DE3417648A1 - Overvoltage suppressor - Google Patents

Abstract

For correct dissipation of the heat from the varistors (1) of overvoltage suppressors, which heat is produced both in the steady-state operating mode and during temporary switching and atmospheric voltage surges, heat-conducting elements (2) are provided. The heat of the varistors (1) is initially introduced into the heat conducting elements (2) in the axial direction, via the end surface (2') of said heat conducting elements (2), and is dissipated radially from said heat conducting elements (2) via their outer surfaces to the insulating housing (6, 6'). In this case, the heat conducting element (2) is connected in a heat-conducting manner to the insulating housing (6). The entire outer surface of the varistor (1) is spaced apart from the inner wall of the insulating housing (6, 6') by means of a space (12) which is filled with insulating gas. The heat conducting element (2) is broadened outside the diameter region of the varistor (1) and is at the same time used for adjusting the varistors (1) during their installation, and additionally prevents accelerations of the varistors (1) when shocks act on the overvoltage suppressor. <IMAGE>

Description

SURGE ARRESTERS

The invention is based on an electrical transition arrester according to the preamble of claim 1.

With this generic term, this invention takes on a prior art Reference as described in U.S. Patent 4,335,417.

Electrical surge arresters are used to protect transformers, Switchgear and apparatus in high and extra high voltage networks against switching and atmospheric overvoltages applied. The surge arrester works like an extremely fast earthing switch. It closes automatically due to the overvoltage a current path to earth and opens it automatically after the overvoltage is discharged again.

Without surge arresters, the would be an economic dimensioning Isolation and a reliable power supply are no longer conceivable. The steady one Further development of the surge arrester with non-linear, ceramic resistance blocks led to varistors based on zinc oxide ceramics, which have an extremely high non-linearity exhibit.

Zinc oxide varistors have a ceramic base compared to conventional ones based varistors have the following advantages: - The resistors can be large Make blocks approx. 8 cm in diameter and several centimeters high. In order to the energy absorption capacity required in the arrester construction can be achieved.

- Under the operating voltage - in the steady state - only flows a predominantly capacitive current of a few milliamperes. With increasing tension - in the non-stationary state - the resistors go into the conducting state without delay State over and limit the further increase in voltage at the terminals. After this When the overvoltage wave drops, the arrester immediately switches to the weakly conductive one State back.

- The production costs can be reduced so far by large-scale technology that an economical application in energy technology is possible.

The constant flowing through the varistor in steady-state operation Electricity can pose a risk of "thermal tipping", i.e. when the rate of heat input is greater than the heat dissipation rate, the power loss of the varistor increases in improper way.

As a result of increased temperature influence and especially in the case of inhomogeneous Temperature distribution in the varistor in the radial direction, is in the non-stationary Operating condition - in the event of overvoltages - the energy absorption capacity of the varistor degraded. This can lead to premature destruction of the varistor itself lead of sufficient dimensioning, since under these conditions its limit energy absorption capacity is exceeded. In addition, instability effects arise under the influence of increased temperature, by taking the ohmic losses Continuous operating voltage with the Operating time and the number of stresses can increase.

To ensure the operational safety of surge arresters in the event of faults in which plasma flashovers develop between the varistors care must be taken that the gas pressure caused by the plasma can be commutated in a controlled manner from the interior of the insulating housing without explosive discharges with unforeseen consequential damage occur. this however, this is only possible if, through appropriate design measures, a possible The resulting plasma is evenly around the outer circumference of the varistors and can also unload there.

In the known electrical surge arrester according to the US-PS 4,335,417 are metallic disks between the end faces of the varistor disks intended to dissipate heat from the varistor disks, which in the event of overvoltages cause a heat reduction.

This varistor metal disk assembly, which is made of a thermally conductive Sheath is surrounded, but is arranged eccentrically in the cavity of the insulating housing and is only partially in direct thermal contact with the insulating housing, and part of the heat has to go through the neighboring, poorly heat-conducting air envelope are delivered to the insulating housing. This will reduce the heat transfer from the varistors reduced to the insulating housing. There is also the heat transfer from the varistor disks to its environment in the non-stationary operating state - in the event of overvoltages - both in the axial direction to the heat-reducing metal disks as well as in the radial direction In the direction of the insulating housing, there is a higher one in the center of the varistor disks Temperature level present than in the outer areas.

