APPLICATION OF SUPERCONDUCTIVITY IN ELECTRIC POWER SYSTEM
1.0. INTRODUCTION
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.
2.1 ADVANTAGES
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.
3.2 ADVANTAGES
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.
4.1 ADVANTAGES
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.
5.0 CONCLUSION
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.
BIBLIOGRAPHY
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
S.K. Sen.
4. POWER SYSTEMS : Proceeding of VI National Conference
M.V. Hariharan & Jyoti Parikh
5. POWER SYSTEMS : Proceeding of VII National Conference
S.K. Basu
