WO2013167200A1

WO2013167200A1 - A high voltage fibre reinforced bushing having a lengthwise weakening ensuring controlled rupture at internal overpressure

Info

Publication number: WO2013167200A1
Application number: PCT/EP2012/058800
Authority: WO
Inventors: Tomas AHLBERG; Thomas Eriksson
Original assignee: Abb Technology Ag
Priority date: 2012-05-11
Filing date: 2012-05-11
Publication date: 2013-11-14

Abstract

The present invention relates to an insulator unit for a high voltage apparatus. The insulator unit comprises a tubular part (2b) made of composite material including fibers arranged in at least two different directions, and two end fittings (3a-b) connected to the tubular part. The tubular part and the end fittings form an enclosure for enclosing a voltage carrying part and a medium for insulating the voltage carrying part. The tubular part comprises an intermediate section (12b) provided with at least one weakening (40) arranged along the length of the intermediate section. The tubular part is designed to be able to withstand a specified type test pressure and to rupture at a certain overpressure above the specified type test pressure.

Description

A HIGH VOLTAGE FIBRE REINFORCED BUSHING HAVING A

LENGTHWISE WEAKENING ENSURING CONTROLLED RUPTURE AT INTERNAL OVERPRESSURE

Field of the invention

The present invention relates to an insulator unit for a high voltage apparatus, the insulator unit comprises a tubular part made of an insulation material, two end fittings connected to the tubular part, the tubular part and the end fittings form an enclosure for enclosing a voltage carrying part and for housing a medium for insulating the voltage carrying part. The present invention also relates to a high voltage apparatus including a voltage carrying part and an insulator unit for insulating the voltage carrying part. With high voltage is meant voltages above lkV.

Prior Art

There exist many types of electrical apparatus for high voltage applications including an insulation unit filled with a medium for insulating the voltage carrying part. Examples of such apparatus are cable terminations, insulation units, tap chargers, surge arrestors, breakers, instrument transformers, bushing and similar high voltage apparatus. The insulating medium is, for example, a solid material, such as a field grading material, a gas, such as SF6, or oil. A problem with such apparatus is that a short circuit inside the insulator unit causes an internal arc. The internal arc will produce a large amount of heat which gasifies everything in the near vicinity of the arc inside the insulator, which causes a high pressure inside the insulator. If the pressure inside the insulator becomes too high there is a risk that the tubular part will explode uncontrolled and debris will be spread over a wide area.

For cable terminations, several customers require that the cable terminations pass internal arc tests. Internal arc tests means that a high powered short cut is created within the termination inside the cable insulation unit, for example 50kA, 20kV during 500ms. No debris from the termination is allowed to end up more than a predetermined distance, for example three meters, from the termination after the test.

Many methods have been developed to prevent an explosion of the cable insulation unit. For example, the end fittings, also named top and bottom fittings, of the insulation unit have been provided with one or more rupture discs to release the inner over pressure that builds up during the internal arc. The positions of the rupture discs are restricted to the end fittings. However, the most frequent location of the short circuit is at a distance from the end fittings. If the dissipated power in the arc is too large, the rupture discs cannot let out the overpressure fast enough due to their relative small area and distance from the internal arc.

US 3,931,451 discloses a high voltage apparatus having a voltage carrying part enclosed by housing. In order to prevent burn-off of the housing and the voltage carrying part in the event of accidental standing arcs, the wall of the insulation unit in the area most likely to be exposed to an accidental standing arc is provided with reinforcement against burn-off. In order to reduce the pressure increase inside the wall, the reinforcement is provided with means to relieve compressed gas pressure. The reinforced part of the tube is provided with a double wall includes an inner wall and an outer wall. The inner wall is made of a more burn-off resistant material than the outer wall, and a space is formed between the walls. The outer wall is, for example, made of aluminum and the inner wall is made of steel. The inner wall is provided with cutouts to relieve the gas pressure in the housing to the space between the inner and outer walls. However, this device cannot be used as an insulation unit since the housing is made of a conducting material.

Another solutions shown in JP1996-149667, is to arrange a heat and pressure resistant layer on the inside wall of the housing to prevent an escape of the hot insulating gases in the event of a casing melting. The layer is preferably made of an epoxy resin or glass fiber fabric. However, to be able to withstand an arc due to a short circuit in a high voltage apparatus, the heat and pressure resistant layer has to be made very thick.

