|SCHOOL OF COMPUTER AND COMMUNICATION SCIENCES|
|EPFL Applied Superconductivity Power network System study|
PSEL, ABB, BFE, RDPCREE
|Authors:||G. Schnyder, ABB S�cheron SA, J. Rhyner, ABB Corporate Research, D. Politano, ETH Z�rich, M. Sj�str�m, EPF Lausanne|
|Phone +41 01 761 11 16||Email: firstname.lastname@example.org|
|Project period:||First Edition: March 2000||Language: English|
The system study �High Temperature Superconductivity (HTS) in power systems� lists the technical and economical advantages for electric energy production, transmission and distribution systems using components build up with HTS Material considering thestate of the art in knowledge on superconductivity.
Advantages of High Temperature Superconductivity
Besides minimal transmission losses the ability to carry large current densities is an important criterion for superconducting materials to create favourable conditions for applications using this new technology. The current densities of known HTS materials are about 100 A/mm2, which is at least 10 times larger compared to the current densities in conventional aluminium or copper conductors. Another interesting feature is the use of the transition from the superconducting to the non-superconducting state of the material. This property is used for current limiting in power systems.
The advantage of the low energy losses compared with the actual costs of investment and maintenance do not justify an economical application of most superconducting components in power systems today. Therefore, additional benefits seem to be required in order to guarantee a successful implementation of superconductors in the field of electric power applications.
An example of such a benefit is the integration of the current limiter and the superconducting transformer. This solution combines the two elements �superconducting transformer with low energy losses� and �current limiter� in an advantageous manner. The current limiter permits then a decrease of the transformer short circuit impedance, , which on one hand leads to a larger transmission capacity and on the other hand allows for an improved voltage stability at the secondary side of the transformer. These synergies lead to a reduction of the investment costs, to more economical applications due to the integration as well as to an increase of energy efficiency in the transmission and distribution systems.
Results of case studies
Detailed research activities are necessary to show the potential of using superconductivity in the field of energy production, transmission and distribution. Therefore, a set of case studies have been investigated:
- Increase of transmission capacity by reducing impedances
- Increase of meshing of power systems
- Increase of quality and availability of power systems
- Redimensioning of elements used in power systems
- Reduction of energy losses
- Power flow control in meshed power systems
- Neutral point treatment
- Reduction of environmental impacts
- Increase of the dynamic stability
- Integration of power production plants
- Development of new switchgear concepts
- Application of DC-network in power systems
The results of the system studycan be categorized in �ideas�, �solutions approaches� and �economical solutions.
Economical solutions: The results of case studies concerning the integration of current limiters in power systems show the great potential using these elements in a technically efficient manner - independent of the nominal power - in all voltage levels of power systems. The actual production costs are difficult to calculate in detail. It must be assumed that the costs lie in a range between 1 CHF/kVA and 15 CHF/kVA. Investment costs at the upper end of this range allows an economical use, especially in regional distribution and industrial power systems. An important advantage resulting from the introduction of current limiters in distribution systems is the use of load breakers instead of the expensive short circuit breakers as switching devices. When postulating production costs to 4 CHF/kVA for current limiters, economical benefits can be achieved in following cases:
The main benefits of the superconducting transformers are the low energy losses, the decrease in weight and volume as well as the reduction of environmental impacts. Due to it�s the behaviour of the superconducting transformers at restart, its first economical applications are seen in urban cable networks and as block transformers in power plants . The integration of the current limiting function in transformers will increase number of economically advantageous applications.
Solution approaches: The replacement of conventional copper cables by HTS-cables in existing ducts results in the simultaneous effect that in the same space more power with less electrical losses can be transmitted. Due to the larger current densities compared to conventional cables, superconducting cables must be constructed in a new manner. Besides the coaxial construction principle, a concentric construction principle might also be possible. With these two construction variations, the electromagnetic influence outside of the cable can be eliminated. This will be a requirement considering the expected applications of large current density. With the mentioned constructions principles, it would, for instance, be possible to manufacture 110 kV HTS-cables with similar physical parameters and transmission capacity as 380 kV conventional overhead lines.
The use of superconducting cables is most promising for direct current networks. Large current applications imply the possibility to eliminate some voltage levels of an electrical network (see figure). Due to this effect, the DC-superconducting cables may cost about 4 times as much as the conventional cables.
The following conclusions are important from a technical point of view:
Superconducting cables with nominal voltages higher than 20 kV can be realised in a technical efficient manner both for AC- and DC systems.
The use of HTS-DC-cables is economically more attractive than the application of HTS-AC-cables. The essential advantages are the effect of no losses in the duct and no dielectric losses as well as the very compact design.
The application of the high temperature superconducting magnetic energy storage devices (SMES) is not economical compared with the flywheel. Reasons are the physical parameters of existing BiSSCO HTS-materials. These materials have a large decrease of the critical current density in a relatively small magnetic field. If in future the HTS-material YBCO will be available, the comparison has to be repeated because the stability of the magnetic field of this material is much better.
Ideas: The transmission capacity of a network can be increased due to the realisation of a network with low-ohmic, coaxial or concentrically constructed, superconducting cables and transformers.
The high current capability of the HTS-cable gives in selected cases the possibility to exchange the 380 kV voltage level by one of 110 kV. Another possibility is to keep the 380 kV voltage level for the European power system and to transform the power directly from 380 kV to powerful superconducting backbone-lines in the distribution network.
Visions: If governmental requirements change concerning environmental impacts for the realisation of overhead lines, it might be impossible to build new overhead lines and it might be mandatory to replace existing overhead lines by underground cables. HTS-cables could in such a situation be the solution to economically transmit the increasing need of energy in the major centres with a low environmental impact. Possible scenarios are superconducting connections through road- or railway-tunnels in mountain areas with the aim to reduce the number of the overhead lines and to decrease transmission losses, as well as a backbone-solution for the transit of energy through Switzerland.