The present invention relates to an amperometric or differential current transformer equipped with a system capable of improving its cooling.
Moreover, the present invention relates to a protection device of an electrical circuit, for example, a low voltage one, against an overcurrent, a short circuit or a earth leakage current and that comprises such transformer, to a circuit breaker that comprises such transformer and/or such differential protection device, and to an electrical system comprising such circuit breaker.
As known, circuit breakers or similar devices are devices designed for allowing the correct operation of specific parts of electrical systems and of the installed loads. To this end, they are equipped with suitable protection devices, for example, electronic devices protecting against overcurrents, short circuits or differential currents (earth leakage currents or ground fault currents).
Such protection devices, also indicated simply as “relays” or “trip units,” can be realized and used as stand-alone components, or more typically they are inserted inside the shell of an automatic circuit breaker and are operatively coupled to its breaking part. The relays are normally associated with some current transformers or amperometric transformers (TA) or current transformers (CT). Normally, the current transformers provide the protection unit with a signal indicating the circulating current at each pole of the circuit breaker; in addition to, or as an alternative to this function, the current transformers are used to supply power to the same protection devices.
Similarly, also the protection devices, in particular the differential type, also referred to simply as differentials or differential relays, can be produced and used as stand-alone components, or more typically are associated with the shell of an automatic circuit breaker and are operatively coupled to its breaking part. The most common components of the amperometric transformers, whether of the unipolar or differential type, comprise a toroidal core, or shortly toroid, on which the so-called secondary windings are positioned; the core is then positioned in such a way as to be passed through, depending on the type of use, by one or more electrical conductors which constitute the so-called primary conductors or windings, each of which is directly or indirectly connected to a corresponding phase of the electrical circuit inside which the device is inserted.
One of the critical issues related to the amperometric transformers, in particular those applied to the automatic circuit breakers, is that the electrical junctions in the conductors that pass through the toroid cause local increases in electrical resistance with resulting production of heat. The heat generated is damaging to the life of the transformer and in particular the delicate secondary windings and their insulation coating. The heat also negatively affects the toroidal core, causing undesirable alterations of the typical B-H response curves. Also, when the device is inserted inside a circuit breaker, this undesirable heat contributes to increase the temperature of the circuit breaker and then can negatively affect its operation and performance. It also needs to be noted that when the amperometric transformers are connected directly to the output terminals of the circuit breaker, because of thermal conduction phenomena, in practice they result in being exposed to the heat produced by Joule effect on the circuit breaker itself. An excessive increase in the temperature of the circuit breaker can render it necessary to resort to the derating of the circuit breaker itself, i.e to an underuse compared to the nominal data, especially when it is installed inside a switchboard. Besides, it is nevertheless desirable to keep the operating temperature of the circuit breakers at low levels; it is known, in fact, that the higher is the operating temperature, the lower is the life span of the circuit breaker (or of its more sensitive components).
Normally, there is an attempt to solve such problems by increasing the dimensions and the volumes and by using materials that are particularly resistant to heat but are expensive.
Although these known solutions certainly provide some technical benefits, there is room and need for further improvements.
Therefore, the present invention is directed toward addressing the aforementioned problems by improving the cooling of a current transformer, and, in particular, the warmer parts thereof, such as those in proximity to the core, as well as the circuit breaker in which the current transformer is disposed.
In accordance with the present invention, a current transformer intended for use in an electrical circuit is provided. The current transformer includes a toroidal core and at least one electrical conductor having a section passing through the inside of the toroidal core. The current transformer further includes a cooling device having a body made of thermal conducting material and configured in such a way as to have a first portion that is connected to the electrical conductor in a position upstream from the toroidal core and capable of absorbing heat from the electrical conductor, and a second portion separated from the first portion, which is connected to the electrical conductor at a position downstream from the toroidal core and capable of transmitting heat to the electrical conductor. The thermal conducting body includes at least one portion made of an electrically insulating material capable of preventing current from flowing through the cooling device itself.
