US3522472A - Direct current breaker - Google Patents

Description

Aug.- '4, 1970 Filed May 26, 1966 B0 BREITHOLTZ DIRECT CURRENT BREAKER 4 SheetsSheet 2 Fig. 4

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45 65 "E vlWV6WV r i 55 62 I 5 2 44 64 i R 7 6/ 54 43 51/ 4/ W J 63 53 rr rRA/vsEER 5 ELEMENT SWITCH INVENIUR :B R El TH our-2- TTQRNEYS United States Patent 3,522,472 DIRECT CURRENT BREAKER Bo Breitholtz, Vasteras, Sweden, assignor to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Filed May 26, 1966, Ser. No. 553,243 Claims priority, application Sweden, May 26, 1965 6,919/65; Dec. 31, 1965, 17,056/65 Int. Cl. H02]; 7/14; Hb 37/00, 41/00 US. Cl. 315-126 8 Claims ABSTRACT OF THE DISCLOSURE A device for breaking a high voltage direct current circuit includes a series connection of a divided circuit and a main switch. The divided circuit consists of a parallel connection of a resistor which can conduct the entire current, a transfer element which for at least a short moment can conduct the entire current and an auxiliary switch whichis normally closed and conducts the current in the circuit. During the beginning of a breaking process, the auxiliary switch is opened and a starting impulse is given to the transfer element, which rapidly decreases its impedance to a value which is much lower than the value of the impedance of the resistor. When the current through the auxiliary switch is broken, the transfer element increases its impedance to a value which is much higher than the impedance of the resistor. The transfer element is a gas discharge valvein which the arc discharge is controllable from plasma condition with low arc voltage drop to space charging condition with very high are voltage drop.

I BACKGROUND OF THE INVENTION Field of theinvention The prior art During the breaking of inductive direct current circuits it has previously been suitable to use different fundamental principles. According to one of these the field strength of an arc is increased by using forced breaking in oil'or with insulated quenching screens. At lower direct voltages thus-arcs of several meterslength must be effected before the arc voltage reaches the same size as the;netw0rlt voltage. Such devices are, of course, very bulky and imprac- .tical.

According to another principle the' current is broughtto zero in an artificial way by discharging a charged capacitor across the contacts in. an- A-.C. breakernFor high-voltages this methodwould necessitate extremely large'capacitem which makes it practically unusable from the economic point of view. 1

3,522,472 Patented Aug. 4, 1970 Ice SUMMARY OF THE INVENTION The present invention relates to a device for connecting a series resistor to a current circuit in connection with the breaking of a high voltage inductive current circuit and thereby reducing the current in the circuit to such a low value that breaking can take place. The resistor is connected in parallel to a transfer element which is dimensioned so that at least for a short time it can conduct the complete current. The invention is characterized by an auxiliary switch connected in parallel with the transfer element, which auxiliary switch is normally closed and conducts the current in the current circuit and by means for opening the auxiliary switch during the beginning of the switching process and for effecting a starting impulse for the transfer element so that this becomes conducting and takes over the current and by the fact that the impedance of the transfer element is controllable from a low to a very high value during a time which is much shorter than the time which is necessary for the current to fall to a value which permits breaking.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings,

FIG. 1 shows a circuit diagram of a high voltage direct current (HVDC) circuit in which an embodiment of the invention is used. 7

FIG. 2 shows the voltage of the circuit as a function of time during a breaking process.

FIG. 3 shows the current in the circuit as a function of time corresponding to FIG. 2.

FIGS. 4 and 5 show two embodiments of the transfer element indicated by 5 in FIG. 1.

FIG. 6 is a circuit diagram of a second form of HVDC- circuit according to the invention which includes further impedance elements.

FIG. 7 is a schematic circuit diagram of a third form representing a portion of the breaking device of FIG. 6.

