WO2015158544A1

WO2015158544A1 - High voltage switching device with auxiliary nozzle

Info

Publication number: WO2015158544A1
Application number: PCT/EP2015/057018
Authority: WO
Inventors: Jadran Kostovic; Mahesh DHOTRE; Bernardo Galletti; Fredrik Lundqvist; Johan Costyson; Sami Kotilainen; Xiangyang Ye; Gunnar Andrae; Helmut Heiermeier; Timothy Sutherland
Original assignee: Abb Technology Ag
Priority date: 2014-04-15
Filing date: 2015-03-31
Publication date: 2015-10-22
Also published as: EP3132461A1; EP3132461B1

Abstract

The gas-insulated HV switching device comprises a contact arrangement with two arcing contacts, one of which being designed as a contact tulip (21), an auxiliary nozzle (40) enclosing at least partially the contact tulip (21) and a tubular electrostatic shield (42), which protrudes along an axis (A) beyond a free end of the contact tulip (21). In order improve the switching performance of the switching device the tubular electrostatic shield (42) is electrically connected to the contact tulip (21) and is integrated in an electrical circuit comprising a current limiter (resistors 43, 44) and extending from the shield (42) through the current limiter to an end of the contact tulip (21) which is arranged opposite its free end.

Description

High voltage switching device with auxiliary nozzle

Technical field

The invention relates to the field of high voltage (HV) switching technology and concerns a gas-insulated HV switching device according to the first part of the independent claim 1 , particularly for use as an earthing device, a fast- acting earthing device, a circuit breaker, a generator circuit breaker, a disconnector, a combined disconnector and earthing switch, or a load break switch in power transmission and distribution systems. The high voltage usually exceeds 1 kV, and typically concerns nominal voltages like 36, 123 or 365 kV.

Background

Switching devices are well known in the field of medium and high voltage switching applications. They are predominantly used for interrupting a current when an electrical fault occurs. As an example, circuit breakers have the task of opening contacts and keeping them far apart from one another in order to avoid a current flow, even in case of high electrical potential originating from the electrical fault itself. The electrical switching devices, like said circuit breaker, may have to be able to carry high nominal currents of 4000 A to 6300 A and to switch very high short circuit currents of 40 kA to 80 kA at very high voltages of 1 10 kV to 1200 kV. The operation principle of circuit breakers is known and will not be described here in detail.

As known, such electrical switching devices like circuit breakers comprise an arcing contact arrangement used for taking over the current during the opening and closing operation of the device. Amongst others, one type of circuit breakers uses a tulip-shaped arcing contact, comprising contact fingers arranged concentrically around a longitudinal axis of the circuit breaker in a so-called finger cage. This arcing configuration is called a contact tulip. The mating arcing contact is a rod or a tube which is inserted into the finger cage of the contact tulip during a closing operation of the switching device. An auxiliary nozzle which is made of a halogenated polymer, like polytetra- fluoroethylene (PTFE) encloses at least partially the contact tulip. A main nozzle partially encloses the mating arcing contact, which also partially encloses the auxiliary nozzle.

The contact tulip and the mating arcing contact are directly exposed to high electric field stress during switching operations. As known, during closing or opening operations of the switching device these two arcing contacts are exposed to electric arcs causing erosion of the contacts, thus reducing their capability to handle high electrical field strengths. Therefore, the contact tulip has to be partially shielded by the nominal contact. However, this creates a higher probability that a leader may propagate from the mating arcing contact into a heating channel formed by walls of the auxiliary and main nozzle and may cause puncturing of the main nozzle.

One of the circuit breaker's main design parameters is the minimum gas pressure required to interrupt the electric arc thermally. The thermal performance indicator di/dt-limit represents the maximum value of the current slope at current zero for which the arc can be extinguished under a given clearing pressure. A circuit breaker has better thermal interruption

performances than another one, if under the same blowing pressure it is capable of interrupting steeper currents, i.e. has a larger di/dt-limit. On the other hand, the larger the di/dt-limit of the breaker under the same blowing pressure conditions, the lower is the clearing pressure required to interrupt a given current. It is desirable to reduce the minimum clearing pressure required for interrupting electric arcs in order to avoid a too high pressure build-up and the resulting excessive mechanical stresses.

In known solutions the thermal interruption capability can be increased by both increasing the blowing pressure (brute force approach) and by profiling the main nozzle contours and/or the arcing contact contours in a way as to ensure efficient cooling of the gas. The latter approach relies strongly on the nozzle system that can change significantly from shot to shot, which may worsen the interruption capability of the circuit breaker.

Circuit breakers are designed in a way that the insulating and cooling gas is accelerated effectively in the nozzle system. Ideally, the flow between the insulating and auxiliary nozzle should reach sonic conditions on either side of the stagnation point at a comparatively short distance from it and then accelerate to supersonic speed. This flow pattern corresponds to an effective convective cooling of an arc and favors the interruption of the conductive path. Furthermore, it is normally well established for interrupters with nozzles which are as good as new, e.g. typically for the first two to three shots.

