WO1996019816A1

WO1996019816A1 - Electric switching device

Info

Publication number: WO1996019816A1
Application number: PCT/SE1995/001526
Authority: WO
Inventors: Gert Bjarnholt; Anders Björklund; Henrik Breder; Lars-Gunnar Dahlberg; Göran Engdahl; Hans-Olof Kalldin; Mats Leijon; Ming Li; Per Nordlund; Jan-Anders Nygren; Stefan Valdemarsson
Original assignee: Asea Brown Boveri AB
Current assignee: ABB AB
Priority date: 1994-12-22
Filing date: 1995-12-18
Publication date: 1996-06-27
Anticipated expiration: 1997-06-22
Also published as: SE9404455D0; SE9404455L

Abstract

An electric switching device for achieving a rapid simultaneous electric closing of at least three conductors (13) intended for high current and high voltage. The switching device comprises a container (3) which is limited by two plates (1, 2) and a cylindrical casing (4). The container (3) is filled with gaseous insulating medium and encloses an electrically insulating fire tube (7). Insulators (6) are connected to the casing (4), through which insulators the conductors (13) are connected to contacts (17) arranged in the fire tube (7). A missile (10) of conductive material rests at one end of the fire tube (7) and a damping block (11) is arranged at the other end thereof. By the compressive force from an explosive charge (20), the missile assumes a contact position in which it is wedged in between the contacts (17).

Description

Electric switching device

TECHNICAL FIELD

The present invention relates to an electric switching device, and more particularly a high-speed circuit closer, for achieving a rapid mechanical and electrical short circuiting of a multi-phase network. The switching device is preferably intended to be used as arc eliminator in cubicle-enclosed switchgear for the low voltage and medium voltage range up to about 40 kV, but also other fields of use are feasible. The switching device is intended to handle high currents, for example in the range of up to about 50 kA.

BACKGROUND ART

The safety in switchgear is not only dependent on the contents of a specified standard. It is also highly dependent on the capacity of the switchgear to eliminate or minimize the damage caused by a short-circuit arc.

In switchgear the electric current is conducted in an uninsu¬ lated, so-called blank busbar trunking system. This is done for economical reasons since insulation is costly. In case of a fault, an arc may arise which, if it is not quickly extinguished, may cause very great damage. Such faults may arise, for example, in case of bad connection, fire, or a short circuit caused by tools left behind, or by animals. T:. extent of the damage is highly dependent on both the currerr intensity and the arc duration. An arc develops, in a short time, a very large amount of energy in the form of extreme heat and light intensity. The heat in the core of the arc approaches 20,0000°C, which means that steel and copper not only melt but vaporize. This leads to a pressure wave of hot gases. In a switchgear unit with limited space, a pressure increase arises which reaches its maximum after a very short time. For cubicle-enclosed switchgear, the pressure maximum is reached after about 10 s. If the arc duration is longer than this time, such a pressure wave may completely "blow out" a switchgear unit . The gas volume is dependent on the arc duration and the gases, besides being hot, are also poisonous and thus harmful to inhale. For these reasons, cubicle- enclosed switchgear units are equipped with evacuation channels, so-called pressure-relief means, which entail both increased volume and increased cost of production of the switchgear.

At present, many types of switchgear are monitored by short- circuit monitors and arc guards. An arc guard may consist of a photocell, which, when an arc arises, gives a signal which triggers a circuit-breaker placed on the line ahead of the switchgear. With such an arrangement, the arc duration may be limited to about 70 ms. With such long arc durations, the pressure increase m switchgear has already reached its maximum and the switchgear must therefore be dimensioned and equipped to withstand the pressure wave. The hot gases and the intense light may also result in ignition of combustible organic material.

From the patent document SE 326 755, a switching device for high voltage is known, which utilizes an explosive charge for removing a latch, whereby a pneumatically driven piston drives a movable contact to make contact with a stationary contact The Sv itching device does not utilize the fast action of the e>plosive charge but relies on the pneumatically driven piston to rapidly cause a closing operation. A problem with this solution is the relatively large mass in the piston and the contact device which entails too long a closing time. A plurality of other solutions are known wherein a piston running m a cylinder is utilized for bringing two contacts to close or break via a linkage system. Such a known circuit breaker with both a closing and a breaking function is described in the patent specification US 4 358 648. In this circuit breaker, a piston is driven by a powder charge, via a linkage system, to cause breaking or closing of an operating device. Also in this case, the mass inertia of the piston and the linkage system indicates that closing is performed only after a relatively long time. From German published patent application 2 113 798, A high- voltage circuit closer is previously known, which utilizes an explosive charge to briefly achieve contact between two con¬ tacts. The explosive charge drives a flow of ionized gas past the contacts, whereby inside the circuit closer an arc arises, which is capable of carrying current between the contacts. US 5 006 679 describes a similar circuit closer, which also utilizes ionized gas from an explosive charge to bring about contact between two contacts via an arc. A problem with these two inventions is that they can only cause closing between two conductors for a short time, whereafter the contact is again broken.

