US2506938A - Lightning arrester construction - Google Patents

Description

May 9, 1950 R. R. PITTMAN LIGHTNING ARRESTER CONSTRUCTION 2 Sheets-Sheet 1 Filed sept. 20, 194s Lgggsvvwsl/M.

INVENTOR Fig.

May 9, 1950 R. R. PITTMAN LIGHTNING ARRESTER CONSTRUCTION 2 Sheets-Sheet 2 Filed Sept. 20, 1946 FIG. 4

Patented May 9, 1950 UNITED STATES PATENT OFFICE 4 Claims.

|The invention relates to lightning arresters of the type adapted for the protection of electric power lines and connected apparatus from damaging eifects of excessive superimposed voltages, such as may be caused by lightning or switching operations.

The objects of the present invention include (a) the provision of a self-supporting lightning arrester construction in which the structural elements operate electrically to establish a predetermined voltage division among the dielectric elements before sparkover thereof; (b) a construction embodying a series arrangement of a plurality of relatively low voltage arc interrupters to enable their use on high voltage circuits, with the arc interrupters so grouped and coordinated with other supporting elements as to cause advantageous voltage distribution between the arc interrupters; (c) a construction in which the terminal voltage is broken up by certain capacitance relationships and then re-combined in such a manner that the arithmetical sum of the voltages impressed on individual serially-connected arc interrupters may be, prior to sparkover, substantially greater than the voltage measured across the series arrangement; (d) the provision of a voltage-dividing capacitance network in which certain of the capacitances are provided by the inherent capacitance of elements which perform additional useful functions; (e) the provision of a construction embodying a leakage spark gap in series with arc interrupting devices in which the predominate portion of the applied voltage prior to sparkover is expressed across the leakage spark gap, thereby maintaining the conducting elements of the arc interrupting devices at relatively low potential; (f) the provision of means for advantageously increasing that portion of the terminal voltage impressed on each arc interrupter in response to sparkover of the leakage gap to assure formation of relatively long, easily extinguished arcs within the arc interrupters; and (g) the provision of a simple and effective voltage-responsive arc extinguishing structure having a relatively low initial sparkover value, high current-interrupting ability and substantially no impedance drop following sparkover. Other objects and advantages will appear from the description.

High voltage lightning arresters in the present state of the art consist in general of a spark gap structure in series with a current limiting element having resistance for limiting the power follow current to a sufliciently small value to prevent sustained arcing of the spark gap struc- 66 ture following sparkover, The resistance drop attending the passage of the surge current through the resistance element is necessarily impressed on the apparatus the device is intended to protect, resulting in insulation failure if the resistance drop is too great. On the other hand, a low resistance element, for the purpose of limiting the resistance drop to an inconsequential Value, may not prevent continued arcing of the gap structure following sparkover, and the entire structure thereby destroyed.

From the above considerations, it is obvious that a high voltage arrester embodying no resist-ance elements would be an outstanding improvement over` the resistance type now generally used. Expulsion type arresters, embodying no resistance, have heretofore been widely used on relatively low voltage circuits, but their use has been uneconomical at higher voltages, for the reason that both dielectric and mechanical strength requirements necessitate a relatively very thick wall if the device has a sufciently long internal gap to extinguish high voltage follow currents. A plurality of economically designed low-voltage expulsion units arranged in series end-to-end relation, with the necessary leakage gap, and with nothing more, requires too high initial sparkover to provide the desired degree of protection. If however, the voltage in pressed across the arrester is subdivided, and the gaps of the arc interrupters subjected to certain peculiarly added combinations of the component voltages, it is possible to provide the desired low initial sparkover voltage, and extinguish high follow current arcs without the use of an undesirable current limiting resistor. The manner in which this is accomplished will be clear from the following description of the invention.

Fig. 1 of the drawing is an elevation of an ernbodiment of the invention shown partly in section; Figs. 2 and 3 are circuit diagrams showing advantageous voltage relationships of the dielectric elements; Fig. 4 is an elevational View illustrating an embodiment of the invention for use with higher voltage circuits; and Fig. 5 is an elevational view of another embodiment of the invention.

Referring iirst to Fig. l, the mounting base 32, of conducting material, has mounted thereon insulating means in the form of an upstanding capacitor 2li and an upwardly extending spool of conducting material 26. A flat conducting member 22 is rigidly secured to the top plate of the capacitor 2li, by means of the cap screw I9, the respective ends extending outwardly to provide for practical purposes, identical arc interrupters may be used. In any event, it will be apparent that the two important considerations are (1) that the arc interrupters in series be included in the preferential sparkover path and (2) that the overall sparkover of spark gap plus arc interrupters be suiciently low to protect associated apparatus. In accordance with the above considerations the preferential sparkover path from the terminal li to the terminal 24 is from the spark gap electrode I2 to the spark gap electrode l5, thence downwardly through the first arc interrupter l1 to the conducting member 22, thence upwardly through the second arc interrupter ll to the conducting member 2l, and thence downwardly through the third arc interrupter l1 to the conducting member 23 and the terminal 24.

