Superconductors protecting the grid
Superconductors can protect the grid. Up to a certain temperature, they conduct electricity with no resistance. Only above this so-called transition temperature does their resistance increase exponentially. In power engineering, this can be an advantage. When properly designed, high-temperature superconductors work as a kind of safety switch and help connect grids, while protecting them at the same time. Since they are self-triggering, they do not require any detection technology.
But why should grids even be coupled in the first place? Increasing distributed feed-in can lead to a poorer quality of supply. For example, by raising the permissible voltage range given high wind strengths and sunny skies. In extreme cases, this will lead to an area-wide power outage. In order to ensure the familiar high reliability of supply in the distribution networks in the future, grid operators must respond appropriately.
The expansion of the distribution networks is very costly and time consuming. Often, coupling the distribution networks is the cheaper solution, but in this case, the short-circuit power can rise enough to force the replacement or reinforcement of existing operating resources in the entire affected distribution network to a considerable extent, which incurs additional costs. Using conventional inductors is not suitable for limiting short-circuit currents, since conventional inductors deteriorate the voltage quality and stability of the grid due to their constant impedance.
Superconductors in daily use
The aim of the project 'SmartCoil' is to prove the technical functionality of a new type of passive short-circuit current limiting inductor with a variable impedance. This is achieved by way of designing, building and testing a single-phase test rig with an equivalent three-phase power of 10 MVA. The special behaviour of the SmartCoil system is realised through the use of a high-temperature superconductor (HTS) in conjunction with an ordinary inductor. For this purpose, a tripping device from a stack of short-circuited superconducting rings is introduced into the inductor. During nominal operation, it almost fully shields the interior of the inductor. This drastically lowers the undesirable choke impedance. In case of a short circuit, the current exceeds the critical value, the shield is lost and the choking effect of the inductor comes into play.
In effect, the SmartCoil system is an impedance-variable inductor. During normal operation, its impedance is greatly reduced, so voltage quality in the distribution network and grid stability remain unaffected; instead, both are improved compared with conventional inductors. Along with the impedance reduction during normal operation, the manageability and controllability of the distribution networks increases given significant horizontal and irregular feed-in of large amounts of electrical energy. In the event of a short circuit, however, nearly the full choking effect of the inductor is available, so that replacements or reinforcements of existing equipment are not required.
Due to the unique intrinsic properties of the HTS, the SmartCoil system is self-triggering, so it does not need detection electronics or other auxiliary equipment to operate, and it is intrinsically safe as well. This means that the grid is not at risk should the superconductor or the cooling system fail.
Different superconductors tested
In the first half of the project, the design and the specifications of the individual components were defined. High-temperature superconductors of different manufacturers were tested, and a suitable conductor was identified. First superconducting components (rings) were successfully tested with respect to their short-circuit behaviour. The synthetic cryostats required for receiving the superconducting tripping device will be ordered soon. As the project progresses, some final refinements will be made, and the superconducting tripping device will be built and then tested.
The project partners Karlsruhe Institute of Technology (KIT) and Siemens AG, Corporate Technology (CT), are working closely together, each contributing their long-standing expertise in the development of superconducting operating resources. KIT are responsible for material characterisation and design regarding losses in nominal operation. CT are the main contributors both to the development of the conceptual design and to simulating the limiting behaviour, and are responsible for building the superconducting tripping device. Both partners are equally responsible for component development and running the final test.
09/2014 – 08/2017