In the transmission and distribution of electric energy, various known static inductive devices such as transformers, reactors, regulators and the like are used. The purpose of such devices is to allow exchange or control of electric energy in and between two or more electric systems. Such devices belong to an electrical product group known as static inductive devices. Energy transfer or control is achieved by electromagnetic induction. There are a great number of textbooks, patents and articles which describe the theory, operation and manufacture of such devices and associated systems, and a detailed discussion is not necessary.

Conventional electric high voltage control is generally achieved in core type devices by having one or more windings wound on one or more legs of a so called core. In shell type devices a rectangular coil and rectangular core limbs are employed. Known devices use complex methods and expensive materials to produce efficient and low loss components.

Said device uses complex and expensive schemes to cover the windings and protect against overloads. It is desirable to manufacture a simplified shell type toroidal high power inductive device employing an exterior shell which is efficient, has a relatively low operating temperature and which may employ simplified covering techniques.

SUMMARY OF THE INVENTION The invention provides a high power static electromagnetic or induction device with a rated power ranging from a few hundred kVA up to over 1000 MVA with a rated voltage ranging from 3-4 kV and up to very high transmission voltages, such as 400 kV to 800 kV or higher, and which does not entail the disadvantages, problems and limitations which are associated with the prior art power devices.

The invention is based on the discovery that each phase of a device employs a cable surrounded by a shell having a region of relatively low reluctance surrounding a major portion of the cable forming a gap at a region of relatively high reluctance formed in the gap.

In Transformers and multiphase devices an exterior shell surrounds each of the enclosed windings. Certain exemplary embodiments employ a simplified cooling arrangement.

In a particular embodiment, the invention comprises a high power static induction device having a winding in the form of a cable wound as a toroid having a flux bearing path for each phase surrounding the cable. A plurality of such phases are surrounded by an outer flux bearing path. Each of the windings is formed of one or more current carrying conductors surrounded by a magnetically permeable, electric field confining insulating cover.

In another exemplary embodiment, the cover comprises a solid insulation surrounded by an outer and an inner potential-equalizing layer being partially conductive or having semiconducting properties. The electric conductor is located within the inner layer. As a result the electric field is confined within the winding. The electric conductor, according to the invention, is arranged so that it has conducting contact with the inner semiconducting layer. As a result no harmful potential differences arise in the boundary layer between the innermost part of the solid insulation and the surrounding inner semiconductor along the length of the conductor.

According to an exemplary embodiment of the invention, the device has a flux bearing region in the form of a toroidal shell surrounding each winding. The shell is in the form of multiple alternating layers of magnetically permeable material and insulation. The shell has a gap which extends around a central axis of the toroid. The gap is formed of a high reluctance material including air.

Certain exemplary embodiments employ structures which have cooling channels within which the windings are located. One such structure employs radial cooling channels with a concentric winding support structure. Another embodiment employs an air cooled device having horizontal or transverse winding supports. Yet another embodiment employs a water or liquid cooled hollow toroidal structure with transverse winding supports The invention employs windings having a solid insulation and semiconducting layers which exhibit similar thermal properties to the solid insulation as regards the coefficient of thermal expansion. The semiconducting layers may be integrated with the solid insulation so that these layers and the adjoining insulation exhibit similar thermal properties to ensure good contact independently of the variations in temperature which arise in the line at different loads. At temperature gradients the insulating layer and semiconducting layers form a monolithic cover, and defects caused by different temperature expansion in the insulation and the surrounding layers do not arise.

The electric load on the material is reduced because the semiconducting layers form equipotential surfaces and the electric field in the insulating part is distributed nearly uniformly over the thickness of the insulation.