From the patent document US 4 326 167, equipment for high- voltage testing is known, which comprises an explosion-driven circuit closer. This circuit closer utilizes a gas, suddenly produced from an explosion, with a high pressure to drive a contact piece from a rest position to a closed position within a sufficiently short space of time. One problem with this circuit closer is that it is capable of bringing about contact between two conductors only. Another problem is that the contact piece lacks guiding, which means that the closer described must rely on the explosive gases to develop uni¬ formly when the contact piece is driven towards the closed position. An unsymmetrical combustion of the explosive charge may impart rotational force to the contact piece when it leaves the rest position, whereby the contact piece may miss the closed position.

Within the low-voltage range, a plurality of explosion-driven circuit closers are known, for example from patent specifica¬ tions GB 1360921 and US 4150266. The latter describes a miniature circuit closer intended to operate in a missile. A piston resting in a cylindrical shell, and the front of which consists of an insulating cover enclosing a mass of conductive material, is brought by the force from an explosive charge to move from its rest position towards two contacts extending into the end of the tube, causing these contacts to penetrate through the insulating cover and bringing them into electrical contact via the electrically conducting mass. The miniature circuit closer has very small dimensions and is intended for low voltage only. By a scaling of the miniature circuit closer, insulation distances between the conductors and between the casing and the respective conductor may be made sufficiently large. The circuit closer, thus scaled, would result m the piston with the closing medium receiving such dimensions and such a weight that the rapidity would be injuriously deteriorated. An additional problem with the circuit closer described is that its electrically conductive mass cannot transfer high currents. For this, the closing medium must have extremely little electrical resistance and, m addition, be able rapidly to carry off heat.

SUMMARY OF THE INVENTION

The invention aims to provide an electric switching device, a so-called high-speed circuit closer, which is capable very rapidly to bring about electrical contact between several conductors and, m particular, to connect three phases to ground. The closing must be reliable and permanent. The high¬ speed circuit closer shall manage both high voltage and high current. It needs to function only once. This is achieved according to the invention by an explosion circuit closer, whicn ma<es use of an explosive force to fire away a missile whicr. closes a plurality of circuits simultaneously. The explosive circuit closer comprises a closed container filled witn a gaseous insulating medium, through which container a plurality of conductors, via wedge-shaped contacts, are connected to a fire tube placed centrally in the container. At one end of the fire tube, the missile rests, and at the other end tnereof, an energy-absorbing damping block rests. The fire tube is made of an electrically insulating material. The contacts penetrate through the mid-portion of the tube m a normal plane to the tube, whereby those contact surfaces of the contacts which are inclined towards the missile are exposed on the inside of the tube. The missile is made of an electrically conductive material with good thermal conducti¬ vity , for example of metal. By the force from an explosive charge, the missile is brought to be fired against the con¬ tacts exposed in the fire tube. In the contact position, the missile is slowed down, whereby its kinetic energy is absorbed by the damper as well as by the plasticizmg work which is 5 performed when the missile is wedged between the contacts. The wedged position brings about a good electrical connection between the missile and the respective contact.

A high-speed circuit closer according to the invention is 10 especially suitable for use as arc eliminator in air-msulated switchgear, since it may be designed with such a short closing time that the overpressure generated during a short-circuit arc is limited to a level harmless for the enclosure. In this wa^*> , the switchgear need not be provided with any means for 15 pressure relief

The closing time, that is, the time it takes from the detec¬ tion of an arc until closing occurs, may be divided into reaction time and action time. The reaction time comprises the

2C time from the discovery until the contact piece of the circuit closer starts moving. The action time is the time during which the contact piece moves to the contact position. For the pur¬ pose of shortening the closing time, both the reaction time and tne action time must be minimized. The reaction time co -

