APPLICATION OF SUPERCONDUCTIVITY IN ELECTRIC POWER SYSTEM
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APPLICATION OF SUPERCONDUCTIVITY IN ELECTRIC POWER SYSTEM
With the discovery of High Temperature Superconducting materials the possibilities of applications of the Superconducting technology in power system have become very bright.
Superconductivity is characterized by an important feature of zero resistivity, which means that superconductor is perfectly electrical conductor. The other critical parameters are: critical Temperature, Critical Magnetic field and critical current Density It has been recognized that for power system applications, though the most important parameter is the critical current density other two are also having their due importances.
Nevertheless the impacts of the superconductivity in the area of power system need deep consideration. There are three major areas in the electric power system where the emerging.
Superconducting technology will have wide applications :-
i) SUPERCONDUCTING GENERATOR.
ii) SUPERCONDUCTING TRANSMISSION LINE CABLES, and
iii) SUPERCONDUCTING ENERGY STORAGE SYSTEMS.
Over and above to these applications there are possibilities in exploring superconductivity technology advantageously in system dampers, Fault Current Limiters, phase shifters static VAR compensators, power converters, and in sensors.
2.0. SUPERCONDUCTING GENERATOR
The difference between the basic design of a conventional and Superconducting generator will be better appreciated in the light of the fundamentals of generation.
In the generator, mechanical energy is converted into electrical energy by rotating a conductor relative to magnetic field produced usually by an electromagnet. The resulting flow of current in conductor generates its own magnetic field . The final useful electrical output depends upon the interaction of these two magnetic fields.
The electrical and magnetic loadings (current density and flux density) determine the output from a generator. Neither of these can be increased indefinitely due to certain limits.
The electrical loading (amp-conductors per meter ) is limited by the rate at which the heat produced can be removed, so the temperature rise is within the value that the insulation can withstand.
The magnetic loading is limited by magnetic ?saturation' ,which in ordinary steel takes place at 1.4 Tesla. Therefore flux density cannot be increased beyond this level, with using special steels.
These limits can be significantly relaxed by the using superconductors. Field winding will provide at least four to five times higher magnetic field with negligible DC voltage. This is possible because superconductors have zero DC electrical resistance and extremely high (100,000 times more than copper conduction of the same size) current carrying capacity. Thus machines with very high rated capacity are possible with superconductors.
Another very attractive feature of the Superconducting field windings is that due to very high magneto motive force set up, it is not necessary to use magnetic iron in the machine. Due to reduced rotor dimensions, the 'air gap' in the machine can be expanded and greater machine stability could result.
Any breakthroughs in generators can help in such rapid expansion. Superconductors could be one such possibility.
The advantages of Superconducting generators are :
* Fifty percent reduction in size and weight for a given unit size.
* Approximately seventy percent lower transportation costs.
* Easier transportation.
* Cheaper foundations and buildings.
* One percent higher electrical efficiency.
* Higher stability due to lower machine reactance.
3.0 SUPERCONDUCTING MAGNETIC ENERGY STORAGE SYSTEM (SMES)
Apart from the apparent advantages of Superconducting machines and Superconducting transmission lines, the application of Superconducting coils for storage of electrical energy is receiving considerable attentions. Such Superconducting magnetic Energy storage (SMES) coils would be charged during off peak hours by using power from the base load generating systems and then would be discharged during hours of peak demands. The high efficiency ( 95% ) of the SMES system makes possible large scale load leveling which may in turn reduce many peaks generating units in redundancy.
3.1 OPERATING PRINCIPLE
A wire carrying electric current generates a magnetic field. The higher the current, the stronger is the generated field. The current carrying wire, wrapped as a coil is called the solenoid is proportional to the current and the number of turns Superconducting solenoids made by wrapping a Superconducting wire in the coil from are functionally superior to conventional solenoids because of:-
i) ZERO DC ELECTRICAL RESISTANCE
Due to zero resistance of superconductors very high currents of the order of kilo amperes can be passed through an superconductor solenoid using moderate voltage. The intensity of magnetic field generated can then be as high as 30 to 40 Tesla.
ii) NO RESISTIVE LOSSES
Unlike conventional solenoids, where resistive or PR losses increase with current, Superconducting solenoids have no resistive losses thus if two ends of a solenoids are short circuited, the current is in the 'persistent mode' persistent super current generates a constant magnetic field which will last forever. The virtue of 'supporting' constant magnetic field is used for storing electrical in gigantic Superconducting solenoids.
If the high temperature superconductors of required properties
become available, the possible SMES can be operated at 750 higher temperature than the one considered so far :
* IMPROVED GENERATION ECONOMICS,
* DAMPING OSCILLATIONS FOR SYSTEM REEIABILITY,
* IMPROVEMENT OF STABILITY LIMIT; &
* SPINNING RESERVE.