A principal advantage of dividing the terminal voltage and recombining components of the divided voltage to obtain more than S31/3% of the terminal voltage on each one of three arc interrupters in series, resides in the fact that the protective level of the interrupters may be correspondingly lowered, thus providing a correspondingly greater margin of protection without any shortening of length of arc path. If, for example, three 9 kv. arc interrupters, each having a 50 kv., 11/2-40 ms. positive wave sparkover, are connected directly in end-to-end series relation have a total sparkover value of 150 kv., it is possible to arrange the interrupters under the conditions of Fig. 2 so that a series arrangement will sparkover with a terminal voltage of only roughly 50/0.8 kv. or 62.5 kv.

Through utilization of the features of construction described above, it is possible to stack the interrupters in groups of three, if desired, and such a construction is shown at Fig. 4 of the drawing. In this construction, an insulator 34, of small capacitance, supports the spark gap electrode 35, which extends in spaced relationship with the other spark gap electrode 36, the latter being mounted on and electrically connected to the upper terminal member of the rst arc interrupter Il. The direction of current traversing the series arrangement forming the preferential discharge path from the line terminal IU to the ground terminal 25 is same as that in the construction previously described, i. e., downward through the outside interrupters, and upward through the middle or intermediate interrupter. In this structure, the capacitors not only serve to control voltage division and as structural supporting members, but also to vertically space the interrupters so that the gas vents are spaced sufciently to avoid mixing of the gas from two or more vents. The conducting gas discharged from each vent is at a potential diierent from that discharged from any other, and for successful operation, the gases must be isolated in space until cooled to a reduced conductivity by their passage through the air.

In the above description, only capacitance relationships have been regarded in determining the various voltage components. The resistance drop has not been included, although it may if desired be made an important part of the impedance drop, for the reason that it is clear that the capacitance of the dielectric elements incorporated in the invention may vary widely from the specific values stated, since it is relationship therebetween which is the dominating consideration, rather than the absolute values. In addition, the current passing through a capacitor is 6 a function of the rate of change of voltage with time, rather than the magnitude, and it is the rapidly increasing voltages which are most dangerous to apparatus, and I therefore have discussed herein only capacitance relationships in obtaining the desired voltage combinations.

In certain respects the present invention is similar to that described in my copending application Serial No. 666,902 iiled May 3, 1946, now Patent No. 2,427,975, the similarity being found in the circuit diagram rather than in the structure.

The embodiments specically described herein readily suggest other constructions falling within the scope of the invention, and it is therefore intended that this description of the invention will be regarded as illustrative rather than limiting.

I claim, as my invention:

l. A lightning arrester construction comprising spaced line and ground terminals, means providing a preferential discharge path from the line terminal to the ground terminal, said means including rst, second and third arc interrupters disposed in spaced parallel relationship, each of said interrupters including an elongate upstanding tubular insulator, normally insulated conducting terminal members mounted at the respective ends of each insulator, and structural means for causing sparkover between said terminal members to occur within said insulator, conducting means serially connecting said interrupters, said conducting means including a rst laterally extending conducting member rigidly secured to the respective lower terminal members of said first and second interrupters and a second laterally extending conducting member rigidly secured to the upper terminal members of said second and third interrupters, an insulator rigidly mounted on said second conducting member upon which said line terminal is carried, elements constituting a spark gap between said line terminal and the terminal member at the upper end of said first interrupter, and means for impressing a predominate portion of the voltage expressed between said spaced terminals across said spark gap prior to sparkover thereof, said means including a rst capacitor shunting the series arrangement of said rst and second arc interrupters and a second capacitor shunting the series arrangement of said second and third arc interrupters.

2. An arrester construction in accordance with claim 1, characterized by the capacitance of each of the capacitors being greater than the inherent capacitance of either said insulator or any of said arc interrupters.

3. A lightning arrester construction comprising spaced line and ground terminals, means providing a preferential discharge path from the line terminal to the ground terminal, said means including first, second and third arc interrupters disposed in spaced parallel relationship, each of said interrupters including an elongate rupstanding tubular insulator, normally insulated conducting terminal members mounted at the respective ends of each insulator, and structural means for causing sparkover between said terminal members to occur within said insulator, conducting means serially connecting said interrupters, said conducting means including a first laterally extending conducting member rigidly secured to the respective lower terminal members of said rst and second interrupters and a second laterally extending conducting member rigidly secured to the upper terminal members of said second and

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