Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01T—SPARK GAPS; OVERVOLTAGE ARRESTERS USING SPARK GAPS; SPARKING PLUGS; CORONA DEVICES; GENERATING IONS TO BE INTRODUCED INTO NON-ENCLOSED GASES
  • H01T1/00—Details of spark gaps
  • H01T1/02—Means for extinguishing arc
  • H01T1/04—Means for extinguishing arc using magnetic blow-out
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01T—SPARK GAPS; OVERVOLTAGE ARRESTERS USING SPARK GAPS; SPARKING PLUGS; CORONA DEVICES; GENERATING IONS TO BE INTRODUCED INTO NON-ENCLOSED GASES
  • H01T2/00—Spark gaps comprising auxiliary triggering means
  • H01T2/02—Spark gaps comprising auxiliary triggering means comprising a trigger electrode or an auxiliary spark gap

Definitions

• the invention relates to a spark gap and is particularly advantageous in the case of a high voltage spark gap making it possible to transfer a large quantity of electrical charges.
• a spark gap is a device of the closing switch type comprising two remote electrodes, called discharge electrodes, separated by a dielectric (gas, vapor, vacuum ...), between which an electric arc forms when the potential difference between the electrodes is greater than a threshold value.
• this threshold value is greater than a few kV, and the operating voltage of the spark gap (voltage applied to the spark gap, i.e. the potential difference between its electrodes) can go up to 1 MV.
• Such a spark gap makes it possible to transfer a quantity of electric charges ranging from a few hundred milli-Coulombs to several hundreds of Coulombs, corresponding to an electric current passing through the spark gap of intensity between 1 kA and 1 MA.
• a high-voltage spark gap is in particular used to supply a test bench for materials under high pressure or any other device sequentially, says charge.
• a shot Each operation of discharging the capacitor and transferring to the charge, by the spark gap, the electrical charges initially accumulated in the capacitor, is called a shot.
• spark gap comprises two hollow cylindrical discharge electrodes, symmetrical of revolution, which each have an internal radius of the order of 8 cm, an external radius of the order of 10 cm and a length of the order of 20 cm.
• the two cylindrical electrodes are aligned "end to end", that is to say arranged so that their axes of symmetry coincide and that they have axial ends, called closing ends, spaced about 8 mm apart and facing each other in the axial direction (direction of the axes of symmetry).
• connection ends are each connected to a connector for the integration of the spark gap in an electrical circuit.
• one of the connectors can be used to connect a current generator (capacitor) and the other to connect a load.
• a shot is triggered by a third electrode, called the trigger electrode, which initiates the formation of an electric arc between the two discharge electrodes.
• the electric charges which are transmitted by the current generator to the unloading electrode to which it is connected, propagate from the connection end of the electrode towards its closing end in the axial direction, then are deflected, by means of inclined slots formed in the end of the electrode, so as to move in a direction comprising a tangential component (along the circular periphery of the electrode in a transverse plane).
• the electric charges then pass through the space between the electrodes in the axial direction, forming an axial electric arc, then are again deflected tangentially in the closing end of the other discharge electrode, by means of inclined slots (in the direction contrary), to then find a substantially axial propagation towards the connection end of the electrode.
• the invention aims to overcome these drawbacks by proposing a simple and inexpensive spark gap, having performances similar or superior to known spark gaps.
• the invention aims to provide a high-voltage spark gap, capable of operating at voltages similar to or greater than the usual operating voltages of the "RAG-TITAN” spark gap, capable of switching currents of similar or greater intensity than the currents switched by the "RAG-TITAN” spark gap, having a lifespan similar or greater than that of the "RAG-TITAN” spark gap, for a negligible rate of failure and undesirable triggering, and a cost price lower than known spark gaps (and in particular the "RAG-TITAN” spark gap).
