Patent Application
20100090794
The present invention relates to a resistor for electric high-voltage apparatus, especially for connection to a diverter switch in an on-load tap changer for a high-voltage transformer, wherein the diverter switch and the resistor are arranged in a housing with transformer oil. In this context, high-voltage apparatus means apparatus for tens of kilovolts, up to for example 800 kV and voltages there above.
The invention also relates to a method of mounting such a resistor.
A diverter switch included in a tap changer is usually used in connection with a transformer to enable tapping at different voltage levels. This occurs in cooperation with a selector connected to the diverter switch. When the power output from a transformer is to be changed from one voltage level to another, this occurs by first connecting the selector to that tapping point of the transformer winding which corresponds to the new voltage level while the diverter switch is still feeding from the existing voltage level. The connection of the selector thus takes place without current load. When the selector is connected to the tap for the new voltage level, a switching operation takes place with the aid of the diverter switch such that output current is taken out from the new tapping point of the transformer. When a transformer has a plurality of tapping points, switching normally only occurs between two tapping points which are close to each other in terms of voltage. If an adjustment to a more distant location should be required, this takes place step by step. A diverter switch of the kind referred to here is normally used for control of power or distribution transformers. The invention is not, of course, limited to this type of application but may also advantageously be used for control of other types of power transmission or distribution products, such as reactors, phase shifters, capacitors or the like.
The operation of the diverter switch involves commutation from one circuit to another with an ensuing occurrence of an electric arc. To avoid polluting the insulating medium, such as oil, into which the diverter switch is normally immersed, and to reduce the wear of the switch contacts, it is previously known to use vacuum switches for those switching operations where an arc arises. The electrical contact wear will then only arise in the vacuum switch. For an appropriate procedure from an electrical point of view, a diverter switch of this kind is provided with at least one main branch and one resistance branch. Heavy demands are placed on the resistors in such a resistance branch, for the following reasons.
The resistor in an on-load tap changer is connected for a very short period of time in the normal operating position, for example for a period of between 20 and 50 ms. During this time, however, the current that passes through the resistor elements in the resistor is considerable. For example, amperages of up to 2000 A may be achieved as well as a step voltage of 4.5 kV. It is thus required that the resistor wire for a short period may absorb a large energy mass in the form of heat, since it is realized that during the short switching operation the heat is not capable of being dissipated to any greater extent outside the resistor, since the cooling takes place substantially through the passage of oil after the actual switching operation.
Thus, the amount of heat that a resistor element may instantaneously accumulate is, among other things, dependent on the amount of resistance material in the resistor. An example of a dimensioning capacity is a heat-absorption capacity of between 80 J/g and 100 J/g resistance material. An example of resistance material is CuNi44.
The modes of operations of the tap changer are described in greater detail in PCT publication WO 2006/004527 and PCT patent applications WO SE2006/050552 and WO SE2007/050187.
Resistors for tap changers are also known from, for example, GB 1483003 and DE 2220111.
The present invention seeks to provide an improved resistor with regard to capacity, compactness, flexibility (modular design), capacity of being mounted, and reliability of service.
The invention also seeks to provide a method of mounting a resistor in a cost-effective manner.
According to an aspect of the invention there is provided a resistor of the kind described in the preamble to claim 1 by arranging the frames so as to enclose resistor elements, which comprise spring elements formed from spirally wound resistance wire. In this way, the entire circumference of the resistance wire is surrounded by the oil in the channel and the instantaneous heating of the wire occurs uniformly since the whole surface of the resistance wire is in direct contact with the oil. Together with the subsequent effective cooling, this contributes to the high capacity of the resistor according to the invention, which is rapidly cooled down after each switching operation. By designing the resistor element in the form of spring elements, the further advantage is obtained that the length of resistance wire in the spring element may be made large. At the high voltages to which the present invention relates, the distance between the top part of the resistor and the top part of the housing, that is, the distance insulated by the oil, must be sufficient to ensure the necessary electrical insulation.
The transition resistances constitute the highest energized point, in the vertical direction, of the diverter switch. The shape of the resistor element in the form of a spring element according to the invention further provides a uniform electric field, where high field concentrations are prevented, whereby the invention eliminates the need of a separate, so-called shield ring.
Other aspects of the invention are described by means of embodiments according to the appended subclaims.
According to one embodiment, the resistor comprises one resistor module for each phase, the frame exhibits a number of mutually isolated, parallel and open channels, said channels being arranged to enclose the spring elements formed from spirally wound resistance wire, and within a resistor module a number of resistor elements are stacked on top of each other thus forming an integrated unit, wherein each channel in a resistor element with the corresponding channels in adjacent resistor elements forms a substantially open channel. This results in a compact design of the resistor module and the resistor, which is thus advantageous from several points of view. Further, the retaining open channels through the whole resistor module makes possible an efficient circulation of the oil through the channels, among other things through the “chimney effect” that arises.
