Patent Application
20130127581
The present disclosure relates to transformer, such as a current transformer, with an arrangement of windings for eliminating or at least reducing local core saturation.
Known current transformers include a toroidal core inside which a primary conductor passes. A secondary or working winding is wound around the core with regularly displayed turns without a sectional winding, i.e. without dividing a winding into individual sections. This type of layout is subject to influences of outside magnetic fields, or misalignment, deviation, or insufficiencies of a primary conductor. These outside influences cause local oversaturation of the magnetic core, thus resulting in inaccuracies of the current transformer.
Thus, it is desirable to provide a solution which faces these issues and allows further improvements over known devices, and in particular with regard to elimination or at least reduction of local core saturation.
An exemplary current transformer is disclosed comprising: a toroidal magnetic core; and an even number of shielding coils which are wound around said toroidal magnetic core, wherein said shielding coils are arranged two by two in order to form corresponding couples and the two shielding coils of each couple are wound on opposite parts of the toroidal magnetic core, the shielding coils in each couple being connected in parallel to establish a respective magnetic flux in each coil, wherein the magnetic flux in the first shielding coil of each couple of shielding coils has an opposite direction with respect to the magnetic flux in the second shielding coil of each couple of shielding coils, and wherein the couples of shielding coils are connected in series.
Another exemplary current transformer is disclosed comprising: a toroidal magnetic core; and plural pairs of shielding coils, wherein each pair forms a shielding coil couple, wherein each shielding coil is wound around said toroidal magnetic core, corresponding couples and the two shielding coils of each couple are wound on opposite parts of the toroidal magnetic core, and for each shielding coil couple the shielding coils are connected in parallel, wherein a magnetic flux in a first shielding coil of each shielding coil couple has an opposite direction with respect to a magnetic flux in a second shielding coil of each shielding coil couple, and wherein the shielding coil couples are connected in series.
Further characteristics and advantages will become apparent from the description of some but not exclusive exemplary embodiments of a current transformer according to the present disclosure, illustrated only by way of non-limitative examples with the accompanying drawings, wherein:
It should be noted that in the detailed description that follows, identical or similar components, either from a structural and/or functional point of view, have the same reference numerals, regardless of whether they are shown in different embodiments of the present disclosure; it should also be noted that in order to clearly and concisely describe the present disclosure, the drawings may not necessarily be to scale and certain features of the disclosure may be shown in somewhat schematic form.
A solution over the prior art provides an exemplary current transformer having a toroidal magnetic core and an even number of shielding coils which are wound around the toroidal magnetic core, wherein the shielding coils are associated two by two to form corresponding couples and the two shielding coils of each couple are wound on parts of the toroidal magnetic core opposite to each other. The shielding coils in each couple of shielding coils can be connected in parallel to each other to establish a magnetic flux in them. The magnetic flux in the first shielding coil in each couple of shielding coils has an opposite direction with respect to the magnetic flux in the second shielding coil of the same couple of shielding coils, the couples of shielding coils being connected in series to each other.
The fact that a magnetic flux in one shielding coil in a coil couple has the opposite direction with respect to the magnetic flux in the other shielding coil of the same coil couple could be obtained by winding the shielding coils in the same direction or by winding the shielding coils in an opposite direction. For example, in order to achieve such an effect it is possible to wind a coil clockwise, counter-clockwise, from inside the core, or from outside the core, and correspondingly realize the interconnection among the coil tags according to solutions well known or readily available to those skilled in the art and therefore not described in detail herein.
For instance, in an exemplary embodiment, winding around the toroidal magnetic core can be in the same direction.
In exemplary embodiments of the present disclosure, the shielding coils can be arranged around the circumference of a toroidal magnetic core next to each other, or one over the other with an angular offset or overlap, and more preferably with an angular overlap or offset of about 90°, for example.
In another exemplary embodiment of the present disclosure the individual couples of shielding coils form at least a part of the secondary or working winding or the whole working winding of the current transformer.
The number of turns of the shielding coils can be the same for all shielding coils.
The shielding coils of each couple are connected in parallel to each other.
The couples formed are then connected in series.
As shown in
In the exemplary embodiment of
The contact ends of the shielding coil 21 can be connected with respective contact ends of the opposite shielding coil 22 in the couple 31. Similarly, the contact ends of the shielding coil 23 can be connected with respective contact ends of the opposite shielding coil 24 in the couple 32.
As a result of this wiring layout, a magnetic flux in shielding coils 21 to 24 is obtained that has the opposite direction in shielding coils 21 and 22 of the couple 31 of shielding coils 21 to 22 and in shielding coils 23 and 24 of the couple 32 of shielding coils 23 to 24, and the couples 31, 32 of shielding coils 21 to 24 are connected in series.
