US3348100A

US3348100A - Sparkover control circuit for lightning arrester shunt gap unit

Info

Publication number: US3348100A
Application number: US441682A
Authority: US
Inventors: James S Kresge
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1965-03-22
Filing date: 1965-03-22
Publication date: 1967-10-17
Anticipated expiration: 1984-10-17
Also published as: GB1101845A; DE1538509B2; SE322283C; SE322283B; DE1538509A1; CH447341A

Description

@et l?, H9? J. s. KRESGE 39343710@ SPRKOVER CONTROL CIRCUIT FOR LIGHTNING ARRESTER SHUNT GAP UNIT Filed March 22, 1965 y

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5,4 R4 Q4 United States Patent O 3,348,1@ SPARKVER 'CONTROL CHRCUH' FOR LIGHT- NlNG ARRESTER SHUNT GAP 'UNIT .lames S. Kresge, Pittstield, Mass., assignor to General Electric Company, a corporation ot New York lFiied Mar. 22, i965, Ser. No. 441,632 Claims. (Cl. 317-70) ABSTRACT 0F THE DISCLSURE A sparkover controlling network comprising auxiliary gaps, impedances and preionizers for a current switching main multi-gap which shunts extra series valve resistance in a valve type lightning arrester.

This invention relates to lightning arrestors and more particularly to improvements in current switching shunt gap units for valve type lightning arrestors for use 0n high switching surge energy systems such as extra high voltage transmission lines and lower voltage cable circuits.

Alightning arrestor is an electrical analog of a mechanical safety valve in that it prevents the escape of normal electrical pressure (voltage) while automatically permitting the escape or release of excess pressure so as to prevent rupture of the electrical pressure containing means (insulation) of electrical apparatus such as power transmission or distribution circuits yof either the open air line type or the cable type. As indicated by the name lightning arrester, lightning is the historical cause of such excess voltage although switching surges have long been recognized as another cause.

As power system operating voltages have increased progressively through the years to 230 kv., then to 340 kv., and now through 500 kv. to 700 kv., it has been found that the still more rapid accompanying increase in switching surge energy (the latter being proportional to the product of the system capacitance and the square of the system operating voltage) imposes a higher duty on the arrestor than lightning surges. One way to enable an arrestor to discharge the very high switching surge energy lof 500 kv. and 700 kv. systems is to provide it with additional valve resistance material shunted by a current switching gap unit which in general is only sparked over by lightning surges but on occasion may also be sparked over during discharge of lower voltage magnitude, but longer duration, switching surges. In a time which is long compared with the duration of a typical lightning surge but short compared with the duration of a typical switching surge, the shunt gap, if it has become sparked over, develops enough arc voltage to switch practically all the arrester current into the additional valve resistance material so that the latter is not only always available to absorb switching surge energy for practically the entire duration of a switching surge, but it is also always available to limit power frequency current following a lightning surge.

ln order to develop suiciently high arc voltage, it is necessary greatly to elongate the arc and in shunt current switching gap units this is achieved in a reasonable space by having each unit consist of a multiplicity of similar spark gaps connected in series with each other and with a magnetic Coil for providing a common magnetic field for elongating the individual arcs, the sum of whose lengths is the total arc length of the unit. Usually the coil is Vprotected against over-voltage by a shunt gap generally similar to the other gaps for developing an arc voltage for rapidly switching the arrester current into the coil.

A problem with such shunt current switching gap units is to give them a sufficiently low sparkover voltage and Bdld@ Patented Get. l?, 1967 ICC make that sparkover voltage highly stable and independent of the wave shape of the applied voltage.

The current switching shunt gap should have a low enough sparkover voltage setting so that on lightning surges it always sparks over at a low enough voltage to prevent the extra resistance in the arresters from increasing the arrester voltage above the maximum of what it would be if the extra resistance were not present. On the lother hand, the current switching gap should have a high enough sparkover setting so that its high arc voltage will not cause restriking on power follow currents and switching surges. Ordinarily with a plurality of similar spark gaps connected in series, the sparkover voltage of the series circuit is approximately equal to the number of gaps multiplied by each ones individual sparkover voltage. It is, of course, impossible to make the sparkover voltage of the series circuit or combination less than the sparkover voltage of each of its components. Being shunted by a resistor in series with a main gap or gaps, the shunt gap does not have to withstand the system operating voltage and hence can have a sparkover voltage which is independent of system voltage. Anomalous as it may seen, such gap units can have a higher arc voltagethan sparkover voltage because the arc voltage is the sum of the are voltages of the individual serially connected gaps. In the case of a single gap, this would be an impossible contradiction of terms because there can be no single arc whose counter voltage exceeds the voltage creating it.