According to one embodiment, the frame is designed with a number of longitudinal parallel walls, forming outer walls and inner walls, which are connected at their respective ends to transverse short walls forming said channels, one longitudinal edge of said walls exhibiting protruding spacers which are connected to transverse bars, whereby said spacers towards adjacent resistor elements form longitudinal gaps open towards the channels. The bars thus serve as support for the spring elements in the channels and ensure that the spring elements, which are located respectively over and under each other in the channels in the resistor, do not make mechanical contact with each other, but that the insulating distance between the spring elements is ensured. At the same time the spacers form open gaps between the resistor elements, which essentially contributes to the oil circulation within the resistor.
According to one embodiment, one end of each inner wall is formed with at least one recess arranged at the opposite end in relation to the recess at the adjacent inner wall, in which recesses bridging wire elements are arranged for electrical connection of the spring elements within the respective frame. This minimizes the number of recesses while at the same time it is ensured that the spring elements within one frame may be connected electrically in series through said wire elements.
According to one embodiment, the spring elements and the wire elements within the respective frame forming the resistor element consist of an integrated unit. This results in an advantageous mechanical locking of the spirally-wound spring elements in the respective channel after they have been pressed down with low pressure into the respective channels. Since the longitudinal walls, at their sides which are open to the channel, are somewhat resilient, the width of the channel may increase somewhat when the spring element is pressed down into the channel. The spiral design of the spring element also allows for the spring element to be somewhat extended at the points where the bulges are arranged.
According to one embodiment, the outer walls and the side walls along the open side of the respective channel comprise wart-like bulges that are arranged to lock the spirally wound spring elements in their respective channels.
According to one embodiment, the frames are formed with guide members in the form of guide pins and corresponding guide holes for fixing adjacent resistor elements to each other. These contribute to fixing adjacent resistor elements to each other in the lateral direction.
According to one embodiment, the short walls at one end of the outer channels are formed with a recess for a connection member to the spring element, and the outer channels in the frame at one end are formed with a recess for a connection member to the spring element. This provides an opening for applying connection members within the height of the frame and direct towards the end of the spring element, which there exhibits an annular cavity.
According to one embodiment, the recesses for the connection members are diametrically arranged and each frame comprises an odd number of channels. This means that the resistor elements may be formed such that, when being mounted to the resistor module, they may be turned so as to face in either direction in the plane.
According to one embodiment, the number of channels is between 3 and 7, preferably 5. In this way, the necessary quantity of resistance wire may be applied in each resistor element while at the same time the width of the resistor module and hence also the resistor may be limited.
According to one embodiment, the frame is made of an injection-moulded electrically insulating plastic material.
This makes possible a cost-effective manufacture of the frame while at the same time the mechanical and electrical properties of the material are advantageous in this context.
According to one embodiment, the resistance wire consists of CuNi44. This results in the advantage of obtaining a resistor with high-quality resistance material with documented properties.
According to one embodiment, the number of resistor elements forming a resistor module is an even number. This results in the advantage that the external connection to the resistor module will always be on the same side.
According to one embodiment, the resistance elements of the resistor elements within the resistor module are series- and/or parallel-connected. This results in great flexibility for achieving a resistor with the desired electrical performance by the choice of connection between the resistor elements.
According to one embodiment, the resistor comprises three resistor modules for a three-phase system. Here, three modules are arranged in parallel and adjacent each other forming a compact integrated unit.
According to one embodiment, the spiral spring element is connected to a connection member that comprises a connection plug that is formed with a thread-formed contact part and a flange part, said contact part being arranged to be screwed, with a rotary motion, into the spiral spring element, said flange part being configured to be mechanically fixed to the frame. In this way, among other things, a fast, simple, cost-effective and safe mounting of the connection member to the spring element is obtained, without any soldering being required. Further, a possibility of non-destructive dismantling of the connection device is provided.
According to one embodiment, the flange part is provided with a threaded bore, and an electrically conductive foil is arranged, with a screw in the threaded bore of the connection plug, to be brought into electrical contact with and be mechanically locked to the connection plug. This results in a fast and efficient locking with good electrical contact between the parts.
