US2276054A - Self-clearing lightning arrester - Google Patents

Description

Patented Mar. 10, 1942 UNITED STATES PATENT OFFICE SELF-CLEARING LIGHTNING ARRESTER Vania Application October 26, 1939, Serial No. 301,420

Claims.

Our invention relates to lightning-arresters or excess-voltage protective devices, and it has particular relation to` the provision of means for preventing arresters which may have failed in service from locking out lines.

It is well understood that a lightning arrester, when it operates, normally reverts to its normal insulating condition after having been discharged by a surge. Thus, under normal conditions, there is no system-disturbance when a lightning arrester operates. There are conditions experienced in practice, however, under which the lightning arrester may lose its insulating properties. Such cases may be caused, for instance, by the discharge of a lightning-surge of a severity beyond the arresters capacity for handling such' discharges, or by an over-voltage condition on the line or system, or as a result of a cross with a high-voltage line.

If the lightning arrester loses its insulating qualities or is unable to revert t0 its insulating condition after being discharged by a surge, it puts a ground on the system and causes the operation of circuit breakers or of feeder-fuses unless the lightning arrester is cleared from the circuit. If the arrester stays on the system in this non-insulating condition, it becomes impossible to reclose the breakers or to refuse, and hence the line is locked out of service until the failed lightning arrester has been found and removed by hand.

To combat this possibility, some types of distribution-line arresters have, for years, been equipped with a so-called drop out feature. This is incorporated in the bottom of the lightning-arrester case, and the ground-lead is attached to it. In the case of a failure of an arrester, the resultant heat and pressure cause a fracture in the weak porcelain-section of which the drop-out element consists, permitting the ground-lead to drop, thereby clearing the arresters from the circuit. With this operation, then, even though some circuit breakers or fuses may operate during the process of the lightningarrester failure, they can be reclosed because the failed arrester has been removed from the circuit.

This drop-out device has given very satisfactory service on systems where the system faultcurrents are appreciable. During the past few years, however, there has been considerable growth' in rural systems on which the fault-currents are limited, and it has been found that, in many cases, the fault-currents are insufficient to operate this drop-out feature. This has resulted, in some cases, in lines locking out.

Examination of arresters which have failed in service, and have not cleared the drop-out feature, but have become short-circuited, has shown that in many instances the gap-parts are in good condition except for the fact that th'e puncture or flashover of the valve-element part has expelled gas and soot which has become deposited on the porcelain insulation of the series gap, thereby reducing its insulation-strength so that it has been unable to withstand the linevoltage. Thus, the reclosing of a breaker or a feeder-fuse which has tripped when the arrester failed, again causes fault-current to flow through the arrester and results in the fuse or breaker tripping again.

It has been found, by test and experience, that where the system fault-currents are appreciable, several hundred amperes or more, the failed arresters clear, either by operation of the dropout or by shattering of the porcelain housing. For instance, on urban and suburban systems where the fault-currents which the systems are able to deliver are high, the operation of the drop-out feature has been reliable. The lockingout difculties prevail on systems Where the faultcurrents are limited, such as rural systems with small power-sources, and circuits with appreciable impedance.

In a Patent No. 2,135,085, granted November l, 1938, to L. R. Ludwig, W. G. Roman, F. B. Johnson and W. E. Berkey, there was disclosed a special lightning-arrester combination including a switching gap utilizing spaced electrodes of brass or other non-arcing metal, but having an arc-quenching ability which is incapable of coping with the final discharge-current of the serially-connected valve-element, in combination with a resistance-shunted quench-gap element which operates to reduce the final dischargecurrent to a value which can be interrupted by the switch-gap element.

Our present invention relates to an improvement in the combination just described, wherein we utilize some or all of a combination including a drop-out element or rupturable part, which is automatically ruptured in response to a sustained current-flow in excess of a predetermined minimum magnitude in the event of a failure of the valve-element part, a quench-gap part which is capable of substantially interrupting the current-now in the event of a failure of the valveelement part in case the short-circuit current of the line is less than said predetermined magnitude which is required to rupture the drop-out part, and barrier-means for protecting at least some of the series-gap parts from contamination from the destruction-products of a valve-element part in case of a failure of the latter.