4.0 SUPERCONDUCTING TRANSMISSION LINE CABLES
Transmission of electricity through a superconductor is technically possible; however a Superconducting cable has to be cooled to cryogenic temperature and therefor has to be underground. In comparison with the existing underground cable a Superconducting cable has following advantages :
* ZERO RESISTANCE and, therefore, reduced losses.
* LOW VOLTAGE ( 86 KV / Phase ) and high current transmission.
* SMALL PHYSICAE SIZE of the cable due to high current carrying capacity
Reduced size implies very high power density. It would reduced excavation costs by reducing the trench size.
* REDUCED CLEARANCE FOR TERMINAL FACILITIES. Generally, high tension equipment is very bulky. If the amount of HT instrumentation is reduced it could result in space saving.
* NEAR TOTAL ELIMINATION OF RESISTIVE LOSS resulting in substantial saving over the life time of cable.
* QUICK RECOVERY AFTER FAULT Transmission line faults due to insulator flash over are common on a transmission line. A given line can, however, sustain most of these faults without causing any reliability problems in case of major fault however conventional transmission lines may trip Recovery time under certain conditions is also long. Superconducting cables on the other hand are expected within a few milliseconds even from major fault where a conventional line takes hundreds of milliseconds.
* HIGHER RELIABILITY Shorting of transformers and fires due to shorting of HT equipment in Superconducting transmission would result in greater reliability.
* OVERLOAD CAPABILITY In the seemingly unlikely event of failure of 66 percent of the available transmission lines, a Superconducting capacity to sustain the entire fault current and overload current for as long as four hours.
The advantages of Superconducting are :
* Zero resistance and therefore low-loss condition.
* Small conductor cross-section resulting in savings in materials.
* Two to three times higher overload capability over extended periods of time.
* Transmission of large blocks of power ( 5 GW and more ) with only a few circuits.
* No Electro-magnetic interference with communications signals and radar equipments.
* Reduced biological hazards.
Most of the studies undertaken conclude that although the application of Superconducting material in power system did indeed lead to improved efficiencies, the capital cost and the cooling energy requirement were too large and that it was not economically feasible to implement.
1. SUPERCONDUCTORS IN POWER SYSTEMS.
Jyoti Parikh & Madhuri Pai Allied Publishers.
2. HIGH TEMPERATURE SUPERCONDUCTORS
S.V. Subramanyam & E.S. RajaGopal Wiley Eastern Limited
3. Principles of ELECTRICAL MACHINE DESIGN
4. POWER SYSTEMS : Proceeding of VI National Conference
M.V. Hariharan & Jyoti Parikh
5. POWER SYSTEMS : Proceeding of VII National Conference
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INTRODUCTION Superconductors materials that have no resistance to the flow of electricity, are one of the last great frontiers of scientific discovery. Not only have the limits of superconductivity not yet been reached, but the theories that explain superconductor behavior seem to be constantly under review. In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University (shown below ). When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C) , its resistance suddenly disappeared . The Kelvin scale represents an "absolute" scale of temperature. Thus, it was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. Later, in 1913 he won a Nobel Prize in physics for his research in this area.
The next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walther Meissner (above) and Robert Ochsenfeld (above) discovered that a superconducting material will repel a magnetic field (below fig.1). A magnet moving by a conductor induces currents in the conductor. This is the principle on which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as strong diamagnetism and is today often referred to as the "Meissner effect" (an eponym). The Meissner effect is so strong that a magnet can actually be levitated over a superconductive material.
In subsequent decades other superconducting metals, alloys and compounds were discovered. In 1941 niobium-nitride was found to super conduct at 16 K. In 1953 vanadium-silicon displayed superconductive properties at 17.5 K. And, in 1962 scientists at Westinghouse developed the first commercial superconducting wire, an alloy of niobium and titanium (NbTi). High-energy, particle-accelerator electromagnets made of copper-clad niobium-titanium were then developed in the 1960s at the Rutherford-Appleton Laboratory in the UK, and were first employed in a superconducting accelerator at the Fermilab Tevatronin in the US in 1997 .
The first widely-accepted theoretical understanding of superconductivity was advanced in 1957 by American physicists John Bardeen, Leon Cooper, and John Schrieffer . Their Theories of Superconductivity became know as the BCS theory - derived from the first letter of each man's last name - and won them a Nobel prize in 1972. The mathematically complex BCS theory explained superconductivity at temperatures close to absolute zero for elements and simple alloys. However, at higher temperatures and with different superconductor systems, the BCS theory has subsequently become inadequate to fully explain how superconductivity is occurring.
Another significant theoretical advancement came in 1962 when Brian D. Josephson , a graduate student at Cambridge University, predicted that electrical current would flow between two superconducting materials - even when they are separated by a non-superconductor or insulator. His prediction was later confirmed and won him a share of the 1973 Nobel Prize in Physics. This tunneling phenomenon is today known as the "Josephson effect" and has been applied to electronic devices such as the SQUID, an instrument capabable of detecting even the weakest magnetic fields(Below graphic).