However, due to ablation, in the subsequent shots the nozzle cross-section changes and the sonic conditions shift, thereby worsening the convective cooling of the arc and consequently the thermal interruption performance of the circuit breaker. Therefore, the di/dt-limit for a given clearing pressure gets smaller shot after shot and the required clearing pressure must be increased.

A gas-insulated HV switching device of the type defined in the first part of claim 1 is described in EP 1 772 882. The switching device comprises a contact arrangement with two arcing contacts, one of which being designed as a contact tulip, an auxiliary nozzle, a main insulating nozzle and a nominal contact which partially encloses the contact tulip, the auxiliary and the main nozzle and which protrudes along an axis beyond a free end of the contact tulip. The nominal contact acts as a tubular electrostatic shield which screens the contact tulip against the strong electric field which in a current interruption process stresses a gap which is formed after disconnection of the two arcing contacts. The known switching device further comprises a conductive element which is inserted into a cavity of the auxiliary nozzle and which is kept on floating electric potential. The conductive element acts as capacitor and controls the distribution of the afore-mentioned electric field.

EP 1 544 881 describes a further gas-insulated HV switching device in which a conductive element is inserted into a cavity of a main insulating nozzle. The conductive element is kept on the potential of a contact tulip by means of a metallic screw connection which electrically connects the conductive element to the free end of the contact tulip and thus allowing an electric shielding action.

EP 0 081 253 describes a circuit breaker with an auxiliary nozzle as an annular body arranged between the arcing contacts and the nominal contacts. The annular body has a conductive ring at its narrowest location. In this way it is avoided that the electric arc forming during switching processes of the circuit breaker comes into contact with the auxiliary nozzle and deteriorates it.

Description of the invention

The objective of the invention is to improve the switching performance and the reliability of the known switching device.

This objective is achieved with a switching device defined in claim 1 .

According to claim 1 a gas-insulated HV switching device is provided with a contact arrangement extended along an axis and comprising an arcing contact designed as a contact tulip and a further arcing contact. The contact arrangement further comprises an auxiliary nozzle enclosing at least partially the contact tulip, a main insulating nozzle enclosing at least partially the auxiliary nozzle such that a heating channel is formed between the auxiliary and the main insulating nozzle and a tubular electrostatic shield which protrudes along the axis beyond a free end of the contact tulip. The tubular electrostatic shield is electrically connected to the contact tulip and is integrated in an electrical circuit comprising a current limiter and extending from the shield through the current limiter to an end of the contact tulip which is arranged opposite its free end.

The tubular electrostatic shield is kept on the potential of the contact tulip and surrounds the contact tulip with a small distance. Thus the contact tulip is almost completely electrostatically shielded against a strong electrical field which the recovery voltage builds-up in a current breaking process after current zero in a gap being bordered from the separated arcing contacts. Hence the formation of craters and peaks in the contact tube which the switching arc induces in the current breaking process are no more effective and do not reduce the dielectric recovery and thus the switching performance, the reliability and the endurance of the switching device. Since the

electrostatic shield is part of the auxiliary nozzle and surrounds the contact tulip and thus also the damages caused by arc erosion, like craters and peaks on the surface of the contact tulip, the contact tulip is shielded radially but also axially very effectively. Unavoidable wear of the contact tulip no more decreases the dielectric recovery of the switching device. Thus almost a minimum size of the gap ensures a safe breaking of the current and can be achieved without additional safety measures, like a high relative velocity of the arcing contacts during current breaking and/or a maximum size of the contact tulip.

Another advantage is the fact that the auxiliary nozzle together with the contact tulip can be placed further downstream with respect to a shielding of a the nominal contact. This results in a smaller probability of puncturing of the main nozzle due to the fact that a leader is more likely to be directed towards the tulip than into the heating channel, as mentioned above.

The electrical connection of the electrostatic shield to the contact tulip via the current limiter makes the same potential in the contact tulip and in the shield. In this way the electric field is advantageously shifted away from the contact tulip. The contact tulip is consequently subjected to lower electric field stresses. Furthermore, the current limiter keeps away any damaging current flow between the shield and the contact tulip as the case arises that the switching arc roots on the shield. The current limiter then builds up a resistance which is higher than the resistance in a current path with the switching arc rooting on the contact tulip. Thus the switching arc only temporarily roots on the shield and in a short time jumps to the contact tulip.

These advantages lead to an overall higher reliability of the switching device according to the invention. At least a section of the electrical circuit can form the current limiter which in a preferred embodiment of the invention can comprise a semi-conductive material, the specific resistance of which being at least 0,3 [kQ^> m].

The auxiliary nozzle in coaxial arrangement can comprise an insulating body and the tubular electrostatic shield.

