LOSSES

Losses in a superconducting AC cable are-despite the name superconducting - not perfectly zero. Losses occur due to imperfections in several components of the cable, and even a perfect superconducting material does generate losses when it is exposed with AC current. The total losses in a full-scale transmission cable are app. 10 W/m per phase at full load.

The following loss components exist in a superconducting cable:

Thermal heat leak

A superconducting cable is cooled by liquid nitrogen at around 77 Kelvin (-19G°C or -384°F). To maintain the low temperature the cable is insulated by a very efficient thermal insulation. But due to the large surface thermal heat leak occur. Work is done to improve the thermal insulation and thereby lower the heat leak It is expected that the thermal heat leak can be reduced to 0.5 W/m per phase.

AC conductor losses

Due to basic physics the Superconducting material so called AC losses occur. The losses arc highly dependent on the load current.

Induced loss in shield

Superconducting AC cables with room temperature dielectric design has a conventional shield in which resistive losses are induced by the load current. These losses are comparable with the induced losses in conventional cables.

Dielectric losses

In superconducting cables - as well as conventional cables - dielectric losses occur due to the loss factor in the dielectric material. For cables with cryogenic dielectric design the liquid nitrogen cools the dielectric, and the loss factor will be very small. For superconducting cables with high current and reduced voltage the dielectric losses will be reduced.

Circulation of liquid nitrogen and hydraulic loss

Circulating liquid nitrogen in the cable is necessary to provide the needed cooling. The liquid nitrogen flow results in a small hydraulic loss, which have to be provided by a pump circulating the liquid nitrogen.

Induced loss in thermal insulation

The thermal insulation consists of metallic parts. For room temperature dielectric cables the thermal insulation is exposed for a magnetic field, currents are induced and losses occur.

Joints and terminations

Joints and terminations include resistive paths which generates ohmic losses.

Cooling

A cooling machine is needed to remove the disposed heat in the liquid nitrogen. The cooling machine operates with a limited efficiency (due to the Carnot-effect). As a rule of thumb 10 Walt is required to remove one 1 Watt. In the bar graph showing the individual loss components the cooling efficiency is included in the primary loss components e.g. the AC conductor loss.

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	AC losses
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