• the invention relates to a spark gap comprising: - two rigid discharge electrodes, mounted fixed and spaced from each other,
• each electrode being connected to one of the connectors for the connection of said electrodes to an electrical circuit comprising a current generator, and in which:
• each discharge electrode has an elongated conductive portion, called the active portion, having a longitudinal end, called the longitudinal connection end, connected to the connector, and an opposite longitudinal end called the downstream end, - the active portions of the discharge electrodes are adapted so that, when an electric arc is formed:
• At least one discharge electrode has at least one other conductive portion, called passive portion, electrically connected to the connector and / or to the active portion, and having a shape adapted to prevent any untimely spontaneous formation of an electric arc (called auto - priming) under normal conditions of use of the spark gap.
• passive portion electrically connected to the connector and / or to the active portion, and having a shape adapted to prevent any untimely spontaneous formation of an electric arc (called auto - priming) under normal conditions of use of the spark gap.
• the term “elongated”, used to describe the active portion of the discharge electrodes according to the invention, means that said active portion extends mainly along a line, called the guideline. In other words, it has a dimension along this guideline, called length, much greater than its other dimensions. Said guideline can be straight or curved.
• the terms "transverse direction" of the active portion at a point designate a direction orthogonal, at this point, to the guideline (that is to say orthogonal to a direction tangent to said line) of the active portion ; and by "transverse plane” at a point is meant a plane orthogonal at this point to the guideline of the active portion.
• the cross section of the active portion at a point is the section of said portion in the transverse plane passing through this point.
• each discharge electrode of the spark gap therefore comprises an elongated active portion.
• the active portions of the two electrodes are suitable for, when a current is established, channel the current and induce between them a magnetic field which displaces the electric arc along said portions.
• the active portions provide an elongated range of displacement of the arc, which is advantageously free of slots.
• at least one discharge electrode according to the invention also comprises at least one passive portion, specifically provided to avoid any cracking of the spark gap by spontaneous and untimely breakdown under normal conditions of use of the spark gap, and of which the shape and arrangement are adapted to fulfill this function.
• Each passive portion is in particular designed and arranged so as to reduce the intensity of the electric field between the two discharge electrodes.
• the presence of the ⁇ assive portion (s) according to the invention makes it possible to choose and adjust the shape of the active portions so as to increase the performance of the spark gap.
• it makes it possible to choose and adjust the shape of the active portions so as to limit the risks of erosion of said portions by the arc.
• This shape is in particular chosen so as to obtain (when an arc is formed) a high current density along said active portions, which induces a stronger magnetic field giving the arc a greater speed of movement. Rapid displacement of the arc limits the risk of erosion of the active portions.
• condition of use of the spark gap designate all the external parameters of use having an influence on the proper functioning of the spark gap.
• These conditions are said to be “normal” when the values taken by these parameters fall within the usual predefined ranges of use of the spark gap, for which the spark gap has been specifically designed.
• the voltage applied to the spark gap must be within a given operating range, and must in particular be less than a maximum operating value, which can be from 1 kV to 1 MV depending on the spark gap and according to its use. Outside these normal conditions of use, and in particular if a voltage greater than a maximum operating voltage provided is applied to the spark gap, a breakdown of the spark gap is not excluded despite the presence of the ) passive portion (s) according to the invention. Note that, if such a breakdown occurs, the electric arc formed will also appear between the active portions of the electrodes.
• the two discharge electrodes or only one of them have) a passive portion.
• the terms “upstream” and “downstream” are used, for each of the discharge electrodes, with reference to the guideline of the active portion of the electrode and to the direction of displacement of the electric arc along of this active portion.
• the active and passive portions of at least one of the discharge electrodes are separated at least over a fraction of the length of the active portion downstream of the electric arc triggering zone. This characteristic facilitates the channeling of electrical charges in the active portion when a current is established.
• the active and passive portions are distant over this fraction of length and only separated by the gas present inside the spark gap.
• a solid insulating element (made of synthetic material for example) extends between said active and passive portions over this fraction of length.
• the spark gap comprises a triggering device, such as a triggering electrode, capable of initiating the formation of an electric arc in the electric arc triggering zone.
• the voluntary formation of an electric arc is triggered either by applying to the spark gap a voltage greater than a minimum self-ignition voltage, or by modifying the pressure of the gas contained in the spark gap, in order to cause the spark gap to self-prime.