An insulator unit is designed for a maximum service pressure. During normal operation of the termination, the pressure inside the insulator unit is not allowed to exceed the maximum service pressure. To be qualified, an insulator unit made of a composite material has to pass a certain qualifying test, which tests that the insulator unit can withstand a specified type test pressure without any visible damages. For example, a cable termination made of a composite material must be able to withstand 4 times the maximum service pressure for the termination.

Object and summary of the invention

One object of the present invention is to provide an improved insulator unit for a high voltage apparatus having a significantly reduced risk for explosion due to an internal short circuit, and thereby avoids debris from being spread over a large area. This object is achieved by an insulator unit for a high voltage apparatus as defined in claim 1.

The insulator unit comprises a tubular part made of a composite material including fibers arranged in different directions. The tubular part comprises an intermediate section provided with at least one weakening arranged along the length of the intermediate section, and designed to be able to withstand a specified type test pressure and to rupture at a certain overpressure above the specified type test pressure.

The weakening is designed to rupture, when the pressure inside the insulation unit has increased above a certain overpressure well above the specified type test pressure, thereby initiate a crack through the wall of the tubular part and along the length of the intermediate part, thus rapidly letting out at least a part of the built up overpressure through the crack. The crack is initiated by breaking at least some of the fibers in the weakening. Due to the different directions of the fibers in the composite, some of the fibers in the weakening will break and some of the fibers will still be undamaged, which makes it possible to evacuate the gases inside the insulator unit and at the same time prevent the tubular part from splitting into parts and to maintaining the main function of the insulator unit. Thus, no debris will escape from the insulator unit. Further, newly built up pressure from the arc is released successively through the rupture until the arc has been extinguished. Consequently, the overpressure in the insulator unit due to the arc is reduced and an explosion of the entire apparatus is avoided. Only the gases inside the insulator unit, is let out through the rupture. The tubular part will burst in a controlled manner and only the hot gases inside the insulator unit is let out through the rupture. The residual strength and stiffness of the tubular part, which is not damages, is able to withstand the rapture. The invention provides a controlled burst without debris. Thus, invention makes it possible to handle an internal arc in a much safer way.

The invention makes it possible to determine the length and position of the rupture in advance and thus to provide a rupture covering the area most likely to be exposed to an accidental standing arc. It is possible to design the weakening so that a much larger rupture occur, compared to the total area of the rupture discs, and at a position close to the position of the arc. Therefore, the overpressure above the specified type test pressure is released more rapidly.

A weakening is an area having a reduced wall thickness compared to the rest of the tubular part. Preferably, the wall thickness of the at least one weakening is 10 - 90% % of the thickness of the wall of the tubular part. More preferably, the wall thickness of the at least one weakening is 30 - 70% of the thickness of the wall of the tubular part.

The at least one weakening is arranged along the length of the intermediate section in order to achieve an elongated rupture covering the length of the intermediate section. For example, it is possible to have only one weakening covering the length of the intermediate section, for example, in the form of a helical groove. It is also possible to have a plurality of weakenings, which together covers the length of the intermediate section, for example, a plurality of parallel but longitudinally displaced grooves extending in the same direction as the longitudinal axis of the tubular part. It is also possible to reduce the thickness of the entire intermediate section. In that case, the intermediate section of the tubular part has a less wall thickness than the wall thickness of two end sections arranged on each side of the intermediate section. The diameter of the tubular part may vary over the length of the tubular part. Preferably, the length of the intermediate section is at least 50% of the largest diameter of the tubular part.

Preferably, the width of the at least one weakening is larger than 5mm, and less than 25% of the diameter of the tubular part in order to initiate a rupture to let the gases out.

According to an embodiment of the invention, the tubular part comprises two end sections without any weakening, arranged on each side of the intermediate part and connected to the end fittings. By providing end sections without any weakenings close to the end fittings, the tubular part is prevented from cracking along its entire length. Further, leakage of insulating medium between the tubular part and the end fittings is prevented. Preferably, each of said end sections is larger than 30 mm.

Preferably, the tubular part including the weakening is dimensioned to be able to withstand at least four times the maximum service pressure for the insulation unit, and to rupture at pressures above five times the maximum service pressure for the insulation unit.