The term Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect).
The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered. However, in ordinary conductors such as copper and silver, impurities and other defects impose a lower limit. Even near absolute zero a real sample of copper shows a non-zero resistance. The resistance of a superconductor, on the other hand, drops abruptly to zero when the material is cooled below its "critical temperature". An electrical current flowing in a loop of superconducting wire can persist indefinitely with no power source. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It cannot be understood simply as the idealization of "perfect conductivity" in classical physics.
Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. Superconductivity does not occur in noble metals like gold and silver, nor in most ferromagnetic metals.
In 1986 the discovery of a family of cuprate-perovskite ceramic materials known as high-temperature superconductors, with critical temperatures in excess of 90 Kelvin, spurred renewed interest and research in superconductivity for several reasons. As a topic of pure research, these materials represented a new phenomenon not explained by the current theory. And, because the superconducting state persists up to more manageable temperatures (past the economically-important boiling point of liquid nitrogen), more commercial applications are feasible, especially if materials with even higher critical temperatures could be discovered.
BCS Theory: The first widely-accepted theory to explain superconductivity put forth in 1957 by John Bardeen, Leon Cooper, and John Schreiffer. The theory asserts that, as electrons pass through a crystal lattice, the lattice deforms inward towards the electrons generating sound packets known as "phonons". These phonons produce a trough of positive charge in the area of deformation that assists subsequent electrons in passing through the same region in a process known as phonon-mediated coupling. This is analogous to rolling a bowling ball up the middle of a bed. 2 people, one lying on each side of the bed, will tend to roll toward the center of the bed, once the ball has created a depression in the mattress. And, a 2nd bowling ball, placed at the foot of the bed, will now, quite easily, roll toward the middle
Josephson Effect: A phenomenon named for Cambridge graduate student Brian Josephson, who predicted that electrons would "tunnel" through a narrow (<10 angstroms) non-superconducting region, even in the absence of an external voltage. In a normal conductor, electrical current only flows when there's a voltage differential and contiguous electrical connection. It has been theorized that the Josephson Effect arises from the incoherent phase relationships between superconducting electrons in the two (separated) superconductors. The AC Josephson Effect is where the current flow oscillates as an external magnetic field impinged upon it increases beyond a critical value. [at a frequency of 2eV/h, where e is the electron charge, V is the voltage that appears, and h is Planck's constant]
EVOLUTION OF SUPERCONDUCTING TRANSITION TEMPERATURE:
1. Applications of Superconductivity
Transportation (MAGLEV Trains)
1.1. Superconducting Magnets
Type II superconductors such as niobium-tin and niobium-titanium are used to make the coil windings for superconducting magnets. These two materials can be fabricated into wires and can withstand high magnetic fields. Typical construction of the coils is to embed a large number of fine filaments ( 20 micrometers diameter) in a copper matrix. The solid copper gives mechanical stability and provides a path for the large currents in case the superconducting state is lost. These superconducting magnets must be cooled with liquid helium. Superconducting magnets can use solenoid geometries as do ordinary electromagnets.
Superconducting Magnet Wire of Niobium-Titanium
Ohanian's Physics has a photograph of a cross- section of copper wire of diameter 0.7 mm with 2100 filaments of niobium-titanium embedded in it. This is an approximate sketch of the geometry. Although copper is one of the best room- temperature conductors, it acts almost as an insulator between the strands.
Almost10% to 15% of generated electricity is dissipated in resistive losses in transmission lines, the prospect of zero loss superconducting transmission lines is appealing. In prototype superconducting transmission lines at Brookhaven National Laboratory, 1000 MW of power can be transported within an enclosure of diameter 40 cm. This amounts to transporting the entire output of a large power plant on one enclosed transmission line. This could be a fairly low voltage DC transmission compared to large transformer banks and multiple high voltage AC transmission lines on towers in the conventional systems. The superconductor used in these prototype applications is usually niobium-titanium, and liquid helium cooling is required.
Current experiments with power applications of high-temperature superconductors focus on uses of BSCCO in tape forms and YBCO in thin film forms. Current densities above 10,000 amperes persquare centimeter are considered necessary for practical power applications, and this threshold has been exceeded in several configurations.
1.2. Transportation (Maglev Trains)
Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains) using electromagnetic force. This method is faster than wheeled mass transit systems, potentially reaching velocities comparable to turboprop and jet aircraft (900 km/h, 559 mph). These trains use superconducting magnets which allow for a larger gap, and repulsive-type Electro-Dynamic Suspension (EDS).
It is not practical to lay down superconducting rails, it is possible to construct a superconducting system onboard a train to repel conventional rails below it.
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