• the shape and arrangement of the active portions of the discharge electrodes are adapted so that the electric arc formed by self-ignition is formed in the region of triggering of electric arc.
• the active portion of at least one of the discharge electrodes has a guideline, called the longitudinal direction, substantially straight at least downstream of the electric arc triggering zone.
• the active portion of at least one of the discharge electrodes has a curved guideline.
• the active portions of the discharge electrodes extend substantially opposite one another, at least downstream of the electric arc triggering zone, it being specified that one by this is meant that any transverse plane (downstream of the triggering zone) of the active portion of at least one of the electrodes intersects the active portion of the other electrode.
• the transverse direction in which the active portions are opposite is called the unloading direction, taking into account that the electric arc which forms between the active portions extends substantially in this direction.
• the active portions of the discharge electrodes have longitudinal connection ends situated on the same side of the spark gap, and preferably arranged substantially opposite one another.
• the active portions of the discharge electrodes have similar overall shapes.
• the active portions of the discharge electrodes both have substantially straight elongated shapes and substantially straight guidelines (longitudinal directions). Alternatively, they both have curved guidelines substantially following the same (the) curvature (s).
• the active portions can have substantially circular guidelines and open crown shapes. Whether straight or curved, the guidelines of the active portions are preferably substantially parallel, at least downstream of the electric arc triggering zone. The active portions are then opposite one another at least downstream of the triggering zone, and the spacing of said portions is substantially constant over the entire length of the active portions. The electric arc moving along these active portions therefore also has a substantially constant length.
• This preferred version of the invention does not exclude the possibility of providing discharge electrodes, the active portions of which have guidelines forming between them, at a point or at any point, a non-zero angle and / or active portions of which the spacing is not constant.
• the spacing of the active portions can increase downstream (in the direction of movement of the arc).
• the active portion of each discharge electrode has a shape adapted so that the induced magnetic field displaces the electric arc at a speed sufficient to avoid erosion of said active portions by local fusion and / or vaporization (s ) (at the point of impact of the electric arc).
• a speed makes it possible to dispense with the use of expensive alloys (example: copper and tungsten alloys) for the production of these electrodes.
• the active portions - as well as the passive portion (s) - of the discharge electrodes are made of a basic conductive material chosen from steels, stainless steels, brasses, aluminum, copper, certain copper-based alloys ... (this list is not exhaustive).
• each active portion has a surface, called useful surface, of dimensions adapted so that the induced magnetic field displaces the electric arc at a speed sufficient to avoid erosion of the active portions by local melting and / or vaporization, the useful surface of the active portion being defined (geometrically) as the surface portion of the active portion which extends opposite the other electrode downstream of the electric arc triggering zone.
• This useful surface of the active portion may have, for at least one of the discharge electrodes, a substantially constant width, the term "width" designating a dimension in a transverse direction orthogonal to the transverse discharge direction.
• the active portion of at least one discharge electrode has a useful surface of increasing width downstream (in the direction of movement of the arc, towards the downstream end of the active portion).
• This characteristic is advantageous for the following reasons.
• the triggering of an electric arc establishes an electric current through the spark gap, the intensity of which increases in an initial phase, before reaching a maximum value and decreasing towards zero (aperiodic regime) or oscillating in s 'damping (oscillating regime) until the energy initially stored in the capacitor (s) is completely dissipated.
• This initial phase is critical given the low intensity of the current and the low kinetic energy of the electric arc.
• the arc is moved along an upstream fraction of the active portion of the electrode, from its point of formation in the electric arc triggering zone.
• This upstream fraction to move the arc at a high speed, it is therefore necessary to have a strong magnetic field between the electrodes, in particular capable of compensating for the low intensity of the current in the arc.
• the use, for each electrode, of an active portion having a useful surface of small width on such an upstream fraction makes it possible to increase the density of the current circulating in this fraction and to have a strong magnetic field opposite it.
• the width of the useful surface can be greater over a fraction downstream of the active portion along which the displaced electric arc has a high current intensity and / or a certain kinetic energy. In this downstream fraction, the induced magnetic field is indeed sufficient to move the arc at the desired speeds without it being necessary to increase it by using an active portion of small width.