According to one embodiment, the thread-shaped contact part is formed with a screw pitch which somewhat exceeds the pitch of the spiral spring element in a mechanically unloaded state, whereby, during mounting, the resistance spring is stretched within that part of the spring element that surrounds the contact part. In this way, the spring element is stretched when being screwed in, and the spring force contributes to provide a high friction between the parts, in which case it is ensured that the risk of play between the connection plug and the spring element is minimized.
According to one embodiment, the connection plug is arranged to be connected to the spring element formed from resistance wire consisting of CuNi44. In this way, a resistor with a high-quality resistance material with documented properties is obtained.
The invention is also directed to a method of mounting a resistor as described in the preamble of claim 20, whereby the mounting comprises the following steps:
In this way, a fast and cost-effective mounting of the resistor is obtained, and the mounting may be made very flexible since resistors with different capacities may be mounted on the same resistor element, the number of which is varied according to the need. Also, it is simple to connect the resistor elements internally in series or in parallel, and it is easy, where necessary, to replace a resistive element in one frame by a resistive element with a different electrical capacity (resistivity and volume of resistance material, respectively).
The invention will be described in greater detail by description of embodiments with reference to the accompanying drawings, wherein
a schematically shows a side view of the resistor module as seen from its left-hand side shown in
b schematically shows a side view of the resistor module as seen from its righthand side shown in
The above units form a unit that is mounted on a frame structure 14, which in turn is integrated with two beams 15, arranged in parallel and made of an electrically insulating material. At their upper parts the beams are connected to lifting means 16. During mounting, the entire unit is lowered down into a tap changer housing (not shown here), which is filled with transformer oil. The function of the oil is partly to cool the units and partly to function as an electric insulator as well as for lubrication of the mechanically movable components.
The resistor 10 according to the invention is arranged between said beams 15 and rests on the upper side of the frame structure 14 and there forms a compact integrated unit. In a three-phase tap changer, three resistor modules 100 are arranged adjacent to each other, one for each phase. Each resistor module 100 is provided with two connections 17, each being connected to the respective vacuum switch 13, as is described in more detail in the above WO 2006/004527.
When the tap changer is connected to control the voltage of a high-voltage transformer, the voltage may amount to tens of kV, up to 800 kV and more. The electric components in the tap changer lie at said high potential whereas the components on the top part of the housing are at ground potential.
At the high voltages to which the present invention relates, the distance between the top part of the resistor 10 and the top part 18 of the housing, that is, the distance insulated by the oil, must be sufficient to ensure the necessary electrical insulation. Since according to the invention each resistor module 100 comprises a number of resistor elements 101 stacked on top of each other, forming an integrated unit, a compact design of the resistor module 100 and the resistor 10 is obtained, which is thus advantageous. The transitions resistances constitute the highest energized point in the diverter switch in the vertical direction. The shape of the resistive elements 102 in the form of a spring element 105 provides an even electric field, where high field concentrations are prevented. In this way, the need of a separate so-called shield ring is eliminated.
The frame 103 exhibits a number of mutually insulated, parallel and open channels 104, which are configured to enclose a spring element 105 formed from spirally wound resistance wire. By designing the resistive element 102 in the form of spring elements 105, the advantage is achieved, among other things, that the length of resistance wire in each spring element 105 may be made long, while at the same time the spring element per se becomes compact. Examples of dimensions are an outer diameter of the spring element of 12 mm and a wire diameter of 2.5 mm. The outer diameter of the spring element 105 is adapted to the channels 104 in the frame 103.
As further example, a resistor module 100 according to one embodiment is formed with five parallel rows of spring elements 105, each having a length of about 300 mm and may contain, depending on the wire thickness, between 13 and 16 m of resistance wire. The spiral spring element further exhibits a good permeability to the oil and has only point contacts with the frame, which is essential for the cooling function.
As is further clear from
It is realized that when the resistor elements 101 are mounted on top of each other (as is clear from
The resistor 10 in a tap changer is switched in for a very short period of time during a normal operating mode, for example for a period of time of between 20 and 50 ms. During this time, however, the current that passes through the resistive elements 102 is large. For example, amperages of up to 2000 A and a step voltage of up to 4.5 kV may be achieved. Thus, it is required that the resistance wire for a short period may absorb a large energy mass in the form of heat as it is realized that, during the short switching operation, the heat will have no time to be diverted to any greater extent outside the resistor, since the cooling takes place substantially through passage of oil after the actual switching operation.
The amount of heat that a resistive element may instantaneously accumulate is thus, among other things, dependent on the amount of resistance material in the resistor. An example of a dimensioning capacity is a heat-absorption capacity of between 80 J/g and 100 J/g of resistance material. An example of a resistance material is CuNi44.
