DEVICE IN ELECTRICAL SYSTEMS

Abstract: The present invention relates to a booster transformer of the type which is placed around an electric cable (4) for reducing stray currents, where the booster transformer includes a core (K) constructed of magnetic material, which is provided with an internal channel (5) intended to accommodate the cable (4), whereby the core (K) is provided with elements (2, 3) which enable the channel (5) to be opened over its whole length. The novelty is that the body (K) includes at least two groups of plane-parallel laminations (1) which in the plane mainly extend across the direction of the channel and which are arranged in such a manner that they encircle the cable (4), alternatively overlapping one another at the ends to form a closed magnetic circuit, which is openable at least one overlapping place through the separation of the laminations (1) included in the respective group.

Description: DEVICE IN ELECTRICAL SYSTEMS TECHNICAL FIELD The present invention relates to a booster transformer of the type which is placed around an electric cable which is included in a mains system, for reducing stray currents, where the booster transformer includes an elongated core constructed of magnetic material and provided with an internal channel in the longitudinal direction, whereby the booster transformer can be opened and is intended to be placed in such a manner that the cable extends in the channel. PRIOR ART Nearly all electric equipment gives rise to electric and/or magnetic fields. In a building, the fields can come from external sources, e.g. power lines, from electric installations or from electric equipment which is used in the building, e.g. display screens, lighting fittings and office equipment. Most buildings where the magnetic fields have high field strengths are not, however, located near power lines. The source of strong magnetic fields are in most cases so-called stray currents. These currents are due to the fact that, in Sweden, it is normal to have an electric system with four conductors in the main line to the building. Figure 1 shows such a four- conductor system P with three phase lines and a fourth, combined neutral and protective earth PE+N. A current I which arises when a single-phase load L is connected has two paths to the earth connection of a feeding transformer T. The return current can go either via the neutral conductor as intended, l„, or via the protective earth, l v , and an existing metallic conductor, e.g. a water pipe, to the earth connection of the transformer T. When the current is flowing along the water pipe, a magnetic field is generated around it, but also around the power line, because return current is "lacking" so that the magnetic fields from the different lines no longer equal. It is often possible to measure up to ten amperes in water, gas and heating pipes in a house which has such installations. When the stray currents flow in lines under ground, external magnetic fields are also obtained from these lines. It is also usual that there are stray currents in computer networks which, apart from generating magnetic fields, causes communication problems. Work in front of a display screen is made impossible in a room with peaks of over 0.5-1 μT due to picture interference. The best way to avoid magnetic fields from stray currents is to install a five-conductor system. The protective earth conductor is here separated from the neutral conductor which is why the return current cannot but be forced to take the correct path via the neutral conductor of the electric system back to the transformer. Installation of a five-conductor system leads to low magnetic fields in the house and a lower risk of problems with computer communication. In existing houses, it is relatively costly to change to a five-conductor system since it is not only the electric installations of the house which are affected but also the cable to the substation of the electric power station which must be exchanged for five-conductors. In smaller houses like privat houses, the stray currents can often be stopped by breaking up the alternative current path, e.g. by exchanging a short section of a metallic water, gas or heating pipe for a plastic pipe. This is not a practicable method in larger buildings. A cost-effective alternative to changing to a five-conductor system is to install one or more booster transformers in the four-conductor system according to what is shown in Figure 2. The principle of the booster transformer has been used for a long time in electrically powered railways in order to avoid currents from leaking out into the surrounding ground and pipeline systems. The main part of the current I, which is a phase unbalance, is forced by electromagnetic induction to flow through the neutral conductor, l n , and thus the stray current l v is reduced. The booster transformer includes an iron core K provided with a through-opening, which is slipped onto the cable and there forms a single-turn transformer with the four conductors of the cable as windings. To install such a booster transformer, it is thus necessary for the cable, where the transformer is to be placed, to be disconnected at either of the endpoints or, alternatively, to be cut and spliced together somewhere along its extent. This means that parts of the electric system must be made powerless during the installation. In a large electric system, it is often necessary to install booster transformers on a large number of cables. If the cable has a large diameter, it is often difficult to bend, as a result of which the installation can become relatively costly, even if it is considerably less costly than if the cable were to be changed to a five-conductor cable. For these reasons, it is desirable to have