2^C prises fie detection time for a sensor, the signal transport, the e-.plosion process and the build-up of the gas pressure. By utilizing semiconductor technology in the sensor and allowing tne signal to consist of electric or light pulses, the first part or the reaction time may be minimized. A fast explosion ⁿ process may be achieved by a suitable choice of explosive material and a rapid build-up of the gas pressure by making the explosion volume small. The action time may be minimized by reducing the mass of the contact piece, by reducing its path of propagation and by imparting to the contact the

35 greatest possible acceleration. This is achieved by making the diameter of the fire tube small, whereby the mass of the missile becomes small and the distance to the contacts may be made small. To give the missile the greatest possible accele- ratior, the pressure should permanently increase m the enclosure behind the missile. This is achieved by the explo¬ sive charge, through which a constantly increasing gas pressure is obtained. A pressure build-up by connecting, via a valve means, the enclosure to a vessel containing a gas under a high pressure is less effective than a gas expansion from an explosive charge. For one thing, the valve itself constitutes a resistor, and for another, the gas pressure drops when the gas is distributed between the vessel and the volume- increasing enclosure behind the missile.

10

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by descrip¬ tion of an embodiment with reference to the accompanying ι drawings , wherein

Figure 1 shows a section of a high-speed circuit closer according to the invention,

20 Figure 2 shows a cross section of the high-speed circuit closer along the dash-dotted line II-II in Figure 1.

Figure 3 shows a section of an alternative embodiment of the 2Ξ high-speed circuit closer in open position, and

Figure 4 shows the alternatively designed high-speed circuit closer in closed position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A high-speed circuit closer according to Figures 1 and 2 comprises a fire tube 7 in a container 3 with a gaseous insu¬ lating medium under overpressure built up between a lower 35 plate 1 and an upper plate 2 parallel thereto. The container 3 comprises a cylindrical shell or casing 4, which together v/ith the lower plate 1 and the upper plate 2 limits the volume of the container. The casing 4 comprises four plane portions 5 where electric insulators 6 may be connected. The plates and the casing are made of electrically conductive material. The fire tube 7 is made of electrically insulating material, for example glass-fibre reinforced epoxy resin, and is fixed in the centre of the container between the lower plate 1 and the upper plate 2. The fire tube 7 is substantially circular and encloses in its lower end 8 a missile 10 and in its upper end 9 an energy-absorbing damping block 11. The tube is filled with a gaseous insulating medium which communicate with the gas in the container through a number of passageways 12.

Insulators 6 are connected to three of the plane portions of the casing. Through each insulator runs a conductor 13, the outer end 14 of which is intended to be connected to one of the phase conductors in the multi-phase network which is to be short-circuited. The inner end 15 of the conductor is connec¬ ted, via a joining piece 16, to a contact 17 fixed to the fire tube 7. At the fourth of the plane portions 5 of the casing, a grounding conductor 18 is connected, which is electrically connected to the container 3. The grounding conductor 18 is connected, via a joining piece 16, to a contact 17 fixed to the fire tube. Each contact 17 projects into the fire tube and has a wedge-shaped inner part, the tip of which is directed along the fire tube 7 in a direction towards the missile 10, whereby the wedge-shaped part is given a contact surface 19 inclined towards the missile. The four contacts 17 are rotationally-symmetrically placed in a normal plane to the fire tube.

The missile 10 is made of an electrically conductive material, for example copper, and has substantially a cylindrical shape with a conical front portion. The missile is designed with such a fit with respect to the fire tube that it is retained in its rest position and comprises, in its lower part, an explosive charge 20 of, for example, powder. The damping block 11 placed m the upper end of the fire tube has a cylindrical shape with a diameter somewhat smaller than that of the fire tube and consists of a material with a good energy-absorbing capacity, for example a plastic material. A channel 21 is provided through the fire tube 7, at the upper end thereof, which channel may be dimensioned so as to admit gas under resistance which, during the closing movement, is compressed by the missile, thus damping the movement.