This in all radial planes over the entire thickness of the varistor disks prevailing, distorted temperature pro- fil influences the power loss both in the stationary and in the non-stationary operating state in disadvantageous Way and also reduces the energy absorption capacity of the varistor disks in the non-stationary operating state. There is a risk that the limit energy absorption capacity of the varistor is exceeded and it is destroyed.

According to US Pat. No. 4,100,588, the varistors are stacked on top of one another and embedded in a thermally conductive material made of plastic, which in turn again in large-area, direct heat-conducting contact with the insulating housing stands. But since there are no metallic disks between the varistors, the heat dissipation from the varistors is limited in the event of overvoltage. Accordingly the energy absorption capacity of the varistors is reduced and the power loss elevated.

The invention as defined in claim 1 solves the Task to specify an electrical surge arrester, its operational safety while increasing the energy absorption capacity and reducing the power loss can be increased.

The advantages of the invention can be seen in the following: The energy absorption capacity the varistors of the surge arrester in the event of overvoltages in the non-stationary State is increased as the temperature, due to the largely axial heat dissipation from the varistors to the heat conducting element in all radial planes over the entire thickness of the varistors are kept almost constant. This will also limit the energy absorption capacity increased and the performance and operational safety of the surge arrester improved.

- The varistors have a higher resistance to aging because the entire surface area of the varistors in connection with an air or gas space and is therefore not exposed to any chemical reaction.

- Due to the air flow surrounding the entire surface of the varistors or a gas space can form a plasma which may form in the event of high overvoltages Form evenly over the entire circumference of the varistors.

Local gas pressure concentrations during the plasma discharge are largely avoided and thus the risk of explosions in the surge arrester locked out.

- As a result of the large-area heat conduction according to claim 2, once from the varistors to the heat conducting element in the axial direction, on the other hand from the heat conducting element the thermal conductivity of the varistors becomes the insulating housing in the radial direction to the insulating housing and thus at the same time the power loss is reduced and the energy absorption capacity increased.

The advantage of claim 3 is that the heat conducting element can be used with play in the insulating housing and not exactly the inner diameter the cavity of the insulating housing must be adapted.

By filling the remaining annular gap with an elastic thermally conductive layer, a good coefficient of thermal conductivity is achieved. This solution is special advantageous with different expansion coefficients of the heat conducting element and insulating housing, and when using porcelain insulators, their inner wall can possibly be uneven, whereby the unevenness due to the elastic stic Thermally conductive layer are compensated. Thermal stresses and an insufficient Heat conduction between the heat conducting element is avoided.

The development according to claim 4 has the advantage that the heat conducting layer the coefficient of thermal conductivity is further improved by applying the heat to the insulating housing over a larger area can be discharged.

Due to the advantageous embodiment according to claim 5, the space sealed outside the jacket surfaces of the varistors. This is an introduction the heat conducting layer in the annular gap between the outer surfaces of the sealing cylinder and heat conducting element and the inner wall of the insulating housing in liquid form possible and the production of the surge arrester is facilitated.

By designing the heat conducting elements also as adjusting elements according to claim 6 is an automatic adjustment of the \ Iaristors when they are installed possible.

In addition, a stable mounting of the varistors is possible in the event of a effective accelerations in case of vibrations guaranteed.

By attaching a ductile metal washer and a disc spring between the end faces of the varistor and the heat conducting element according to claim 7 a better heat-conducting and electrical transition achieved by the Metal disc Any unevenness in the face of the varistors can be compensated. Partial electrical discharges are avoided in this way. In addition, the varistor and heat-conducting element can be supported by defined contact surfaces an axially symmetrical arrangement of both parts can be achieved.

The advantage according to claim 8 can be seen in the fact that disc springs with stronger pressure can be used, so that also a better one Heat transfer and better electrical contact between the heat conducting element and varistor is guaranteed. Electrical partial discharges are thus also avoided.

With the method according to claim 9, the varistors and the heat conducting elements in a simple way once in optimal thermally conductive contact with the insulating housing to be brought. In addition, the varistors and heat conducting elements are at the same time held mechanically stable within the insulating housing.