If the tubular electrostatic shield is metallic an electrical connection between the shield and the contact tulip then can form the current limiter. Such a current limiter can be designed as resistor which is configured as a section of the insulating body and which can be supported on a carrier of the contact tulip. The current conductor can also be designed as a resistor which can be attached to the insulating body and which can be supported on the carrier of the contact tulip.

To avoid rooting of the switching arc on the auxiliary nozzle the shield can be entirely enclosed in the insulating body.

The material of the insulating body can be a halogenated polymeric material and the shield can be arranged on an internal surface of the insulating body and preferably can start downstream a throat on a conically enlarging portion of the auxiliary nozzle and can extend downstream to a tubular portion of the insulating body. The shield then can comprise a conductive layer which can be applied to the internal surface and which can comprise either a sprayed conductivity lacquer or a metallic material being deposited on the afore- etched internal surface. The shield can further comprises a tubular body of a metallic or semiconductive material which can be kept on the potential of the conductive layer and which can be deposited electrochemically or

mechanically on the conductive layer or which can be plugged in the insulating body. In order to protect the shield against the eroding effect of the switching arc an insulating coating can be applied to the shield or an insulating sleeve can be plugged in the insulating body. The halogenated polymer usually is a polytetrafluoroethylene (PTFE) but can also be another halogenated polymer used in HV technology, like

polytrifluoromonochloroethylene. The material of the shield can be metallic and can comprise aluminum, chromium, copper, iron, silver, tungsten etc. resp. can comprise alloys with one or more of the afore-listed metals, in particular alloys which resist switching arc erosion.

In order to save time and money in a method to manufacture a further embodiment of the switching device according to the invention the aforementioned metallic or semiconductive tubular body can comprise a

semiconductive hollow cylindrical insert which is plugged in an annular recess of the insulating body, and an electrically conducting annular part of the shield preferably can fill an undercut of the recess.

The tubular electrostatic shield can also be formed as a free end of the auxiliary nozzle, wherein the free end can be positioned adjacent to the heating channel or can be separated from the heating channel by an external portion of the insulating body.

The shield can be made of ablation-resistant material. Such a material additionally mitigates the decrease of the switching performance of the switching device since it keeps the optimal flow conditions by using ablation resistant materials in the body of the auxiliary nozzle. By reducing the extent of ablation, the pressure peak in the heating volume is reduced. On the other hand, the use of such a material in the auxiliary nozzle opposes the decrease of di/dt-limit for a given clearing pressure of the switching device during its use because the nozzle comprising non-ablating or ablation-resistant material keeps its shape. It is thus ensured that the section where the gas travels at sonic speed remains at the auxiliary nozzle in all arcing times or shots, thereby ensuring an efficient cooling of the arc. The thermal interruption performance is kept more stable throughout the shot sequence. Thus, no or much less worsening of the circuit breaker ratings is achieved. The invention also includes an auxiliary nozzle for an embodiment of the gas- insulated high voltage switching device according to the invention in which the auxiliary nozzle in coaxial arrangement comprises the insulating body, the tubular electrostatic shield and the current limiter, and in which the auxiliary nozzle further comprises a terminal for electrically connecting the electrostatic shield to an end of the contact tulip which is opposite its free end.

Short description of the drawings

Embodiments, advantages and applications of the invention result from the dependent claims and from the now following description by means of the figures. Thereby it is shown in:

Fig. 1 a side view on a section along an axis A of an arc extinguishing

chamber of a gas-insulated HV switching device according to the invention, in which the switching device above the axis is shown in the current making position and below the axis during current breaking,

Fig. 2 an enlargement of a contact tulip and an auxiliary nozzle of an

embodiment of the switching device according to fig . ,

Fig. 3 a view on a section along a line Ill-Ill through the auxiliary nozzle

according to fig .2, and

Fig. 4 an enlargement of a portion of a free end of an auxiliary nozzle used in a further embodiment of the switching device according to fig . , and

Figures 5 to 8 sectional side views of four further embodiments of the switching device according to the invention. Ways of carrying out the invention

The invention is described for embodiments in which the gas-insulated high voltage switching device is configured as circuit breaker. In the figures are shown in a schematized way the arc extinguishing chambers of five embodiments of the switching device according to the invention. However, the principles described in the following also apply for the usage of the invention in other switching devices, e.g. of the type mentioned at the beginning.

In the following same reference numerals denote structurally or functionally same elements of the embodiments of the invention.