Figure 3 shows an alternative embodiment of the high-speed circuit closer with the missile 10 in the rest position, and Figure 4 shows the missile in the closed position. By a signal generated by a detector, for example an arc guard, the powder charge is brought to explode, for example by an electric firing device. A sudden gas expansion arises between the missile 10 and the lower plate 1, whereby the missile is driven away from its rest position along the fire tube 7. The missile is guided by the fire tube so as to move m a plane movement, thus reaching the four 17 contacts at the same time. hen the missile has reached the contacts, its movement must be braked and brought to a stop. The propellant gases are evacuated through the passageways 12 in the lower part of the fire tube after these have been exposed after the passage of the missile 10 (Fig. 4) . The kinetic energy of the missile s transformed into heat when the cone-shaped front portion of the missile 10 hits the contacts 17 and becomes wedged there¬ in Finally, the missile hits the damping block 11, whereby the movement of the missile is stopped by an energy absorption while the damping block is being deformed.

P\ placing the fire tube 7 in a container filled with an insulating gas, for example SF6, the distances between the contacts 17 and the distances between the contacts and the missile 10, as well as the diameter of the fire tube, may be made small. With a diameter of about 30 mm, the missile is given a small mass, while at the same time the distance between the rest position and the closed position of the missile may be limited to about 30 mm. A small diameter gives -. faster pressure build-up of the explosion gases. A smaller mass of the missile also increases the rapidity in that its mass inertia decreases, while at the same time less energy is needed to stop the missile. Ultimately, the distances are determined bv the risk of flashover. In the embodiment according to Figures 3 and 4, the fire tube 7 comprises two coaxially placed tubes of the same construc¬ tion. The wedge-shaped inner part of the contacts may then rest against the outer tube and be locked by rotation of this tube. Such an arrangement also results in the contacts 17 being easily replaceable after a closing operation. The shape of the damping block 11 is not locked to that of the cylinder but may be, for example, conical or have the shape of an accordion. The body itself may be solid or hollow and may comprise one material or a composition of materials.

In an improvement, the front portion of the missile may be given the shape of the tip of a Phillips® screwdriver, in which case only those parts of the missile which encounter the wedge-shaped contacts are bevelled. Such a shape would give the missile an improved guiding capacity in the fire tube. The explosive charge may advantageously be placed on or in a recess of the lower plate, or in a seat, arranged for direc¬ ting the explosive force, between the missile and the lower plate. For fixing the fire tube, a circular recess is suitably arranged in each of the upper plate and the lower plate, into which recess the fire tube may be pressed and fixed by a drive fit.

Claims

1. An electric switching device for achieving a rapid simultaneous electric closing of at least three conductors (13) intended for high current and high voltage, characterized in that the switching device comprises a container (3) which is filled with gaseous insulating medium and in which the conductors (13) are connected to contacts (17) arranged in an electrically insulating fire tube (7) , at one end of which there rests a connecting member (10) which, by the compressive force from an explosive charge (20) , is adapted to assume a closed position in which the connecting member (10) is wedged between the contacts (17) .

2. A switching device according to claim 1, characterized in that the container (3) comprises a cylindrical casing (4) limited by two substantially parallel plates (1, 2) between which the fire tube (7) is fixed.

3. A switching device according to any of the preceding claims, characterized in that the fire tube (7) comprises evacuation channels (12) for relieving the pressure from the explosive charge.

4. Λ .-.-.■.^•i cn r.g device according to any of the preceding cia rr: , characterized in that at that end of the fire tube XT' which is opposite to the connecting member (10) , a kinetic damping device (11) for the connecting member (10) is arranged.

5. A sv.'itching device according to any of the preceding claims, characterized in that the fire tube (7) comprises a plurality of tubes arranged coaxially inside one another.

6. A switching device according to any of the preceding claims, characterized in that an explosive charge (20) is arranged between the lower plate (1) and the connecting member

(10) .

7. A switching device according to any of the preceding claims, characterized m that the front portion of the connecting member (10) exhibits a substantially conical shape.

8. A switching device according to any of the preceding claims, characterized m that the contacts (17) exposed on the inside of the fire tube (7) are wedge-shaped and exhibit contact surfaces (19) which are inclined with respect to the connecting member (10) .

9. A method for a rapid simultaneous electric closing of at least three conductors (13) intended for high current and high voltage, characterized in that a connecting member (10) , resting m an electrically insulating fire tube (7) in a container (3) filled with a gaseous insulating medium, is caused by the force from an explosive charge (20) to move to a closed position, in which the connecting member (10) is simultaneously wedged between those contact members (17) of the conductors which are exposed on the inside of the fire tube (7) , thus obtaining a permanent short circuit.

10. Use of a switching device according to any of the preceding claims as an arc eliminator in a switchgear unit.