. 'A'further principle. is based on afr'nercury arc rectifier with a controlfgrid whichnorrnally conducts'the current in the circuit. A resistor of approximately the same size as the'resistance of the complete circuit isconnected in parallel with the mercury arcrectifierand is dimensioned A direct current circuit where the device according to so that it can conduct the complete current. A large capac itor in series with a breaker is also connected in parallel to. the mercury arcrectifier- During operation the breaker isopen and a large charging resistor is also connected over one side of the capacitor to the jminuspole of the net- Work, while the other sid'e.of thecapacitor is co nnected FIGS. 8a and 8b show modifications of parts of FIG. 7. FIG.'9 shows a part of'a fourth form of circuit representing a modification of FIG. 6.

FIG. 10 is a schematic circuit diagram showing a part of a fifth formin which the transfer element in FIGS. 6 and 7 is replaced by another element;

FIGS. 11 and 12 show schematic diagrams of sixth and seventh forms of apart of the circuit in which the resistor in FIG. 6 is replaced by a number of part resistors and connected across the transfer element by theaid of spark gaps.

DESCRIPTION OF THE PREFERRED 1 EMBODIMENT the invention is connected, comprises a voltage source vV and a load resistor R The line inductance is'denoted .byL and consists to agreat extent ofthe anode reactors necessary in connection with mercury arc rectifiers..Between the points 2 and 3 in the line 1 an. auxiliary switch t,'a transfer element 5 (shown in detail in FIGS. 4 and 5) with controllable inner impedance and a resistor 6 are connected in parallel to each other. The auxiliary switch can be of the isolating circuit opening type or the like, because it is not required tobreak the current circuit, but is only required to set up an arc with a reasonable arc voltage drop. The transfer element 5 can be a trollable from plasma condition with an arc voltage drop of some tens of volts to space charging condition with very high are voltage drop. For a certain type such gas charging means involves the fact that the low arc voltage drop can only be maintained if the condition R-n. A is met, where R is the effective radius of the gas discharge means, n is the density of neutral atoms in the gas and A is a magnitude which depends on the type of neutral atoms in the gas and a possible axial magnetic field through the discharge channel. The arc voltage of the discharge channel drop can consequently be controlled to the desired value by varying one or several of these parameters. The effective radius in the discharge section can be varied by means of moving screens which function as an iris diaphragm. The density of the neutral atoms in the gas can be varied by varying the pressure in the discharge section in one way or another. The value of A can be influenced in the desired direction for example by means of an axial magnetic field passing through the discharge section.

The method of connecting a resistor parallel to a breaking section is well-known, partly with ordinary A.C. breakers and partly with tap-change gears, but in these cases their function is completely different from that in the present invention.

In one form of the transfer element 5 shown in FIG. 4, this is connected to a vacuum pump at which normally keeps the pressure in the discharging section between the anode 11 and the cathode 12 so low that the inner impedance is very high and the discharge distance is non-conducting. By means of a plasma gun 13 placed at the side of the discharging section a plasma 14 can be inserted into the discharging section either continuously or intermittently, so that the discharge is started at a relatively low voltage between anode and cathode and the arc voltage drop is kept at a low value for such a long time that the breaking section in the auxiliary switch has time to be completel deionized and the arc is then extinguished. At the side opposite to the plasma gun the discharge means is provided with a possibly cooled condenser surface 15 where ions and electrons are recombined. The neutral gas is removed by the vacuum pump.

The transfer element shown in FIG. 4 can also be modified in the way shown in FIG. 5. The anode and cathodeare annular and the plasma gun is arranged so that it can emit a plasma along the discharge tube with such a frequency and pulse length that there is always a plasma between the anode and cathode. In the righthand end of the transfer element there is a space constituting a condenser and deionizing chamber where the ions are recombined'and the neutral gas is either condensed on cold surfaces 15 or is pumped away by a vacuum pump 10.-Possibly the right-hand part of the means can comprise a large space 16 in order that the gas which cannot be immediately pumped away shall not return into the discharge tube.

' By using gas which is already ionized when it enters the discharge section, asin the means shown and described above, by means of electromagnetic forces the gas can be made to move very much more quickly than if non-ionized gas were used. The impedance change of the transfer element can thereb be made very quickly.