The arc extinguishing chambers shown in the figures comprise an axial symmetry and are filled with an insulating gas with arc-extinguishing properties, like SF6, N2, CO2, air or mixtures of such gases with each other. Typical filling pressures are a few, typically 5 or 8 bar. Further filling gases may comprise an organofluorine compound being selected from the group consisting of: a fluoroether, an oxirane, a fluoroamine, a fluoroketone, a fluoroolefin, and mixtures and/or decomposition products thereof. Herein, the terms "fluoroether", "oxirane", "fluoroamine", "fluoroketone" and "fluoroolefin" refer to at least partially fluorinated compounds. In particular, the term "fluoroether" encompasses both fluoropolyethers (e.g. galden) and

fluoromonoethers as well as both hydrofluoroethers and perfluoroethers, the term "oxirane" encompasses both hydrofluorooxiranes and perfluorooxiranes, the term "fluoroamine" encompasses both hydrofluoroamines and

perfluoroamines, the term "fluoroketone" encompasses both

hydrofluoroketones and perfluoroketones, and the term "fluoroolefin" encompasses both hydrofluoroolefins and perfluoroolefins. It can thereby be preferred that the fluoroether, the oxirane, the fluoroamine and the

fluoroketone are fully fluorinated, i.e. perfluorinated.

In embodiments, the dielectric insulation medium is selected from the group consisting of: a (or several) hydrofluoroether(s), a (or several) perfluoro- ketone(s), a (or several) hydrofluoroolefin(s), and mixtures thereof. The dielectric insulation medium can further comprise a background gas or carrier gas different from the organofluorine compound (in particular different from the fluoroether, the oxirane, the fluoroamine, the fluoroketone and the fluoroolefin) and can in embodiments be selected from the group consisting of: air, N2, O2, CO2, a noble gas, H2; NO2, NO, N2O; fluorocarbons and in particular perfluorocarbons, such as CF₄; CF3I, SF6; and mixtures thereof.

Each arc extinguishing chamber comprises a housing 10 and a contact arrangement comprising two contact members 20 and 30 which are arranged in the housing 10 and can be moved relative to each other along an axis A. As shown in fig .1 a mechanical drive D can move the contact member 20 along the axis A, whereas the contact member 30 is mounted stationary.

The housing comprises a hollow cylindrical insulator 1 1 which on its left end supports a hollow metal body 12 and on its right end a hollow metal body 13. The hollow body 12 resp. 13 surrounds the contact member 20 resp. 30. Furthermore the hollow metal body 12 resp. 13 is electrical conductively connected to the contact member 20 resp. 30 and serve as one of the two current terminals of the contact assembly. Ends of the metal bodies 12, 13, which are arranged face to face, border an axially extended range of the insulator 1 1 which during an opening or closing operation has to withstand a strong dielectric stress. In the embodiment according to fig .1 the switching device is a dead tank breaker and thus comprises a gas-filled metal case which is connected to ground (not shown in fig . ). The metal case encloses the housing 10 in a gas-tight manner and receives exhaust gases which escape the housing 10 when the switching device breaks a current. Thus the before-mentioned ends of the hollow metal bodies 12, 13 are designed as stationary hollow shields 14, 15 and control the electric field in a gap T which during opening or closing the contact assembly is arranged between the free ends of the two contact members 20, 30. The shields 14, 15 can be eliminated if the circuit breaker is designed as a live-tank breaker.

The two contact members 20, 30 can be moved relative to one another along the axis A. Each contact member 20 resp. 30 comprises an arcing contact 21 resp. 31 and a nominal contact 22 resp. 32 which surrounds the

corresponding arcing contact 21 resp. 31 coaxially. The arcing contact 21 is designed as a contact tulip and comprises a plurality of contact fingers which can be arranged in a finger cage or which can be cut in a suitable sleeve. The contact tulip 21 and the nominal contact 22 in an electrical conducting manner are fixedly secured to a cylindrically designed contact carrier 23 of the contact member 20. A sliding contact 16 supported on the hollow metal body 12 in a current conducting manner connects the body 12 to the contact carrier 23 resp. to the arcing 21 resp. to the nominal contact 22. The arcing contact 31 is rod-shaped in this embodiment, but can also be designed as a contact tulip. The arcing contact 31 and the nominal contact 32 are in an electrical conducting manner connected to the hollow metal body 13. If required the arcing contact 31 can be arranged such that it can be moved axially within the stationary contact member 30.

The housing 10 further encloses an auxiliary nozzle 40 and a main insulating nozzle 50 which partly encloses the auxiliary nozzle. Both nozzles are manufactured of a halogenated polymer, typically on the basis of a

polytetrafluorethylene (PTFE). Both nozzles 40, 50 are fixedly secured to the contact carrier 23 and are kept in coaxial arrangement between the contact tulip 21 lying inside and the hollow nominal contact 22 lying outside. The auxiliary nozzle 40 surrounds the contact tulip 21 and protrudes beyond the free end of the same. The auxiliary nozzle 40 and the surrounding main nozzle 50 border an annular heating channel 61 which connects an arcing zone to a heating volume 60 or to a puffer chamber.