Because of the invention, the entire circumference of the resistance wire is surrounded by the oil in the channel and the instantaneous heating of the wire takes place uniformly since the entire surface of the resistance wire is in direct contact with the oil. This fact, and the subsequent efficient cooling, contributes to the high capacity of the resistor according to the invention, which is rapidly cooled down after each switching operation.
As will be clear from
The spacers 109 at the same time form open gaps 201 between the resistor elements, as is clear from
As is clear in more detail from
According to one embodiment, the resistive element 102 in a frame 103 is constituted by an integrated unit of the spring elements 105 and the wire elements 112, as is shown in
By providing the outer walls 106 and the side walls 107 along the open side of the respective channel 104 with wart-like bulges 113, according to one embodiment shown in
By one embodiment, the frames 103 are formed with guide members in the form of guide pins 114, shown in
Additional guiding and locking of the resistor blocks to each other and to the frame structure 14 in the tap changer occur by bolt joints (not shown) running through bolt holes 117 provided in the frames.
According to one embodiment, the outer channels 104 in the frame 103 are formed, at one end, with a recess 116 for a connection member to the spring element 105, which is clear, inter alia, from
According to one embodiment, the frame 103 is made of an electrically insulating material, preferably of a plastic material that is suited for compression moulding, which is favourable from the point of view of manufacture.
According to one embodiment, the number of resistor elements 101 forming a resistor module 100 is an even number, suitably a number between 2 and 12. In case of resistors for very high voltages, the number of resistor elements 101 may be considerably higher. Since the number is even, the two outer connections of the resistor module to the electric apparatus will be on one short side of the resistor module 100, as shown in
On the short side of the resistor module according to
Correspondingly, at the other short side of the resistor module according to
The invention also relates to a connection member 30 in a resistor according to the above. As previously mentioned, the resistance wire consists of CuNi44. This material is relatively hard, which implies that brazing with a silver solder is the normal manner of connecting connections to the material. However, soldering is a very time-consuming and costly process from the material point of view. It also entails considerable difficulties to connect the resistance wire to connections, using some form of clamping joint, because of the hardness of the resistance material.
As is clear, among other things, from
According to one embodiment, the flange part 311 of the connection plug 31, at its end opposite to the contact part 310, is provided with a threaded bore 312. An electrically conductive foil 313, shown in
According to one embodiment, the threaded contact part 310 is formed with a pitch that somewhat exceeds the pitch of the spiral spring element in a mechanically unloaded state, whereby, during mounting, the resistance spring is stretched within that part of the spring element which surrounds the contact part 310.
The spring force together with the friction between the threaded contact part ensures that the risk of a play between the connection plug 31 and the spring element 105 is minimized.
It is realized that the mechanical and electrical contact of the contact plate 313 to the connection plug 31 may be additionally ensured by applying a locking washer (not shown) between the screw 314 and the foil 313.
According to one embodiment, the flange part 311 of the connection plug 31 is formed with two flanges 311a and 311b with an intermediate spacer 311c, which is formed with a rectangular cross section, the width of which is somewhat smaller than the width of the recess 116 arranged in the frame, which provides the advantage that when the connection plug 31 is applied in the recess it is locked for rotation whereas the two flange parts 311a and 311b, which are located on respective sides of the short wall 108 at the recess 116, lock the plug in the longitudinal direction. Taken together, this facilitates the mounting and fixing of the connection plug 31 to the spring element and to the frame, as well as the mounting of said foil 313 with said screw 314. Also the square-shaped flanges 311a and 311b contribute to locking against rotation in that they are supported against the edge in the recess 116 in the frame 103.
The invention also relates to a method of mounting a resistor 10 for electric high-voltage apparatus, especially for connection to a diverter switch in a tap changer for a high-voltage transformer of the kind described above.
According to one embodiment, the mounting operation according to the invention comprises the following steps:
It is realized that the mounting need not be performed exactly according to the order of these individual steps but that certain steps may be made in a different order within the scope of the invention. The connection plugs 31 may, for example, be mounted to the spring elements 105 before these are placed in the channels 104, and the resistor elements 101 for the respective resistor module 100 may be mounted direct onto the bolts of the bolt joint, or the internal connection of the connecting foils 313 within a resistor module 100 may take place before this module is mounted on the bolt joint.
In those cases where the resistor comprises several resistor modules 100, these may be mounted together to form a resistor 10 before being placed on the frame structure 14.
Further, the invention is not limited to the described and shown embodiments, but the person skilled in the art may, of course, modify it in a number of ways within the scope of the invention as defined by the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0601742-0 | Aug 2006 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE07/50578 | 8/24/2007 | WO | 00 | 12/7/2009 |