Further characteristics and advantages will become more apparent from the description of some preferred but not exclusive embodiments of the transformer according to the invention, illustrated only by way of non-limiting examples with the aid of the accompanying drawings, wherein:
In the following description, for the purpose of the present invention, the same or technically equivalent elements are indicated with the same reference numbers in the various figures.
In particular, illustrated in
As illustrated in such figures, the transformer 1 comprises a toroidal core 2, which is usually made of a ferromagnetic material on which the secondary windings are positioned (not illustrated in
In the exemplary embodiments of
In the example in
Transformer 1 comprises, for each phase of the line or electrical circuit inside which it will be used, at least one electrical conductor, indicated in all cases by reference number 4, which is flown through by the current circulating in the associated circuit.
In particular in the case of a current transformer 1 of the differential type (
Therefore, in the following description, for the sake of simplicity, reference will be made to a single phase of the circuit or line inside which transformer 1 is used; such description is clearly to be understood to be applicable in entirely analogous manner to all the phases of the line or circuit in which the current is detected.
As better represented in
The conductor 4 can be made up of a single electro-conducting element, or more commonly, of several elements connected to each other in series, as illustrated in the attached figures; in particular, in the example embodiments illustrated in
In the example illustrated in
Both the second conductor 5 and the other sections/components that contribute to form the conductor 4 can be realized by means of rigid elements such as rods, or by flexible elements, such as bare braids, insulated cables or by a combination of rigid and flexible elements, and can, for example, be made of copper, aluminium, etc.
Advantageously, the current transformer 1 according to the invention comprises one cooling device, overall indicated by reference number 10, having a body made of a thermal conducting material and configured in such a way as to have: a first portion that is connected to electrical conductor 4 at a first position (A) upstream (with respect to the flow of the current circulating within the electrical conductor from element 6 to element 7) of toroidal core 2 and capable of absorbing heat from electrical conductor 4; and a second portion, separated from the first portion, that is connected to the electrical conductor at a position (B) downstream (with respect to the flow of the current circulating within the electrical conductor from element 6 to element 7) from the toroidal core 2 and is capable of transmitting heat to the conductor element 4.
Furthermore, the thermal conducting body of the cooling device 10 comprises at least one portion 30 made of material that is electrically insulating but thermal conducting and capable of preventing the current flow through the cooling device itself.
In particular, the thermal conducting body may have a structure made predominantly of electrically conducting material, for example, copper, aluminium or any other commercially available material suitable for the purpose, inside which a portion 30 made of a thermal conducting but electrically insulating material is inserted, for example, ceramics, or a plastic material resistant to high temperatures; alternatively, the body of device 10 could be made completely of a thermal conducting and electrically insulating material, whether this be ceramics or a plastic material resistant to high temperatures, or any other material suitable for the purpose.
Preferably, as illustrated in the attached figures, the cooling device 10 is positioned with the thermal conducting body positioned completely external to the toroidal core 2.
Preferably, the thermal conducting body of cooling device 10 comprises a hermetically sealed cavity 11 (indicated by dashed lines in the figures) which contains a cooling fluid; preferably the cavity 11 comprises a small quantity of vaporizable liquid, for example, water.
Preferably, the walls of the sealed cavity 11 have porous or ribbed internal surfaces.
Advantageously, the thermal conducting body of device 10 is operatively coupled to the electrical conductor 4 such that the hermetically sealed cavity 11 has a first surface positioned in proximity to said position (A) upstream from the toroidal core 2, and a second surface positioned in proximity to said position (B) downstream from the toroidal core 2.
In particular, as illustrated in the examples of
Preferably, the device 10 also comprises two suitably shaped plates 13, 14, which are also made of thermal conducting material, such as for example, aluminium or copper. The two plates 13 and 14 are connected to opposite ends of tubular element 12 and can be equipped, one or both, with suitable holes capable of receiving fastening means, such as screws 15, 16, with one of the components of conductor 4.
In particular in the examples illustrated in
In the example of
In the various exemplary embodiments, the hollow tubular element 12, which can be of a rectilinear design (
Furthermore, the hollow tubular element 12 comprises at least one portion 30 made of an electrically insulating material, for example, ceramic. In the illustrated examples, this portion 30 may be constituted by a collar or cap positioned at one end of the tubular element 12 at the plate 13 (
This portion 30, made of electrically insulating material, prevents the current flow through the device 10; in this way, the detection of the currents is not affected by the device 1.