When the current circuit is closed the device will have low impedance. During breaking its impedance will for a short time rise to a high but finite value, by means of which the current falls to such a low value that a normal switch or an isolating circuit opener can effect final breaking as described previously. Normally the current circuit is closed by both the auxiliary switch 4 andthe main switch 7 being closed (these however being shown opened in the drawings). The transfer element 5 as well as the resistor 6 has such a high resistance that thercuring opened. Thus an arc is generated and the arc voltage drop is sufficient to start a' discharge process through the transfer element 5. The arc voltage drop in the transfer element 5 is considerably lower than the arc voltage drop across the auxiliary switch and consequently the arc across the auxiliary switch quickly expires, and the current goes completely through the transfer element. As soon as the arc in the auxiliary switch 4 has expired the impedance of the transfer element is raised so that the arc voltage drop increases and so that a considerably greater part of the current passes through the resistor 6. In order that a transfer of the current from the transfer element 5 to the resistor 6 can be'made, the impedance of the transfer element must be much greater than that of the resistor.-'In order that the total breaking time of the direct current circuit shall be as short as possible, the impedance of the device should increase with the time so that the voltage during the whole time is maximum, that is, equal to the highest voltage permissible from the point of view of insulation resistance, which can be achieved if the resistor 6 is of non-linear type. Usually the highest permissible voltage is equal to twice the operating voltage.

In FIGS. 2 and 3 the voltage and the current respectively are shown as a function of the time during a breaking process. Before the time t-O the operating voltage V prevails. When the auxiliary switch 4 opens and the transfer element 5 takes over the current, the voltage rises somewhat, but this increase which is of some tens of ,volts in side is completelynegligible compared with the operat ing voltage of about 100 kv. At the time t0 the inner impedance of the transfer element rises and after the time athis impedance has increased so much that the total impedance of the device is substantially equal to the load impedance, which is achieved by making the impedance of the resistor 6 approximately equal to the load impedance, and the voltage of the system has risen to twice the operating voltage, since the current does not have time to alter to any extent during this short time. Since the impedance of the transfer element increases, more and more of the current will pass through the resistor and finally the voltage across the transfer element will have risen so much that practically all the current passes through the resistor 6 and practically none through the transfer element 5. In order that the voltage across the device shall be constant when the current falls andrin order that it shall give a short breaking time, the resistor must have a non-linear current voltage characteristic. After the time T, such a large impedance has been connected into the circuit that the current has dropped to such a low value as shown in FIG. 3 that final breaking can be effected with the main switch 7. The function of the transferelement 5 is thus that at the opening of the auxiliary switch 4 it takes over the current and then transfers it to'the resistor 6 under such conditions that the voltage of the system does not rise above the permissible connection over-voltage. Together with the current being taken over by the resistor 6 the inner impedance of the trailer will be able to be practically infinite.

-If the parallel resistor 6 has linear current-voltage characteristics the current will' be halved after one switch operation, since the maximum resistance which is con- I nected in is approximately equal to the load resistance,

, rent to a value acceptable for final breaking. If, on the rent passes completely through the two switches 4 and other hand, the resistor is of nonlinear type, with one or two devices the current can be reduced from thousands of amps to a fewamp s. i The device-is inthe first place intended for equipment for high voltage direct current, but is in principle also suitable for low voltage direct current circuits, for alternating current networks and above all when there is need to effect a current limitation in a short time.

If the current circuit is strongly inductive a high voltage will exist over the transfer element for some tens of microseconds when the current through this element is cut off and transferred to the resistor. It is evident from this that there is a great risk of re-ignition in the transfer element.

In order to eliminate this risk of re-ignition the described device can be modified as hereinafter described in connection with FIG. 6 by connecting a switch in series with the transfer element and by connecting parallel to this series connection a series connection of two impedances for determining the voltage ratio between the switch and the transfer element.

In order to reduce the slope of the voltage which occurs over the device when the current through the transfer element and switch is cut off, a capacitor can be connected parallel to the resistor. If the two impedances are composed of sufficiently large capacitors these can replace the said capacitor.

By means of this connection it is possible to control the voltage over the two series connected breaking elements so that the flash-over strength in at least one of them has time to rise so quickly that a re-ignition over the complete break distance is prevented.