• the surface useful of the active portion of each discharge electrode preferably has a length between 5 and 200 cm, and a width less than 50 cm over this length and less than 7 cm at least over an upstream fraction of this length.
• the width of the useful surface can advantageously be less than 2 cm at least over an upstream fraction of this length if the spark gap is intended to transfer amounts of charge less than 20 C. In all cases, the speed of movement of arc obtained allows the use of electrodes in a basic and inexpensive material such as copper or stainless steel.
• the spark gap also has one or more of the following characteristics:
• - at least one of the discharge electrodes has an active portion having the shape of a cylindrical rod (such a rod has a straight guide line), at least between the zone of triggering of electric arc and its downstream end, - at least one of the discharge electrodes has an active portion having the shape of a rod (of straight or curved guide line) of circular cross section, at least between the arc triggering zone and its downstream end.
• the width of the useful surface of such an active portion corresponds to the diameter of the rod.
• Said rod has a cross section of substantially constant diameter, preferably less than 2 cm if the quantities of charges to be transferred are less than 20 C.
• said rod has a cross section of increasing diameter (gradually or not) downstream, said diameter being less than 2 cm in the arc triggering zone electric if the quantities of charges to be transferred are less than 20 C, - the active portion of at least one discharge electrode has an electrically isolated downstream longitudinal end.
• At least one discharge electrode has a passive portion.
• Each passive portion preferably extends substantially opposite the active portion of the electrode in the transverse unloading direction.
• each passive portion extends along at least one fraction upstream of the active portion of the electrode; said passive portion projects from a longitudinal edge of the active portion and so as not to pass through an intermediate space extending between the active portions of the two electrodes.
• the passive portion must in particular extend along at least an upstream fraction of the active portion in which the shape of said active portion (small width and / or small radius of curvature of the surface of the active portion which is oriented towards the 'other electrode ...), thus chosen to induce a high magnetic field in this area, also generates a strengthening of the electric field capable of triggering the unexpected formation of an electric arc (self-ignition) under normal conditions of use of the spark gap.
• the passive portion can also extend over the entire length of the active portion.
• each passive portion has a surface, called useful surface, having a minimum radius of curvature which is greater than a threshold radius below which, under normal conditions of use of the spark gap, the intensity of the electric field between the discharge electrodes is greater than a minimum value of self-ignition of the spark gap (defined as the minimum electric field intensity causing the spontaneous formation of an electric arc).
• the useful area of the passive portion is defined as the surface part of the passive portion which extends opposite the other electrode. Note that according to this definition, the useful surface of the passive portion may possibly extend upstream of the electric arc triggering zone (unlike the useful surface of the active portion as defined above).
• the passive portion of at least one discharge electrode has a flat useful surface (that is to say of infinite radius of curvature).
• At least one of the discharge electrodes comprises:
• an active portion which has, at least downstream of the electric arc triggering zone, the shape of a cylindrical rod of circular section, called the active rod,
• a passive portion having the shape of a hollow cylindrical tube, said passive tube, of section greater than that of the active rod, said tube having a longitudinal slot opposite which extends the active rod, said passive tube and said active rod being arranged so that the rod extends between the tube and the other discharge electrode.
• the longitudinal downstream end of the active rod is advantageously carried by a longitudinal downstream end of the tube, to which it is connected by preferably electrically insulating fixing means,
• At least one of the discharge electrodes comprises, on the one hand, an active portion having, at least downstream of the electric arc triggering zone, the shape of a cylindrical rod and, on the other hand, a passive portion in the form of a flat plate, said plate and rod being spaced from each other and arranged so that the rod extends between the plate and the other discharge electrode, parallel to said plate and close to it.
• the spark gap comprises a housing inside which the discharge electrodes are placed.
• Said housing may include at least one conductive wall acting as a passive portion (in the form of a flat plate) of a discharge electrode.
• At least one of the discharge electrodes comprises an elongated flat plate, one longitudinal connection end of which is connected to the connector.
• the active portion of the electrode consists of a downstream fraction of said plate and at least one ba ⁇ e or rod of length and width respectively smaller than those of the plate, fixed to an upstream fraction of said plate.
