US2721960A - Vapour electric discharge apparatus - Google Patents

Description

Oct. 25, 1955 D. L. SMART 2,721,960

VAPOUR ELECTRIC DISCHARGE APPARATUS Filed Feb. 25, 1952 2 Sheets-Sheet 1 A FIGZ.

FIG. 3.

2| Inventor QM Kiwi Attorney:

Oct. 25, 1955 D. 1.. SMART VAPOUR ELECTRIC DISCHARGE APPARATUS 2 Sheets-Sheet 2 Fi led Feb. 25, 1952 United States Patent Ofiice 2,721,960 Patented Oct. 25, 1955 VAPOUR ELECTRIC DISCHARGE APPARATUS David Lorimer Smart, Stafford, England, assignor to The English Electric Company Limited, London, England, a British company Application February 25, 1952, Serial N 0. 273,284 18 Claims. (Cl. 315-258) This invention relates to vapour electric discharge devices.

In the operation of such devices with parallel discharge paths, it is desirable to provide some means for ensuring substantially equal load sharing between the paths concerned.

One conventional method of achieving this result is to include a choke, known as a drop choke, in series with each anode or, inthe case of single anode devices, in series with each cathode. These chokes are designed so that, say, to volts are induced in them with the normal rate of rise of current. With such an arrangement, any path not taking its fair share of current will have a lower voltage induced in its choke and the difference in choke voltages consequent upon the respective rates of rise of current will appear as an additional positive voltage applied to that path tending to make it take more current. Similarly, during the period in each cycle when the current is at its full value, the chokes tend to prevent any change in current of the discharge paths with which they are associated. If, however, discharge is not initiated in any one path until part of the way through the commutation period, the other paths will already be carrying some current and only the remaining rise in current will be equally shared. Furthermore, if one path fails altogether, the current will be equally shared between the remainder.

A further method sometimes used for a pair of discharge paths is a balance choke in which connection to the external circuit is made at a centre tap of the choke, the ends of the choke being connected to the respective paths. With this arrangement there is no voltage drop in normal operation since the current flows in opposite directions in the same winding and a much higher value of inductance is obtainable. However, as with the case of a drop choke, late initiation of one discharge path causes unbalance and a failure of one path can cause high voltages to be set up where the choke has a high inductance.

In either case the use of grid control for firing delay gives rise to much more rapid commutation over a shorter period and the importance of simultaneous initiation of the paths is therefore greatly increased as is also the danger of over-voltages in the case of high inductance balance chokes.

According to one aspect of the present invention, an electric discharge device installation having parallel vapour discharge paths is connected to an external circuit through one or more inductive devices arranged so as to increase the ettective voltage across any discharge path not taking its full share of the load and a resistance path is provided between said paths at those ends thereof with which said inductive devices are associated.

The resistance path which is provided according to the invention may, if desired, be composed of one or more resistors of the non-linear type. Thus if a resistor is used of the kind whose resistance varies inversely with the voltage applied across it, a higher balancing voltage can be produced on small amounts of unbalance than would otherwise be the case, without exceeding a predetermined maximum balancing voltage on large amounts of unbalance.

Other aspects and features of the invention will appear from the following description, reference being had to the accompanying drawings in which:

Figs. 1 and 2 are diagrams illustrating two known arrangements for parallel operation of mercury arc rectifiers;

Fig. 3 shows the basic arrangement according to the invention applied to a balance choke for operating a pair of parallel discharge paths;

Fig. 4 shows a modification of the arrangement of Fig. 3 for operating three discharge paths in parallel;

Fig. 5 shows the application of the invention to three discharge paths in conjunction with drop chokes, and

Figs. 6 and 7 show the application of the invention to the special case of discharge tubes which have high voltage potential dividing grids.

Referring first to Fig. 1, four anodes 1, 2, 3, 4, which are assumed to be associated either with a common cathode or each with a cathode in separate rectifier bulbs, are connected to a line 5 through individual chokes 6, 7, 8, 9. As has already been mentioned, this arrangement sullers from the disadvantage that if one of the anodes 1, 2, 3, 4, does not pick up until part of the way through the commutation period, the others will be already carrying some current and the tendency of the chokes to provide equal load sharing will be applied only to the remaining rise in current.

Referring now to Fig. 2 where a pair of anodes 10, 11 are connected through a balance choke 12 to a line 13, the same disadvantage is again present and, in addition, complete failure of one anode may result in undesirably high voltages being set up in the choke which, in the case of a balance choke, is usually of higher inductance.