The electrodes of the switching-gap part of the lightning-arrester combination may be separated by insulators made of porcelain or other materials which are adapted, by their composition and physical shape, to reliably continuously withstand the normal rated voltage of the lightning arrester, thus cutting off, or switching off, 1

the other parts of the arrester-combination from the line-voltage during normal operating conditions When there is no excessive voltage to be discharged by the lightning arrester. At the same time, the quench-gap part can be made, as is almost necessary if adequate quenching properties are to be obtained, with the use of insulating spacers which are unsuited to reliably continuously withstand the normal rated voltage of the protector-device.

The valve-element part is a variable-resistance member or assembly which automatically changes from a normal high resistance to a discharging-state low resistance in response to a predetermined excess-voltage, and which automatically returns to a high-resistance condition when the impressed voltage returns to the normal line-Voltage or the rated voltage of the arrester. Various forms of lightning-arrester valve-type parts are known, answering this dention.

With the foregoing and other objects in view, our invention consists in the apparatus, structure, parts, combinations, systems and methods hereinafter described and claimed, and illustrated in the accompanying drawing, wherein Figure 1 is a vertical longitudinal view through an arrester embodying our invention in` a prei'erred form of embodiment, and

Fig. 2 is a similar View illustrating a modi-ncation.

In the illustrative form of embodiment which is shown in Fig. 1 of the drawing, our invention comprises a cylindrical porcelain housing I which houses the various elements of the arrester.

Starting at the bottom, it will be noted that the arrester shown in Fig. l comprises two arrester-blocks 2 which constitute the valve-type part of the combination, as above described.

These blocks may consist of molded blocks ol` silicon-carbide crystals and a binder, or other equivalent device, as well-known in the art.

The arrester-blocks 2, in the form of embodiment of our invention as illustrated in Fig. 1,` are surmounted by a flexible-washer barrierelernent 3, which may consist of an assembly of a flexible washer, held between riveted metal plates, to seal off the space within the bore of the porcelain housing I. A suitable form of such a washer is described and claimed in an application of F. B. Johnson, Serial No. 247,422, led December 23, 1938, for Lightning arresters, assigned to the Westinghouse Electric & Manufacturing Company. In the particular form of our invention which is shown in Fig. l, the nexible-washer assembly 3 is surmounted by a metal tube 4 which is surrounded by a suitable gum or wax 5 which seals the annular space between the tube 4 and th bore of the porcelain insulator I, thereby supplementing the sealing action of the exible-washer assembly 3, the washer 3 being underneath the wax so as to prevent it from falling down away from the metal tube 'I.

In Fig. 1, the metal tube 4 is surmounted by a quench-gap part 6 which consists of a stack of dished electrode-plates 'I of brass or other nonarcing metal, separated by insulating washers 8 which may be built up from laminations of mica and other suitable insulating sheet-material suitably bonded together, as shown in the quenchgap part of the aforementioned Patent No. 2,135,085, except that our quench-gap part 8 may be made up entirely of the plates 1 and insulating washers Ii, Without a shunting resistor as advocated in the aforesaid patent. It is desirable, in carrying out our invention, that the quench-gap part 6 shall be capable of substantially interrupting the short-circuit current if the short-circuit current of the protected line is of a small value, such as an alternating current of something like 50 amperes root-mean-square value.

Surmounting the quench-gap part 6, in the embodiment of our invention shown in Fig. 1, is the switch-gap part 9, illustrated as being made up entirely of two spaced electrodes I I and I2, spaced from each other by a ring I3 of porcelain or other insulation which is of such dimensions and material as to be able to safely, reliably and continuously withstand the full rated line-voltage of the arrester, although it will be readily understood that a plurality oi said gap-elements may be utilized in series with each other, if the line-voltage is high.

In the form of our invention shown in Fig. l, the entire assembly, comprising the valve-type part 2, the barrier-means 3 5, the quench-gap part 6 and the switch-gap part 9, is surmounted by a spring i5 which holds the parts in` firm contact with each other. For electrical purposes, it is preferable to have the spring I5 shunted by a flexible shunt IG.