• the passive portion of the electrode essentially consists of the upstream fraction of the plate.
• the rod preferably extends away from the passive portion at least downstream of the electric arc triggering zone, so that, when an electric arc is triggered and an electric current is established, the electric charges flowing in the electrode are channeled and concentrated in the rod - at least as long as the electric arc extends between the rod and the other electrode - in order to induce a high magnetic field.
• the spark gap comprises several pairs of discharge electrodes, said pairs being arranged in parallel.
• the quantity of charges conducted can be multiplied by the number of pairs of electrodes operating in parallel.
• at least one of the discharge electrodes of each pair is connected to a connector of the spark gap which is specific to it.
• the spark gap comprises at least, on the one hand, a connector per pair of discharge electrodes, and on the other hand, a single connector or a connector per pair of electrodes.
• Electrical decoupling means inductive, resistive, temporal, etc.
• spark gap comprises only two connectors
• spark gap incorporates means of electrical decoupling (inductive, resistive, temporal, etc.) between one of the two connectors and one of the electrodes of each pair.
• electrical decoupling inductive, resistive, temporal, etc.
• the invention also relates to a spark gap characterized in combination by all or some of the characteristics mentioned above and below.
• FIG. 1 is a schematic sectional view along a longitudinal plane of a first spark gap according to the invention
• FIG. 2 is a schematic sectional view along a transverse plane of the first spark gap according to the invention
• FIG. 3 is a schematic sectional view along a first longitudinal plane of a second spark gap according to the invention
• FIG. 4 is a schematic sectional view along a second longitudinal plane AA -orthogonal to the first longitudinal plane- of the second spark gap according to the invention.
• the first spark gap according to the invention illustrated in FIGS. 1 and 2 comprises a parallelepipedal conductive housing 1 made of stainless steel, a first discharge electrode 2, a second discharge electrode 3 and an electric arc triggering electrode 4.
• Each discharge electrode 2, 3 has a generally elongated and straight shape, defining a longitudinal direction of the electrode. It includes a straight elongated active portion, described below, whose longitudinal direction (guideline) coincides with that of the electrode.
• the electrodes extend opposite one another in a transverse direction Z, called the discharge direction, so that their longitudinal directions are parallel and define a common longitudinal direction X.
• the trigger electrode 4 extends between the discharge electrodes, in a transverse direction Y, orthogonal to the longitudinal direction X and to the discharge direction Z. It extends more precisely between the active portions (described below) ) discharge electrodes, near the connection ends of said active portions. It defines a zone 21 for triggering an electric arc.
• the discharge electrode 2 comprises, on the one hand, a passive portion comprising a hollow cylindrical tube 9, called a passive tube, which has an internal radius of the order of 55 mm, an external radius of the order of 75 mm and a longitudinal slot 22 over almost its entire length.
• Said passive portion has a downstream longitudinal end produced by an end piece 16, electrically isolated (both from the housing 1 and from the active portion of the electrode).
• the passive portion is also connected to a connector 11 via a tip 17 and a tube portion 50 (of short length) extending the passive tube 9 upstream, said tip and tube portion forming part of the active portion (described below).
• the connector 11 passes through the housing 1 for the connection of the electrode 2 to an electrical circuit, and in particular to one or more high voltage capacitor (s).
• the connector 11 comprises a conductive rod 12, one longitudinal end of which is welded into the end-piece 17, and a sleeve 13 of electrically insulating material for embedding the connector in the housing 1.
• the passive portion of the discharge electrode 2 has a useful surface 23 oriented towards the electrode 3, formed by the part of the external surface of the passive tube 9 located “below” (in FIGS. 1 and 2) of a "horizontal" median plane - orthogonal to the direction of discharge - passing through a diameter of the tube.
• the discharge electrode 2 further comprises an active portion comprising a cylindrical rod 10, called the active rod, of circular section having a substantially constant diameter of the order of 10 mm, and of length substantially corresponding to that of the passive tube.
• the active portion has a longitudinal connection end which extends said active rod 10, and which comprises the end piece 17, the tube portion 50 and a lug 15 for fixing the rod 10 to said tube portion 50.
