US2897404A - Grid-controlled mercury-arc rectifier - Google Patents

Description

July 28, 1959 D. CSILLINGH v GRID-CONTROLLED,MERCURY-ARC RECTIFIER Filed Sept. 22, 1955 2 Sheets-Sheet 1 IN V EN TOR.

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United States Patent GRID-CONTROLIJED MERCURY-ARC RECTIFIER Dezs Csillingh, Budapest, Hungary, assignor to Licencia Talalmanyokat Ertkesitii Vllalat, Budapest, Hungary, a Hungarian company Application September 22, 1955, Serial No. 535,929

Claims priority, application Hungary September 27, 1954 8 Claims. (Cl. 315-205) This invention relates to grid-controlled mercury-arc rectifiers, especially to high-power polyphase-rectifiers provided with a mercury-pool cathode and an anode for each phase, adapted to carry heavy currents, each of said anodes being provided with a grid at least partly surrounding the said anode, and also relates to the circuits used in connection with such rectifiers for controlling their direct current voltage and ensuring safety against shortcircuiting or arcing back and both short-circuiting and arcing back.

In the known rectifiers of the type specified grid control is used for ensuring the right moment of ignition of the anode or anodes, for accelerating de-ionization and for ensuring protection against short-circuiting caused by arcing back. For this purpose, the grids of these rectifiers are supplied with a separate extraneous grid potential produced by special auxiliary apparatus and circuits. These auxiliary additional means for producing said grid potential comprising auxiliary transformers, rectifiers, and

'other accessories, are intricate, expensive, and consume quite appreciable currents, impairing thereby the efiiciency of the rectifier and also being often the source of trouble or breakdown in the service of the rectifiers. Furthermore, in actual practice, it is also necessary to provide overload switches on the DC. side of the rectifiers, these switches being frequently used also for remote interruption or control of the DO' supply lines. These switches, having to interrupt heavy direct currents at usually quite high voltages, constitute heavy and expensive apparatus necessitating special mounting and much additional floor space, and being very noisy in operation when interrupting line currents of some hundreds or even thousands of amperes or short-circuit currents of even higher ratings with nerve-wracking, explosion-like reports, cause a heavy strain on the workers.

it is an object of the present invention to provide a new and improved grid-controlled mercury-arc rectifier needing neither a separate extraneous grid potential nor expensive auxiliary means for producing such potential, but providing its grid potential by deriving it from the potental of the anode surrounded by the grid in question by means of a simple and inexpensive electrical circuit, thereby obviating the necessity of installing and using expensive and intricate auxiliary means for supplying an extraneous separate grid potential.

It is a further object of the invention to provide an improved rectifier of the type specified consuming no noteworthy power for its grid voltage supply and ensuring thereby an improvement in the efficiency of the rectifier, and other advantages.

It is another object of the invention to provide an improved rectifier of the type specified, which in itself constitutes an overload and distant-control switch for the direct current produced by said rectifier, ensuring also perfect safety against short-circuiting, whether caused by arcing back or other causes, and being perfectly noiseless in operation.

It is a further object of the invention to provide an improved rectifier of the type specified, which allows easy and accurate control of the direct-current voltage of the rectifier by means of the grid potential derived from the anode potential.

Other objects and advantages of the invention will be apparent from the following description explaining the invention.

According to the invention, the rectifier of the type specified comprises a grid assemblage for its anode or, in the usual case of poly-phase rectifiers, for each of its anodes, constructed and disposed in a manner to screen said anode in such a manner that ignition to the anode cannot occur even at the peak voltage of the AC. supply and in the normal state of working of the auxiliary anodes if said grid assemblage is not provided with a suitable grid potential. The term grid assemblage is used here inafter and in the appended claims to mean a single grid or a plurality of grids surrounding and thereby screening its anode in the manner specified, thereby rendering ignition to the said anode impossible when the grid assernblage receives no suitable grid voltage ensuring ignition. It has been found that such grid voltage does not need to consume appreciable current compared to those consumed by the grids of the known rectifiers of the types specified. It has further been found that by using a grid voltage derived from the anode voltage instead of a separate extraneous grid voltage produced by auxiliary means, the danger of arcing back is substantially reduced, ing almost nil in actual working practice. it is believed that this favorable effect is due to the fact that the asymmetry of the extraneous grid voltage in respect of the voltage of the anode is absent in case of a grid potential derived from the anode voltage, but the invention is untouched by the correctness or eventual incorrectness of this theory, formed as a result of investigations relating to the causes of arcing back and seeming to show that the perfect symmetry of the grid potential derived from the anode voltage has also a favorable smoothing effect to the unavoidable asymmetries of construction apt to cause trouble in actual working practice. in order to show the substantial saving in grid current achieved by using the new rectifier, it may be mentioned that a known rectifier of usual construction having a DC. output of about 700 amperes at a voltage of 3000 volts or an output of about 3000 amperes at a voltage of 500 volts consumed about 60 to milliamperes of grid current for each of its anodes, whereas the same rectifier when constructed according to the invention consumed only about 2 miiliamperes of grid current per anode.

