Superconductors are materials that offer no resistance to the flow of DC electricity when cooled below a critical temperature Tc which, depending on the material, can range from near absolute zero (0 degrees Kelvin/-273 degrees Celsius) to as high as 135 degrees Kelvin. The cooling of the materials is achieved using liquid nitrogen for temperatures in the range of 77 degrees Kelvin/-196 degrees Celsius, or liquid helium for even lower temperatures. Mechanical refrigerators can also be used.

Practical superconductors can be grouped into two main types:

LTS materials, which are usually metals, have critical temperatures below about 40 degrees Kelvin (−233 degrees Celsius), and for most applications, they need to be cooled to about 4K, which is achieved by using liquid helium.

An electrical current flowing round a loop of superconductor wire can continue to flow indefinitely, producing some of the most powerful electromagnets known to man. The major commercial application of these magnets is use in clinical magnetic resonance imaging (MRI scanners) and this remains as the major market for LTS even today.

Other applications of LTS include magnets for high energy physics, for tokamak-based fusion reactors and for a variety of research equipment involving high magnetic fields.

Efforts to develop superconductors at higher temperatures using many materials have been conducted in past decades, and the outcome of these experiments resulted in the discovery of High Temperature Superconductors in 1986.

HTS materials were first discovered by two IBM scientists. The transition temperatures of this group of materials range up to 135 degrees Kelvin (−138 degrees Celsius), and this discovery opened up a wealth of new potential applications for superconductors. These HTS materials can be cooled by liquid nitrogen, which liquefies at 77 degrees Kelvin, approximately 20x the operating temperature of most LTS. Liquid nitrogen is a widely accessible industrial cooling medium, quite inexpensive and can be easily handled. Mechanical refrigerators, called cryocoolers, can also be used.

Experiments on many materials and compounds showed superconducting properties at different critical temperatures, and the most popular materials among them are bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO).

After years of development, robust wires have been developed that can carry up to 150 times more power than copper wire of same dimensions. Fig.1 compares the amount of copper and the amount of AMSC's AmperiumTM HTS wire required to carry 1000 A.

Present day HTS wires have been demonstrated in many power grid and military applications such as AC power cables, fault current limiters, transformers, motors and generators. HTS current leads are now being used in the Large Hadron Collider (LHC) at CERN to supply current to the magnets steering the proton beam while minimizing heat leak and dramatically reducing refrigeration requirements.

New opportunities for these remarkable materials include HVDC cables, which can be used to carry huge amounts of power long distances with 100% efficiency, and low size and weight yet high power generators for wind turbines (10MW and higher).

Some of the present day commercial opportunities for HTS are discussed below.

One of the most significant application opportunities for HTS is in AC power cables which have the potential to change the way that power is transmitted and distributed in the power grid.

India is experiencing unprecedented growth in power generation, transmission and distribution segments:

The agencies and industries involved in these segments are facing many challenges in absorbing the above growth. Some of the important ones are

HTS power cables address many of the above issues. New cable installations where high power and limited right of way are issues may find underground superconductor cables more effective than those with conventional conductors.

The three most significant advantages of the superconductor cable over conventional power lines are:

Superconductor power cables use HTS wire in place of conventional copper/aluminium wire. These wires need to be cooled below their critical temperatures and liquid nitrogen is used as coolant. The liquid nitrogen is made to flow through a cryostat or tube with thermal isolation from the exterior of the cable. Typically, HTS cables carry much higher currents than conventional copper cables, and so, for comparable power, the voltage can be reduced significantly. This simplifies the dielectric requirements and, importantly, the permitting requirements.

Engineers have designed three different architectures of the cables (see Fig. 2) to meet a variety of requirements:

The main components of the HTS power cable system (see Fig. 3) are:

With the Government of India giving an impetus to the accelerated growth of power generation, a large number of Ultra Mega Power Projects with generation capacity of 4000 MW and more are coming up. Also many private players are coming up with power generation plants of thousands of MWs. This accelerated growth of power generation is bringing new challenges for the power system operators. The Transmission operator PGCIL has already interconnected the Northern, Eastern, Northeast and Western Power Grids and is now planning to add the Southern Power Grid to make a National Grid for transmission and exchange of huge amounts of power. This high density growth of power generation and the interconnected operation of the power system are leading to high fault currents in the Indian network. The power equipment in the existing network is rated for a fault current of 40 kA at 400 kV and 220 kV levels. With the growing addition of power generation, there will be many pressure, and flow rate such that the cable operates in its superconductive state HTS cable Demonstrations Examples of each design of HTS cable demonstrations successfully commissioned (See Fig. 4) are: substations in the country where the fault currents will exceed the rated capability of the existing equipment. It may not always be economically feasible to replace the existing equipment and hence suitable measures need to be adopted for limiting the fault currents. Though introduction of series reactors to reduce the fault current levels is an option, the planning engineers do not prefer this option as they offer high resistive losses, are bulky and contribute to grid instability through the voltage drop. A superconductor fault current limiter offers a perfect solution for this problem due to its unique ability to switch from a conductive to a resistive state.

A fault current limiter is a device which limits the prospective fault current when a fault occurs (e.g., in a power transmission network). See Fig. 5.