• the active portion also has a downstream longitudinal end 18, carried by the longitudinal end downstream 16 of the passive portion to which it is connected by a plug 14 of electrically insulating material.
• the active rod 10 extends opposite, in the unloading direction Z, from the slot 22 of the passive tube 9, so as to project slightly (along said unloading direction Z) from the useful surface 23 of the passive portion and to extend between this useful surface and the discharge electrode 3.
• the active portion of the discharge electrode 2 has a useful surface 24 formed by the part of the external surface of the rod 10 which is oriented towards the electrode 3 (this part is cylindrical of semi-circular section) and which s' extends, in the longitudinal direction, between the trigger electrode 4 and the downstream end 18 of the rod.
• the unloading electrode 3 comprises on the one hand a passive portion formed by a plate or wall 6 of the housing 1, called the passive wall. Said wall 6 of the housing is connected, at one end 27, to a connector 7 making it possible to connect said wall to ground.
• the connector 7 has a conductive rod passing through the wall 6 (which provides the connection between said connector and the passive wall 6), and a mortise for receiving the connection end of the active portion (described below) of the electrode 3.
• the discharge electrodes 2 and 3, provided with their respective connector 11, 7, are arranged so that said connectors are facing each other in the transverse discharge direction.
• the passive portion of the unloading electrode 3 has a flat useful surface 25 formed by the part of the internal face (oriented towards the electrode 2) of the wall 6 of the housing which extends, in the longitudinal direction, between the tip 17 and the tip 16 of the electrode 2.
• the unloading electrode 3 also comprises a single-piece active portion consisting of a cylindrical rod 5, called the active rod, of circular section having a substantially constant diameter of the order of 10 mm.
• the active portion has, on the one hand, a longitudinal connection end formed by the longitudinal end 28 of the active rod, welded in the mortise of the connector 7, and on the other hand, a downstream longitudinal end formed by the opposite longitudinal end 20 of the active rod 5, carried by a wall 29 of the housing 1 in which it is fixed by a pin 8 of electrically insulating material.
• the downstream end of the active portion of the electrode 3 is thus electrically isolated.
• the active rod 5 of the electrode 3 extends parallel to the passive wall 6 and close to the latter; it also extends parallel to the active rod 10 of the electrode 2.
• the active portion of the discharge electrode 3 has a useful surface 26 formed by the part of the external surface of the rod 5 which is oriented towards the electrode 2 (this part is cylindrical with a semi-circular section) and which s' extends, in the longitudinal direction, between the trigger electrode 4 and the free endpiece 16 of the electrode 2.
• the connector 11 is connected to one or more capacitors, the connector 7 and the housing 1 being connected to ground.
• the discharge electrodes 2 and 3 are thus brought to separate potentials, the difference of which can amount to 50 kV.
• the electric charges are distributed over the useful surfaces of the active and passive portions of the discharge electrodes, and an electric field appears between the two electrodes.
• the useful surfaces 23 and 25 of the passive portions of the electrodes act as reducers of the electric field, thus limiting the risks of self-ignition of the spark gap under normal conditions of use thereof. this.
• an arc is then initiated between the active rods 10 and 5 of said electrodes, in the zone 21 for triggering an electric arc, by bringing the trigger electrode 4 to a given given potential.
• the presence of the triggering electrode when it is brought to this potential, locally increases the electric field and causes a breakdown in the electric arc triggering zone.
• a current is thus established between the conductive rods of the connectors 11 and 7.
• This current essentially flows in the active portions of the electrodes: the electric charges propagate in the nozzle 17, the tube portion 50, the fixing lug 15 and the active rod 10 of the discharge electrode 2; they are transferred to the discharge electrode 3 by the electric arc formed between the active rods 10 and 5, which arc extends substantially in the transverse direction of discharge; then they propagate in the active rod 5 of the discharge electrode 3 towards the connector 7.
• the current is channeled in the active rods 10 and 5.