The grid assemblage used according to the invention to screen its anode in the manner specified may consist of one or more grids electrically connected to each other and surrounding at least partly the same anode, and may even comprise in some cases a further grid insulated from the other grid or grids and also from all other electricaliy conducting parts of the rectifier, such insulated grid being usually the outermost of the grids of the grid assemblage, i.e. the grid positioned farthest from the anode. The desired screening effect of the grid assemblage is obtained by suitably choosing the individual and total cross-sections of the perforations of the grid or grids which may consist of graphite or iron or any other suitable material, and their disposition in relation to each other or to the anode, the actual construction and arrangement being best determined by experiments. When such grid assemblage receives no grid potential, it becomes loaded to an oscillating potential by capacitive effects, this potential ensuring such screening of the anode that no ignition can occur to the said anode.

The grid potential fed to the grid assemblage is derived from the anode voltage, according to the invention, by interconnecting between the anode and its grid assemblage a suitable impedance, which may "consist of a simple ohmic resistance, a capacitance, or an inductance or a suitable combination of two or three of these. The nature and electrical dimensioning of this impedance depends mainly of the capacity of the gridassemblage and can be determined by experiments; As a general rule, in case of substantial capacities of the grid assemblage, the impedance ought to be slightly inductive, in case ofimean capacities anohmic resistance will generally be suitable, whereas in case of small capacities of the grid assemblage the impedance should be capacitive.

As it has been stated above, the rectifier according to the invention has its anode oranodes screened by the grid assemblage in such a manner, that ignition to the anode, and therefore functioning. of the rectifier is possible only if the grid assemblage receives its suitable grid voltage derived from the anode voltage, so that the function' of the rectifier is instantly stopped when the con-- nection between the anode and grid assemblage established by the interconnection of the impedance specified above is broken. This fact enables the new rectifier to constitute in itself an overload or distant control switch for the direct current, by providing a circuit-breaking contact in the grid potential circuit, or in case of polyphase rectifiers, such a contact in each grid potential circuit. If these contacts are actioned to open the grid circuit, which may be done by a suitable electromagnetic device controlled by the alternating current of the rectifier, or a manual or distant control, the rectifier instantly stops its functioning and therefore the direct current disappears without the noise and other troublesome phenomena caused by interrupting'it by means of a D.C; ovcrload switch. As it has been already mentioned, the grid currents to be interrupted for this purpose are only of the order of magnitude of a milliampere, and therefore the contacts breaking them may be light, small and inexpensive and function very quickly, thus ensuring instantaneous and complete protection against short-circuiting and arcing back, without. necessitating the use of separate auxiliary grid potentials and means producing these potentials forthis purpose.

According to a further feature of theinvention', the rectifier may be provided with a very accurate, convenient and cheap control of the producediD'.C. voltage by means of the grid potential by providing in the grid circuit or in case of poly-phase rectifiers in each of the grid circuits, in addition to the contact-breaking device specified above, a special device for periodically influencing the grid potential. By this periodical influencing of the grid potential the absolute discharge time of the anodes may be altered from the possible maximum to nil, according to the setting of the device, thus ensuring acontrol of the effective DC. voltage, this voltage being the maximum when the gridreceives' the grid potential necessary to enable the discharge to the anode during all the time of the half-periods during which the anode is positive, and being nil if the grid receives no potential during these periods, any other setting being also possible between these limits. This control therefore functions by varying the natural discharge times of the anode or anodes according to the setting of the grid potential influencing device. The means for periodically influencing the grid potential may either be mechanical, and consist of a ro tary or other suitable contact which opens and shuts the grid circuit periodically and may be adjusted to vary the relation of the times during which said circuit is shut and open, or electrical, consisting in this case of means for generating a voltage periodically counteracting the grid voltage, the extent and phase of such counteracting being adjustable in order to effect the desired control. In this relation it is to" be remarked that it is not necessary to drive the mechanical means opening and closing the grid circuit by a synchronous motor or other similar device ensuring phase conformity between the anode potential and the contact-making and breaking means, as ignition to the anode cannot take place during the half-periods when the anode is negative, and it is therefore immaterial whether the grid receives voltage during these half-periods or not, and accordingly the drive may either be a synchronous or an assynchronous drive. In case of electrical influencing-means consisting preferably of a phase shifting transformer or other similar device, the secondary winding of which is connected into the grid circuit, phase conformity isusually assured by feeding the'primary winding of the said transformer from the mains supplying the anode voltage.