Preferably, the weakening is a machined weakening. According to an embodiment of the invention, the material of the tubular part is a fiber enforced polymer. This material is suitable to machine in order to form a weakening that ruptures at a certain overpressure. Further, this material is strong enough to withstand the pressure and mechanical loads, even though weakening has ruptured. According to an embodiment of the invention, at least one of the end fittings is provided with one or more overpressure releasing unit, such as rupture discs and pressure relief valves. The over pressure releasing units in the end fitting cooperate with the at least one weakening in the tubular part to let out the overpressure, and accordingly the overpressure is let out faster.

According to an embodiment of the invention, the overpressure releasing unit is dimensioned to release at a lower overpressure than the weakening in the tubular part. This means that the overpressure releasing unit reduces the pressure if there is a small or medium overpressure in the tubular part due to the electric arc. The at least one weakening in the tubular part ruptures only at a high pressure which cannot be handled by the overpressure releasing unit. Thereby, the tubular part is kept unbroken at small or medium overpressure in the tubular part.

According to an embodiment of the invention, the at least one weakening comprises a groove with a flat bottom. Preferably, the width of the bottom is larger than 5mm, and less than 25% of the diameter of the tubular part. The flat bottom enables the weakening to rupture and initiate a crack with a certain width. The width of the bottom determines the width of the crack. The width and length of the crack determines how fast the overpressure is let out from the insulator unit.

The invention is particularly suitable for cable terminations. However, the invention can be used for many types of insulated high voltage apparatus, such as, insulation units, tap chargers, surge arrestors, breakers, instrument transformers, and bushings. The invention can be used for AC as well as DC apparatus.

Brief description of the drawings The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 shows an external view of an example of an insulator unit for a high voltage apparatus with a tubular part, two end fittings, and silicon sheds arranged along to the tubular part.

Fig. 2 shows a cross sectional view of the insulator unit as shown in figure 1.

Fig. 3 shows a cross sectional view of an insulator unit for a high voltage apparatus with a tubular part, two end fittings and overpressure releasing units arranged in one of the end fittings. Fig. 4 shows a perspective external view of the tubular part provided with a weakening according to a first embodiment of the invention.

Fig. 5 shows a perspective external view of the tubular part provided with a weakening in the form of a straight groove according to a second embodiment of the invention.

Fig. 6 shows a perspective external view of the tubular part provided with a weakening in the form of a helical groove according to a third embodiment of the invention.

Fig. 7 shows a perspective external view of the tubular part provided with a plurality weakening in the form of parallel and longitudinally displace grooves according to a fourth embodiment of the invention.

Fig. 8 shows a cross section through the tubular part shown in figure 5. Detailed description of preferred embodiments of the invention Figure 1 shows an external view of an example of an insulator unit 1 enclosing a voltage carrying part 4. Figure 2 shows a cross sectional view of the insulator unit shown in figure 1. The insulator unit 1 includes a tubular part 2, in the form of a hollow composite insulator, and two end fittings 3a^_b. In this embodiment, the insulator unit is a part of a cable termination and the voltage carrying part 4 is a cable. The tubular part and the two end fittings form an enclosure 6. The enclosure 6 is formed for housing an insulating medium, such as a field grading material, pressurized gas or oil. The insulating medium is used for insulation of the voltage carrying part. The Maximum Service Pressure (MSP) depends on the size of the high voltage apparatus and the type of the insulating medium, and is usually between 1 - 10 Bar. If the insulating medium is a solid material or oil, the specified pressure is significantly lower than if the insulating medium is a pressurized gas.

The function of the insulator unit is to control the electric field from the cable. The electric arc most likely to occurs in connection with a stress cone, which is located in the middle of the tubular part. The stress cone is arranged in order to reduce the electric field from the cable, which has been stripped from its outer layers. A large electric arc will produce a large amount of heat of about 20 000 degrees in Celsius, which gasifies everything in the near vicinity of the arc inside the tubular part and thus causes a large overpressure. Each of the two end fitting has an opening to receive the voltage carrying part 4 and includes a flange. In this embodiment, the two end fittings are made of aluminum. The tubular part is tubular shaped and has a wall. The tubular part might include of an inner tube 2 covered with an external housing 10. The material of the tubular part 2 and 10 is preferably an electrical insulator. The tubular part 2 is made of a composite material including reinforcing fibers, for example fiber reinforced polymer or a thermoplastic material including reinforcing fibers. The fiber is, for example, bundles of glass fiber, aramid fibers or basalt fibers. The glass fiber is preferable in this case. The polymer is, for example, an epoxy resin. The fiber bundles are arranged into an optimized a pattern of fibers which might be woven and wound. The fibers should be arranged in at least two different directions, preferably perpendicular to each other, as in a woven material. The external housing 10 is preferably made of silicon with sheads.