• the trigger electrode extends near the connection ends of the active portions of the two discharge electrodes, slightly downstream (and not opposite) of said connection ends. Consequently, when a current is established, it circulates in the active rod of each electrode over a length which coincides, at the time of establishment of the current, with the distance - according to the longitudinal direction - between the end of connection of the active portion and the arcing zone. As soon as it is established, the current therefore has a component in the longitudinal direction X, immediately upstream of the electric arc.
• the current flows towards the downstream end 18 of the rod 10, while it flows in the opposite direction in the discharge electrode 3, towards the connection end 28 of the rod 5
• the flow of current in each of the active rods 10, 5 induces a magnetic field with substantially circular field lines in the vicinity of the rods.
• the resulting magnetic field (sum of the fields induced by the two electrodes) has a direction substantially orthogonal to the longitudinal and transverse directions of discharge, and a "re-entering" direction in FIG. 1.
• the resulting induced magnetic field displaces the electric arc in the longitudinal direction towards the downstream ends 18 and 20 of the active rods 10 and 5, along said rods which realize a straight range of displacement of the arc.
• the terms "upstream” and “downstream” are defined with reference to this direction of displacement of the electric arc.
• the active rods 10 and 5 having small diameters, their useful surfaces 24 and 26 have a small width.
• the density of the current flowing in the active rods along these useful surfaces 24 and 26 is therefore particularly high, so that the induced magnetic field is strong and the resulting Laplace force large.
• the speed of displacement of the arc obtained is sufficiently high to considerably reduce, or even avoid, the damage due to the erosion of the electrodes by the electric arc.
• the second spark gap according to the invention illustrated in FIGS. 3 and 4 comprises: a parallelepipedal housing 30, conductive or not, made of a steel or any synthetic material; two identical discharge electrodes 31 and 32; and an electrode 42 for triggering an electric arc.
• each discharge electrode 31, 32 has a generally elongated and straight shape, defining a longitudinal direction of the electrode.
• Each electrode comprises a straight elongated active portion, described below, whose longitudinal direction (guideline) coincides with that of the electrode.
• the electrodes are arranged parallel to each other and symmetrically; they extend opposite one another in the transverse unloading direction Z, and their parallel guidelines define a common longitudinal direction X.
• the trigger electrode 42 also extends in the longitudinal direction X, and has, between the discharge electrodes, a free end defining in its vicinity an area 41 for triggering an electric arc.
• the trigger electrode 42 is mounted on a wall 48 of the housing 1 by means of a sleeve made of insulating material, which makes it possible both to fix the electrode trigger 42 on the housing 1, to isolate the electrode from the housing, and to protect the fraction of the electrode which extends outside the housing 1.
• Each discharge electrode 31, 32 comprises an elongated flat plate 33 and a rod 34, the respective longitudinal directions of which coincide with the longitudinal direction X of the electrode.
• the rod 34 is fixed to the flat plate by a fixing flange 46 and means with screws or bolts, so as to extend opposite an upstream fraction of said plate.
• the flat plate has a length of the order of 700 mm and a width (dimension in a transverse direction orthogonal to the longitudinal direction X and to the discharge direction Z) of the order of 100 mm.
• the rod 34 has a length of the order of 200 mm and a width of the order of 25 mm.
• each discharge electrode 31, 32 is formed by the rod 34 and by a downstream fraction 44 of the plate which extends in the extension of a downstream longitudinal end 47 of the rod 34 towards a free end 35 of the plaque.
• This active portion has a longitudinal connection end formed by an upstream longitudinal end 40 of the rod 34, and a downstream end formed by the free end 35 of the plate.
• each rod 34 is slightly curved in a plane containing the two rods (plane of formation and displacement of the electric arc), so that the spacing between the two rods is variable: it is minimal in the zone 41 triggering of an electric arc, then increases downstream, towards the ends 47 of the rods 34.
• the electric field induced between the rods 34 is therefore maximum in the area of arcing. The triggering of an electric arc is facilitated.
• each electrode 31, 32 is formed by an upstream fraction 45 of the plate 33, which extends from an upstream end 36 of the plate 33 to the downstream end 47 of the rod 34. Said passive portion is directly connected to a connector by its upstream end 36.
