GB2332558A

GB2332558A - A fault current limiter

Info

Publication number: GB2332558A
Application number: GB9725319A
Authority: GB
Inventors: Jan Brangefaelt; Udo Fromm; Christian Sasse; Thorsten Schuette
Original assignee: Asea Brown Boveri AB
Current assignee: ABB AB
Priority date: 1997-11-28
Filing date: 1997-11-28
Publication date: 1999-06-23
Also published as: WO1999029006A1; AU2267499A; JP2001525650A; GB9725319D0; DE19882840T1

Abstract

A fault current limiter (4) for a power system having an electrical current carrying path (3), comprising a superconducting device (6), for disposition in the current carrying path (3), which has superconductor means exhibiting superconducting properties, solid electrical insulation (9) surrounding the superconductor means for confining the electric field and cooling means (12) for cryogenically cooling the superconductor means below its critical temperature in use of the fault current limiter (4).

Description

2332558 1 A Fault Current Limiter

Technical Field

This invention relates to a fault current limiter for a power system having an electrical current carrying path, the f ault current limiter being of the kind comprising a superconducting device, for disposition in the current carrying path, which has superconductor means exhibiting superconducting properties, electrically insulating means surrounding the superconductor means and cooling means for cryogenically cooling the superconductor means below its critical temperature in use of the fault current limiter. The invention also relates to a power system incorporating such a fault current limiter and capable of operating at high voltages, e.g. such as 400 to 800 kV or higher.

Background of the Invention

Superconducting fault current limiters of the kind referred to are already known and operate by the superconductor means passing from a superconducting state to a non-superconducting state on the occurrence of a fault in the power system. Normally a resistor or inductive coil is connected in parallel with the superconducting device and the current is commiltated to the resistor or coil when the superconductor means switches to the non- superconducting state. The electrically insulating means of a fault current limiter of the kind referred to needs to have high electrical breakdown strength. Conventional insulating means comprising paper/oil or polypropylene/paper/oil tends, however, to deteriorate under high voltage. It also is liable to suffer from partial discharges due to its structure. Therefore the long-term reliability of known superconducting fault current limiters is determined by the electrical insulation.

Summary of the Invention

An aim of the present invention is to provide a superconducting fault current limiter having an improved electrical insulation system.

According to one aspect of the present invention a fault current limiter of the kind referred to is characterised in that the electrically insulating means comprises solid plastics material within which the electric field is confined in use of the fault current limiter.

According to another aspect of the present invention a fault current limiter of the kind referred to is characterised in that the electrically insulating means is of substantially unitary construction comprising an inner layer of semiconducting material in electrical contact with said superconductor means, an outer layer of semiconducting material at a controlled electrical potential along its length and an intermediate layer of electrically insulating material between the said inner and outer layers.

In this specification the term "semiconducting material" means a material which has a considerably lower conductivity than an electric conductor but which does not have such a low conductivity that it is an electrical insulator. Typically, but not exclusively, the semiconducting material should have resistivity of from 1 to 105 ohm-cm, preferably from 10 to 500 ohm-cm and most preferably from 50 to 100 ohm-cm.

The electrically insulating means is of unitary form with the layers either in close mechanical contact or, more preferably, joined together, e.g. bonded by extrusion. The layers are preferably formed of plastics material having resilient or elastic properties at least at ambient temperatures. This allows the cable forming the winding to be flexed or shaped into a desired form if desired. By using 3 for the layers only materials which can be manufactured with few, if any, defects having similar thermal properties, thermal and electric loads within the insulation are reduced. In particular the insulating intermediate layer and the semiconducting inner and outer layers should have at least substantially the same coefficients of thermal expansion (ci) so that defects caused by different thermal expansions when the layers are subjected to heating or cooling will not arise. ideally the layers will be extruded together around the superconductor means.