The invention is explained in more detail below with the aid of exemplary embodiments explained: FIG. 1 shows a vertical section through a surge arrester, 2 shows a partial vertical section through a surge arrester in one first embodiment, Fig. 3 is a partial vertical section through a Surge arrester in a second embodiment, FIG. 4 an enlarged, partial section according to FIG. 3, FIG. 5 shows a partial vertical section through a surge arrester in a third embodiment, Fig. 6 shows an enlarged, partial section according to FIG. 5, FIG. 7 shows a partial section Vertical section through a surge arrester in a fourth embodiment, and FIG. 8 shows a partial horizontal section through a surge arrester according to FIG. 7.

Fig. 1 shows a vertical section through a surge arrester. Disk-shaped varistors are located within a cavity of a porcelain insulator 6 1 with also disc-shaped heat conducting elements 2 alternately and centrally stacked on top of each other. The end faces of the varistors 1 and the heat conducting elements 2 are arranged transversely with respect to the longitudinal axis of the insulator 6. The heat conducting elements 2 consist of a first, within the diameter range of the varistors 1 lying part, and a second outside the diameter range of the varistors 1 lying part, the second part comparing to the first in its thickness is expanded. The outer diameter of the heat conducting elements 2 is smaller than that the inner wall of the insulator 6 and the remaining annular gap between the outer surfaces the heat conducting elements 2 and the inner wall of the insulator 6 is with a heat conductive layer 3 filled. This layer 3 extends over the entire Height of the stack of varistors 1 and heat conducting elements 2. As can be seen from FIG. 1, only the heat-conducting elements 2 are in direct heat-conducting contact with the porcelain housing 6, while the varistors 1 with their entire surface area through an exposed space 12 filled with insulating gas from the heat-conducting layer 3 or from the insulator 6 are spaced.

The heat dissipation from the varistors 1 to the porcelain housing 6 takes place for the most part in the axial direction from the varistors 1 to the heat-conducting elements 2, over their mutually abutting end faces.

After that, the heat in the radial direction is first from within the diameter range of the varistors 1 lying part of the heat conducting elements 2 to the extended part 2 ″ lying outside the varistors 1 and then via the Jacket surfaces of the heat conducting elements 2 passed on to the heat conducting layer 3 ' and delivered by this to the inner wall of the porcelain insulator 6.

Finally, the heat is spread over a large area via the cooling fins of the porcelain insulator 6 dissipated to the environment of the surge arrester. A small proportion of Heat is mainly generated by convection over the jacket surfaces of the varistors 1 the surrounding air / gas space removed.

The extended part 2 ″ of the heat conducting elements 2 takes over in addition to the Heat conduction still another function of adjusting the varistors 1.

As can be seen in Fig. 1, the diameter of the first part of the heat conducting elements 2 selected so that it exactly matches the outer diameter of the varistors 1 corresponds. Outside the jacket surface of the varistors 1 is the expanded one Part 2 ". With such a design of the heat-conducting element 2, the installation of the varistors 1 during assembly by automatic adjustment of the varistors 1 and in the event of any shocks that affect the surge arrester can act, an acceleration of the varistors 1 by a fixed bracket, which is guaranteed by the heat conducting elements 2, completely eliminated. In the heat conducting elements 2 openings 5 are provided, which isolate the exposed gas Interconnect spaces 12, whereby a pressure relief channel 17 is formed, the over the entire Height of the stack of varistors 1 and heat conducting elements 2 up to an upper 11 and a lower sealing rupture disk 11 '. These Sealing rupture disks 11, 11 'are made by means of an unspecified adhesive attached to the end faces of the porcelain insulator 6.

The porcelain insulator 6 is at both ends with an upper 9 'and a lower housing part 9 by means of a connecting material 13, for example Cement bonded. Inside the housing parts 9, 9 'are each cylindrical compression springs 10, 10 'arranged, the varistors 1 and heat conducting elements 2 in each operating state hold the surge arrester tightly braced.

In the event of a possible plasma formation, as a result of too large Overvoltage or in the event of an unforeseen malfunction in the surge arrester, the associated gas pressure can initially be increased via the pressure relief duct 17 and after penetrating the sealing bursting disks 11, 11 'via the pressure relief chambers 14, 14 'in the housing parts 11, 11' and through the openings 15, 15 'into the Free escape.

In the following figures, the same parts are given the same reference numerals as indicated in FIG.

Fig. 2 shows a partial vertical section through a surge arrester, wherein the varistors 1 and the heat conducting elements 2 within an elastic insulating material housing 6 'are arranged. In the one shown in Fig.