In the closed position of the circuit breaker (shown in fig .1 above the axis A) the two contact members 20, 30 contact one another, wherein the contact tulip 21 receives a head of the arcing contact 31 which is shaped as a plug.

In order to break a current the contact members 20, 30 are disconnected by means of the drive D. After the separation of the nominal contacts 22, 32 the current to be interrupted commutates into a electrical circuit which includes the arcing contacts 21 , 31 . When the commutation of the current is finished, the arcing contacts 21 , 31 separate and a switching arc S is struck between the contact tulip 21 and the head of the plug-shaped arcing contact 22 (shown in fig.1 below the axis). The arc S generates pressurized arcing gases which via heating channel 61 flow into the heating volume 60 in which they are stored as arc extinguishing gas. As soon as the plug head of the arcing contact 31 releases the heating channel 61 the stored arc extinguishing gas - where necessary additionally pressurized by means of a puffer device - passes the arcing zone and afterwards a throat 45 of the auxiliary nozzle 40 as arc extinguishing gas flow P (only shown in fig.2) and blows the arc S beyond current zero until the current is interrupted.

At current zero the current is interrupted and charge carriers which the switching arc S had generated are widely removed from the gap T. After the interruption of the current the recovery voltage is applied to the gap T and stresses the gap and the nozzles 40 and 50 dielectrically. Erosion of the free end of the contact tulip 21 increases the dielectric stress considerably and thus reduce the dielectric recovery of the switching device. A root of the switching arc S favors the erosion and thus the formation of undesired craters and peaks. in the surface of the contact tulip 21 .

Figures 2 and 3 show an embodiment of the auxiliary nozzle 40 which considerably amends the dielectric recovery of the switching device according to the invention. The auxiliary nozzle in coaxial arrangement comprises an insulating body 41 and a tubular shield 42 which protrudes along the axis A beyond a free end of the contact tulip 21 . The shield 42 is electrically connected to the contact tulip 21 and is integrated in an electrical circuit which comprises a current limiter and which extends from a section of the shield 42, which protrudes the free end of the contact tulip 21 , across the current limiter to an end of the contact tulip 21 which is arranged opposite its free end. The shield 42 is part of the auxiliary nozzle 40 and surrounds - as shown in fig.2 - the contact tulip 21 and thus also damages caused by arc erosion, like craters and peaks on the surface of the contact tulip 21 . Thus the contact tulip 21 is screened radially but also axially very effectively. Unavoidable wear of the contact tulip 21 no more decreases the dielectric recovery of the switching device. Thus a minimum size of the gap T, which ensures a safe breaking of the current, can be achieved without additional safety measures, like a high relative velocity of the contact tulip 21 and the arcing contact 31 during current breaking and/or like a maximum size of the contact tulip 21 . Usually the thickness of the shield 42 is only a few 100 μιτι. For reason of its small thickness and its arrangement in the auxiliary nozzle 40 the space

requirements of the shield 42 are negligible. Thus the auxiliary nozzle 40 can replace the auxiliary nozzle of a switching device according to prior art and thus can improve the dielectric recovery of the switching device without any additional place requirements.

The auxiliary nozzle 40 of the switching device according to figures 1 and 2 comprises a metallic shield 42. The electrical connection between the shield 42 and the contact tulip 21 forms the current limiter. The current limiter can be configured as a resistor 43 resp. 44.

The resistor 43 is configured as a section of the insulating body 41 and then can be supported on the carrier 23 of the contact tulip 21 (embodiment according to fig.2 above the axis A). An end of the resistor 43 then forms a current terminal 47 of the shield 42 which ensures that the shield 42 and the resistor 43 are electrically connected to the contact carrier 23 and thus to the contact tulip 21 .

The resistor 44 is configured as a resistor modul and is set in an electrically conducting manner on an end of the insulating body 41 and is supported on the carrier 23 of the contact tulip 21 (embodiment according to fig.2 below the axis A). The end of the resistor 44 which is positioned on the contact carrier 23 forms the current terminal 47 of the shield 42 which ensures that the shield 42 and the resistor 44 are electrically connected to the contact carrier 23 and thus to the contact tulip 21 .

Each resistor 43 resp. 44 comprises an electrically conducting polymer, in particular a PTFE having a specific resistance of at least 0,3 Dm, and limits a current through the shield 42 which can be fed during current breaking with charge carriers generated from the switching arc S.

The shield 42 is arranged on an internal surface of the insulating body 41 and preferably starts downstream a throat 45 of the auxiliary nozzle, which the arc extinguishing gas flow P passes, that means on a conically enlarging portion of the auxiliary nozzle 40. The shield 42 extends downstream to the resistor 44 resp. to a tubular portion of the insulating body 23 which comprises the resistor 43. The resistor 43 resp. 44 is integrated in an electrical circuit which extends from the shield 42 to an end of the contact tulip 21 which is arranged opposite its arc-exposed free end. The design of the electrical circuit and the resistor 43 resp. 44 ensure that the root of the switching arc S is kept away resp. is quickly removed from the shield 42 and is kept on the free end or within the interior of the contact tulip.

