What are they?
A High Temperature Superconducting (HTS) power cable is a wire-based device that carries large amounts of electrical current. There are two types of HTS cables.
Warm Dielectric Cable
The warm dielectric cable configuration features a conductor made from HTS wires wound around a flexible hollow core (figure 1). Liquid nitrogen flows through the core, cooling the HTS wire to the zero resistance state. The conductor is surrounded by conventional dielectric insulation. The efficiency of this design reduces losses.
Figure 1. Construction of a warm dielectric HTS cable.
Cryogenic Dielectric Cable
The cryogenic dielectric is a coaxial configuration comprising an HTS conductor cooled by liquid nitrogen flowing through a flexible hollow core and an HTS return conductor, cooled by circulating liquid nitrogen. This represents an enhancement to the warm dielectric design, providing even greater ampacity, further reducing losses and entirely eliminating the need for dielectric fluids.
Figure 2. Construction of a cryogenic dielectric HTS cable.
Where and How are They Used?
HTS transmission cables would be used for power transmission and distribution in urban areas throughout the United States and the world.
What are the Benefits?
- Can meet increasing power demands in urban areas via retrofit applications carrying two to five times more power than conventional cable
- Eliminates need to acquire new rights of way
- Replaces overhead transmission lines when environmental and other concerns prohibit their installation
- Enhanced overall system efficiency due to exceptionally low losses
- Increased utility system operating flexibility
- Reduced electricity costs
With an estimated 80,000 miles of existing underground cable throughout the world, High Temperature Superconducting (HTS) cables will provide enormous benefits to a utility industry that is poised for growth and is faced with an ever rising demand for electricity faced with an ever rising demand for electricity and tightening constraints on siting flexibility.
Conventional underground power transmission cables are utilised to transmit large amounts of power to congested urban areas. Conventional (copper-based) cables are capable of transmitting power (40 to 600 MVA) at high voltages (40 to 345 kV) through integrated underground duct systems. Existing duct systems limit the size of the conventional cables and the amount of power that can be transmitted through them.
When will HTS Power Transmission Cables be Available?
• First HTS cable installation in a utility network is scheduled for the year 2000.
• The first HTS coaxial HTS cable demonstration is scheduled for that same year.
• The first commercial sales of HTS cable wires are expected shortly after 2001.
More about HTS Cables
Superconducting cables can provide 2 to 5 times more power than conventional cables of the same size. Much more power can be moved using the existing ‘right of way’ duct system in densely populated urban areas. No expensive and disruptive excavation and construction are required.
Figure 3. Underground superconducting wires for domestic usage can increase current carrying capacity over conventional copper-based power lines.
A promising application for HTS cables will be to provide a high capacity electrical “highway” solution to over-stressed transmission networks. With more power supply markets opening up due to utility deregulation, coupled with an ever-increasing demand for electricity, better electrical highways can be built to streamline the transportation of electricity from low cost generation suppliers to dense populated cities using smaller, higher capacity cables.
Construction of HTS Cables
Construction of the HTS cable uses traditional stranding techniques and equipment to wind HTS wires around a hollow core. Once liquid nitrogen is run through the hollow core to cool the HTS material, it becomes superconducting with significantly more current transport capacity than conventional copper-based cables.
Why HTS Cables will become the Norm
HTS cable systems will have the most immediate market acceptance for the following applications:
• Replacement of older cable systems past their rated life or with loads approaching the rating of the cable
• Replacement of existing overhead transmission lines with underground cable
• Improvement of conventional transmission service links from ‘Generation’ suppliers to ‘Open Access’ customers
High Power Distribution
Another promising future application for HTS Cables will be ‘High Power Distribution’. Today, to increase a power supply to a urban area, utilities have to install transmission level voltage cables and utilize step-up (down) transformers at new substations. With stringent siting requirements and the unfavourable view of new substations in urban areas, an HTS cable will be able to transmit the same amount of power at distribution level voltages and eliminate the need for new substations.
Types of HTS Cable Applications
By design there are two types of HTS cable applications:
• Existing system upgrades
• New Systems
Existing System Upgrade
In order to minimize replacement costs, the ‘warm dielectric’ HTS cable is designed to fit within an existing duct system. This cable design uses conventional dielectric insulation and is sized to easily install in the existing duct system. With the HTS wires surrounding the hollow core, the cable effectively becomes superconducting with a significantly higher ampacity rating.
The decision to choose an underground transmission cable rather than an overhead line system may include many factors such as; close proximity to heavy residential areas, size of the towers, fear of electro-magnetic frequencies (EMF’s), and limitations on the existing infrastructure. With new duct systems, a ‘coaxial’ cable system (comprised of one ‘supply’ HTS conductor surrounding a flexible hollow core, and one ‘return’ HTS conductor surrounding a cryogenic dielectric insulation) provides the best option with greater ampacity and reduction in losses.