The embodiment shown in FIG. 2 is the lateral surfaces of the heat conducting elements 2 in direct thermally conductive contact with the inner wall of the housing 6 '.

This assumes that the diameter of the heat conducting elements 2 is exactly that of the cavity of the housing 6 must correspond. Radial thermal expansion are taken up by the elastic housing 6 'and thermal tensions do not occur. In Fig. 2, the is within the diameter range of the varistors 1 lying part of the heat conducting elements 2 with dl and the extended, edge side, outside of the varistors 1 lying part designated by d2. The ratio of dl: d2 is chosen so that an optimal coefficient of thermal conductivity from the heat conducting elements 2. to the insulating housing 6 'is reached. Between the respective adjoining end faces 1 ', 2 'of the varistors 1 and the heat conducting elements 2 are arranged contacting means, to which reference is made in more detail in FIG. 4. The outer surface of the varistors 1 is surrounded by an insulating layer 18, which increases the dielectric strength the varistors 1 is used.

In Fig. 3, the diameter of the heat conducting elements 2 is smaller than that of the cavity of the insulating housing 6 'selected. The remaining annular gap is filled with the heat conducting layer 3, the hollow cylinder once to the Jacket surface of the heat conducting disk 2 and on the other hand on the inner wall of the housing 6 applies and thus produces good heat conduction between the two parts 2, 6. The insulating housing 6 'could also not be in the present exemplary embodiment be elastic porcelain insulator 6, since the radial thermal expansion of the heat conducting element 2 taken up by the elastic heat-conducting layer 3 and not on the porcelain insulator 6 are passed on. Thermal stresses do not occur.

FIG. 4 shows a partially enlarged section according to FIG. 3. Between the end faces 1 'of the varistor 1 and the end faces 2' of the heat conducting element 2 each have an annular metal disc 16 and a plate spring 8 are arranged. The metal disk 16 is made of a ductile metal, preferably aluminum and serves to compensate for minor unevenness on the Face 1 'of the varistors 1 - due to their manufacturing process - can be present. The plate spring 8 is also for better contact between varistor 1 and Heat-conducting element 2 is provided because the plate spring 8 also has bumps on the end faces 1 'of the varistors 1 can be compensated.

Fig. 4 shows the operational state of the surge arrester, whereby the end faces 1 ', 2' of the \ Iaristor 1 and of the heat conducting element 2 against each other are pressed.

It goes without saying. even that the end faces 1 'of the varistors 1 are metallized in order to make electrical contact under precisely defined conditions to ensure.

The embodiment according to FIG. 5 differs from that as has been described in FIGS. 2 and 4 by the fact that once different contacting means provided between the end faces 1 ', 2' of the varistors 1 and the heat conducting elements are and that on the other hand, the heat conducting layer 3 'extends over the entire height of the stacked varistors 1 and heat conducting elements 2 extends as a hollow cylinder. This heat-conducting layer 3 'can easily be attached and the heat can not only in the radial but also in the axial direction, i.e.

larger area are given to the porcelain insulator 6 so that the coefficient of thermal conductivity increases.

In Fig. 6 the contacting means is as it is in the exemplary embodiment 5 applied in accordance with FIG. 5 is shown in more detail in an enlarged, partial section.

In the end face 2 'of the heat conducting element 2 is a central one Recess provided in which a plate spring 8 'with a greater compressive force is arranged.

The plate spring 8 'extends over a large part the End face 2 of the adjacent varistor 1 and with this embodiment can a very good contact between varistor 1 and heat-conducting element 2 can be achieved.

The disc springs 8 'ensure that the clamping force exactly in the axial direction on the individual varistors 1 and heat conducting elements 2 acts. This results in the varistors 1 being destroyed under continuous mechanical stress completely avoided by non-axial forces. You can see their effect through Strengthen the stacking of several disc springs 8 to form spring columns.

In Fig. 7 is a further embodiment according to the invention shown. The heat conducting element 2 has in both end faces 2 'in the area 2 " Outside the end faces 1 'of the transistor 1 there is an annular recess 7 each on, in each of which a sealing cylinder 4, which surrounds the varistors 1 at a distance, intervenes.