In this example, the diameter Dl of one side of the tubular part is larger than the diameter D2 of the other side. However, the invention can as well be used if the diameter is equal along the length of the tubular part. The larger diameter part extends to a certain distance from an end of the tubular part, and the diameter diminishes its size gradually at a certain interval to D2. The smaller diameter part continues to extend to other end. Both of the larger diameter part and the smaller diameter part of the tubular part are connected to the end fittings. The end fittings are hollow shaped. The end fitting connected to the larger diameter part has higher thickness than the other end fitting. The higher thickness end fitting part is hollow shaped. Silicon sheds on the housing 10 are disc shaped and provided along to the tubular part. The silicon sheds prolong the path travelled by the electric field and provides necessary creepage distance and protects the tube from the environment.

According to the invention, the tubular part comprises an intermediate section with a length L provided with one or more weakenings arranged along its length L. The intermediate section includes at least one weakening, but may include a number of weakenings. The one or more weakenings are designed to be able to withstand at least the specified type test pressure of the insolation unit and to rupture along its length at a certain overpressure, which is higher than the specified type test pressure. Figures 4 - 7 show different examples on how the weakenings can be arranged. The tubular part 2 is designed to able to withstand a specified type test pressure. The at least one weakening of the intermediate section is dimensioned to rupture at pressures above the specified type test pressure. The wall thickness of the at least one weakening is between 10 - 90%, preferably 30 - 70% of the wall thickness of the rest of the tubular part. For example, the wall thickness of the at least one weakening is about 50% of the wall thickness of the tubular part.

Figure 3 shows an insulator unit according to an embodiment of the invention including a tubular part 2 provided with one or more weakenings as previously described, and overpressure releasing units 30, such as rupture discs or over pressure valves, arranged in one of the end fittings 3a. Preferably, the overpressure releasing units are dimensioned to release at a lower overpressure than the weakening in the tubular part. Thus, the overpressure units are dimensioned to release at small and medium overpressure due to a small electric arc and the weakenings in the tubular part are designed to rupture at high overpressure due to a large electric arc. The overpressure releasing units in the end fittings prevent the tubular part from rupture at low and medium over pressures.

Figure 4 shows a perspective view of a tubular part 2a provided with a weakening according to a first embodiment of the invention. The tubular part includes an intermediate section 12a and two end sections 13a and 14a. The surface of the intermediate section 12a has been machined, for example by lathing, to reduce its thickness. The two end sections 13a and 14a have not been machined. Accordingly, the intermediate section 12a has a smaller wall thickness than the end sections 13a^_14a. The entire intermediate section is a weakening of the tubular part. The length 1 of the end sections is preferably larger than 30cm, normally about 50cm. The length L of the intermediate section may vary dependent on the type of the high voltage apparatus and the application. Figure 5 shows a perspective view of a tubular part 2b provided with a weakening according to a second embodiment of the invention. Fig. 8 shows a cross section through the tubular part shown in figure 5. The tubular part includes an intermediate section 12b and two end sections 13b and 14b. The surface of the intermediate section 12b has been provided with at least one weakening in the form of a groove 40. The groove is straight and extends along the length of the intermediate section 12b and is parallel with the longitudinal axis of the tubular part. In this embodiment the groove 40 is provided with a flat bottom, and the width of the groove is larger than 5mm and it is less than 25% of the largest diameter Dl of the tubular part. Fig. 5 shows only one straight groove on the intermediate section, however, a plurality of parallel straight grooves can be arranged on the intermediate section. In an alternative embodiment, the groove for example is designed with a round bottom. Figure 6 shows a perspective view of a tubular part 2c provided with a weakening according to a third embodiment of the invention. The tubular part 2c includes an intermediate section 12c and two end sections 13c and 14c. The surface of the intermediate section 12c has been provided with a weakening in the form of a helical groove 50. The helical groove 50 extends along the length of the intermediate section 12b at the same time as it winds around the longitudinal axis of the tubular part. The helical groove on the surface of the intermediate part has a certain ascent. The ascent of the helical groove may be synchronized to an ascent of the wound fiber strips forming the fiber reinforced material of the tubular part. However, it may be advantageous to have a different ascent compared to the ascent of the wound fiber strips, to achieve a tubular part that can withstand a larger pressure. The groove 50 may have the same cross sectional shape as the groove 40.