access to an openable booster transformer which, at the same time, is effective with respect to volume, which can be assembled directly around an existing cable without the cable having to be disconnected or spliced. It is thus not effective to use a core which has been sawed in half and which, after having been slipped on, is put together cut surface against cut surface since the magnetic coupling is then poor and the transformer must then be made considerably larger in order to obtain good efficiency. DESCRIPTION OF THE INVENTION The above problems are solved by a booster transformer such that the core includes at least two groups of plane-parallel laminations which in the plane mainly extend across the direction of the channel and which are arranged in such a manner that they encircle the cable, alternately overlapping one another at the ends to form a closed magnetic circuit, which is openable at at least one overlapping place through the separation of two groups of laminations. Outstanding advantages of the invention are that stray currents can be reduced in existing four-conductor installations in a cost-effective way through simple and quick installation and without a break in operation, which, in turn, leads to reduced technical interference and magnetic fields. In a very simple embodiment of the invention, at least one group of laminations has in its respective plane an arc shape and is arranged with a second group with overlapping ends so that the concave side of the arc faces towards the enclosed cable. This embodiment has the advantage of having only two "joints", but has the disadvantage that it is not as effective with magnetically oriented material. In another variant, the laminations have in their respective plane a mainly elongated, straight extent and are arranged in at least three or more groups with a corresponding number of overlapping sections. With this simple embodiment of the laminations, polygons can be constructed, so that using laminations of only one size, booster transformers can be made of a size which is adapted to the thickness of the cable which is to be encircled. If the laminations with straight, elongated shape, where the magnetic field will extend along the laminations, are made of magnetically oriented sheet-metal plate, whereby the orientation of the direction with magnetically optimized characteristics is along the laminations, the effectiveness of the booster transformer is increased considerably. Certainly, magnetically oriented sheet-metal plate can also be imagined in laminations with another shape. DESCRIPTION OF THE FIGURES The invention will be described in greater detail below and will be explained in connection with the figures shown on the attached drawings, in which: Figure 1 shows a diagrammatic illustration of a known 4-conductor system, Figure 2 shows a diagrammatic illustration of a 4-conductor system with a booster transformer installed, Figure 3 a-d shows a diagrammatic illustration of the structure of a booster transformer according to the invention, Figure 4 a-b shows a four-sided variant of the invention with rounded corners, and Figure 5 a-e shows, in sequence, how a booster transformer according to the invention is installed around a cable. PREFERRED EMBODIMENT As shown in Figure 2, the result with which the invention deals is achieved in that a booster transformer K is mounted on a power system P as in Figure 1 with stray currents between a transformer T and a load L. It is easy to establish whether a system has stray currents by detecting the magnitude of the magnetic field around the conductors. The booster transformer forces a relatively large proportion of a phase unbalance current l n through electromagnetic induction in a known way to pass into the combined protective earth and neutral conductor PE+K, which reduces the stray current l v . With a booster transformer according to the invention, the power system does not need to be shut down during the installation. The installation is done by the core of laminations included in the transformer being opened at one of the overlapping places of the laminations, slipped over the cable/cables and then closed by overlapping rejoining as will be described below. This creates an effective, closed magnetic circuit around the cable where the arrangement is installed without shutdown. The joining of the laminations to form a core can be likened to building a house, see Figures 3 a-d. Four groups of plane-parallel laminations M with rectangular shape are placed in such a manner that they form a quadrangle with a continuous opening in the centre. The laminations are placed with overlap at corner sections H where the laminations thus rest against each other while there are spaces R between the laminations at the intermediate sections of the laminations. The laminations are covered in a known way with an electrically insulating surface layer (not shown) which prevents eddy currents from being formed when they rest against each other. Each sheet-metal plate in itself can consist of a number of thinner sheet-metal plate which are joined to each other. The construction shown in Figure 3 achieves the following: - The arrangement can utilize the good magnetic characteristics of magnetically oriented transformer sheet-metal plate since the laminations are produced in such a way that the optimum magnetic characteristics and flux always coincide in their longitudinal direction. - By being assembled with standardized laminations, the booster transformer sizes can be adapted to different cable diameters and also cable runs constructed at relatively low cost. - Booster transformers of different size and different capacity can be constructed simply by "stacking" a different number of layers of laminations onto