Power Conversion for High Energy Storage

Abstract

Very high current conversion is not restricted to industrial applications. Complications arising from greater penetration of photo-voltaic and wind generation, and increased diurnal load variability have seen renewed interest in large-scale electrical load-levelling technology, to remove the dependence on gas-fired peaking plant during high demand periods and to better utilize cheaper base load generation when demand is low.Load-levelling schemes utilizing pumped storage have been successful in over 150 installations worldwide, but with efficiencies as low as 60-70%, owing to the conversion to an intermediate medium for storage. Better efficiencies are reached with battery, flywheel and compressed air energy storage, though they currently lack the energy density necessary for levelling on a large scale.Superconducting magnetic energy storage (SMES), a technology first proposed during the 1970s oil crisis to reduce the use of expensive oil peaking plant, uses a high-power AC/DC converter to store energy directly from the AC system in the magnetic field of a supercooled inductor. High energy density is made possible with large inductances, and extremely high DC currents (in excess of 100 kA); projected storage levels of 5000 MWh were reported at the time.Because the energy storage requires no change from one medium to another (other than AC/DC conversion), the SMES exhibits very low losses, with round trip efficiencies of 90-95%. The fast bidirectional operation of the converter permits power-flow direction change in a few cycles, which makes SMES technology suitable for improving voltage stability, providing spinning reserve and in short-duration very high-current applications, such as pulse lasers and fusion reactors.Early SMES designs had problems with limited converter controllability and involved coils with considerable civil complexity. Although some prototypes were designed [1] and Self-Commutating Converters for High Power Applications