ROOM TEMPERATURE DIELECTRIC (RTD) DESIGN

The cable is characterised by having a conventional electrical insulation that operates at room temperature, just as a conventional cable. A detailed graph is shown in the figure.

Schematic of RTD type superconducting cable.

 

Description of the components of the system:

Former

The former is basically a tubular construction that serves as a mechanical support for the superconductor, which is wound on it, and a container for the coolant. The former may be a composite construction containing metals and/or composites. The former may be tight (closed former) to the coolant in which case the room between the superconductor and the inner cryostat wall is either evacuated or filled with an exchange gas. In the case of an open former the coolant may also circulate up to the inner cryostat wall, thus immersing the superconductor. The inside of the former is the main flow channel.

Superconductor

The superconductor is currently available in the form of a multi-filamentary tape that contains superconducting filaments in a normal conducting matrix. This tape is wound onto the former in a spiral fashion in several layers. The winding angles are adapted in order to fulfil mechanical and electrical requirements. The number of layers needed is determined by the power rating of the cable. Typically it is chosen so that the critical current of all the superconducting layers is sufficiently above the operating current (taking into account the sine shape of the ac) in order to satisfy requirements such as low loss at operating current and tolerance to short circuit currents.

Cryostat

The cryostat is the thermal insulation of the cable. In order to obtain the low thermal loss of order 1.5W/m - which corresponds to a thermal conduction coefficient of 0.0008 W/(K m) - some efforts have been made. Thermal heat transfer occurs by three means:

• conduction through solid materials exposed to a temperature gradient,

• convection of gaseous or liquid materials,

• Radiation.

All three means of thermal leak need to be minimised in order to achieve optimal performance.

The cryostat consists of a pair of concentric corrugated tubes, the inner and outer cryostat wall. The space between them is evacuated to less than 0.001-mbar pressure in order to eliminate heat transfer by gas convection. The space between the walls is then filled with layers of multi-layer insulation (MLI) that eliminate radiation transfer of heat. The LMI typically consists of metallised sheets of thin Mylar that reflect radiation. The main source of heat leak is the spacer (distancer) that keeps the inner and outer walls from touching.

Dielectric

The dielectric layer is used for electrical insulation of the current carrying parts of the cable. In the RTD design the cryostat is at high electrical potential. However, since the outer cryostat wall is virtually at room temperature (due to the excellent thermal insulation) a conventionally extruded dielectric can be used. This consists of a first inner semiconducting layer that is co-extruded with the PE or XLPE dielectric and the external semiconducting layer. This part of the technology is completely conventional and corresponds to that in a conventional copper cable.

Screen and sheath

Just as in conventional cables a copper screen and plastic sheath are added that serve as ground connection and environmental protection, respectively.

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