one another. - The contact area at the corner sections H between the laminations makes the design relatively insensitive to any play between the plates. The core does not need to be assembled with the same high precision as in the case of an ordinary transformer where the edges of the laminations also constitute a contact area between the laminations. This makes the installation uncritical and easy to carry out. No special tools are required. - The intermediate spaces between the laminations allow an air flow through the arrangement which counteracts heating of the arrangement and the cable around which the arrangement is installed. Apart from cables, the invention can be used e.g. on water pipes or district-heating pipes where it is desired to counteract stray currents without cutting and rejoining the pipe. The effect of the booster transformer is then that it constitutes an inductance for the current in the pipe, with the result that the current is reduced. Conceivable variants of the invention as shown in Figure 3 can be, for example, that some of the laminations H are constructed in such a manner that they make up more than one of the sides of the booster transformer, with the result that the booster transformer is assembled around the cable with L-, I- and/or U-shaped laminations. These then do not overlap each other in all corners of the booster transformer. Figure 4 a,b shows a variant of the booster transformer according to the invention which, in comparison with Figure 3, is closer to the final production stage; a) in the direction of the through-channel, b) across the direction of the channel. The ends of the laminations 1 are rounded and drilled through so that screws 2 can be threaded through the corners of the booster transformer and provided with nuts 3 for a bolt connection. When the bolt connections are drawn together, the contact areas are pressed together and ensure good magnetic contact between the laminations 1. Figure 5 a-e shows how the booster transformer according to Figure 4 is installed on a cable 4. During installation of the booster transformer, one of the screws 2a with its nut 3 is removed out of its hole 6 according to Figure 5a. A nearby screw 2b is suitably also loosened so that the affected group of laminations 1 which constitute one of the sides of the transformer can fold out according to Figure 5b. The transformer is then applied around the cable 4 according to Figure 5c by introducing the cable into the channel 5 through the opening produced. The laminations 1 are then folded back according to Figure 5d. The screw 2a is threaded back into the holes and united with its nut 3, after which the bolt connection is tightened according to Figure 5e. This concludes the installation. The invention is not limited to the illustrative embodiments shown and described here, but a number of modifications are possible within the scope of the patent claims following. For example, if the laminations have an adequate thickness, the outermost lamination located on the opposite side of the core relative to the head of the screw could have a threaded hole which matches the screw whilst the other laminations have larger holes, so that no nut would be required for tightening. To become more manageable and aesthetically pleasing, the core can also be provided with a cover of colour or plastic (not shown). It is also conceivable to have at the overlap places which do not need to be opened, a connection of a type which is cheaper (but not non-destructively openable), e.g. a rivet connection, instead of a bolt connection,

Claims: PATENT CLAIMS 1. Booster transformer of the type which is placed around an electric cable (4) for reducing stray currents, where the booster transformer includes a core (K) constructed of magnetic material, which is provided with an internal channel (5) intended to accommodate the cable (4), whereby the core (K) is provided with elements (2,3) which enable the channel (5) to be opened over its whole length, characterized in that the core (K) includes at least two groups of plane-parallel laminations (1 ) which in the plane mainly extend across the direction of the channel and which are arranged in such a manner that they encircle the cable (4), alternately overlapping one another at the ends, so-called overlapping places (H), to form a closed magnetic circuit, which is openable at at least one overlapping place through the separation of the laminations (1) included in the respective group. 2. Booster transformer according to Claim 1 , characterized in that at least one group of laminations (1 ) has in its respective plane an arc shape and is arranged in such a manner that the concave side of the arc faces towards the enclosed cable (4). 3. Booster transformer according to Claim 1 , characterized in that the laminations (1) have in their respective plane a mainly elongated, straight extent and are arranged in at least three or more groups with a corresponding number of overlapping sections (H). 4. Booster transformer according to any of the preceding claims, where the laminations have a mainly elongated shape, characterized in that the material in the laminations (1) is magnetically oriented sheet-metal plate, whereby the orientation of the direction with magnetically optimized characteristics is along the laminations. 5. Booster transformer according to one of the preceding claims, characterized in that the laminations are held pressed together at the overlapping places (H) by a bolt connection (2,3) extending across the plane of the laminations through holes (6) in the laminations (1).

Inventors: JOHANSSON BENGT, JOHANSSON, BENGT
Applicants: ENVIROMENTOR AB, JOHANSSON BENGT, JOHANSSON, BENGT
Application year: 2000
Main classification: H01F38
IPC codes: H01F27