Accordingly, it is an object of the invention to provide a new and improved sparkover control circuit for lightning arrester gap units.

It is another object of the invention to provide a new and improved sparkover control circuit for lowering the sparkover voltage of a multiple gap unit to a desired value anywhere between the sparkover voltage of one gap of the unit and the sum of all of the gaps of the unit.

It is a further object of the invention to provide a sparkover control circuit for current switching gap units 'which is substantially independent of wave shape of the applied voltage.

It is an additional object of the invention to provide a new and improved sparkover control circuit in which certain elements provide the dual function of preioning associated main gaps and also controlling the sparkover voltage distribution between the main gaps so as to provide cascading sparkover of the individual gaps.

Briefly described, the invention utilizes a novel auxiliary network which applies a greater than pro rata proportion of the applied voltage to an individual main gap until it sparks over, after which it progressively applies nearly the full applied voltage to the remaining main gaps sequentially so as to cause them to break down or spark over sequentially in a cascading manner. The invention also includes simple means for adjusting the voltage at which the ysparkover sequence commences and further provide-s separate means for compensating the circuit for the effect of changes in wave shape of the applied voltage.

The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing,

FIG. 1 is a block diagram of a lightning arrester in which the invention is adapted to be used,

FIG. 2 is a circuit diagram of an embodiment of the invention, and

FIG. 3 is a circuit diagram of a modification thereof.

Referring now to the drawing and more particularly to FIG. l, there is shown therein a module 1 of a lightning arrester adapted for use on extra high voltage circuits such as circuits of 500 kv. or 700 kv., it being understood that in practice such a lightning arrester would consist of a large number of such modules connected in series. The module l consists of a main series gap unit 2 which may either be of the current limiting or the noncurrent limiting type, the former being one which develops an arc voltage soon after sparkover comparable to its sparkover voltage and the latter being an ordinary current interrupting or are quenching gap unit which does not develop substantial arc voltage but rapidly develops recovery voltage after normal current zero. Connected in series with the main gap unit is a main series valve resistor 3 ordinarily of well-known material having a negative resistance characteristic. Also connected in series with the arrester is an additional shunted valve resistor 4 generally similar to 3 which is shunted by a shunt gap unit 5 of the current switching type which rapidly develops an arc voltage after spark over comparable to its sparkover voltage so as to switch the arrester current into the additional valve resistor material, the function of such additional resistor material being to absorb switching surge energy and limit power follow current.

Inasrnuch as the shunt gap unit 5 does not have to withstand applied power frequency voltage, it does not have to have as high a sparkover level as the main series gap unit 2, and in fact it is necessary that it have a comparatively low sparkover voltage level so as to take the additional valve resistor material out of the circuit to limit the discharge voltage of the arrester to the desired protective levels while discharging lightning surges. l

Referring now to FIG. 2, which shows a circuit of a shunt gap unit 5 in accordance with the present invention, the unit is provided with end terminals 6 and 7 between which there are connected in series a plurality of main gaps, four being shown by way of example and being designated G1, G2, G3 and G4 respectively, and also included serially in the circuit is a magnetic coil 8 which is preferably physically and electrically near the middle of the series circuit so that its magnetic eld will have substantially equal strength between the electrodes of all of the gaps. A gap G5 generally similar to the other main gaps is connected across the coil 8 for protecting it from overvoltages during discharge current operation of the arrester and for switching the discharge current into the coil for increasing its magnetic eld and hence arc propelling action on the gaps G1 through G5. The gaps G1 through G5 may be of any suitable physical construction and are preferably of the familiar horn gap construction so that they inherently tend to elongate their arcs, and the polarity of the coil S is made such that its magnetic field reinforces the individual magnetic fields of the gaps.