• the connection ends 40 and 36 of the active and passive portions of each electrode are traversed by the conductive rod 38 of a connector 37.
• the mechanical connection thus produced is electrically conductive; it allows the electrode to be connected to an electrical circuit.
• one of the connectors 37 can be connected to one or more capacitors and the other connector to a load.
• the conductive rod 38 of the connector is surrounded by an insulating sleeve 39, for its embedding in a wall of the housing 30 and its attachment to said wall.
• the trigger electrode 42 when brought to a given potential, locally modifies the electric field in the electric arc triggering zone and initiates the formation of an electric arc between the rods 34.
• the established current, channeled in the rods 34 flows in the longitudinal direction X, downstream -that is to say towards the free end 35- in the electrode connected to the generator, and towards the upstream -that is to say towards the connection end 40- in the electrode connected to the load.
• the established current induces a magnetic field between the electrodes, the direction of which in the plane of the arc is orthogonal to the longitudinal direction and to the direction of discharge.
• the induced magnetic field displaces the arc towards the free ends 35 of the active portions.
• the discharge of a capacitor comprises an initial period during which the current passing through the spark gap presents an increasing intensity (initially zero).
• the active portion of each electrode in the vicinity of the electric arc triggering zone is advantageously formed by the rod 34, the useful surface of which has a small width in order to concentrate the electric charges, increase the current density and thus generate a high magnetic field in this area despite the low current intensity at the start of the discharge.
• the induced field is sufficient to move the electric arc at a speed capable of limiting erosion.
• the rods are advantageously dimensioned so that, as long as the intensity of the current is not sufficiently high, the moving electric arc still extends between the rods.
• an element 43 of electrically insulating material is arranged between the active rod 34 and the passive upstream fraction 45 of the plate, downstream of the electric arc triggering zone. This element 43 makes it possible to channel the established current into the rod 34, at least as long as the moving electric arc has not reached the downstream end 47 of the rod.
• the electrical insulation that the element 43 provides can also be obtained by leaving a space between the rod 34 and the upstream passive fraction 45, that is to say by removing the element 43, the gas contained in the housing 1 making an insulator.
• the speeds obtained for displacement of the electric arc between the rods 34 and between the downstream fractions 44 of the plates are sufficiently high to limit the erosion of said rods and downstream fractions to the point of authorizing the use of a basic material ( copper or any steel for example) for their manufacture, or to allow the transfer of quantities of charges and / or current intensities greater than those usually transferred.
• a spark gap comprising an unloading electrode devoid of a passive portion is in accordance with the invention, since the other electrode has one.
• a spark gap comprising two identical discharge electrodes, similar either to electrode 2 or to electrode 3 shown in the figures, is in accordance with the invention.
• a spark gap comprising one of the electrodes 2 or 3 shown, associated with an electrode such as the electrode 31, is in accordance with the invention.
• spark gap illustrated in Figures 1 and 2 could be used by connecting one of the electrodes to one or more capacitor (s) and the other electrode to a load, by means of a modification of the electrode 3 (such as the addition of an insulating sleeve around the connector 7) in order to isolate the latter from the housing 1.
• the electric arc triggering means are not limited to the triggering electrodes shown.
• a needle-shaped electrode which crosses (in the unloading direction) without contact the active portion of one of the unloading electrodes. When brought to a given suitable potential, such an electrode creates a plasma in its vicinity, which propagates so as to form an electric arc.
• the spark gap does not have a trigger electrode. It is closed either by applying a voltage greater than the minimum self-ignition voltage to it, or by temporarily creating, between its discharge electrodes, an overvoltage greater than said self-ignition voltage.
• the gas pressure inside the spark gap housing is reduced (by opening a co ⁇ esponding adjustment valve).
• the shape and structure of the electrodes are not limited to those illustrated.
• the active portions of the electrodes may have a curved guideline, until they form, for example, an open (or even possibly closed) turn or circular crown.
• the passive portions of the electrodes may have various shapes, as soon as these shapes are adapted (in particular by the extent and arrangement of their useful surface) to avoid any untimely self-priming of the spark gap.

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