A POWER CURRENT BOOSTER TRANSFORMER

Abstract: A power current booster transformer is wound with a flexible cable (10) comprising inner (12) and outer coaxial (18) conductors separated by an insulating layer (16), one of said conductors forming a primary of the transformer and the other of said conductors forming a secondary of the transformer. Two such flexible cables may provide three windings, two of which are formed by the two inner conductors and the third by the outer conductors connected in parallel.

Description: A Power Current Boosterr Trans ormer BACKGROUND TO THE INVENTION The present invention relates to a power current booster transformer. The term "power" means that the transformer has a rated output from a few hundred kVA to more than 1000 MVA and a rated output from 3-4 kV to very high transmission voltages, e.g. 400-800 kV or higher. The use of booster transformers (BTs) in traction power supplies is described in R.J. Hill, "Electric Railway Traction - Part 3 - Traction Power Supplies", Power Engineering Journal, December 1994, pp 275-286. In the preferred arrangement, BTs rated at about 100 to 150 kVA are placed at intervals of approximately 3 km, for a 25 kV, 50 Hz supply, or approximately 5 km, for a 16 kV, 16 % Hz supply, along a catenary . The primary winding of each BT is connected across a gap in the catenary. A return conductor is provided in parallel with the rails and the secondary of the BT is connected into the return conductor. The turns ratio of the BT is unity and traction return current is forced from the rails and earth to flow through the return conductor and transformer secondary to equalise the Ampere-turns in the core set up by the primary current. This arrangement provides a low feeding impedance and reduces earth currents which might otherwise cause interference with adjacent telecommunications circuits. Our co-pending British Patent Application No. 9725336.3 describes current booster transformers and autotransformers wound with a cable such as that described in our International Patent Application No. 097/45923. The cable comprises conductive strands surrounded in turn by semiconductive, insulating and semiconductive layers. A BT wound with such a cable is advantageous in that it need not be filled with oil, and is therefore safer and more environmentally friendly, and no complicated insulation system is required. - 2 - In GB 9821696.3, traction supply systems are described which require a transformer having three windings and forcing the current in one of the windings to be equal to the sum of the currents in the other- two windings. SUMMARY OF THE INVENTION It is one object of the invention to provide a power BT having a very low impedance. Accordingly, the present invention provides a power current booster transformer wound with a flexible cable comprising inner and outer coaxial conductors separated by an insulating layer, one of said conductors forming a primary of the transformer and the other of said conductors forming a secondary of the transformer. In addition to a very low impedance, such a transformer has a transformation ratio of almost exactly 1:1, as the stray fluxes are negligible. Preferably, each of said inner and outer coaxial conductors comprises a plurality of strands of wire. In the preferred embodiment, the inner conductor is surrounded by a first semiconductive layer which is surrounded in turn by the insulating layer, a second semiconductive layer and the outer conductor. The outer conductor is surrounded by a further insulating layer. The semiconductive layers may be formed from a base polymer mixed with carbon black or metallic particles and the insulating layer is preferably of cross-linked polyethylene or ethylene propylene rubber. The BT of the invention has a very simple construction and is very cheap. In an embodiment of the invention which meets the requirements of GB 9821696.3, there are at least three windings, at least two of said windings being formed by - 3 - inner and outer coaxial conductors, such that the current in one of said windings is substantially equal to the sum of the currents in the other of said windings. In a preferred transformer according to this embodiment, three windings are formed by flexible coaxial cables, each having an inner conductor forming one of the windings and an outer conductor, the outer conductors being connected in parallel to form the third winding. Alternatively, one of the windings may be formed by a separate cable. If one of the windings is formed from a separate cable, it may be constructed with the insulation system described above but omitting the outer conductor and second insulating layer. -BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: - Figure 1 is a transverse section through a cable for use in a transformer according to one embodiment of the invention; Figure 2 is a schematic perspective view of a transformer including the cable shown in Figure 1; Figure 3 is a schematic perspective view of a transformer according to another embodi ent of the invention; and Figure 4 is a circuit diagram of the transformer of Figure 3. - 4 - DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a flexible cable 10 comprising an inner conductor 12 formed from strands, foαr example of copper, the majority of which are insulated, surrounded by a first semiconductive layer 14. A first insulating layer 16, for example of cross-linked polyethylene or ethylene propylene rubber, surrounds the first semiconductive layer 14 is in turn surrounded by a second semiconductive layer 18 , an outer conductor 20 formed from further strands, and a second polymeric insulating layer 22. The semiconductive layers 14 , 18 are formed from a base polymer mixed with carbon black or metallic particles and have a resistivity of between 1 and 10 5 Ωc , preferably between 10 and 500 Ωcm. Suitable base polymers for the layers 14, 18 (and for the insulating layers 16, 22) include ethylene vinyl acetate copolymer/nitrile rubber, butyl grafted polythene, ethylene butyl acrylate copolymer, ethylene ethyl acrylate copolymer, ethylene propylene rubber, and polyethylenes of low density. Figure 2 shows a BT 30 having a coil wound from the cable 10 around a core 32. The primary of the BT is formed from the inner conductor 12 of the cable and connections can be made thereto through bushings 34. The secondary of the BT 30 is formed from the outer conductor 20 of the cable and terminals 36 for the secondary are connected to the outer conductor at the ends thereof where the second insulating layer 22 is absent. Whilst for clarity, Figure 4 shows a single coil wound around a limb 31a of the core 32, in practice the transformer winding is usually distributed into two or more coils, for example one coil on each of opposite limbs 31a, 31b. The BT of the invention can be provided with a single winding, or a plurality of windings connected in series, parallel or series-parallel. As very high currents may - 5 - flow, parallel connected windings are preferred. The BTs 30 can be used to replace conventional oil- filled BTs. In a three-winding embo iment of the invention shown in Figure 3 , the windings are formed from two coaxial cables of the type shown in Figure 1, first and second windings being formed from the inner conductors of each cable and being terminated through bushings 3. A third winding is formed from the two outer conductors, connected in parallel, having terminals 4 each connected to one end of both cables where the second insulating layer is absent. Figure 4 shows the transformer's circuit diagram. In a particular application for a traction power supply system, the inner conductors 12 form the windings for catenary (e.g. 16kV) and high voltage (e.g. 66kV) conductors, the respective first insulating layers 16 being given an appropriate thickness. The parallel-connected outer conductors 20 form the winding of a return conductor in which the current is forced to be equal to the sum of the currents in the other two windings. In order to cool the transformer windings, either the coil(s) or the entire transformer can be contained in a liquid-filled vessel. Each cable can then include an outer barrier layer which may be of extruded lead or a corrugated metal. The vessel is built so as to allow heat exchange with its surroundings. The cooling liquid may for example be a mixture of water and a low freezing-point liquid such as ethylene glycol. The present invention provides an inexpensive transformer which can safely withstand very high voltages and which does not contain insulating oil - which may be a pollutant and a fire hazard.

