COOLING

Critical temperature

All superconducting materials have to be cooled the critical temperature, to be superconducting. The critical temperature is characteristic for the material in question, and although some materials are called "High-Temperature Superconductors" (HTS), they actually have to be cooled substantially below room temperature, namely roughly two hundred degrees, to be in their superconducting state.

Optimal cooling temperature

Concerning a cable, the HTS material has to be cooled to 80 Kelvin (K) or below, in order to perform satisfyingly well. For superconducting materials in general, the critical current increases for decreasing temperature, and for a 2000 A cable, the increase is approximately 100 A (5%) per Kelvin.

Thus, from this point of view, it is advantageous to cool the conducting part of it as much as possible. However, wheres it is relatively easy and inexpensive to cool to temperatures above the freezing point for nitrogen (which is at 63.3 K), by using liquid nitrogen (LN2) as the refrigerant, it is much more cumbersome and economically much less attractive to cool further down, because below the freezing point for nitrogen, only the expensive gases helium and neon can be used as refrigerants. The total cost to remove l W above the freezing point of nitrogen is of the order of 10 W, but this penalty factor increases noticeably for lower temperatures.

Cable cooling

In each phase of a cable system, the superconducting tapes have been stranded around the former, which can be flexible tube of stainless steel. LN2 is circulated through the formers and in this way, the tapes are cooled indirectly by the LN2.

Cooling system

For the demonstration project in Copenhagen the cooling system forms a closed loop, which consists of a refrigerating plant with circulation pump, transfer lines, cable terminations, and the formers. A storage tank for LN2 is connected to the cooling system.

The refrigerating plant is a commercially available cooling station. It has a total cooling power of 2.2 kW, and it is water cooled. Also, during operation, it is possible to build up pressure throughout the circulation system. This way, bubble formation due to boiling in the circulating LN2 can be prevented.

Heat inflow

The cable conductors are mounted in vacuum-insulated cryostats, which are inside the electric high-tension insulators. Also, the transfer lines and the cable terminations have vacuum-insulated walls. In this way, the heat transfer from the surroundings to the cooled parts of the cable is a few Watts per meter. However, the six current leads are made of copper, with relatively large cross sections and short lengths, and therefore, the major contribution to the total heat inflow is related to the current leads.

Cooling installation at the 30 m demonstration in Copenhagen.

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	Triax
	Cable designs
	RTD design
	3-phase design
	Cooling
	Standard and norms
	Critical current
	Losses
	AC losses
	Overcurrent