The sealing cylinder 4 is used to seal the air or gas space, when the heat conducting layer 3 'in liquid form in the hollow cylindrical space between the outer surfaces of the heat conducting element 2 and the inner wall of the insulating housing 6 is introduced. The thermally conductive layer 3 'is on the one hand in the intimate thermally conductive Contact with the sealing cylinder 4 and on the other hand with the inner wall of the insulating housing 6th

Fig. 8 shows a horizontal section through the surge arrester 7. There are openings 5 in the heat dissipation element 2 can be seen in this embodiment are each arranged offset from one another by 1200. These openings 5 connect the air or gas spaces 12 and form the pressure relief channel 17th

the axially symmetrical arrangement of the openings 5 ensures a Perfect commutation of the gas pressure in the event of a plasma discharge.

In the following, the production will be based on two exemplary embodiments of the surge arrester according to the invention are explained in more detail: 1. Example: First of all, a first varistor 1 is placed on a first heat-conducting element 2, wherein beforehand a metal disk 16 and / or a plate spring between their end faces 1 ', 2' 8 were inserted. A sealing cylinder 4 is then inserted into the annular recess 7 of the first heat conducting element 2 is inserted. On the end face 1 'of the first varistor 1 is in turn a metal disc 16 and / or a plate spring 8 is placed and on this in turn a second heat conducting element 2, the sealing cylinder 4 in the Recess 7 of the second heating element 2 engages in a sealing manner and thus the air-gas space 12, which completely encloses the jacket surface of the first varistor 1, is formed will. Then further parts 1, 2, 4, 8, (16) put together to form a stack. This stack is then braced and held coaxially in the cavity of the insulating housing 6, 6 '. Then in the remaining hollow cylindrical gap between the outer surfaces of the heat conducting elements 2 and the sealing cylinder 4 and the inner wall of the insulating housing 6 is a curable elastic, thermally conductive and electrically insulating material cast in, which the heat-conducting layer 3 'forms. After partial hardening of the liquid material the upper 9 and the lower housing 9 'with the upper and lower ends of the Insulating housing 6, 6 'joined together and also by means of a connecting material 13 firmly connected. The helical compression springs 10, 10 'hold the varistors 1 and heat conducting elements 2 in every operating state in a firmly clamped position.

2nd example: First the varistors 1 and the heat conducting elements 2 alternately stacked on top of one another and inserted into the cavity of the insulating housing 6 and held coaxially to one another on a turntable. After that, the, the heat conducting layer 3 'forming material in liquid form between the outer surfaces of the heat conducting elements 2 and the inner wall of the insulating housing 6 and the turntable in Rotation offset. As a result of centrifugal force and / or gravitational force, the inner wall becomes of the insulating housing 6 over the entire height of the varistor unit 1, 2 with one layer of the liquid material coated, the layer thickness being dimensioned in such a way that that the gap between the outer surfaces of the heat conducting elements 2 and the inner wall of the insulating housing 6, completely closed over the entire height of the stack will. The rotation of the turntable is continued until the material at least is partially cured and the stack is fixed in the insulating housing 6. The others The process steps are now the same as those given in the 1st example.

In Figures 1 to 8, the varistors 1 were in a central position, with respect to the cavity of the insulating housing 6, 6 'and to the heat conducting elements 2, shown.

However, it is just as conceivable that the varistors 1 are eccentric to the cavity of the insulating housing 6, 6 ′ and to the heat conducting elements 2 could be.

In each of the exemplary embodiments shown, a heat-conducting element was used 2 arranged between two adjacent varistors 1. But this is not necessarily the case necessary.

Two or more varistors 1 can be connected directly one behind the other before a heat-conducting element 2 is again inserted into the stack.

The space 12, which surrounds the lateral surfaces of the varistors 1, is in Normally filled with air. Insulating gases can just as well enter space 12 to complete. It is advantageous for the resistance to aging of the varistors 1 that the insulating gas contains oxygen.

The material which is preferred for the heat-conducting layers 3, 3 ' is used is a polymerizing, cold-curing, cold-curing silicone elastomer, which inorganic fillers, for example aluminum oxide in parts by weight from 60 to 70 ° Ó.

The material of the heat conducting elements 2 is steel or aluminum.

The varistors 1 are metal oxide varistors, preferably zinc oxide varistors.

The surge arrester according to the invention is dimensioned in such a way that that both the heat from the varistors 1 in the steady operating state as well dissipated properly in the event of temporary switching and atmospheric overvoltages can be.