Conveniently the electrically insulating intermediate layer comprises solid thermoplastics material, such as low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), cross-linked materials, such as cross-linked polyethylene (XLPE), or rubber insulation, such as ethylene propylene rubber (EPR), ethylene -propylene-diene monomer (EPDM) or silicone rubber. The semiconducting inner and outer layers may comprise similar material to the intermediate layer but with conducting particles, such as particles of carbon black or metallic particles, embedded therein. Generally it has been found that a particular insulating material, such as EPR, has similar mechanical properties when containing no, or some, carbon particles.

The screens of semiconducting inner and outer layers form substantially equipotential surfaces on the inside and outside of the insulating intermediate layer so that the electric field is confined between the inner and outer layers in the intermediate layer. In the case of concentric semiconducting and insulating layers, the electric field is substantially radial and confined within the intermediate layer. In particular, the semiconducting inner layer is arranged to be in electrical contact with, and to be at the same potential as, the superconductor means which it surrounds. The semiconducting outer layer is designed to act as a screen to prevent losses caused by induced voltages. Induced voltages in the outer layer could be reduced by increasing the resistance of the outer layer.

is The resistance can be increased by reducing the thickness of the outer layer but the thickness cannot be reduced below a certain minimum thickness. The resistance can also be increased by selecting a material for the layer having a higher resistivity. On the other hand, if the resistivity of the semiconducting outer layer is too great, the voltage potential midway between adjacent spaced apart points at a controlled, e.g. earth, potential will become sufficiently high as to risk the occurrence of corona discharge in the insulation with consequent erosion of the insulating and semiconducting layers. The ser.1conducting outer layer is therefore a compromise 'between a conductor having low resistance and high induced voltage losses but which is easily connected to a controlled potential, typically earth or ground potential, and an insulator which has high resistance with low induced voltage losses but which needs to be connected to the controlled potential along its length. Thus the resistivity p. of the semiconducting outer layer should be within the range p.,i.<p,<p-.,, where pi, is determined by permissible power loss caused by eddy current losses and resistive losses caused by voltages induced by magnetic flux and p. is determined by the requirement for no corona or glow discharge.

The superconducting device may comprise a transmission cable having an outer conductive shield or protective sheath, e.g. of lead. Alternatively the superconducting device may comprise a coil. In this latter case, if the semiconducting outer layer is earthed, or connected to some other controlled potential, at spaced apart intervals along its length, there is no need for an outer metal shield and protective sheath to surround the semiconducting outer layer. The diameter of the cable is thus reduced allowing more turns to be provided for a given size of coil.

Preferably the fault current limiter includesan impedance in parallel with the superconducting device. In the event of fault current occurring in the electrical - 5 current carrying path in use of the limiter, the resistance of the superconductor means increases as it becomes nonsuperconducting and the current flow is commutated from the superconductor means to the impedance. The impedance may comprise a resistive device (in which case the current limiter is a resistive fault current limiter) or an inductive coil (in which case the current limiter is an inductive fault current limiter). In the case of a resistive fault current limiter, the resistance of the resistive device is dimensioned so that, under normal operating conditions, its resistance is large compared with that of the superconductor means but, under fault conditions, its resistance is small compared with that of the superconductor means. In the case of an inductive fault current limiter, the equivalent ohmic resistance (frequency x inductance) of the inductive coil is large under normal operating conditions but small under fault conditions compared with that of the superconductor means.

The cooling means may comprise insulated container means, e.g. a cryostat, through which cryogenic fluid, e.g.

liquid nitrogen, is passed and in which the superconducting device is contained. With an inductive fault current limiter, the inductive coil may have solid electrical insulation similar to that of the electrically insulating means, in which case the inductive coil may also be included in the insulated container means addition, Alternatively, or in the cooling means may comprise part of the superconductor means. For example, the superconductor means may comprise a support tube through which cryogenic coolant fluid is passed and on which elongate superconducting means is wound.