The secondary or working winding of the current transformer can either be formed only by shielding coils 21 to 24 in couples 31, 32 or there can be another supplementary winding 4 connected in series with couples 31, 32 of shielding coils 21 to 24; in the latter case, the working winding of the current transformer then comprises (e.g., consists of) couples 31, 32 of shielding coils 21 to 24 plus the supplementary winding 4 which is also wound around the toroidal magnetic core 5.
The four shielding coils 21 to 24 and the supplementary winding 4 can be interconnected in the same way as it is shown in
In the exemplary embodiment of
In this arrangement the first shielding coil 21 and the second shielding coil 22 of the first couple 31 are positioned on the core 5 opposite to each other with respect to a reference axis 200. In turn, the first shielding coil 23 and the second shielding coil 24 of the second couple 32 can be positioned on the core 5 opposite to each other with respect to a reference axis 300.
The first couple 31 formed by the shielding coils 21 and 22 can occupy the whole circumference of the toroidal magnetic core 5. Similarly, the first shielding coil 23 of the second couple 32 can be wound on one half of the toroidal magnetic core 5, while the second shielding coil 24 of the second couple 32 can be wound on the other half of the toroidal magnetic core 5. From this arrangement, the second couple 32 formed by the shielding coils 23 and 24 can occupy the whole circumference of the toroidal magnetic core 5. The windings or shielding coils of the first couple 31 and of the second couple 32 are wound on each other with an angular overlap of about 90° for example. The four shielding coils 21 to 24 and the supplementary winding 4 are interconnected in the same way as it is shown in
In an exemplary embodiment of the present disclosure reference is made, for example, to the axis 200 and counting either clockwise and/or counter-clockwise, the ends of each shielding coil in a first couple of coils, e.g. the ends of the shielding coil 21 and/or of the shielding coil 22 of the couple 31, are offset at about 90° with respect to the ends of each shielding coil in a second couple of coils, e.g. the ends of the shielding coil 23 and/or of the shielding coil 24 of the couple 32.
It should be understood that such an angular offset or overlap can have a different value other than 90°.
In operation, if for example the primary conductor is not in the centre of the current transformer 1, the magnetic field of the primary conductor can increase the magnetic flow in the magnetic material of the toroidal magnetic core 5 up to the saturation point. At the same time, the magnetic field of the primary conductor induces a current in the shielding coils 21 to 24 the value of which is increasing with the closeness of the primary conductor to the respective shielding coil 21 to 24. The bigger the deviation of the primary conductor from the centre of the toroidal magnetic core 5, the bigger the induced current in the closest one of the shielding coils 21 to 24. The magnetic flow induced in the toroidal magnetic core 5 by the shielding coils 21 to 22 is in the opposite direction with respect to each other. Both opposite shielding coils 21 and 22 or 23 and 24 mutually cooperate—namely one of them adds a magnetic flow where it is missing or low in the toroidal magnetic core 5 and the other one reduces the magnetic flow on the other side of the toroidal magnetic core 5, where the magnetic flow is excessive.
In other words, the electric flow in the primary conductor induces an electric tension in the shielding coils 21 to 24. As the shielding coils 21 and 22 are connected in parallel and similarly the shielding coils 23 and 24 are connected in parallel, the electric tension on the shielding coil 21 is identical with that on the shielding coil 22 and the same electric current flows through both shielding coils 21 and 22; likewise, the electric tension on the shielding coil 23 is identical with that on the shielding coil 24 and through both shielding coils 23 and 24 flows the same electric current. The magnetic flow induced in the shielding coils 21 and 22 and in the shielding coils 23 and 24 adds a magnetic flow where it is missing or low in the toroidal magnetic core 5 and reduces the magnetic flow on the other side of the toroidal magnetic core 5, where the magnetic flow is excessive.
Shielding coils 21 to 24 form only parts of the working winding and they are connected in series with the supplementary winding 4. The number of turns is the same in all shielding coils 21 to 24 and in this exemplary embodiment each shielding coil 21 to 24 has x turns, where x is a number in the order of hundreds to thousands, while the supplementary winding 4 has y turns, where y is in the order of thousands, depending on the specified transformer ratio.
An exemplary current transformer according to the present disclosure gives some improvements over the existing devices, allowing the disclosed embodiments to overcome the issues of the prior art previously mentioned, since it makes possible to at least reduce if not completely eliminate the problem of local core saturation.
The exemplary current transformer disclosed herein is susceptible of modifications and variations, all of which are within the scope of the inventive concept including any combination of the above described embodiments; for example, the number of shielding coils 21 to 24, that is four in the described exemplary embodiment, could be any even number, and their positioning is used for optimisation of the whole set-up based on the specified level of saturation.
All details may further be replaced with other technically equivalent elements; in practice, the materials, so long as they are compatible with the specific use, as well as the individual components, may be any according to the specifications and the state of the art.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