In practice, the plate 13 acts as a heat collector at position (A) inside which is located, for example, the junction of the collector 6, which, being connectable to the contacts of the circuit breaker, represents a particularly critical point for the heating; the first surface of the sealed cavity 11 absorbs (directly or indirectly) heat produced by the area of position (A) 21 and conveys it to the second surface of cavity 11. The second surface transmits heat to plate 14, which acts as a diffuser and transmits heat (directly or indirectly) to the downstream electrical system; with particular reference to
In conclusion, this is a thermal circuit that has: a warmer section immediately upstream of toroidal core 2 and which is found in proximity to the conductors that can be placed in direct contact with the real breaking part inside the circuit breaker, that is with the part of the circuit breaker that can reach high temperatures; and a “cooler” section separated from the warmer section that can be found at any point of the path of the electrical connection downstream from the toroidal core 2 wherein the temperature does not have a particular effect on the operation of transformer 1, as well as the protection device or circuit breaker inside which it may be used. The warmer section acts as an evaporator for the cooling fluid placed inside the sealed cavity, while the cooler section acts as a condenser; basically, a “thermal short circuit” is achieved between the two sections (A) and (B) of the electrical chain characterized by very different temperatures, wherein the device 10 absorbs heat at its warmer section, transferring it to the cooler section which then transfers it to the areas in contact with it (towards the electric line).
It has been observed in practice how the transformer 1, according to the invention, allows to accomplish the intended scope by providing several significant improvements with regard to the known solutions; in fact, the cooling device 10 keeps the toroid 2 much colder than the known solutions.
Furthermore, the transformer 1 has a simple structure that is easy-to-use in any electrical system as a stand-alone component or associated with any type of protection device, for example, an electronic relay, even just to supply it with electrical power, or with a circuit breaker.
Therefore further objects of the present invention include: a device for protecting an electrical circuit against failures, for example, because of overcurrent or short circuit or earth leakage current, characterized in that it comprises a current transformer 1 as previously described and defined in the appended claims; a circuit breaker, for example, of the low-voltage type, characterized in that it directly comprises a current transformer 1 as previously described and defined in the appended claims, or comprising a protection device, as defined above, having in turn a current transformer 1; or finally, an electrical system, for example, of the low-voltage type, characterized in that it comprises a current transformer as previously described and defined in the appended claims or characterized in that it comprises a protection device as defined above comprising such transformer 1, or again characterized in that it comprises a circuit breaker comprising such transformer 1 or such protection device having the transformer 1 itself.
In this way, all conditions being equal, the use of a transformer 1 with cooling device 10 allows to have in particular a circuit breaker with improved performance and which can be used with a rating potentially higher than an equal circuit breaker which is not provided with such a transformer 1.
The transformer 1 thus conceived is susceptible to numerous changes and variants, all of which are within the scope of the inventive concept; additionally, all details can be replaced by other equivalent technical elements. For example, for each phase, the number of tubular elements as well as their configuration, e.g. rectilinear, curved, or mixed, can be varied; plates 13, 14 can be shaped differently and can be formed by several pieces connected to each other; the device may comprise a connection element that consolidates the assembly of the components intended for each phase and makes device 10 a single block that can be applied as a separate module. Also the methods for fastening plates 13 and 14 to the conductors of phase 4 can be selected according to technical and economic convenience (for example, screws, bolts, rivets or welds). Moreover, it is possible to carry out any combination of the illustrated examples described at the outset. To this end,
In practice, the materials, as well as the dimensions, can be of any kind according to the requirements and state of the art.
Number | Date | Country | Kind |
---|---|---|---|
BG2009A000031 | May 2009 | IT | national |
This application claims priority under 35 U.S.C. §119(a)-(d) to Italian Patent Application Number BG2009A000031, filed on May 28, 2009, the entire contents of which are hereby incorporated by reference.