The direct current line 1 in FIG. 6 comprises a voltage source V, the load resistor R and the line inductance L. The circuit breaking device is inserted between two points 2 and 3 and comprises a nuumber of branches parallel connected between said points. One of these branches comprises the auxiliary switch 4, another comprises the transfer element 5 in series with a switch 8 which will be called the series switch or circuit opening means. Said transfer element and series breaker 8 are connected together at a point 9. A third branch comprises two impedances 10 and 11 in series and they are connected together at a point 12. The points 9 and 12 are connected so that they have the same potential. The impedances 10 and 11 can consist of capacitors or a combination of capacitors and resistors. Which type of impedance is to be used depends on the characteristics of the line and the transmitted power and must be determined from case to case. if In the device according to FIG. 6 a fourth branch consists of a capacitor 13 which serves to take up the energy stored in the line which is set free during the breaking process. If the impedances 10 and 11 consist of capacitors and if it is possible to dimension them so that they not only act as a voltage divider for the two series connected elements 5 and 8 but can also take up the released inductive energy, the capacitor 13 can be eliminated. A fifth branch consists of the resistor 6. .With a connection according to FIG. 6 there can be no complete breaking because the resistor 6 allows a certain current to How in the circuit between the points 2 and 3. For final breaking a main switch 7 is necessary. During the breaking process the auxiliary switch 4 and the series switch 8 can be synchronised so that the series switch is closed until the current has been completely transmitted from the auxiliary switch to the branch with the transfer element and the series switch. Before the impedance change occurs in the transfer element 5, the contacts of the series switch are opened. When the cutoff of the current through the transfer element occurs, the current through the branch will be zero or practically zero and the current is transferred to the capacitor 13 which begins to be charged. If the transfer element can maintain its high impedance for 5-10 microseconds, the series switch will have reached such a high strength that ,it can withstand a slope of some 100 V./,u,S. of the returningvoltage and a possible re-ignition in the transfer element only means that the energy in the impedance 10 is discharged into the transfer element and the whole voltage across the capacitor 13 and across the capacitor 14 and the resistor 6 for a short time exists across the impedance 11 and the series switch 8.

In FIG. 7 a circuit arrangement is shown which gives complete breaking between points 2 and 3. In this the resistor 6 is series connected to a capacitor 14 which can then replace the capacitor 13 in FIG. 6.

If the transfer element 5 after repeated re-ignition itself can finally effect complete breaking, the series switch 8 can be eliminated. The capacitor 13 or 14 in such cases serves two purposes, it prevents on the one hand the voltage oscillations in the network generated by the re-ignition from becoming too great, on the other hand the transfer element from receiving too great a voltage bias after the current has been cut olf.

In the embodiment shown in FIG. 7 the capacitor 14 must be dimensioned so that it can absorb all the remaining magnetic energy without too high a voltage being produced. The resistor 6 is also intended to dissipate little energy, so that the capacitor can be made smaller than when the capacitor does not have a resistor such as 14 in series with it. With this arrangement, which otherwise operates substantially like that of FIG. 6, with the capacitor 14 performing the function of capacitor 13 of FIG. 6, immediately after the current is cut off a voltage will occur across the transfer element with the series switch. If this should not be suitable an extra capacitor can be connected in parallel, in the same way as shown in FIG. 6.

The series switch 8 must have very high returning voltage strength. During a cutting off in the transfer element it is not absolutely certain that the current will be exactly zero and therefore the series switch must be certain to break the residual current.

The circuit opening means can be a vacuum breaker with for example, tungsten contacts which are able to extinguish the arc spontaneously for currents less than approximately 50 amps. It is also possible to use a gas blast circuit breaker which operates with compressed air or an electronegative gas by means of which a forced breaking at 10 to 20 amps is produced.

If the series switch is not able to break the resistance current, an artificial zero passage of the current must be obtained and FIGS. 8a and 8b show two variations of a series switch arrangement for this purpose, these showing a modification of a part of FIG. 7. According to the variation shown in FIG. 8a an oscillating circuit consisting of a capacitor 15 and a coil 16 is arranged parallel to the series switch 8. When the arc voltage increases upon current reduction, a current in the series switch is induced with such a direction that an artificial zero passage is obtained.