The superconductor means may comprise low temperature semiconducting materials, but most preferably comprises high-temperature superconducting (HTS) materials, for example elongate HTS means, such as HTS wires or tape, helically wound on an inner tube. HTS wire or tape conveniently comprises silver-sheathed BSCCO-2212 or BSCCO- 2223 (where the numerals indicate the number of atoms of each element in the [Bi, Pb] 2 Sr2 Ca2 CU3 0, molecule) and hereinafter such HTS wires or tapes will be referred to as NBSCCO wire(s) or tape(s)". BSCCO tapes are made by encasing fine filaments of the oxide superconductor in a silver or silver oxide matrix by a powder-in-tube (PIT) draw, roll, sinter and roll process. Alternatively the tapes may be formed by a surface coating process. in either case the oxide is melted and resolidified as a final process step. Other HTS tapes, such as TiBaCaCuO (TBCCO-1223) and YBaCuO, (YBCO- 123) have been made by various surface coating or surface deposition techniques. Ideally an HTS wire or tape should have a current density beyond jC_105 ACM-2 at operation temperatures from 65 K, but preferably above 77 K.

The filling factor of HTS material in the matrix needs to be -2 high so that the engineering current density j,: 104 Acm. jc should not drastically decrease with applied field within the Tesla range. The helically wound HTS wire or tape is cooled to below the critical temperature Tc of the HTS material by a cooling fluid, preferably liquid nitrogen, passing through the inner support tube. Alternatively, or in addition, the cooling means may comprise the insulated container means referred to above.

According to a further aspect of the present 2S invention there is provided a power system having an electrical current carrying path in which there is connected in series a circuit breaker and a fault current limiter according to said other aspects of the invention.

Brief Description of the Drawing

Figure 1 is a circuit diagram illustrating a power system incorporating one embodiment of a fault current limiter according to the invention; Figure 2 is a circuit diagram illustrating a power system incorporating another embodiment of a fault current limiter according to the invention; and - 7 Figure 3 is a schematic sectional view of part of a superconducting device of a fault current limiter according to the invention.

Description of Preferred Embodiments

Figure 1 shows part of a power utility system 1 comprising a high voltage source 2, an electrical current carrying path 3 and, arranged in series in the current carrying path 3, a superconducting resistive fault current limiter, generally designated 4, and a circuit breaker S.

The fault current limiter 4 comprises a superconducting device 6 and, arranged in parallel therewith, a resistor 7. The superconducting device 6 is contained in a thermally insulated container 12 (see dashed lines in Figure 1), e.g. a cryostat, cooled by a cryogenic fluid, e.g. liquid nitrogen.

The superconducting device 6 may comprise a coil (not shown) wound from a cable 8 (see Figure 3) comprising elongate inner superconducting means 10 having an inner metal, e.g. copper or highly resistive metal or alloy, support tube 21 and an HTS wire 22, e.g. of BSCCO wire, wound helically around the tube 21 and embedded in a layer 23 of semiconducting plastics material. Electrical insulation 9 is arranged outwardly of, at a small radial spacing 24 from, the layer 23. This electrical insulation 9 is substantially void-free and comprises an inner semiconducting layer 25, an outer semiconducting layer 26 and, sandwiched between these semiconducting layers, an insulating layer 27. The layers 25-27 preferably comprise thermoplastics materials solidly connected to each other at their interfaces. Conveniently these thermoplastics materials have similar coefficients of thermal expansion (a) and are preferably extruded together around the inner superconducting means 3. Preferably the layers 25-27 are extruded together to provide a monolithic structure so as to minimise the risk of cavities and pores at the interfaces of the electrical insulation. The presence of such pores and cavities in the insulation and at its interfaces is undesirable since it gives rise to corona discharge in the electrical insulation at high electric field strengths.

The outer semiconducting layer 26 is connected at spaced apart regions along its length to a controlled potential, e.g. earth or ground potential, the specific spacing apart of adjacent earthing points being dependent on the resistivity of the layer 26.