Halogenated polymers, like PTFE, comprise an anti-adhesive surface. Thus a portion of the internal surface of the insulating body 42 which holds the shield 42 comprises a roughened and adhesive surface structure. Such a structure can be manufactured using a known reduction method in which the a portion of the anti-adhesive surface of the PTFE is etched with a reducing agent, in particular an alkali metal, like natrium. The reducing agent is applied to the anti-adhesive surface of the insulating body 42 either as vapour (US

4,885,018 A), as solution (US 5,874,154 A) or as solid (US 201 1/0236567 A1 ). The roughened, anti-adhesive surface can be coated currentless with copper and then forms a conductive layer 42a. The conductive layer is shown in fig.3 and can be used as shield 42. In order to improve the electrical and mechanical properties of such an embodiment of the shield 42 an additional conductive layer 42b can be deposited in an electroplating process, in which for example an additional copper layer with a thickness of a few 100 μιτι can be deposited from an acid copper solution.

The conductive layer 42a can also be manufactured in a spray or paint process in which a lacquer comprising a metal or semiconductor, like a lubricant on the basis of M0S2, is deposited as a layer on the internal surface of the insulating body 41 . After drying the conductive layer 42a can be used as shield 42. In order to improve the electrical and mechanical properties of such a shield an additional conductive layer 42b can be deposited in an electroplating process on the layer 42a. Furthermore the additional conductive layer 42b can be configured as tubular sleeve of a metallic or semiconductive material which is plugged in the insulating body 41 and which contacts the layer 42a in order to keep the sleeve 42b on the potential of the conductive layer 42a.

In order to protect the shield 42 against the switching arc S an insulating layer 46 is applied to the shield 42. The layer 46 typically comprises a halogenated polymer, like PTFE, and can be deposited in a sputtering process as described in US 201 1/0236567 A1 . The protection can also be achieved with an insulating sleeve which is plugged in the insulating body 41 and which covers the shield 42 instead of or additional to the insulating layer 46. The thickness of the insulating layer 46 resp. of the insulating sleeve can be some millimeters, typically 1 to 5 millimeters.

In contrast to the embodiment according to figures 2 and 3 the additional conducting layer 42b of the embodiment according to fig.4 comprises a semiconductive hollow cylindrical insert 70 made of an electrically conductive halogenated polymer, preferably a PTFE filled with an electrically conducting powder, like carbon black. An annular section 71 of the insert 70 which protrudes the free end of the contact tulip 21 (shown in fig . ) fills a recess 72 cut in the insulating body 41 . The section 71 is configured in a manner to form together with the stationary hollow shield 14 and the nominal contact 22 (only shown in fig . ) an equipotential surface which in the open position of the switching device is largely spherical. In order to facilitate the manufacture of the auxiliary nozzle 40 two undercuts of the recess 72 are each filled with an electrical conducting annular part 73 resp. 74 of the shield section 71 in a manner to form a recess with coaxially arranged and axially extended faces 75, 76. The parts 73, 74 are electrically connected to the layer 42a and each comprise a convex profile which is designed in a manner to improve the screen function of the insert 70. The conducting layer 41 a covers the recess 72 and electrically contacts the insert 70 when in a manufacturing process the insert 70 is plugged in the recess comprising the faces 75, 76. The thickness of the additional conducting layer 42b can be some millimeters, typically 1 to 5 millimeters.

Fig. 5 and the following figures show sectional views of further embodiments of the switching device according to the invention. Also these embodiments of the switching device are rotationally symmetric around the longitudinal axis A and only an upper half of the sectional views is shown. Only the elements of the switching devices, in the following referred to as circuit breakers, which are related to the present invention will be described in the following, whereas other elements, e.g. nominal contacts, enclosure, etc. are not shown in the figures.

The contact tulip 21 comprises multiple fingers arranged in a finger cage. The arcing contact 31 is rod-shaped in this embodiment. For the sake of simplicity, only one finger of the contact tulip 21 shown in Fig. 5 and the following figures.

The auxiliary nozzle 40 is arranged partly around the contact tulip 21 . In other words, the auxiliary nozzle 40 encloses the contact tulip 21 concentrically and protrudes beyond it, as can be seen in the figure. Accordingly, the arcing contact 31 is enclosed by the main nozzle 50, which partly encloses the auxiliary nozzle 40. A purpose of the auxiliary nozzle 40 and the main nozzle 50 is to form a path, here the heating channel 61 , for guiding the insulating fluid into and out of the before-described arcing zone Z, which in fig.6 and the following figures is denoted with the reference numeral Z.