In the variation shown in FIG 8b the oscillating circuit is series connected to a spark gap 17. The capacitor can be charged from the beginning and when the spark gap is ignited because of the rise of the arc voltageacross the series switch above a certain. value, an artificial zero passage is induced and the switch breaks. The spark gap can also be ignited by some external ignition pulse. The capacitor can also be uncharged from the beginning.

In the circuit shown in FIG. 6 the series switch 8 must be closed when the transfer element 5 ignites. Only afterwards will the series switch open. This takes about 10 msec. and during this time the transfer element will conduct. When the transfer element cuts off the current the series switch can be quickly de-ionized.

If the series switch is a mechanical breaker of the types previously mentioned, the transfer element must conduct for about 10 msec.

If it is necessary to shorten the conductive time of the transfer element 5, this can be done in several ways.

According to one way of doing this, the contact arm in the series switch 8 is provided 'with a device which pulls an ignition wire when the series switch is opened and thereby the series switch can begin to open before the auxiliary switch is fully open and de-ionized. Consequently there is a short conduction time at the transfer element and a quicker breaking process. In order to get sufficiently low resistance in the series during the commutation, the wire must be thick, as otherwise the compressed air blasting will not start before the current has commuted from the auxiliary switch to the transfer element and the series switch.

According to another way the switch can be modified as shown in FIG. 9. Here the auxiliary switch 4 is connected only across the transfer element. The advantage with this connection is that the two switches can be opened simultaneously. Here the transfer element only needs to conduct for the time which is needed for an arc in the auxiliary switch to quench, that is, some hundreds of as. Another advantage is that the auxiliary switch only needs to break approximately 10 kv., instead of full voltage with the switch according to FIG. 6. This is due to the voltage divider connection 10, 11.

As a further variation of the circuit shown in FIG. 1, a capacitor can be connected between the points 2 and 3 in order to reduce the steepness of the recovery voltage.

The invention also comprises (as shown in FIG. 10) a modification of the device shown in FIGS. 6 and 7. The transfer element is constituted by a vacuum discharge device which is not suitable for conducting the complete current continuously but only during the breaking process itself. According to the proposed modification the auxiliary switch 4 in parallel to the series connection of the transfer element 5 and the series switch 8 are replaced by a continuously conducting low pressure discharge device 18 in series with a high speed circuit opening device or switch 19. The high speed circuit opening device can be a gas blast circuit opener which operates with compressed air or a suitable electronegative gas, for example SP It can also be constructed as a rectifier valve with a blocking grid.

During a breaking a current instability will be induced in the discharge device 18 (transfer element) so that it cuts off, that is, so that its current carrying properties are quickly reduced or completely disappear. Before this cut-off is started the series switch 19 will be opened. If the cutoff is of such a long duration as 5-10 as. the series switch will have time to be de-ionized and recover a part of its voltage strength. Upon the cut-off the complete current is connected over to charge the capacitors 10, 11 and 13. If the capacitor 13 is 5 uf., the voltage between the points 2 and 3 will rise by about 500 V./,uS., which is a normal value for an air blast circuit opening device. By means of capacitors 10 and 11 this voltage will be divided in a suitable way between the discharge device (transfer element) and the series switch. When after several hundred microseconds the voltage has reached the network voltage, the complete current can be conducted by the parallel resistor 6, so that the magnetic energy stored in the circuit disappears in the resistor and the current is reduced. Final breaking is carried out with the switch 7. If the resistor 6 is eliminated or series connected to a capacitor as in FIG. 7 a complete breaking can be obtained without the main switch 7, but then the capacitor 13 must be large so that it can deal with the remaining magnetic energy without too great overvoltages occurring.

