Current limiter could keep grids in step with renewables

Unlike conventional current limiters and fuses, the superconducting ceramic devices designed by researchers at the Karlsruhe Institute of Technology (KIT) are not destroyed by surges and maintain power throughout.

The team first grew a one-micron-thick YBCO crystal (yttrium, barium, copper, oxygen) layer directly on a stainless-steel strip of a few millimetres in width.

When the device is cooled below 90K (-183°C) the material becomes superconductive. However, superconductivity collapses abruptly when the current in the conductor exceeds predetermined design limits.

In the case of current peaks in the grid, the superconductor loses its conductivity within fractions of a second and the current then flows through the stainless-steel strip only, which has a much higher resistance and, thus, limits the current. The resultant heat is removed by the cooling system of the superconductor. A few seconds after the short circuit, it is returned to normal operation in the superconducting state.

‘The standard solution in the grid is a kind of fuse that burns through and has to be replaced by a service… you have to manage the power connection, repair everything or feed the current around other lines,’ project lead Dr Wilfried Goldacker told The Engineer.

He said that compact superconducting current limiters could enhance the operational stability of future power grids and allow for a simplification of the grid structure. Furthermore, the various components in future grids — transmission lines, transformers, generators — could be designed for smaller peak currents, increasing overall efficiency.

‘For the transformer you can reduce the size and weight by a factor of two or three. Also you have protection oil [in existing transformers] and in the case of failure very often these transformers are destroyed by fire — in the superconducting version you have only the coolant,’ he said.

In terms of renewable-energy generation, using superconducting limiters could reduce the size of generators in wind turbines, for example, but also create a more stable overall grid design.

‘In Germany we have severe problems with our wind-energy generation in the North sea and Baltic sea — we need 8,000 to 10,000km of transmission lines to bring energy down to the centre to industrial areas and about one quarter of the wind energy cannot be fed into the grid because of instability, and we fear in Germany that we need more than 10 years to extend the grid and prepare for the upcoming increase in wind-power generation.’