In other words, the arcing contact 31 is enclosed by the main nozzle 50, which partly encloses the auxiliary nozzle 40 such that the heating channel 61 is formed between the main nozzle 50 and the auxiliary nozzle 40, which channel 61 fluidly connects the arcing zone Z of the circuit breaker with the heating volume 60 resp. a puffer chamber of the switching device. As mentioned analogously before the arcing zone Z is a region in which the contact tulip 21 is moved back and forth for closing or opening the circuit breaker and in which the switching arc S develops between contact tulip 21 and the arcing contact 31 during an opening and closing procedure, thereby heating up the fluid located in the arcing zone Z.

The auxiliary nozzle 40 comprises inside its body the shield 42. The shield 42 is electrically connected to the contact tulip 21 . The material of the shield 42 is conductive or is semiconductive. Particularly, the shield is made of a material which is similar to the surrounding auxiliary nozzle 40, which typically is made of a PTFE. An appropriate material is a PTFE- carbon compound. This has the advantage that the semiconducting shield 42 has similar mechanical properties like the surrounding material of the auxiliary nozzle 40.

In this embodiment, the shield 42 is fixedly embedded in the body of the auxiliary nozzle 40 and is entirely enclosed by the auxiliary nozzle 40.

However, in another embodiment the shield 42 is a distinct element insertable into the body of the auxiliary nozzle 40.

The shield 40 is electrically connected to the contact tulip 21 by a

semiconductor ceramic or metal connection 48 having a predefined electrical resistance and acting as the afore-described current limiter.

The shape of the shield 42 is shown only exemplarily in Fig. 5. It is chosen depending on the dimensions of the auxiliary nozzle 40 and is chosen in such a way that electric field stresses are minimal.

Depending on its shape, the shield 42 may be inserted into the auxiliary nozzle 40 as an independent element or it may form a part of the auxiliary nozzle 40, as mentioned above. In the second case the shield 42 may be manufactured first and the auxiliary nozzle 40 may be formed around it in known ways, e.g. by injection moulding of polytetrafluoroethylene.

Preferably, the shield 42 is elongated with respect to the axis A, and in particular has a tubular or substantially tubular shape concentric to the axis. By using a shield 42 with said shape the shielding of the contact tulip 21 is improved and thus the electrical field strength on the side of the contact tulip 42 does not depend on the condition of its surface. Instead, the electrical field strength on the side of the contact tulip is given by the electrical field strength of the shield 42.

Different to the embodiment according to fig.5 in the embodiment according to fig .6 the auxiliary nozzle 40 comprises a shield 42 which is adjacent to the heating channel 61 and which consists of ablation-resistant material. The term "ablation-resistant" is understood as referring to a material exhibiting a minimum mechanical ablation in the presence of a switching arc, as compared to other available conductive or semiconducting materials, particularly as compared to nozzles made of PTFE. It encompasses also non- ablating materials. The shield 42 is electrically connected to the contact tulip 21 . The material of the shield 42 is conductive or semi-conductive.

Particularly, the material of the shield comprises metal, e.g. tungsten or wolfram or steel, and/or a ceramic semiconductor. It may also comprise or be made of PTFE with an appropriate filler. In this embodiment the shield 42 is only formed by the front part of the auxiliary nozzle 40. The rest of the auxiliary nozzle 40 comprises the insulating body 41 , usually PTFE.

The shield 42 is electrically connected to the contact tulip 21 by a

semiconductor ceramic or metal connection 48. In this embodiment the connection 48 is formed by a line embedded in the insulating body 41 .

Different to the embodiment according to fig.6 the auxiliary nozzle 40 of the embodiment according to fig.7 is made of ablation-resistant material. In this case the material comprises a semiconducting ceramic and/or a metal, which ensure the ablation-resistance and the limitation of a possible current from the auxiliary nozzle 40 to an end of the contact tulip 21 which is opposite its free shielded end. The connection 48 between the auxiliary nozzle 40 and the contact tulip 21 is made of metal or ceramic. In both cases the semiconductor ceramic or metal connection 48 is dimensioned to have a pre-defined electrical resistance in order to avoid high current flow through the auxiliary nozzle 40. For example, TIG-welding processed ceramic material can be used for this connection 48.

Fig. 8 shows yet another embodiment of the switching device according to the invention. This embodiment only differs from the embodiment according to fig.7 in that the shield 42 is formed by the free end of the auxiliary nozzle 40 and is separated from the heating channel 61 by the insulating material of the body 41 .

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may otherwise variously be embodied and practised within the scope of the following claims. Therefore, terms like "preferred" or "in particular" or "particularly" or "advantageously", etc. signify optional and exemplary embodiments only.

The electrical switching device is preferably used as an earthing device, a fast-acting earthing device, a circuit breaker, a generator circuit breaker, a disconnector, a combined disconnector and earthing switch, or a load break switch.

