, either by the use of complicated equipment or by over-rating of components.

There is a growing concern that as potential fault currents levels continue to increase they will soon exceed the protection capabilities of existing equipment.

As a solution, cable industry major Nexans has developed superconducting fault current limiters (SCFCLs) for medium voltage (MV) power networks. These devices, it said, can provide an almost instantaneous response to fault currents - limiting the current to prevent damaging overloading of switchgear and other network components

SCFCLs use a non-linear ’high-temperature’ superconducting (HTS) ceramic rather than electronic, electromechanical, mechanical or pyrotechnic components.

When the HTS element is cooled below its critical temperature of -196°C – a temperature that can be obtained using relatively inexpensive and readily available liquid nitrogen – it loses all electrical resistance, thereby allowing normal load current to flow with negligible losses.

Either the increased current density caused by the passage of fault current, or the loss of the liquid nitrogen cooling medium causes the?temperature of the superconducting material to rise with the result that the material reverts to a normal resistive state.

This added resistance has the effect of reducing the fault current to a lower, more acceptable level. This process is referred to as ’clamping’ because it effectively sets a limit above which the fault current will not rise.

The SFCL is said to operate in a few milliseconds, after which its resistance remains high until the fault current is cleared by a circuit breaker. Operation is sufficiently fast to ensure that the first peak of the fault current is limited – vitally important when considering the closing of a circuit breaker onto a section of faulty network.

The degree to which the subsequent current is limited can be set at the design stage to suit a specific application. It will, in many cases, be useful to choose this level such that existing protection arrangements do not need to be adjusted.

First deployments

The first field test of a Nexans SCFCL was carried out at an ENW (Electricity North West) substation in the UK – at Bamber Bridge, Lancashire, where it was live on the grid from October 2009 to June 2010.

This location was selected for two reasons. Firstly, there was plenty of space for the installation and secondly, the site provides an example of where an SFCL might be installed in response to a real need.

The two 33/11kV transformers feeding the substation had been recently upgraded, with the result that the fault level increased to above the making and breaking capacities of the existing circuit breakers.

It was therefore necessary to build a new substation and install a new 11kV switchboard of primary distribution circuit breakers comprising 10 feeders, two incomers and one bus-section.

So, while the fault level problem was addressed in a conventional manner, the situation allowed the design of the SFCL to be determined according to realistic criteria, as it was actually used to provide a solution to the fault level issue.

At the end of 2009, Nexans commissioned the world’s first SCFCL to be installed in a power plant.

In this pilot project for Vattenfall Europe Generation AG, the SFCL was used to provide short-circuit protection for the internal medium voltage power supply that feeds coal mills and crushers in the Boxberg brown coal power plant in Saxony, Germany.

The 12-month project enabled Vattenfall’s experts to gain valuable hands on experience with innovative SCFCL technology that they believe will offer significant benefits in personnel and plant safety.

This the first time that this type of device had ever been used in a power plant, said Nexens, adding that the project was implemented without public grants.

Vattenfall’s SFCL, designed for a rated current of 800 A, received live testing by daily routine operation in a feeder bar of the 12 kV power supply for rebound hammer mills (used for crushing coal).

It was designed and built by Nexans according to the specifications from Vattenfall and the Brandenburg Technical University in Cottbus (Germany), which provided scientific support for the project.

The device could limit a 63 kA prospective short circuit current to less than 30 kA immediately and to about 7kA after 10 milliseconds. A second field test is now planned using a new superconductor tape.