Superconducting-based systems on the electric grid require three enabling technologies: the superconducting wire or tape, a cryogenic cooling system, and high-voltage cryogenic dielectrics. Most of the R&D effort is going into development of the superconductor, second generation (YBCO) tape in case of HTS. This is understandable as there are no grid-based applications without a cost effective and capable superconductor.

Little resources, however, are being applied to development of the other two areas: cryogenic cooling and high voltage dielectrics. In part, this is due to the perception that there are available systems and materials that can be made to work, at least in short term demonstrations. Lurking below the surface, however, are several technical issues that need to be addressed for utility acceptance of HTS grid devices.

In the cryogenics area the three major issues are reliability, efficiency and cost. In the most important area, reliability, the performance of cryogenic systems to date has ranged from 95 to 99% reliable. This needs to improve significantly to 99.5 to 99.9% if the HTS devices are to be seamlessly inserted into the US grid.

The needed improvements in efficiency are also substantial. Present closed-cycle cryogenic cooling systems have thermodynamic efficiencies from 10 to 15% of ideal Carnot efficiency. This needs to about double if the overall HTS system efficiency gains due to superconductivity are to be realized.
 

Finally, the cost per watt of cooling needs to be reduced by a factor of 2 to 4, depending on the application. This can best be done with the economy of scale that should come from a large production base.
 

Improved Dielectrics and Characterization Needed
 

Conventional dielectrics have grown with the grid over the last 120 years to higher voltage levels, now approaching 1MV in some cases, with high component reliabilities and proven materials. In the cryogenics dielectrics area there have been a number of failures of LTS and HTS grid prototypes due to electrical breakdown from design and materials related issues. This performance has indicated more focused development is needed in cryogenic dielectrics if HTS applications are to impact the grid in the next decade.
 

Most of the low temperature experience base is in the area of large DC-type magnets (e.g. MRI, accelerator dipoles, etc.) where the concern is transient kV-level voltages during occasional quenching. These voltage levels are much lower than grid voltages and there are no partial discharge or aging effects.
 

In general, work is required in two areas: materials R&D and conservative design techniques. The number of available materials that can provide high voltage electrical insulation in typical thermal gradients from 300K to the 30 to 80K range, and which are mechanically compatible with typical conductors and structural materials, is not large.
 

Materials currently used include some epoxies, G-10/11 composites, cable dielectric tapes, Ultem™ and vacuum. Even these materials are not completely characterized. There are typically little data on partial discharge and impulse (lightning) performance. Significant partial discharge will eventually produce a breakdown in AC applications which experience nearly 1011 cycles in a 30 year lifetime.

Vacuum as an electrical insulation medium is not reliable and subject to the Paschen breakdown as it degrades due to external leaks and outgassing. More fully-characterized, cryogenic dielectric materials are needed to allow designers to make engineering tradeoffs in real devices and understand volume scaling as one advances from scaled models to full-scale prototypes.

Proven techniques need to be developed to allow designers to integrate these materials (in some cases with liquid or gaseous nitrogen also serving as a dielectric) in a reasonable and robust insulation package that can meet stringent IEEE requirements (AC withstand, partial discharge, BIL and surge transients) and function at the operating voltage for 20 to 40 years.