Also serially connected between the terminals 6 and 7 are auxiliary gaps S1, S2, S3 and S4 which not only act as preionizers for the respective main gaps G1, G2, G3 and G4 but also function to control the sequential application of sparkover voltage to the main gaps. Thus auxiliary gap S1 is mounted physically adjacent gap G1 so that when it sparks over it ionizes or illuminates the space between the electrodes of G1 and the same is true for S2 with respect to G2, S3 with respect to G3 and S4 with respect to G4. Serially connected with the gaps S1 through S4 are a pair of resistors R1 and R2, the former being provided for the purpose of impressing substantially all of the applied voltage across gap G1 which, as will be explained hereinafter, is the lirst main gap to be sparked over and which consequently can be called the trigger gap. The resistor R2 is provided primarily for determining the level of applied Voltage at which the trigger gap G1 sparks over. As shown, the resistor R1 is between S1 and S2 but this is not essential and the relative positions of S1 and R1 can be reversed. However, it is important that R1 in effect be ahead of S2, that is to say between S2 and terminal 6. Resistor R2 is connected between S2 and S3 for reasons which will become apparent hereinafter in explaining the operation of FIG. 2. A resistor R3 is connected across or in shunt with S1 and R1 and a resistor R4 is connected across S4, these being provided for the purpose of producing a desired sequence of the sparkover of the gaps S1 to S4 inclusive.

Coupling resistors R5, R6 and R7 are provided for the purpose of intermediate coupling of the circuit of the S gaps to the circuit of the G gaps, R5 being connected between the junction of G3-G4 and the junction of S13-S4; R3 being connected between the junction of G3-G5 and the junction of R2-S3, and R1 being connected between the junction of G1-G2 and the junction of R1-S2. A capacitor C1 connected in shunt with R2 is provided for the purpose of making the sparkover voltage of the gap unit substantially independent of the wave shape of the applied voltage and a capacitor C2 connected in shunt with the coil 8 is provided for the purpose of eilfectively short circuiting the coil until after gaps G1 through G4 have been caused to spark.

Th operation of FIG. 2 is as follows, it being assumed that G1 through G5 have essentially the same electrode spacing and individual sparkover voltage and the spark gaps S1 through S4 have an equal electrode space of somewhat less than half the spacing of the electrodes of the G gaps so that the S gaps each have asparkover voltage somewhat less than half the sparkover voltage of each of the G gaps. Assume now that the voltage between terminals 6 and 7 is increasing. In this connection it makes no diilerence which is the higher voltage terminal but, for the purpose of explanation, it will be assumed that terminal 7 is the low voltage terminal which is maintained at ground potential and the terminal 6 is the one whose potential is increasing relative to ground. It will also be assumed for the present that R2 and C1 are short circuited or that R2 has substantially zero resistance which amounts to the same thing. Inasmuch as the eifective resistance of the gaps S2 and S3 is very much higher than the resistance of R3 and R4 practically all of the applied voltage will be across S2 and S3 so that when the applied voltage attains a value somewhat less than the sparkover voltage of a G gap S2 and S3 will spark over. When S2 and S3 spark over, their arc voltage is very much less than the voltage across R3 and R4 so that practically all of the applied voltage will then be across S1 and S4 and they will spark over. At that time practically all of the applied voltage will be across R1, it being assumed for the time being that R2 has negligible resistance. The sparked over gaps S1 through S4 now ionize their correspondingly numbered G gaps and when the applied voltage reaches a value such that the voltage across R1 equals the sparkover voltage of G1, G1 will spark over and its initial arc voltage will be low so that instantaneously the voltage on the upper electrode of G2 as viewed in the drawing will be very close to the voltage of terminal 6 whereas the voltage of the lower electrode of G2 will be very close to the voltage of terminal 7 because the right-hand terminal of coupling resistor 6 is at substantially the potential of terminal 7 because of the low arc drops in S3 and S4 and consequently the lower electrode of G2, which is tied to this point through R3 and C2 or coil 8 will also be substantially at the same potential as terminal 7, Consequently practically all of the applied voltage will then be across G2 and it will spark over. As the voltage across the capacitor C2 cannot change instantaneously, the voltage of the upper electrode of G3 will immediately become substantially the voltage of the terminal 6 while the lower electrode of G3 remains substantially at the p0- tential of terminal '7 to Which it is connected through sparked over gap S4 and coupling resistor R5. Consequently substantially full applied voltage will now be applied to G3 and it will spark over which in turn applies substantially full voltage across G4 whose upper electrode will then be at substantially the potential of terminal 6 and whose lower electrode is connected to terminal G4 sparked over, practically the entire applied voltage will be across G and it will spark over thus completing the spark over of the entire gap unit 5 in a cascading manner at an applied voltage not much more than the sparkover voltage of the G `gaps individually.