In an exemplary embodiment, a transformer according to the invention employs a plurality of adjacent windings arranged as coaxially around a central axis perpendicular to the plane of each coil. Each winding has one or more turns and a high resistance outer shell forming a flux bearing region surrounding a major portion of the turns. Each cover has a region of relatively high reluctance in the gap and thereby forming at least one continuous turn or flux bearing region around a central axis of each winding.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the accompanying drawings, wherein Fig. 1 shows the electric field distribution around a winding of a conventional inductive device such as a power transformer or reactor; Fig. 2 shows an embodiment of a winding in the form of a cable in a high power inductive device according to the invention; Fig. 3A shows a schematic cross-sectional illustration of a generalized three phased power transformer employing laminated exterior shell type flux bearing region according to the invention; Figs. 3B and 3C are enlarged fragmentary details of the flux bearing region shown in Fig. 3A; Fig. 4 is a respective illustration of a three phase transformer according to the invention; Fig. 5 is an enlarged fragmentary sectional detail of the transformer of Fig. 4; Figs. 6A, 6B, 6C are illustrations of a multiphase toroidal inductive device employing radial cooling channels and concentric winding supports shown respectively in perspective, end and fragmentary sectional views; Figs. 7A and 7B are illustrations of a toroidal inductive device employing traverse cooling channels shown respectively in perspective and end views; and Fig. 8A, 8B and 8C are illustrations of a multiphase toroidal inductive device having transverse fluid cooling channels shown respectively in perspective, end and enlarged fragmentary sectional views.

DESCRIPTION OF THE INVENTION The inventive concept which forms the basis of the present invention is applicable to various static inductive devices including, power transformers, reactors and regulators. As is known, the devices herein may be designed as single-phase and three-phase systems.

Fig. 1 shows a simplified and fundamental view of the electric field distribution around a winding of a conventional static induction device such as a power transformer/reactor 1, including a winding 2 and a core 3. Equipotential lines 4 show where the electric field has the same magnitude. The lower part of the winding is assumed to be at earth potential. The core 3 has a window 5 and carries a flux ¢.

The potential distribution determines the composition of the insulation system since it is necessary to have sufficient insulation both between adjacent turns of the winding and between each turn and earth. In Fig. 1 the upper part of the winding is subjected to the highest dielectric stress. The design and location of a winding relative to the core are in this way determined substantially by the electric field distribution in the core window 5.

Fig. 2 shows an example of an exemplary cable 6 which may be used in windings which are included in high power inductive devices according to the invention. Such a cable 6 comprises at least one conductor 7 including a number of strands 7A with a covering 8 surrounding the conductor. The covering 8 includes an inner semiconducting layer 9 disposed around the strands 7A. Outside of this inner semiconducting layer is the main insulation layer 10 of the cable in the form of a solid insulation, and surrounding this solid insulation is an outer semiconducting layer 11. The cable 6 may be provided with other additional layers for special purposes, for example for preventing too high electric stresses on other regions of the device. In the exemplary embodiment, the majority of the strands 7A of the conductor 7 are mutually insulated. However, one or two strands may be formed without insulation so as to be in conductive contact with the inner semi-conducting layer 9.

From the point of view of geometrical dimension, the cables 6 in Fig. 2 will generally have a conductor area which is between about 30 and 3000 mm2 and an outer cable diameter which is between about 20 and 250 mm.

In the embodiment illustrated, the outer semiconducting layer 11 exhibits such electrical properties that potential equalization along the conductor is achieved. The semiconducting layer does not, however, exhibit such conductivity properties that the induced current causes an unwanted thermal load. Further, the conductive properties of the layer 11 are sufficient result in that an equipotential surface. Exemplary thereof, the resistivity, p, of the semiconducting layer 11 generally exhibits a minimum value, pmin = 1 Qcm, and a maximum value, pmax = 100 kQcm, and, in addition, the resistance of the semiconducting layer per unit of length in the axial extent, R, of the cable generally exhibits a minimum value Rmjn = 50 Q/m and a maximum value R ,, = 50 MQ/m.

The inner semiconducting layer 9 exhibits sufficient electrical conductivity in order for it to function in a potential-equalizing manner and hence equalizing with respect to the electric field outside the inner layer. In this connection, the inner layer has such properties that any irregularities in the surface of the conductor are equalized, and the inner layer forms an equipotential surface with a high surface finish at the boundary layer with the solid insulation. The layer may, as such, be formed with a varying thickness but to ensure an even surface with respect to the conductor and the solid insulation, its thickness is generally between 0.5 and 1 mm. However, the inner layer 9 does not exhibit such a great conductivity that it contributes to induce voltages. Exemplary thereof, for the inner semiconducting layer, thus, Pmin = 1 o-6 Qcm, Rmjn = 50 IlQ/m and, in a corresponding way, Pmax = 100 kQcm, Rmax = 5 MQ/m.