Figure 7 shows a perspective view of a tubular part 2d provided with a plurality of weakenings according to a fourth embodiment of the invention. The tubular part includes an intermediate section 12d and two end sections 13d and 14d. The surface of the intermediate section 12d has been provided with a plurality of weakening in the form of elongated and parallel grooves 60. The grooves are parallel with the longitudinal axis of the tubular part and are longitudinally displaced relative each other so that they cover the entire length of the intermediate part. The grooves 60 have the same cross sectional shape as the groove 40.

The silicon sheds 10 will be provided on the processed surface of the tubular part. The at least one weakening is arranged before the silicon sheds are provided.

The weakenings may be manufactured by machining of the surface of the tubular part, or by other methods such as by winding strips of a composite material in certain patters to achieve weakenings on the tubular part.

The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For example, the weakening can be designed in many other different shapes and numbers.

Claims

1. An insulator unit for a high voltage apparatus, the insulator unit (l) comprises a tubular part (2, 2a^_d) made of a composite material including fibers arranged in at least two different directions, two end fittings (3a^_b) connected to the tubular part, the tubular part and the end fittings form an enclosure (6) for enclosing a voltage carrying part (4) and for housing a medium for insulating the voltage carrying part, characterized in that the tubular part comprises an intermediate section (l2a^_d) provided with at least one weakening arranged along the length of the intermediate section, and designed to be able to withstand a specified type test pressure and to rupture at a certain overpressure above the specified type test pressure.

2. The insulator unit according to claim 1, wherein the tubular part (2, 2a^_d) comprises two end sections (l3a^_d, 14a^_d) without any weakening, the end sections are arranged on each side of the intermediate part (l2a^_d) and connected to the end fittings (3a^_b).

3. The insulator unit according to any of the claims 1 - 2, wherein the material of the tubular part (2, 2a^_d) is a fiber enforced polymer.

4. The insulator unit according to any of the claims 1 - 3, wherein said at least one weakening is dimensioned to withstand at least four times the maximum service pressure for the insulation unit, and to rupture at pressures above five times the maximum service pressure for the insulation unit.

5. The insulator unit according to any of the claims 1 - 4, wherein the wall thickness of the at least one weakening is 10 - 90%, preferably 30 - 70% of the wall thickness of the tubular part (2, 2a^_d).

6. The insulator unit according to any of the claims 1 - 5, wherein the length (L) of the intermediate section (l2a^_d) is at least 50% of the largest diameter of the tubular part (2, 2a^_d).

7. The insulator unit according any of the claims 1 - 6, wherein the width of the at least one weakening is larger than 5mm.

8. The insulator unit according any of the claims 1 - 6, wherein the width of the at least one weakening is less than 25% of the largest diameter of the tubular part (2, 2a^_d).

9. The insulator unit according to any of the claims 1 - 8, wherein the at least one weakening of the intermediate section (l2a^_d) is a machined weakening.

10. The insulator unit according to any of the claims 1 - 10, wherein at least one of the end fittings (3a^_b) is provided with an overpressure releasing unit (30).

11. The insulator unit according to claim 10, wherein the overpressure releasing unit (30) is dimensioned to release at a lower overpressure than the weakening in the tubular part (2, 2a^_d).

12. The insulator unit according to any of the claims 1 - 11, wherein the at least one weakening comprises a groove (40, 50, 60).

13. The insulator unit according to claim 12, wherein said groove (40, 50, 60) has a flat bottom and the width of the bottom is larger than 5mm, and less than 25% of the diameter of the tubular part (2, 2a^_d).

14. A high voltage apparatus comprising a voltage carrying part (4) and an insulator unit (l) according to any of the claims 1 - 16 enclosing the voltage carrying part.

15. The high voltage apparatus according to claim 14, wherein the high voltage apparatus is a cable termination.