By using a finite resistor R3 `a smaller percentage of the total applied Voltage will appear across R1 when all the S gaps have sparked over so that the greater the resistance of R3 the greater the applied voltage needed for sparking over the trigger gap G1. The value of R2 thus determines the sparkover setting of the gap unit 5. If R2 were equal in resistance to R1 then the voltage would be divided equally between G1 and G3 (before either of them became sparked) and they both become trigger gaps. With the arrangement shown in FIG. 2 then the maximum sparkover level of the gap unit cannot exceed twice that of an individual G gap. If higher sparkover levels are desired they can be achieved by adding resistors in series with S3 above the junction with R5 and also in series Capacitor C1 functions to increase the current through R1 as the rate of rise of applied voltage between terminals 6 and 7 increases because capacitor current is proportional to rate of change of voltage applied across its terminals. Consequently the time lag effect of steep front applied voltages on the spark over of G1 is compensated for by the fact that the extra capacitor current on steep front applied voltages flows through R1 and thus increases the voltage applied to the trigger gap G1.

An example of suitable numerical values for FIG. 2 is 44 mil sparkover spacings of the G gaps giving a sparkover voltage of those gaps of about 5.25 kv. The S gaps can have a spacing of about -20 mils with an accompanying sparkover voltage of 2 or 3 kv. R1, R5 and R1 can have a resistance of about 10,000 ohms each, R2 can have :a resistance of about 2200 ohms and R5, R5 and R1 can have a resistance of about 5600 ohms each. C1 can have a capacitance of about 200 picofarads and C3 can have a capacitance of about 750 piocfarads. The above mentioned component values result in `an overall gap unit sparkover voltage of about 7.7 kv.

Referring now to the modification shown in FIG. 3, gaps G1 through G5, coil 8 and preionizer gap S1 correspond respectively to the similar elements in FIG. 2. Resistor R1 corresponds generally to R1 in FIG. 2 but the one in FIG. 3 ordinarily will have a very substantially higher ohmic value. I1, I3 and I3 are simple corona type preionizers for main gaps G1, G3, G3 and G1 and are shown connected thereacross. Resistors R2, R3 and R4 connected respectively across the corona perionizers I1, I2 and I3 together with R1 and the coil 8 form a series resistive type voltage divider network which after S1 sparks over, as will be described hereinafter, divides the total applied voltage in the desired ratio amongst the gaps G1 through G1. Capacitors C1 and C2 connected respectively across R3 and R4 provide a more favorable voltage distribution among the main gaps for steeply rising applied voltage waves so as to compensate the sparkover control circuit for the volt-time lag characteristic ofthe main gaps. Capacitor C3 connected across coil 8 prevents any appreciable part of the applied voltage from appearing across the coil and G5 until after gaps G1 through G1 have been sparked over as will be described hereinafter.

The operation of FIG. 3 is as follows. As the applied voltage rises, a value will be reached at which gap S1 sparks over thereby providing ionization for gap G1 and placing the total applied voltage across the resistance voltage divider network. R1 has a resistance many times the sum of the resistances of R3, R3 and R1 plus the resistance of coil 8. Consequently, most of the total applied voltage will then he across gap G1 and when the voltage across gap G1 reaches its sparkover v-oltage (typically about 5.25 kv.) G1 sparks over. Resistor R1 is now effectively short circuited and a sharp rise in voltage appears across R2 through R1. Most of this voltage rise appears across R3,

however, because the voltage across capacitors C1 through C3 cannot be changed instantaneously and R2 has a resistance many times the total resistance of R3 plus R1. Consequently ionizer I1 suddenly provides ionization for G3 which is now overvoltaged and it immediately sparks. In a like manner gap G3 is next to spark followed by G4 and finally G5. Thus the gaps G1 through G5 are forced to spark over in a cascading manner in an overall applied voltage only a little above the sparkover level of any of the individual G gaps.

The capacitors serve a dual function. First, as already described, they force the cascading by forcing gaps G2 through G5 to be highly overvoltaged one at a time after G1 has been sparked. Second, for steeply rising applied waves, they modify the division of voltage in the resistor network (before G1 is sparked) in such a manner that G1 sees a higher proportion of the applied voltage than it would for slowly rising waves. This compensates for a time lag effect of gap G1 and makes the sparkover level ofthe overall gap assembly quite independent of applied waveshape down to very short times.

An example of suitable numerical resistance and capacitance values for the circuit 0f FIG. 3 is as follows: R1 equals 68,000 ohms, R3 equals 22,000- ohms, R3 equals 6,800 ohms and R4 equals 2,200 ohms; capacitor C1 equals picofarads, C3 equals 400 picofarads and C3 equals 1500 picofarads.