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Claims (9)

PATENT CLAIMS Electrical surge arrester with a hollow insulating gas-filled insulating housing (6), with electrical connections at the ends, with at least two varistors (1), with at least one heat conducting element (2), the is arranged between the end faces (1 ') of the varistors (1), the one with the varistors (1) and the insulating housing (6) is in electrical and heat-conducting contact, wherein the varistors (1) and the heat conducting element (2) within the insulating housing (6) are arranged and connected in series between the electrical connections, and with a pressure relief channel (17) extending from one end to the other end of the varistors (1) and the at least one heat conducting element (2) extends, characterized in that the entire jacket surface of the \ Iaristor (1) from the inner wall of the insulating housing (6) through an insulating gas-filled Space (12) is spaced and that the heat conducting element (2) in an outside the end faces of the varistor (1) lying area at least one breakthrough (5). 2. Surge arrester according to claim 1, characterized in that that the heat conducting element (2) on the edge in the area outside the end faces (1 ') of the varistor (1) is widened so that the ratio of the thickness (dl) of the heat conducting element (2) between the varistors (1) to the thickness (d2) at the transition for the inner wall of the insulating housing (6) in the range from 1: 1.3 to 1: 5, in particular is selected in the range from 1: 1.5 to 1: 3.5 (Fig. 1, 2, 4, 6, 8). 3. Surge arrester according to claim 1 or 2 with a heat conducting layer (3) made of an elastic, thermally conductive material between the thermally conductive element (2) and the inner wall of the insulating housing (6), characterized in that the Jacket surface of the heat conducting element (2) on all sides through an annular gap from the inner wall of the insulating housing (6) is spaced, and that this annular gap through the heat conducting layer (3) is completely filled out (Fig. 2). 4. Surge arrester according to claim 1 or 2 with a heat-conducting layer (3 ') made of an elastic, thermally conductive material between the thermally conductive element (2) and the inner wall of the insulating housing (6), characterized in that the Jacket surface of the heat conducting element (2) on all sides through an annular gap from the inner wall of the insulating housing (6) is spaced apart that the heat-conducting layer (3 ') as a cylinder from one end to the other of the varistors (1) and the at least a heat-conducting element (2) extends and that this annular gap through the heat-conducting layer (3 ') is completely filled out (Fig. 4). 5. Surge arrester according to one of the preceding claims, characterized in that the heat conducting element (2) in the two end faces (2 ') in the area (2 ") outside the end face of the varistor (1) at least each has an annular recess (7), and that a sealing cylinder (4) is provided which surrounds the varistor (1) at a distance and seals into the recesses (7) engages (Fig. 6). 6. Surge arrester according to one of the preceding claims, characterized in that the heat conduction element (2) outside the end faces of the varistor (1) a widening with an inclined adjustment surface (2 "') and that the varistor (1) is adjustable (Fig. 1, 2, 4, 6, 8). 7. Surge arrester according to one of the preceding claims, characterized in that between the end face (1 ') of a varistor (1) and the end face (2 ') of the heat conducting element (2) an annular metal disc (16) is arranged, the outer diameter of which is equal to that of the \ Iaristor (1) is that a disc spring (8) is provided within this metal disc (16), whose outside diameter is equal to the inside diameter of the metal disc (16), and that the metal disc is made of a ductile material, in particular aluminum, consists (Fig. 3). 8. Surge arrester according to claim 1 and 6, characterized in that that within a central recess in the end face (2 ') of the heat conducting element (2) a plate spring (8) is arranged (Fig. 5). 9. Process for the production of a surge arrester with a Thermally conductive layer (3 ') according to claim 3, characterized in that a stack, consisting of at least two varistors (1) and at least one heat conducting element (2), which is arranged centrally between the end faces of the varistors (1) is held in the cavity of the insulating housing (6) that the stack (1, 2) and the insulating housing (6) are set in rotation that then on the front side in the remaining gap between the one lateral surface of the heat conducting element (2) and the inner wall of the insulating housing (6) is a curable, elastic, electrical insulating and thermal good conductive material in liquid Shape is introduced until the gap between the outer surfaces of the heat conducting elements (2) the full height of the stack is completely closed and that the rotation is continued until the material of the heat conducting layer (3 ') at least is partially cured.