Electrically connected to the top of the arrester-assembly, or to the exible shunt I6 in the form of embodiment of our invention shown in Fig. l, is a line-lead I7, which may be connected to a long rural distribution-line I8 which is illustrated as being energized through a circuit-breaker IJ.

We prefer to retain the drop-out feature consisting of a frangible part, as heretofore known in the art, so that our arrester will be adapted for use both on lines having a high short-circuit current and on lines having a low shirt-circuit current. In the form of embodiment of our invention shown in Fig. 1, this drop-out feature is provided, at the bottom of the porcelain housing I, in the form of a weakened porcelain part 2| which is automatically broken in response to the heat and pressure which is caused by a sustained current-now in excess of a predetermined magnitude such as 50 amperes, or other convenient value.

In operation, the switch-gap part is set to have a breakdown voltage which is only slightly higher than the highest contemplated peakvalues of the line-voltage, so that it will readily and quickly break down at the very beginning of a steep wave-front or excess-voltage-Isurge. The valve-type part 2 is designed to have a critical change in resistance, at voltages which are somewhat higher than the normally rated line-voltage of the arrester, so that this part of the arrester normally has a high resistance, but converts itself into a relatively low-resistance discharging-path in the presence of an excessvoltage surge. When the excess-voltage condition has been dissipated, so that the voltage returns toward normal, the resistance of the valvetype part 2 automatically increases, so that the discharge-current is reduced to a value which can readily be interrupted by the quench-gap part 6, whereupon the arc through the switchgap part 9 is also intercepted, lso that the switchgap insulator I3 normally withstands substantially the entire line-voltage, insulating other parts of the arrester from the line,

It has been found that our quench-gap assembly 6 is able to interrupt currents of more than 50 amperes, depending somewhat upon the power factor oi the system. Therefore, on systems where the fault-current is limited to such magnitudes, the gap will clear the circuit after a lightning-discharge, even without the presence of the Valve-type blocks 2. in other words, an arrester-assembly such as we have shown and described, if installed on such a system, would still operate as an arrester, even though the valvetype blocks 2 may have failed, or were omitted altogether, and consequently the likelihood of locking out lines on such a system is prevented, or at least reduced, by our device.

If the condition should arise where the blocks 2 fail, and the drop-out section 2l fails to operate, and the quench-gaps 6 do not interrupt the current, then there will be a short-circuit on the system, which will be cleared by a feeder-fuse or the circuit breaker I9. When the feeder-fuse or circuit breaker I9 clears, the circuit-current ceases to iiow through the arrester in which the blocks have failed. Now if the porcelain insulation I3 maintains its insulating qualities, then if the breaker I9 is reclosed, the arrester will still be an insulator and the line will not be locked out, even though the arrester has lost its valvecharacteristics. On successive discharges by lightning, the breaker I9 may again trip, but this is much less troublesome than a short-circuit on the line which would prevent reclosing of the breaker.

Tests have indicated that our barrier-means 3 5 is very effective in maintaining the insulating qualities of the series-gap parts B and 8 in the event of a failure of the blocks 2, preventing contamination of the gap-insulation by the gases and other destruction-products of the blocks.

In the illustrated arrester-construction, the drop-out section 2i has been retained to provide circuit-clearing in the case `of system-overvoltage conditions, crosses with high-voltage lines, and those cases in which the fault-currents are high. Our novel series-gap assembly, consisting of a number of quench-gaps 6, a switch-gap 9, and a barrier 3-5 between the valve-type blocks 2 and at least some of the gap-parts, prevents the locking-out of lines in which the fault-currents are limited.

In Fig. 2, we illustrate a slight modiication of our invention in which we omit the metal tube 4 and the sealing gum 5 of Fig. 1, utilizing the flexible-washer element 3 alone, as the barriermeans, and designating this element by the numeral 3 in Fig. 2. In Fig. 2, the order of arrangement of the parts is also changed, so that the quench-gap part 6 is placed immediately above the valve-element part 2, the spring I5 is placed over the quench-gap part 6, and the flexible-washer barrier 3 is placed over the spring I5, and between said spring and the switch-gap part 9.