The device shown in FIG. 10 for inducing a current instability in the discharge device 18 consists of a separate chargeable capacitor 20, a switch 21 and a suitable impedance 22. This device is connected in parallel across the discharge device. When the switch 21 is closed and the capacitor 20 is discharged over the discharge device (transfer element), the current through this will increase so that a cut-off occurs. The extra current from the capacitor must be so great that the instability lasts so long that-the series switch 18 has time to be deionized. If the discharge device 18 consists of an arc which burns in mercury vapor the dimensions of the vessel, the gas pressure and type of gas are chosen so that the discharge can be maintained only if an extra gas inlet is arranged in the vicinity of the anode. When this-gas inlet is closedthedischarge is quenched in the discharge and the current is cut off. i

If the high speed switch 19 of FIG. 10 consists-of-a continuously burning low pressure discharge device of the rectifier valve type, the breaking device will not contain any mechanical parts and can thus be very quick. In such a case a current instability is induced in the one valve so that both the valves can be currentless for a short time. Th valve which has been normal-conducting is deionized quickly and can thus break the circuit even if the first valve should re-ignite after the instability. If the series switch is provided with blocking grids, this is given a negative potential when the first valve cuts off. The cut-off must be of such a long duration that the grid has time to block the current path. The series switch can also be without grids, but then the cut-off must be of such a long duration that the de-ionization can be sufliciently great.

Another way to shorten the conductive time of the trans fer element is to consitute the series switch of one or several triggered spark gaps. At the same time that the transfer element receives an ignition impulse the spark gap is also ignited. The transfer element now only needs to conduct for the time which the seriese switch needs to quench. In order that this device shall function it is however necessary that the sum of the resistances of the transfer element 5 and the series switch 8 is much less than the resistance of the auxiliary switch 4, otherwise no commutation will be carried out.

Likewise, if the circuit is strongly inductive, it can be practically impossible or at least unsuitable to dimension the resistor for all the magnetic energy which is stored up in the circuit.

FIGS. 11 and 12 show both of these possibilities. In them the re-resistor means is built up of a number of partial resistors which by means of capacitors and spark gaps are connected in parallel to the transfer element. Each of the capacitors is provided with a discharge resistor connectible over an auxiliary spark gap. In the embodiment according to FIG. 11 the first partial resistor 60 is connected between the points 2 and 3 by means of a spark gap 50 and a capacitor 40. A second partial resistor 61 is in a similar way connected by means of a spark gap 51 and a'capcitor 41. The capacitor 40 can be discharged by means of a discharge resistor 63 and an auxiliary spark gap 53. In the same way the capacitor 41 can be discharged over a discharge resistor 64 and an auxiliary spark gap 54. All the spark gaps are controllable so that their time of ignition can be determined.

When the voltage rises over the device due to the reversal of the transfer element 5 from low to high ohmic condition, the spark gap 50 will ignite at the maximum permissible voltage value over the device. The capacitor 40 then begins to be charged with a time constant which is determined by the break current and the capacitor 40, while at the same time a part of the magnetic energy is used in the resistor 60. When the capacitor begins to be charged, the current ceases to flow in these branches and the voltage over the device begins to rise again. As soon as the voltage once again reaches the maximum permissible value, the spark gap 51 is ignited and the current begins to fiow through the branches 41, 51, 61, so that the capacitor 41 is charged and energy is dissipated in the resistor 61. As soon as the current has ceased to go through the circuit 40, 50, 60 the spark gap 50 becomes extinguished and the auxiliary spark gap 53 is ignited so that the capacitor 40 is discharged across the resistor 63. The circuits are dimensioned so that the time constant for 63, 40 is less than for 61, 41. By this means the capacitor 40 is discharged before the capacitor 40 is fully charged.

When the capacitor 41 begins to be fully charged, the voltage begins to rise again. Then 50 will be ignited so that the current is connected back to the branches 45, 50, 60. The spark gap 51 becomes extinguished and the spark gap 54 receives an ignition impulse so that the capacitor 41 is discharged over the resistor 54 which is to be carried out in a time which is shorter than the time which the capacitor 40 needs to be charged. Then the operation described is repeated until the inductive energy which is left can be stored in one of the capacitors 40 and 41. The circuit is then broken. 4

If it is found to be suitable several branches with capacitors, spark gaps and resistors can be connected one after the other, so that the current can be broken without the first connected branch needing to be used again.