Where discharge devices operated in parallel are subject to grid control for delaying the firing point, these disadvantages are considerably accentuated since the commutation is more rapid. With such an arrangement, therefore, it is desirable to avoid the quite serious disturbance in load sharing which can be caused by even quite small differences in pick-up time of the respective discharge paths. Furthermore, the voltage difierence between anodes should not exceed a predetermined maximum especially in the case of a device operating on high voltages, e. g. of the order of 520 kv. By way of example, in the case of a pair of grid controlled rectifier tubes operating in parallel, it may be desirable to limit the voltage diiference between the anodes to volts with anode currents of about 20 amperes (peak) per tube and the maximum voltage difference should preferably be of the same order both with and without delay by grid control. The arrangement shown in Fig. 3 is intended to achieve this kind of result and operates in the following manner:

The anodes 14 and 15 of a pair of grid controlled rectifiers operating in parallel are connected through a balance choke 16 to a line 17 and a resistor 18 is connected across the ends of the choke 16. The resistance value of the resistor 18 is 5 ohms. If now we assume that the anode 14 has picked up and is operating at full current before the anode 15 has started, it will be seen that there will be a current of approximately 20 amps. flowing directly to the anode 14 through the left-hand half of the choke 16 and a further current of approximately 20 amps. flowing through the right-hand half of the choke 16 and through the resistor 18 to the anode 14. There will thus be a voltage drop along the resistor 18 of 100 volts and this will appear as an additional voltage on the anode 15. It now the anode 15 picks up, the current in the resistor 18 will fall to Zero 3 I and balance will be established. It will be seen that with this arrangement the balancing voltage is dependent upon the value of current rather than upon its rate of rise. The inductance and core section of the choke need be designed only to prevent a serious unbalance building up in the two circuits in any reasonable time of delay, say to 18 or 22 amperes in one-tenth of a cycle. Thus di/dt 2 amps/2 In. secs.

Fig. 4 shows an arrangement using the principle of that shown in Fig. 3 applied to the operation of three parallel discharge paths. In this case anodes 19, 20 and 21 are connected to a line 22 through balance chokes. The arrangement of the choke windings is such that each anode is fed through two choke windings each of which is inductively associated with one of the choke windings through which a different one of the anodes is fed. Thus the anode 19 is connected to the line 22 via a winding 23 and a winding 24. The anode 20 is connected to the line 22 through a winding 25 and a winding 26, and the anode 21 is connected via a winding 27 and a winding 28. The windings 23 and 26, 25 and 28, 24 and 27 respectively are arranged on common cores. These cores may be entirely separate or may be limbs of a common core after the manner of a three-phase transformer. Resistors 29, 30 and 31 are connected respectively to the anodes 19, 20 and 21 and to a common point which may, if desired, be connected to the line 22. The circuit would still operate with only two resistors in circuit, for example, between anodes 19, and 20 and between anodes 20 and 21, but the balancing voltage available would not be the same for each anode.

Fig. shows the same principle applied to drop chokes and in this case three anodes 33, 34 and 35 are connected to a line 36 through chokes 37, 38 and 39 respectively. Resistors 40, 41 and 42 are connected to the anodes 33, 34 and 35 respectively and to a common point.

In the case of parallel operation of discharge paths which have high voltage potential dividing grids which in single operation would be allowed to float in potential, a further difficulty arises in that, if one path fires before the others, the changes in anode potential of the other path or paths may drive the potential dividing grid or grids negative by capacity effects and, in the case of a high voltage device, these changes in anode potential are very large compared with the choke voltage. In this case the arrangement shown in Fig. 6 may be used which shows in diagrammatic form a pair of rectifier tubes each having an anode 43, a high voltage potential dividing grid 44, acontrol grid 45, an excitation and ignition electrode 46 and a cathode 47. In this case the potential dividing grids 44 are connected together either directly or through an impedance 48 and a balance choke 49 is connected in the cathode circuit and is shunted by a resistor 50. The control grids 45 are connected through resistors 51 and 52 to a common source 53 of control voltage. Thus when one tube fires, its cathode becomes more positive than the other. This ensures that both the control grid 45 and the potential dividing grid 44 on the non-firing tube are positive with respect to the cathode 47 since the grids on the firing tube can supply reverse grid current from the ionisation in the tube. Thus by this method of interconnection with the choke in the cathode circuit, each electrode is supplied with a small current providing it with a sufficiently positive potential to enable it to fire and provide sufficient ionisation for the next electrode to pick up. It will be appreciated, of course, that the choke 49 may be replaced by drop chokes and that a corresponding arrangement may be used where more than two parallel discharge paths are involved.

Fig. 7 shows a further arrangement of this kind but in this case it is assumed that the control grids are excited by separate synchronised supplies and that a potential dividing network is used to control the potentials of the three potential dividing grids in each discharge path. The network 56 is connected between an incoming line 54 and an outgoing line 55. This potential dividing network may consist of either resistance or capacitance or, as shown, by a combination of the two. Grids 57, 58 and 59 in each tube are connected to various points along the network 56 and appropriate pairs of grids are connected together through paralleling impedances 57a, 58a and 59a. In other respects the circuit'is similar to that of Fig. 6 except that separate synchronised supplies 60 are provided for the control grids. The paralleling impedances 57a, 58a, 59a are not essential but if provided would normally be of low values in relation to the impedances constituting the potential dividing network 56.