The use of HTS wire in rotating machines offers the following advantages as compared to conventional machines:

AMSC has produced several HTS rotating machines of various ratings. The world's first 36.5 MW (49,000 horsepower) superconductor ship propulsion motor designed and built by AMSC in collaboration with Northrop-Grumman is shown in the Fig. 6. The motor is less than half the size of conventional motors and can reduce ship weight by nearly 200 metric tons. It can help make new ships more fuel-efficient and free up space for additional equipment and armaments. Further discussion of generator opportunities for off-shore wind turbines is given below separately.

Transformers form an important part in electric transmission and distribution systems in stepping up and down the voltage so that the power is transmitted and distributed at appropriate voltage with less loss. However there are always losses in the transformer itself, and these systems present potential hazards like fire & explosion due to the presence of transformer oil.

Replacing the copper coils in a conventional transformer with HTS wire can provide significant benefits:

With the successful demonstration of HTS transformers, India is poised to take up challenges with the development and manufacture of bigger rated transformers to meet the energy growth. However, more progress needs to be made in decreasing AC loss.

Other applications of HTS include: warship degaussing, which have been tested onboard US Navy ships, magnetic resonance imaging, and current leads which are being successfully used in the Large Hadron Collider at CERN.

With the huge success of HTS in many applications as above, many organizations and the engineers are excited and are on constant look out for innovative uses for new applications. Some of them are HTS HVDC cables and low weight high power generators especially for off-shore wind turbines. Brief details on the work being done in these areas are given below:

In the power grid, it is always a challenge to transmit huge amounts of electricity over long distance, as often generating stations are far from the load centres and located near to their input resources like coal mines, rivers for thermal and hydro plants; away from the population for nuclear plants and wide spread windy areas for wind farms etc. Among the most important challenges are

In India, power transmission companies are addressing these issues through the use of high voltages for transmission corridors at 400kV or 765kV and now are looking at using 1200 kV to achieve the above objectives. They have also used 800kV HVDC, which has also proven to be a big success, and more of these lines are being constructed. Though the above solutions are able to address the first challenge to a great extent, they do very little in terms of minimizing rights of way. This is causing project delays and cost increases. Avery innovative solution to the above challenges is use of HVDC superconductor cables.

Apart from addressing the above mentioned challenges, these systems also address additional issues faced by transmission companies, including integration of multiple sources of energy including renewables like wind and solar which are often distributed over a wide geographical area, multiple destinations, better security compared to overhead lines, better flow control and cost allocations.

One of the most important and interesting benefits of HVDC superconductor cables is the dramatically reduced right-of-way they requires. Fig. 7 shows the comparison of transmitting 5 GW of electricity by way of overhead power lines or by the alternative HVDC superconductor cables.

With the present cost structure, HVDC superconductor cables are ideally suited for transmission of moderate (1–5 GW) to High Power (>5 GW) for moderate (150 to 650 km) to long (>650 kms) distances.

As the growth of wind power generation throughout the world is increasing, the ratings of the wind turbines also are increasing to maximum wind power production and reduce overall project cost. The weight and sizes of the turbines are growing, making the structures bigger and installation more and more challenging, especially at offshore sites. A wind turbine utilizing a superconductor generator offers a solution for this challenge, which is especially severe for offshore wind turbines.

A superconductor wind generator being developed by AMSC is rated for 10MW and will only weigh as much as a 5MW conventional wind generator, making it compact and practical to install.

In this concept, different grids can be interconnected through a superstation using DC superconductor power cables for exchange of large power between grids. The idea here is to increase overall power reliability and lower energy cost. Such superstations can be built to interconnect the grids across the states, countries, etc. Tres Amigas is a superstation being developed in the USA to interconnect the Western, Eastern and Texas grids.

In order to reduce greenhouse gas (GHG) emissions, plug-in hybrid electric vehicles (PHEVs) with rechargeable batteries are being introduced in developed nations and will soon be used all over the world. These vehicles will be plugged into the electric grid to charge the batteries. At present the PHEVs have been used basically for passenger cars. Time is not far off when many more commercial vehicles-trucks, buses, trains-will be made as plug-ins to check the GHG emission and reduce the dependency on fuels like petrol/diesel. When these large number of PHEVs start using electric grids to charge their batteries, the utilities will face potential challenges to adjust to their demands, especially in highly populated areas. The concerns include not only the availability and distribution of the power; but also upgrading power grid equipment. One of the solutions is to interconnect the substations to manage/share the loading during uneven demands. Superconductor power cables would be the best choice to interconnect the substations due to their much reduced right-of-way requirements and their fault limiting features. They also offer high capacity and are relatively easily installed in a dense urban environment.

High temperature superconductors are currently being used successfully in many applications in the field of electricity, and they have demonstrated their capabilities and suitability very well. The benefits they offer to the utilities in the transmission and distribution sectors will make them more and more useful for the entire society. Though the HTS cable system itself may be higher cost than a conventional cable system because of the extra cost of the refrigeration system, this cost is often outweighed by the much reduced right of way requirements and the correspondingly reduced cost of installation. With the increased usage of superconductors, one can one look forward to increased innovation and application in the fields of electricity generation, efficiency, transmission and distribution.

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