Claims: - 6 - CLAIMS 1. A power current booster transformer wound with a flexible cable comprising inner and outer coaxial conductors separated by an insulating layer, one of said conductors forming a primary of the transformer and the other of said conductors forming a secondary of the transformer. 2. A transformer according to claim 1, wherein each of said coaxial conductors comprises a plurality of strands of wire. 3. A transformer according to claim 1 or 2 , wherein the inner conductor is surrounded in turn by a first semiconductive layer, then by the insulating layer, a second semiconductive layer and the outer conductor. 4. A transformer according to claim 1, 2 or 3, wherein the outer conductor is surrounded by a further insulating layer. 5. A transformer according to any preceding claim, comprising at least three windings, at least two of said windings being formed by the inner and outer coaxial conductors of said cable. 6. A transformer according to claim 5, wherein two of the windings are formed by respective inner conductors of said cable and a further coaxial cable, a third winding being formed by outer conductors of the said cable and of said further coaxial cable connected in parallel. 7. A transformer according to any preceding claim, wherein the windings are contained in a liquid-filled vessel.

Inventors: CARLSON AAKE, FROMM UDO, SASSE CHRISTIAN, SCHUETTE THORSTEN, CARLSON, AAKE, FROMM, UDO, SASSE, CHRISTIAN, SCHUETTE, THORSTEN
Applicants: ABB AB, CARLSON AAKE, FROMM UDO, SASSE CHRISTIAN, SCHUETTE THORSTEN, CARLSON, AAKE, FROMM, UDO, SASSE, CHRISTIAN, SCHUETTE, THORSTEN
Application year: 1999
Main classification: H01F27
IPC codes: H01F27