The semiconducting layer 26 acts as a static shield and as an earthed outer layer which ensures that the electric f ield of the superconducting cable is retained within the solid insulation between the semiconducting layers 25 and 26. Losses caused by induced voltages in the layer 26 are reduced by increasing the resistance of the layer 26. However, since the layer 26 must be at least of a certain minimum thickness, e.g. no less than 0.8 mm, the resistance can only be increased by selecting the material of the layer to have a relatively high resistivity. The resistivity cannot be increased too much, however, else the voltage of the layer 26 mid-way between two adjacent earthing points will be too high with the associated risk of corona discharges occurring.

The radial spacing 24 provides an expansion/contraction gap to compensate for the differences in the thermal coefficients of expansion (a) between the electrical insulation 9 and the inner superconducting means 10 (including the metal tube 21). The spacing 24 may be a void space or may incorporate a foamed, highly compressible material to absorb any relative movement between the superconductor and insulation system. The foamed material, if provided, may be semiconducting to ensure electrical contact between the layers 23 and 25. Additionally or alternatively, metal wires may be provided for ensuring the necessary electrical contact between the layers 23 and 25.

By way of example only, the solid insulating layer 27 may comprise crosslinked polyethylene (XLPE). Alternatively, however, the solid insulating layer may comprise other cross-linked materials, low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), or rubber insulation, such as ethylene propylene rubber (EPR), ethylene-propylenediene monomer (EPDM) or silicone rubber. The semiconducting material of the layer 23 and of the inner and outer layers 25 and 26 may comprise, for example, a base polymer of the same material as the solid insulating layer 27 and highly electrically conductive particles, e.g. particles of carbon black or metallic particles, embedded in the base polymer. The resistivity of these semiconducting layers may be adjusted as required by varying the type and proportion of carbon black added to the base polymer. The following gives an example of the way in which resistivity can be varied using different types and quantities of carbon black.

Base Polymer Carbon Black Carbon Black Volume Type Ouantity (%)_ Resistivity Q-C Ethylene vinyl EC carbon black -15 350-400 acetate copolymer/ nitrite rubber P-carbon black -37 70-10 Extra conducting -35 40-50 carbon black, type I Extra conducting -33 30-60 black, type 11 Butyl grafted -25 7-10 polyethylene Ethylene butyl Acetylene carbon -35 40-50 acrylate copolymer black P carbon black -38 5-10 Ethylene propene Extra conducting -35 200-400 rubber carbon black The superconducting device 6 may comprise a transmission cable (not shown) instead of a coil. If the device is a transmission cable, an outer electrically - 10 conductive shield and protective sheath, e.g. of lead, is suitably provided around the outer layer 26 of semiconducting material.

Under quiescent operating conditions of the utility system 1, the RTS wire 22, cooled to below its critical temperature by the liquid nitrogen in the container 12, is in its superconducting state and current passes along the wire with virtually no losses. No current flows through the resistor 7 because of the virtually zero resistance path provided by the RTS wire in its superconducting state. In the event of fault current occurring in the current carrying path 3, the critical current density ic of the ETS wire changes so that the wire becomes non- superconducting and becomes a series resistor having a large resistance compared with the much smaller relative resistance of the resistor 7. The circuit breaker 5 opens on occurrence of the fault.

Figure 2 shows an alternative power utility system 40. The same reference numerals have been used in Figures I and 2 to identify the same or similar features. The main differences of the two systems are that in system 40, a superconducting inductive fault current limiter 41 is provided. The fault current limiter 41 has a superconducting device 6 arranged in parallel with an inductive coil 42 (instead of the resistor 7 of system 1) which may or may not, include a core (not shown). the coil 42 is wound from a cable (now shown) having inner, conventional (i.e. non- superconducting) conductor means surrounded by an electrical insulation system similar to the electrical insulation 9 of the superconducting device 6.

The thermally insulated container may contain only the conducting device 6 (as indicated by dashed line 12) or may contain both the conducting device 6 and the coil 42 (as indicated by dashed line 44).