List of reference numerals

10 housing

1 1 hollow cylindrical insulator

12, 13 hollow metal bodies,

14, 15 hollow shields,

16 sliding contact,

20 contact member

21 arcing contact, contact tulip

22 nominal contact

23 contact carrier

30 contact member

31 arcing contact

32 nominal contact

40 auxiliary nozzle

41 insulating body

42 shield

42a, 42b conductive layers

43, 44 resistors

45 throat

46 insulating layer

47 current terminal

48 electrical connection

50 main insulating nozzle

60 heating volume

61 heating channel

70 semiconductive hollow cylindrical insert

71 annular section of the shield 42

72 recess

73, 74 parts of the annular section 71

75, 76 axially extended faces of a recess

A axis

D drive

P arc extinguishing arc flow

S switching arc

T gap

Z arcing zone

Claims

1 . A gas-insulated high voltage switching device with a contact

arrangement extended along an axis (A) comprising:

a first arcing contact designed as a contact tulip (21 ) and a second arcing contact (31 ),

an auxiliary nozzle (40) enclosing at least partially the contact tulip (21 ), a main insulating nozzle (50) enclosing at least partially the auxiliary nozzle (40) such that a heating channel (61 ) is formed between the auxiliary (40) and the main insulating nozzle (50), and

a tubular electrostatic shield (42) which protrudes along the axis (A) beyond a free end of the contact tulip (21 ),

characterized in that the shield (42) is electrically connected to the contact tulip (21 ) and is integrated in an electrical circuit comprising a current limiter and extending from the shield (42) through the current limiter to an end of the contact tulip (21 ) which is arranged opposite its free end.

2. The switching device according to claim 1 , wherein at least a section of the electrical circuit forms the current limiter.

3. The switching device according to claim 2, wherein the current limiter comprises a semi-conductive material.

4. The switching device according to claim 3, wherein the specific

resistance of the semi-conductive material is at least 0,3 [kQ ^> m].

5. The switching device according to one of claims 2 to 4, wherein the shield (42) is metallic, and wherein an electrical connection (48) between the shield (42) and the contact tulip (21 ) forms the current limiter.

6. The switching device according to one of claims 1 to claim 5, wherein the auxiliary nozzle (40) in coaxial arrangement comprises an insulating body (41 ) and the tubular electrostatic shield (42).

7. The switching device according to claim 6, wherein the current limiter is a resistor (43) which is configured as a section of the insulating body (41 ) and which is supported on a carrier (23) of the contact tulip (21 ).

8. The switching device according to claim 6, wherein the current limiter is a resistor (44) which is attached to the insulating body (41 ) and which is supported on a carrier (23) of the contact tulip (21 ).

9. The switching device according to one of claims 6 to 8, wherein the shield (42) is entirely enclosed in the insulating body (41 ).

10. The switching device according to one of claims 6 to 9, wherein the material of the insulating body (41 ) is a halogenated polymeric material, and wherein the shield (42) is arranged on an internal surface of the insulating body (41 ) and preferably starts downstream a throat (45) on a conically enlarging portion of the auxiliary nozzle (40) and extends downstream to a tubular portion of the insulating body (41 ).

1 1 . The switching device according to claim 10, wherein the shield (42) comprises a conductive layer (42a) which is applied to the internal surface and which either comprises a sprayed conductivity lacquer or a metallic material being deposited on the afore-etched internal surface.

12. The switching device according to claim 1 1 , wherein the shield (42) further comprises a tubular body (42b) of a metallic or semiconductive material which is kept on the potential of the conductive layer (42a), and which is deposited electrochemically or mechanically on the conductive layer (42a) or which is plugged in the insulating body (41 ).

13. The switching device according to claim 12, wherein the tubular body (42b) comprises a semiconductive hollow cylindrical insert (70) which is plugged in an annular recess (72) of the insulating body (41 ), and wherein preferably an electrically conducting annular part (73, 74) of the shield (42) fills an undercut of the recess (72).

14. The switching device according to one of claims 6 to 13, wherein an insulating coating (46) is applied to the shield (42) and/or wherein an insulating sleeve is plugged in the insulating body (41 ).

15. The switching device according to one of claims 6 to 14, wherein the shield (42) is formed as a free end of the auxiliary nozzle (40) and wherein the free end is adjacent to the heating channel (61 ) or is separated from the heating channel (61 ) by an external portion of the insulating body (41 ).

16. The switching device according to claim 15, wherein at least the shield (42) is made of ablation-resistant material.

17. An auxiliary nozzle (40) for the gas-insulated high voltage switching device according to one claims 6 to 16, wherein the auxiliary nozzle (40) in coaxial arrangement comprises the insulating body (41 ), the tubular electrostatic shield (42) and the current limiter, and

wherein the auxiliary nozzle (40) further comprises a terminal (47) for electrically connecting the electrostatic shield (42) to an end of the contact tulip (21 ) which is opposite its free end.