FIG. 12 shows a circuit which comprises a spark gap without further ignition devices and where the auxiliary spark gaps for the discharge circuits of the capacitors are made with three electrodes with the center electrode connected to a branch lying close by. The ignition voltages for the spark gaps 50, 51 and 52 should be somewhat different so that V V V When 50 has ignited and 40 has become fully charged, the voltage rises over the ignition voltage for 51 so that this ignites and the current is led over to the branch 41, 61. Thereby an ignition pulse occurs over the spark gap 53 through the capacitor 43 with a certain time displacement. This displacement should be so great that 50 has time to close before 53 ignites. As soon as 41 is fully charged, 52 ignites and and after a delay 44 ignites so that 41 is discharged. In the same way the other branches are connected in one after the other. Finally the current is so low that the remaining inductive energy is stored in the branch which is connected in last.

With this connection the discharge of the capacitors does not need to be carried out particularly quickly, but the time constant for the discharge circuits can be made so long that the complete breaking process is accomplished before the first connected capacitor is discharged.

If a smaller number of branches is used, a reconnection from the last to the first branch can be made, as indicated with the broken line and the capacitor 45 in FIG. 12. In this way the current can be connected in several times to each branch. In this case the time constant for the discharge of the capacitor is chosen so that a certain capacitor does not need to be completely discharged until just before its branch is to be connected into the circuit, as otherwise its spark gap will be ignited too soon.

Devices of the type now described are known and used within the power industry for the protection of equipment against overvoltages. According to the present invention their main object is not to protect equipment but to take care of and dissipate the inductive energy which is stored in a transmission for high voltage direct current in order to make possible a safer breaking of the current.

I claim:

1. A device for breaking a high voltage inductive electric circuit, preferably a line for high voltage direct current, said device comprising a series connection of a main switch and a divided circuit, said divided circuit comprising a parallel connection of resistor means for conducting the entire current in the inductive electric circuit, a transfer element which at least momentarily can conduct the entire current and an auxiliary switch which is normally closed and conducts the current in the circuit, said divided circuit being provided with means for opening the auxiliary switch at the beginning of a breaking process and for creating a starting impulse for the transfer element,

whereby said transfer element will be conducting with an impedance which is substantially lower than the impedance of the resistor means, said transfer element having means for rapidly increasing its impedance to a value which is substantially higher than the impedance of the resistor means, whereby the current is forced to pass through the resistor means and thereby is reduced to such a low value that the main switch can cut off the current.

2. A device according to claim 1, in which said switch element comprises a gas discharge means, in which the arc discharge is controllable from plasma condition with a very low voltage drop to space charge condition with a very high arc voltage drop.

3. A device according to claim 2, in which the ignition voltage of said gas discharge means is so low that the voltage which occurs across the discharge means when the auxiliary breaker is opened is sufficient to start a current flow through the gas discharge means.

4. A device according to claim 2, said gas discharge means being provided with a vacuum pump which, when the auxiliary switch is closed, keeps the pressure at such a low value that the arc voltage drop is very high, said gas discharge being provided with a plasma gun forintroducing a plasma into the discharge section of the discharge means, whereby the discharge means becomes conducting with a very low arc voltage drop.

5. A device according to claim 1, a circuit opening means, said transfer element being series connected with said circuit opening means, and a series connection of two impedances being connected in parallel to the series connection of said transfer element and said circuit opening means.

6. A device according to claim 1, the resistor means comprising a number of parallel branches, each branch comprising a series connection of a partial resistor, a spark gap and a capacitor.

7. A device according to claim 6, each capacitor being connected in parallel with a series connection of a discharge resistor and an auxiliary spark gap.

8. A device according to claim 1, the resistor being non-linear.

References Cited UNITED STATES PATENTS 2,445,836 7/ 1948 McCrosky 317-26 2,888,639 5/1959 Petermichl et al. 324-28 3,312,889 4/1967 Gold 320-36 2,849,659 8/ 1958 Kesselring 31711 3,309,570 3/1967 Goldberg 317-11 3,310,707 3/1967 Boksjo 315-123 3,252,014 5/1966 Sevlen 307-136 OTHER REFERENCES Applicant citation: Lowder Fast Gas Valve pp. 1236- 1238 of Review of Scientific Instruments, November 1962.

JOHN W. HUCKERT, Primary Examiner S. BRODER, Assistant Examiner US. Cl. X.R.

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