In operation of the system 40, during a fault condition, all the energy is dissipated in the superconducting device 6 and thus the latter must have a - 11 large thermal mass. The current through the HTS wire 2 is commutated to the inductive coil 42. The coil 42 is dimensioned so that its equivalent ohmic resistance (frequency x inductance) is large compared to that of the HTS wire 2 when the latter in superconducting but is small compared to that of the HTS wire 2 under fault conditions when the latter is non-superconducting.

The electrically insulating means of a fault current limiter according to the invention is intended to be able to handle very high voltages and the consequent electric and thermal loads which may arise at these voltages. By way of example, a fault current limiter according to the invention may be designed for a rated power of a few hundred kVA up to more than 1000 MVA and with a rated voltage ranging from 3-4 kV up to very high transmission voltages of 400-800 W. At high operating voltages, partial discharges, or PD, constitute a serious problem for known insulation systems. If cavities or pores are present in the insulation, internal corona discharge may arise whereby the insulating material is gradually degraded eventually leading to breakdown of the insulation. The electric load on the electrical insulation surrounding the superconductor means is reduced by ensuring that the inner layer of the insulation is at substantially the same electric potential as the inner superconductor means and the outer layer of the insulation is at a controlled, e.g. earth, potential. Thus the electric field in the intermediate layer of insulating material between the inner and outer layers is distributed substantially uniformly over the thickness of the intermediate layer. Furthermore, by having materials with similar thermal properties and with few defects in the layers of the insulating material, the possibility of PD is reduced at a given operating voltages. The fault current limiter can thus be designed to withstand very high operating voltages, typically up to 800 kV or higher.

Claims

1. A fault current limiter for a power system having an electrical current carrying path, comprising a superconducting device, for disposition in the current carrying path, which has superconductor means exhibiting superconducting properties, electrically insulating means surrounding the superconductor means and cooling means for cryogenically cooling the superconductor means below its critical temperature in use of the fault current limiter, characterised in that the electrically insulating means comprises solid plastics material within which the electric field is confined in use of the fault current limiter.

2. A fault current limiter according to claim 1, characterised in that the electrically insulating means is of substantially unitary construction comprising an inner layer of semiconducting material in electrical contact with said superconductor means, an outer layer of semiconducting material at a controlled electrical potential along its length and an intermediate layer of electrically insulating material between the said inner and outer layers.

3.

A fault current limiter for a power system having an electrical current carrying path, comprising a superconducting device, for disposition in the current carrying path, which has superconductor means exhibiting superconducting properties, electrically insulating means surrounding the superconductor means and cooling means for cryogenically cooling the superconductor means below its critical temperature in use of the fault current limiter, characterised in that the electrically insulating means is of substantially unitary construction comprising an inner layer of semiconducting material in electrical contact with said superconductor means, an outer layer of semiconducting material at a controlled electrical potential along its length and an intermediate layer of electrically insulating material between the said inner and outer layers.

4. A fault current limiter according to claim 2 or 3, characterised in that the semiconducting outer layer has a resistivity of from I to 105 ohm-cm.

5. A fault current limiter according to claim 2 or 3, characterised in that the semiconducting outer layer has a resistivity of from 10 to 500 ohm-cm, preferably from 50 to 100 ohm-cm.

6. A fault current limiter according to any one of claims 2 to 5, characterised in that the resistance per axial unit length of the semiconducting outer layer is from 5 to 50,000 ohm.m-1.

7. A fault current limiter according to any one of claims 2 to 5, characterised in that the resistance per axial unit of length of the semiconducting outer layer is from 500 to 25,000 ohm.m-1, preferably from 2,500 to 5,000 ohm.m-1.

8. A fault current limiter according to any one of claims 2 to 7, characterised in that said controlled electrical potential is at or close to ground potential.

9. A fault current limiter according to any one of claims 2 to 8, characterised in that the said intermediate layer is in close mechanical contact with each of said inner and outer layers.

