Patent EA-001096-B1 (EA)
Title: A dc transformer/reactor
Inventor: LEIJON MATS
IPC: H02K9/19
Filed: 1997-05-27
Abstract: 1. A DC transformer/reactor comprising a magnetic circuit, wherein the magnetic circuit comprises a magnetic core and at least one winding, characterized in that the winding comprises at least one current-carrying conductor and each winding comprises an insulation system, which comprises on the one hand at least two semiconducting layers, wherein each layer constitutes substantially an equipotential surface, and on the other hand between them is arranged a solid insulation. 2. A DC transformer/reactor according to claim 1, characterized in that at least one of said semiconducting layer has in the main equal thermal expansion coefficient as said solid insulation. 3. A DC transformer/reactor according to claim 2, characterized in that the potential of the inner one of said layers is essentially equal to the potential of the conductor. 4. A DC transformer/reactor according to claim 2 or 3, characterized in that an outer one of said layers is arranged in such a way that it constitutes an equipotential surface surrounding the conductor. 5. A DC transformer/reactor according to claim 4, characterized in that said outer layer is connected to a specific potential. 6. A DC transformer/reactor according to claim 5, characterized in that said specific potential is ground potential. 7. A DC transformer/reactor according to any one of claims 1 to 6, characterized in that at least two of said layers have substantially equal thermal expansion coefficients. 8. A DC transformer/reactor according to any of the previous claims, characterized in that the current-carrying conductor comprises a number of strands, only a minority of the strands being non-isolated from each other. 9. A DC transformer/reactor according to any one of the preceding claims, characterized in that, each of said two layers and said solid insulation is fixed connected to adjacent layer or solid insulation along substantially the, whole connecting surface. 10. A DC transformer/reactor comprising a magnetic circuit, wherein the magnetic circuit comprises a magnetic core and at least one winding, characterized in that the winding comprises a cable (1) comprising at least one current-carrying conductor (2), each conductor (2) comprises a number of strands, around said conductor (2) is arranged an inner semiconducting layer (3), around said conductor (2) is arranged an inner semiconducting layer (3), around said inner semiconducting layer (3) is arranged an insulating layer (4) of solid insulation, and around said insulating layer (4) is arranged an outer semiconducting layer (5). 11. A DC transformer/reactor according to claim 10, characterized in that said cable also comprises a metal shield and a sheath. 12. A DC transformer/reactor according to claim 11, characterized in that the cable has a diameter comprised in the approximate interval 20-250 mm and a conductor are comprised in the approximate interval 80-3000 mm<2>. 13. A DC transformer/reactor according to any of claims 1-12, characterized in that windings on different potentials are wound in direct contact to each other. 14. A DC transformer/reactor according to any of claims 1-13, characterized in that a winding on DC potential versus the core is wound directly on, or very near the core. 15. A DC transformer/reactor according to any of claims 1-14. characterized in that the magnetic circuits exposed for DC magnetisation comprise at least one zone with lowered permittivity in the magnetic main flux. 16. A DC transformer/reactor according to claim 15, characterized in that the zone with lowered permittivity is accomplished with an air gap arranged in said core. 17. A DC transformer/reactor according to claim 15, characterized in that the zone with lowered permittivity is accomplished with a series of small air gaps arranged in said core. 18. A DC transformer/reactor according to claim 15, characterized in the zone with lowered permittivity is accomplished with a gap arranged in said core, wherein said gap is made of a material with a relative permeability, μr, which satisfies the expression  1 <= μr <= μcore 19. A DC transformer/reactor according to any of claims 15-18, characterized in that said core comprises an essentially rectangular outer member having opposing side sections and connecting end sections and further comprises a cylindrical center member that is continuous in the longitudinal direction, said core end sections having a pair of aligned circular apertures extending completely there through into which opposite ends of said cylindrical center member extend to define between at least one end section and the respective end of said center member a radial magnetic air gap of constant predetermined length for controlling the permeability of said core. 20. A DC transformer/reactor according to any of claims 16, 17, or 19, characterized in tat each air gap is compensated by a capacitively loaded compensation winding. 21. A DC transformer/reactor according to any of claims 1-20, characterized in that said transformer/ reactor also comprises a housing including at least thyristor valve. 22. A DC transformer/reactor according to claim 21, characterized in that all of said thyristor valves are of the integrated type. 23. A DC transformer/reactor comprising a magnetic circuit, wherein the magnetic circuit comprises a magnetic core and at least one winding, characterized in that the winding comprises at least one current-carrying conductor, and also comprising an insulation system, which comprises at least two semiconducting layers, wherein each layer constitutes substantially an equipotential surface, which insulation system in respect of its thermal and electrical properties permits a voltage level in said HVDC transformer/reactor exceeding 36 kV. 24. An integrated back-to-back station, characterized in that said station comprises two transformers/groups of DC transformers according to any of claims 1-23. 25. An integrated arrangement for transformation of high electric power from one DC voltage level to another DC voltage level, characterized in that said arrangement comprising a DC transformer according to any of Claims 1-23, wherein the DC transformer comprising first valve windings and second valve windings, wherein the first valve windings are connected to a first valve bridge and the second valve windings are connected to a second valve bridge, whereby the first valve bridge is operated as an inverter and the second valve bridge is operated as a rectifier. 26. An integrated arrangement according to claim 25, characterized in that said first valve bridge comprising at least one six-pulse inverter bridge which includes a plurality of self-commutated thyristors and further comprising a plurality of diodes, each diode being connected antiparallel to a self-commutated thyristor, and said second valve bridge comprising a six-pulse rectifier bridge which includes a plurality of diode valves. 27. A DC transformer/reactor according to claim 13 or 14, characterized in that the cable is specially adapted for mixed voltage. 28. A reactor for DC plants according to claim 20, characterized in that said compensation winding is loaded with a variable capacitance so as to be able to vary the inductance of the reactor. 29. A DC transformer/reactor according to any of claims 10-22, 27, characterized in that said outer semiconducting layer (5) is cut in a number of parts, each of which is connected to ground potential. 30. A DC transformer/reactor according to any of claims 1-22, 27, and 29, characterized in that the DC transformer/reactor also comprising at least one sensor/transducer for monitoring and diagnostics.