Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
2001-06-22
2004-06-01
Mayo, III, William H. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
Reexamination Certificate
active
06743984
ABSTRACT:
FIELD OF THE INVENTION
In general terms, the present invention is related to an electrical power transmission system using superconductors which is compatible with conventional transmission systems.
BACKGROUND OF THE INVENTION
As is known, superconductors are metals, alloys, oxides, and, in general, compounds which, below a temperature normally referred to as the critical temperature, show a fall in resistivity to practically zero values.
In particular, a superconductor will remain superconducting only below its critical temperature, below a critical magnetic field, and below a critical current density.
Superconducting materials may be of the low-temperature type, which are generally metals such as alloys of niobium and titanium, or of the high-temperature type, which are generally ceramics such as those based on bismuth, strontium, calcium and copper oxides (BSCCO) or yttrium, barium and copper oxides (YBCO).
Reference may be made, by way of example, for one of these materials and for its preparation, to the description in European Patent EP 646 974 held by the present Applicant.
In the field of superconductors and for the purposes of the present description, the term “low-temperature superconducting materials” denotes materials having an operating temperature of the order of 4°K (approximately −269° C.), and “high-temperature superconducting materials” denote materials having an operating temperature of the order of 70-77°K (approximately −203/−196° C.).
In order to operate at these temperatures, these superconductors are cooled with suitable coolant fluids, such as liquid helium for the low temperatures and liquid nitrogen for the high temperatures.
For the purposes of the present description, “conventional cable” denotes a non-superconducting cable using electrical conductors with non-zero resistance, in particular a cable which has at least a significant portion with characteristics of non-zero electrical resistance. An electrical power transmission or distribution network generally comprises a set of connecting lines consisting of cables or overhead lines, connected in different ways (in terminal load, loop, or mesh configuration) and capable of carrying energy between units connected to interconnection nodes (of the connecting lines of the network) or to terminal nodes of the network, such as sub-stations supplied by electrical power plants, transformer stations and user loads.
Transmission networks may occasionally be subjected to overcurrents, in other words currents having a value higher than the operating value, which occur in the presence of faults and particularly in the presence of short circuits of the equipment and particularly of the lines. In the cables, these overcurrents may cause not only electrodynamic forces capable of damaging parts not securely fixed to the structures, but also an excessive temperature rise which, if persistent, may result in the burning of insulators and fires in combustible materials close to the insulators (transformer oil, for example).
In installations with conventional networks, overcurrent protection is provided by the use of automatic circuit breakers which, by means of an automatic cut-out and reconnection device, open the circuit at a current value equal to a set value and reclose the circuit when the overcurrent ceases.
For the protection of these circuit breakers or other equipment present in an installation, such as transformers, it is possible to use, among other systems, current limiting devices which may be of the induction or resistance type.
The current limiter, installed in series with the equipment to be protected, has a low impedance during normal operation, but when an overcurrent occurs in the network it increases its impedance in such a way as to limit the current to below a threshold value so as not to damage the circuit breaker or transformer. There are known overcurrent limiters, comprising inductances, which make use of the superconductivity characteristics of the materials. Under normal conditions, these limiters, or parts of them, are in a superconducting state and are designed in such a way that they have a low impedance. In the presence of overcurrents, they leave the superconducting state and behave in such a way as to have a high impedance.
Limiters of this type are described, for example, in the patents U.S. Pat. No. 5 140 290, U.S. Pat. No. 5 546 261 and EP 336 337.
The book “Impianti elettrici”, by Filippo Tiberio, Published by Vanini, Brescia, 1953, describes, for conventional non-superconducting networks, the use of reactance coils which are connected either to the busbars (in series between two sections of bar) or to the lines (in other words between the bars and the lines departing from the power plant).
SUMMARY OF THE INVENTION
The Applicant has observed that superconducting cable installations are typically intended to be provided within a conventional network, for example by replacing a conventional cable with a superconducting cable between two nodes of the network, or by inserting a new section.
The Applicant has observed that the problem of the compatibility between transmission systems using superconductors and transmission systems using conventional conductors has not been tackled in the prior art.
In particular, the Applicant has tackled the problem of the behavior of transmission systems using coaxial superconducting cables inside a conventional network in case of a short circuit.
The Applicant has noted that the introduction of a coaxial superconducting cable into a network might lead to an increase in the value of the short-circuit current in the branch in question as a result of the lower value of the characteristic impedance of the coaxial superconducting cable by comparison with that of a conventional cable.
It has also noted that the line comprising the coaxial superconducting cable, having a lower characteristic impedance than that of conventional lines, forms a preferential path for the short-circuit currents, involving the lines close to it which might have to withstand a higher current than a conventional line.
The low characteristic impedance of coaxial superconducting cables is due to their low resistance and also to their low reactance. The latter value is the one which has most effect on the absolute value of the impedance. The reactance of a coaxial superconducting cable is low owing to its coaxial structure, which comprises a phase superconductor and a return superconductor which carries in the opposite direction to the phase conductor a quantity of current equivalent to that carried by the latter. In conventional non-coaxial cables, however, the reactance is a function of the geometrical characteristics of the cable and also of the relative positioning of one cable with respect to the others.
At this point, the Applicant tackled the problem of ensuring the compatibility of a coaxial superconducting cable, in the presence of overcurrents, with the whole network.
The Applicant has also observed that short circuits create problems for superconducting cables. In particular, it has observed that in the presence of a short circuit the superconductor passes from the superconducting state to the state of normal conduction, in other words the resistive state; in this state, the emission of heat by the Joule effect increases considerably, and consequently there is an increase in the temperature of the cable with potential evaporation of the coolant liquid. When normal operating conditions are restored, in other words at the end of the short circuit, the superconductor must return to the nominal operating temperature, in other words it must cool down, to return to the superconducting state. This means that the superconducting cable cannot operate correctly immediately on the restoration of the short circuit, since it is necessary to wait for it to cool and return to the superconducting state.
Given the aforementioned problems in a network using conventional cables and coaxial superconducting cables, the Applicant has realized that the problems can be resolved,
Ladie′ Pierluigi
Nassi Marco
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Mayo III William H.
Pirelli Cavi E Sistemi S.P.A.
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