While we have illustrated and described our invention in connection with only two preferred forms of embodiment, we wish it to be understood that various changes may be made, by those skilled in the art, without departing from the broader aspects of the invention. We desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

We claim as our invention:

l. An excess-voltage protective device for discharging heavy surge-currents at an over-all voltage limited to a value suitable for the protection of lines having a limited short-circuit current, the short-circuit current of the line being at least of the order of 50 amperes, said device comprising a switch-gap part and a serially connected quench-gap part, said switch-gap part comprising at least one pair of spaced electrodes and an insulating separating member therebetween, said duench-gap part comprising a stack of closely spaced electrodes of non-arcing metal and insulating spacers therebetween, said closely spaced electrodes and spacers of the quench-gap part being capable of substantially interrupting the short-circuit current of the protected line, the insulating separating member or members of said switch-gap part being of a type suitable for reliably continuously withstanding the normal rated voltage of the protective device, and the insulating spacers of said quench-gap part being of a type unsuited to reliably continuously withstand the normal rated voltage of the protective device.

2. An excess-voltage protective device com.- prising a switch-gap part, a serially connected quench-gap part, a serially connected valve-element part, and means for protecting at least one of said gap-parts from contamination from the destruction-products of said valve-element part in case of a failure of the latter; said valve-element part being a variable-resistance member or assembly which automatically changes from a normal high resistance to a discharging-state low resistance in response to a predetermined excessvoltage, and which automatically returns to a high-resistance condition when the impressed voltage returns to normal; said switch-gap part comprising at least one pair of spaced electrodes and an insulating separating member therebetween, said quench-gap part comprising a stack of closely spaced electrodes of non-arcing metal and insulating spacers therebetween, said closely spaced electrodes and spacers of the quench-gap part being at least capable of substantially interrupting an alternating current of 50 amperes root-mean-square value; the insulating separating member or members of said switch-gap part being of a type suitable for reliably continuously withstanding the normal rated voltage of the protective device, and the insulating spacers of said quench-gap part being of a type unsuited to reliably continuously withstand the normal rated voltage of the protective device.

3. An excess-voltage protective device comprising a switch-gap part, a serially connected quench-gap part, a serially connected valve-element part, an insulating casing enclosing said parts, and barrier-means within said casing for protecting at least one of said gap-parts from contamination from the destruction-products of said Valve-element part in case of a failure of the latter; said valve-element part being a variable-resistance member or assembly which automatically changes from a normal high resistance to a discharging-state low resistance in response to a predetermined excess-Voltage, and which automatically returns to a high-resistance condition when the impressed voltage returns to normal; said casing having a rupturable part which is automatically ruptured in response to a sustained current-flow in excess of a predetermined magnitude in the event of a failure of said valve-element part; and said quench-gap part being at least capable of substantially interrupting an alternating current of approximately said predetermined magnitude in the event of a failure of Said valve-element part.

4. An excess-voltage protective device comprising a valve-element part, a series-gap part, an insulating casing enclosing at least said Valveelement part, and means for prctecting at least a portion of said series-gap part from contamina- Lion from the destruction-products of said valveelement part in case or" a failure of the latter; said Valve-element part being a variable-resistance member or assembly Which automatically changes from a normal high resistance to a discharging-state low resistance in response to a predetermined excess-voltage, and which automatically returns to a high-resistance condition when the impressed voltage returns to normal; said casing having a rupturable part which is automatically ruptured in response to a sustained current-flow in excess of a predetermined magnitude in the event of a failure of said valve-element part; and said series-gap part being at least capable of substantially interrupting an alternating current of approximately said predetermined magnitude in the event of a failure of said valve-element part.

5. A lightning-arrester having a valve-type excess-voltage-responsive part, a series-gap part, and a rupturable casing-part, said rupturable casing-part being rupturable in response to a continuous current-flow of a predetermined magnitude or more, and said series-gap part being capable of substantially interrupting current of said predetermined magnitude or less.

LEON R. LUDWIG. EDWARD F. W. BECK.

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