What I claim as my invention and desire to secure by Letters Patent is:

1. An electric discharge device installation having separate electrodes terminating parallel vapour discharge paths at at least one common end thereof, an inductive device connected in series between each of said electrodes and an external circuit, and a substantially non-inductive resistor inter-connecting at least two of said electrodes.

2. Apparatus according to claim 1 wherein electrodes terminating two parallel vapour discharge paths at a common end thereof are connected together through a single inductive device and to an external circuit by means of a centre tap on said inductive device, the inductive device being shunted by a resistor.

3. An electric discharge device installation according to claim 1, wherein the value of resistance of said resistor is such that whatever the distribution of the total discharge current between the electrodes inter-connected thereby the efiective voltage across any such discharge path will not be increased by more than one-tenth of the mean efiective voltage across any of the discharge paths in the non-conducting state.

4. An electric discharge device installation including separate parallel vapour discharge devices and having an inductance in series with each such device, and a resistance path between at least two of said devices at those ends thereof which are connected to an inductance.

5. An electric discharge device installation having separate electrodes terminating parallel vapour discharge paths at at least one common end thereof, an inductive device connected in series between each of said electrodes and an external circuit, and a resistor connected between each of said electrodes and a common point.

6. An electric discharge device installation including separate parallel vapour discharge devices and having an inductance in series with each such device, a resistor connected between the junction of each of the discharge paths and its associated inductance and a common point.

7. An electric discharge device installation having separate electrodes terminating three parallel vapour discharge paths at at least one common end thereof, said electrodes being connected to an external circuit through an arrangement of balance chokes said arrangement consisting of three chokes each having two windings one of which is connected to the appropriate one of said electrodes and the other of which is connected in series with the first mentioned winding of the choke appropriate to a different one of said electrodes and a resistor con nected between the point of connection of said first mentioned winding with its appropriate electrode and a common point;

8. An electric discharge device installation including separate parallel vapour discharge devices and having an inductance in series with each such device at the cathode end thereof, a resistance path between at least two of said cathodes, and interconnected potential dividing grids in said discharge devices.

9. Apparatus according to claim 8, wherein control grids for the respective discharge paths are connected through resistors to a common source of control voltage.

10. An electric discharge device installation having separate electrodes terminating parallel vapour discharge paths at at least one common end thereof, wherein each said electrode is connected to an external circuit through an inductive device, the arrangement being such as to increase the efiective voltage across any discharge path not taking its full share of the load, connections between said electrodes, and a voltage dependent resistor in each of said connections,

11. Apparatus according to claim 10 wherein electrodes terminating two parallel vapour discharge paths are connected together through a single inductive device and to an external circuit by means of a centre tap on said device, the device being shunted by a resistor.

12. An electric discharge device installation including separate parallel vapour discharge devices and having an inductance in series with each such device, a resistance path including at least one voltage dependent resistor etween at least two of said devices at those ends thereof which are connected to an inductance.

13. An electric discharge device installation having separate electrodes terminating parallel vapour discharge paths at at least one common end thereof each of which electrodes is connected to a load through an inductance, a voltage dependent resistor connected between the junction of each of the electrodes and its associated inductance and a common point.

14. An electric discharge device installation including separate parallel vapour discharge devices and having an inductance in series with each such device, a voltage dependent resistor connected between the junction of each of the discharge paths and its associated inductance and a common point.

15. An electric discharge device installation having separate electrodes terminating three parallel vapour discharge paths at at least one common end thereof and connected to an external circuit through an arrangement of balance chokes said arrangement consisting of three chokes each having two windings one of which is connected to the appropriate one of said electrodes and the other of which is connected in series with the first mentioned winding of the choke appropriate to a different one of said electrodes and a voltage dependent resistor connected between the point of connection of said first mentioned winding with its appropriate electrode and a common point.

16 Apparatus according to claim 15, wherein said common point is connected to the load.

17. An electric discharge device installation including separate parallel vapour discharge devices and having an inductance in series with each such device at the cathode end thereof, a resistance path including at least one voltage dependent resistor between at least two of said cathodes, and interconnected potential dividing grids in said discharge devices.

18. Apparatus according to claim 17, wherein control grids for the respective discharge paths are connected through resistors to a common source of control voltage.

References Cited in the file of this patent UNITED STATES PATENTS 2,016,148 La Roque et al. Oct. 1, 1935 2,140,736 Demontvignier Dec. 20, 1938 2,418,161 Campbell Apr. 1, 1947

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