For a better. understanding of the present invention, together with other advantages and details thereof, reference'is hadto' the following detailed description of some embodiments thereof to be considered as examples, and to the accompanying diagrammatical drawings.

Referring to the drawings,

Fig; 1 shows a circuit diagram of a six-phase rectifier ofthe type specified embodying the invention, and

Fig. 2 shows a circuit diagram of a rectifier unit according: to the invention provided with direct-current voltage control.

Referring to Fig. 1, this shows the circuit diagram of a six-phase rectifier consisting of six separate units or cells, for example of the ignitron type. It is however to be pointed out that the invention may also be used in single phase rectifiers or poly-phase rectifiers of other types, for example those containing a plurality of anodes in a com mon cell provided with a common mercury-pool cathode.

For the sake of simplifying Fig. 1. and ensuring its clearness, of the six units formingthe rectifier shown on this figure, only the unit marked with the character of reference I is shown in detail, the other five units marked If, III, IV, V and VI being only indicated by oblongs, as they are. identical with each other and with the unit I.

In Fig; 1: the primary windings of the conventional transformer 1. can be connected to the three-phase feeding. mains in the usual manner by means of the switch 2. Each of the six secondary windings of the transformer 11 is connected'to the anode of one'of the six units forming the rectifier, the connection to the anode 6 of the cell 3' of unit I being shown on this figure in detail. This cell 3' is provided with a mercury-pool cathode 4 of any convenient design, the starting or igniting means being diagrammatically indicated at 5. It is to be understood that these means may be of any known or desired kind, and may comprise auxiliary anodes, or the like, as known to those skilled in the art, these means being immaterial from the point of view of the invention. Thegrid assemblage 7 shown on this figure consists of three grids electrically connected to each other, but may also consist of a single grid, two interconnected grids, or, in addition to these, a further insulated grid surrounding the said grids, as mentioned above- The grid assemblage 7 receives its grid voltage derived from the voltage of its anode6 through the impedance 8' connected between said anode 6 and: the grid assemblage 7. This impedance 8 consists, in the example shown by Fig. 1, of the ohmic resistance R connected in parallel with the capacitance C and the inductance I, but as already mentioned, this impedance 8' may consist of either of the elements R, C and I singly or in any other suitable combination. The grid circuit which is normally closed to enable the functioning of the rectifier can be opened by the contact-breaking device 9 common for all the six units and provided with suitable contacts for the grid circuits of each unit, as indicated on this figure. All of the six grid circuits are simultaneously opened if any' of the magnet coils 10, 12 or 14 is energized, whereby, in accordance to what has already been stated, the rectifier ceases to function instantly. The coil 10 is connected to the current transformer II and the coil 12' to the current transformer 13'. Each of these current transformers is connected into" a separate phase of the primary of the transformer 1, and in consequence thereof any predetermined overload or any short-circuit actuates the contact-breaking device 9 irrespectively of whether it occurs in the direct current circuit outside of the rectifier, or inside any of the units or a plurality of the units constituting the rectifier. The coil 14 connected to the mains may be energized by closing the switch 15, thereby also causing the rectifier to stop its operation, and therefore this system constitutes the manual control or distant-control device, whereas the coils 10 and 12 together with the transformers 11 and 13 and their connections constitute the overload and safety device already mentioned above. Owing to the very weak currents to be interrupted by the device 9 this device may be light, small and cheap, and very quick of operation. The coil 14 may also be used for actuating the contact-breaking device 9 in case of small overloads or back currents. For this purpose an auxiliary contact not shown on the drawing is connected in parallel with the contact 15 and actuated by an overload and back current relay.

Referring now to Fig. 2, the rectifier unit shown on this figure diifers from that shown on Fig. 1 only in that its grid assemblage 7 consists, in this example, only of a single grid, and in that it is provided with direct current voltage control means 16 and 17, either of which may be connected into the grid circuit by means of the changeover switch '18. In actual practice it is usually quite sufiicient to install and use only one of these means constantly connected into the grid circuit in series with the contacts of the contact-breaking device 9.