Fig. 3A shows, in a schematic cross-section, a high power inductive device in the form of a three phase external shell type transformer 12 in accordance with the present invention. The transformer 12 comprises three windings 13-1,13-2, and 13-3, one for each phase, each formed of multiple turns of the cable 6 in Fig. 2. Each winding 13-1, 13-2, and 13-3 has a corresponding flux bearing shell 15-1,15-2, and 15-3 including a region of relatively low reluctance 16 and a region of high reluctance 18. It should be understood that the suffixes herein designate the position of the corresponding element, and are otherwise not used when the position is not relevant to the discussion.

The region of low reluctance 16 may comprise a high resistance metal, and the region of high reluctance may comprise a non-magnetic dielectric. In the arrangement illustrated, the region of low reluctance 16 may further comprise insulated layers of a conductor 24 and an insulator or dielectric 26 formed in the gap 22 as shown in Fig. 3B. The region of relatively low reluctance surrounds a major portion of the winding and has endpoints 20 in relatively close proximity which form a gap 22 therebetween as shown in Fig. 3A. The region of high reluctance 18 is formed in the gap 22 and may comprise air, but in the exemplary embodiment comprises a solid dielectric or a number of layers of dielectric 28 as shown in the enlargement of Fig. 3B.

The transformer 12 has an outer flux bearing region 30 which may be similarly formed with respective relatively high and low reluctance regions 32 and 34 in the same way as described with respect to regions 16 and 18 respectively. Figs. 4 and 5 illustrate an exemplary embodiment of a toroidal transformer 40 comprising three windings 41-1,42-1, and 42-3 having central axis 44-1,44-2, and 44-3 forming vertices of an equilateral triangle 46. Each winding 42 has one or more turns of the insulated cable as shown in Fig. 2 and may be provided with a power cable termination or coupler 43 Each cable is formed with conductors and an insulated cover as described above.

Each winding is surrounded by a flux bearing region 48-1,48-2, and 48-3. The entire structure is likewise surrounded by a outer main or mutual flux bearing region 50. Each of the windings 42 produce a corresponding flux which is carried by the corresponding flux bearing region 48 and by the outer flux bearing region 50. As discussed with respect to Figs.

3A-3C, the various flux bearing regions have relatively low and relatively high reluctance areas 54 and 56 formed in the manner described hereinabove. In the devices described herein, the high reluctance region 56 protects the device from excessive magnetic fields induced by high currents which may be caused by a fault or short. It should be understood that any one winding, for example, 42-1 could be operated alone as a separate reactor if desired, and one need only make a single winding with the high and low reluctance cover.

Such an arrangement need not be separately shown in Figs. 4 and 5, but is part of the invention as exemplified in Figs. 7A and 7B.

Fig. 6A, 6B and 6C are illustrations of an air cooled three phase transformer 60 according to the invention formed with radial channels 62 forming pie shaped phases 64-1, 64-2, and 64-3 each of which has a flux bearing region 66. The transformer also has an outer flux bearing region 68 surrounding the phases 64. The flux bearing regions have regions of high and low reluctance including a gap similar to the arrangement described above.

Each phase has a corresponding winding 70-1,70-2, and 70-3 supported in a series of toroidal concentric ring-like segments 72-1, 72-2, and 72-3. The segments 72 may each be formed of a pair of closely spaced curved metal sheet elements 74 having welded spacers 76 for supporting the elements in confronting relation as shown. The segments establish cooling channels 78 therebetween. The channels may be adapted to carry any of a variety of working or cooling fluids such as air, gas or liquids.

Fig. 7A and 7B illustrate a single phase inductive device 80 having a winding 82 surrounded by a flux bearing number 84 of the type as hereinabove described. In the arrangement, transverse supports 86 are provided in the form of parallel annular rings forming a toroidal structure. The supports 86 may be formed of confronting parallel sheets 88 with spacers 90 forming cooling channels 92 for carrying a cooling fluid.

Figs. 8A, 8B and 8C illustrate a three phase transformer formed with three devices 80- 1,80-2 and 80-3 of the type similar to the arrangement of Fig. 7A-7B, surrounded by a outer or mutual flux bearing member 96 and forming a fluid cooled three phase transformer according to the invention.

In accordance with the invention, the various cooling structures herein described may be coupled to earth or appropriately grounded.

While there have been provided what are considered to be exemplary embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications therein may be made without departing from the invention, and it is intended in the appended claims to cover such changes and modifications as fall within the true spirit and scope of the invention.