THE micro-SMES PROJECT

A SMES (Superconducting Magnetic Energy Storage) is a device simply consisting of a superconducting coil. The magnetic energy stored in the system is related to the circulation of a DC current inside the coil. This current flows permanently thanks to the ability of the superconductors to transport DC current without losses. This energy can be exchanged with an eternal system by means of static electronic converters. The first prototype of SMES was built in the early seventies. SMES systems were initially conceived as large devices to be used for the leveling of load fluctuations in electric power grids. Several other applications were later found. Nowadays small scale SMES (micro-SMES) are used for the local and short term control of power quality of critical loads, by reducing flicker effects and preventing brief voltage sags. The Figure below shows possible future applications for SMES.

The main drawback for the application of SMES systems is their cost. The aerogram shows how various components contribute to the total cost of the system.

Superconducting coils and refrigerators cover almost the whole cost. Recent significant advantages in superconducting materials and technologies are expected to allow drastic reduction of the costs and to facilitate the diffusion of SMES systems in the market place.

·       A micro-SMES system for Power Quality applications has been designed and partially realized at the University of Bologna. The main design parameters are the followings:

 E = 200 kJ, P = 35 kVA

I = 150 A, Vdc = 750 V, Vac = 380 V.

 

·       The power conditioning system has been designed, realized and tested at the University of Bologna in cooperation with Elettronica Santerno. It can connect the superconducting coils to a three-phase, 380 V grid and let the SMES operate as active filter and UPS.

·       The superconducting coils system has been designed at the University of Bologna in cooperation with Ansaldo Superconductors S.p.A. in Genova and is now in the manufacturing phase. The SC systems is of the active screening type and consists of two solenoidal coaxial coils realized by means of 12 km of a NbTi strand which was offered at low price by Europa Metalli S.p.A.. The main data of the strand are the followings:

 strand diameter 0.825 mm, SC filament diameter 6 mm

number of SC filaments 6534, Ic 429 A @ 4.2 K, 6 T.

 

·       A low-loss criostat is in the designing phase in cooperation with Ettore Zanon S.p.A. in Schio. A cryocooler system should recondensate helium vapors in order for the SMES to operate in a close cycle way.