10. A fault current limiter according to any one of claims 2 to 8, characterised in that the said intermediate layer is joined to each of said inner and outer layers.

11. A fault current limiter according to claim 10, characterised in that the strength of the adhesion between the said intermediate layer and the semiconducting outer layer is of the same order of magnitude as the intrinsic strength of the material of the intermediate layer.

12. A fault current limiter according to claim 10 or 11, characterised in that the said layers are joined together by extrusion.

13. A fault current limiter according to claim 12, characterised in that the inner and outer layers of semiconducting material and the insulating intermediate layer are applied together over the superconductor means through a multi layer extrusion die.

14. A fault current limiter according to any one of claims 2 to 13, characterised in that said inner layer comprises a first plastics material having first electrically conductive particles dispersed therein, said outer layer comprises a second plastics material having second electrically conductive particles dispersed therein, and said intermediate layer comprises a third plastics material.

15. A fault current limiter according to claim 14, characterised in that each of said first, second and third plastics materials comprises an ethylene butyl acrylate copolymer rubber, an ethylene -propylene - diene monomer rubber (EPDM), an ethylene-propylene copolymer rubber (EPR), MPE, HDPE, PP, XLPE, EPR or silicone rubber.

16. A fault current limiter according to claim 14 or 15, characterised in that said first, second and third plastics materials have at least substantially the same coefficients of thermal expansion.

17. A fault current limiter according to claim 14, 15 or 16, characterised in that said first, second and third plastics materials are the same material.

18. A fault current limiter according to any one of the preceding claims, characterised in that the superconducting device comprises a transmission cable.

19. A fault current limiter according to claim 18, characterised in that said transmission cable has an outer electrically conductive protective sheath.

20. A fault current limiter according to any one of the preceding claims, characterised in that the superconducting device comprises a coil.

21. A fault current limiter according to any one of the preceding claims, characterised in that the fault current limiter includes an impedance in parallel with the superconducting device. '

22. A fault current limiter according to claim 21, characterised in that the impedance comprises a resistive device.

23. A fault current limiter according to claim 22, characterised in that the resistance of the resistive device, under normal operating conditions, is large compared with that of the superconductor means but, under fault conditions, is small compared with that of the superconductor means.

24. A fault current limiter according to claim 21, characterised in that the impedance comprises an inductive coil.

25. A fault current limiter according to claim 24, characterised in that the equivalent ohmic resistance of the inductive coil is large under normal operating conditions but is small under fault conditions compared with that of the superconductor means.

26. A fault current limiter according to any one of the preceding claims, characterised in that the cooling means comprises insulated container means, e.g. a cryostat, through which a cryogenic fluid is intended to be passed and in which the superconducting device is contained.

27. A fault current limiter according to claim 26 when dependent on claim 24 or 25, characterised in that the inductive coil has solid electrical insulation and in that the inductive coil is contained in the insulated container 5 means.

28. A fault current limiter according to any one of the preceding claims, characterised in that the superconductor means comprises high-temperature superconducting (HTS) material.

29. A fault current limiter according to claim 28, characterised in that the said HTS material comprises elongate means, e.g. wires or tape, helically wound on an inner tube.

30. A fault current limiter according to claim 29, characterised in that said elongate means comprises silversheathed BSCCO tapes.

31. A fault current limiter according to any one of the preceding claims, characterised in that the said electrically insulating means is designed for high voltage, suitably in excess of 10 W, in particular in excess of 36 kV, and preferably more than 72.5 kV up to very high transmission voltages, such as 400 kV to 800 kV or higher.

32. A fault current limiter according to any one of the preceding claims, characterised in that the said electrically insulating means is designed for a power range in excess of 0.5 MVA, preferably in excess of 30 MVA and up to 1000 MVA.

33. A power system having an electrical current carrying path in which there is connected in series a circuit breaker and a fault current limiter as claimed in any one of the preceding claims.