The mechanical voltage control means 16 shown on Fig. 2 as an example consist, in principle, of the adjustable contacts 19 and 20, adapted to be electrically connected with each other by means of the conductive segment 21 rotatable around and by means of the shaft 22 driven by a suitable motor not shown on the diagrammatic drawing. Voltage-control may be effected, in the sense of what has been said above, by varying the relative angular distance of the contacts 19 and 20, or the angular extension of the segment 21 or the speed of rotation of the shaft 22, it being of course also possible to combine these methods with each other in any suitable or desired manner. The motor driving the shaft 22 may be a synchronous or asynchronous motor, or any other suitable motor provided with speed regulating means. Instead of the rotatable contact-making and contact-breaking means 16 it is also possible to use oscillatory means of any suitable type, as apparent to those skilled in the art.

The electrical voltage control means 17 shown on Fig. 2 consist of a phase-shift transformer, the secondary winding 23 of this being connected into the grid circuit in series with the contacts of the contact-breaking device 9, and the primary winding 24 being fed from the A.C. anode voltage of the anode 6. Voltage controlling by this device 17 is effected by suitably adjusting the phase-shift transformer to induce in its winding 23 a voltage partly or wholly counteracting the voltage supplied to it through the impedance 8, and thereby periodically altering the effective voltage on the grid assemblage 7. By this arrangement, voltage control is also effected by means of a voltage taken from the circuit of the anode in question, thereby obviating the disadvantages caused by introducing an extraneous voltage and its undesirable effects into the system, as has already been stated above.

Instead of the phase-shift transformer described above, other electrical control means may also be used, based on other principles, it being only essential to feed these devices from the voltage of the anode the effective D.C. voltage of which is being controlled by means of the grid voltage periodically influenced by said electrical control means.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is therefore aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention. In the claims, no modification is intended to be claimed with the exclusion of others, and the right subsequently to claim all modifications not covered by the claims is expressedly reserved.

What is claimed is:

l. A grid-controlled mercury vapor rectifier system comprising a polyphase transformer having a primary and a plurality of secondary circuits, a polyphase supply line connected to said primary circuit, a mercury vapor rectifier having its main discharge path in each of the secondary circuits, each rectifier having an anode, a cathode, and grid means screening the anode, each rectifier having its anode connected in series with the corresponding secondary circuit and its cathode connected to a common load, a circuit for each rectifier connected between the anode and grid for deriving a voltage from a voltage on the anode and applying it to the grid, a contactor in each grid-anode circuit, and means responsive to an overload in any portion of the system for simultaneously actuating the contactors in the grid-anode circuit of each rectifier to open the grid-anode circuits.

2. A grid-controlled mercury vapor rectifier system comprising a polyphase transformer having a primary and a plurality of secondary windings, a polyphase supply line connected to said primary winding, a mercury vapor rectifier for each secondary winding having an anode, a. cathode, and grid means screening the anode, each rectifier having its main discharge path connected in series with its corresponding secondary winding and a common load, a circuit for each rectifier connected between the anode and grid means for deriving a voltage from a voltage on the anode and applying it to the grid means, a contactor in each grid-anode circuit, and means responsive to an overload in any portion of the system for simultaneously actuating the contactors in the grid-anode circuit of each rectifier to open the grid-anode circuits.

3. A rectifier system according to claim 2 in which the means for actuating the contactors comprises electromagnetic actuating windings each energized by a separate phase of the primary of the transformer upon passage of an overload through the same.

4. A rectifier system according to claim 2 further comprising means separately operable for actuating the contactors to simultaneously open them.

5. A rectifier system according to claim 2 further comprising means for periodically controlling the voltage applied to the grids for automatically regulating the direct current produced by the system.

6. A rectifier system according to claim 5 further comprising means in each of the grid-anode circuits for developing a voltage opposing periodically the grid voltage, said means embodying an adjustable phase shift transformer having a primary winding connected across the secondary of the polyphase transformer and a secondary winding in the grid-anode circuit.

7. A rectifier system according to claim 5 further comprising means for intermittently opening and closing the grid-anode circuit.

8. A rectifier system according to claim 7 further comprising means for varying the frequency of operation of the opening and closing means and the ratio of the time the circuit is closed to the time it is open.

References Cited in the file of this patent UNITED STATES PATENTS 1,550,877 Chafl'ee Aug. 25, 1925 2,083,701 Fouquet June 15, 1937 2,426,054 Rose Aug. 19, 1947 2,550,477 Herskind Apr. 24, 1951

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