It will be observed that the resistors vary in substantially geometric progression having a common ratio of three and that the capacitors vary in a substantially geometric progression having a common ratio of four. The above mentioned component valuesresult in an overall gap unit sparkover voltage of about 7.7 kv.

In the FIG. 3 circuit having the above numerical resistance and capacitance values S1 carries in the order of 75 milliamperes at the time G1 sparks over. At this current level it acts as a very etiicient preionizer and so the spark over of G1 is very stable. The remaining ionizers I1 through I3 are called upon to operate on a very steep voltage rise after G1 sparks and hence they also are very efficient. As a result of the ethcient preionization and severe overvoltage of gaps G3 through G5 the cascading sparkover action occurs with negligible time delay. Spark over of this gap assembly occurs over a very narrow band for applied waves reaching sparkover level at any time from a few tenths of a microsecond to several thousand microseconds. While FIG. 2 is capable of achieving a lower sparkover level than FIG. 3 the latter has the advantage that it is simpler, uses fewer components and the power dissipation in its resistors is less leading to possibly greater reliability in service.

While there have been shown and described particular embodiments of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and therefore it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A valve resistor shunting current switching gap unit for a lightning arrester comprising, in combination, at least two main spark gaps, a magnetic coil, said main gaps and coil being serially connected with the coil in the middle, a spark gap connected across said coil, said spark gaps all having about the same individual sparkover voltage, and means for sequentially sparking over said gaps at an applied voltage across the series circuit which is between the individual sparkover voltage of said gaps and the sum of the sparkover voltages of all said gaps, the sequence being such that one main gap sparks over before the coil shunting gap sparks over.

2. A unit as in claim 1 in which the sequence is such that one main gap sparks over first and the coil shunting gap sparks over last.

3. A gap as in claim 1 in which said means comprises an impedance network connected to each of said gaps.

4. A gap as in claim 1 in which said means includes a capacitor for making the value of said applied voltage at which the first gap sparks over substantially independent of the wave shape of said applied voltage.

5. A gap as in claim 1 in which said means includes individual preionizers for said gaps.

6. A gap as in claim l in which said means comprises an impedance network connected to each of said gaps and includes a capacitor for making the value Iof said applied voltage at which the rst gaps spark over substantially independent of the wave shape of said applied voltage, said means also including individual preionizers for said gaps.

7. An extra high voltage lightning arrester current switching shunt gap unit comprising, in combination, a pair of end terminals, four main spark gaps G1, G2, G3 and Gg and a magnetic coil connected in series with the coil between G2 and G3 and with G1 and G1 connected respectively to said end terminals, a spark gap G5 connected across said lcoil, said spark gaps all having substantially t'ne same sparkover lever, and means comprising a network of a plurality of graded resistance resistors con- "nected between said end terminals and to points between `certain of said gaps including at least one preionizing spark gap S1 for main gap G1 connected in series with a highest resistance resistor across G1 and a capacitor connected across another resistor to force sparkover of the gaps G1 through G5 in a cascading manner in numerical order at a highly constant over-all applied voltage Value slightly exceeding their individual sparkover level and which voltage value is substantially independent of the wave shape of the applied voltage.

8. A gap unit for a lightning arrester comprising, in combination, a pair of end terminals, four main spark gaps G1, G2, G3, and G4 respectively, a common magnetic arc propelling coil for said gaps, said gaps being serially connected in numerical sequence between said terminals with said coil connected serially therewith between G3 and G2, a spark gap G5 connected in shunt with said coil, all of said spark gaps having substantially the same sparkover voltage level, a separate preionizer spark gap S1, S2, S3 and S4 for each of the gaps G1, G2, G3 and G4 respectively, said preionizer spark gaps all having a lower sparkover voltage level than said main spark gaps, all of said preionizer spark gaps being serially connected between said end terminals with a resistor R1 serially between S1 and S2 and a resistor R2 serially between S2 and S3, a resistor R3 connected in shunt with S1 and R1, a resistor R4 connected in shunt with 5,1, a resistor R5 connected between the junction of G3 and G4 and the junction of S3 and S4, a resistor R5 connected between the junction f G3 and the Icoil and the junction of R2 and S3, a resistor R7 connected between the junction of G1 and G2 and the junction of R1 and S2, a capacitor C1 connected in shunt with R2 fand the capacitor C2 connected in shunt with the coil, R1, R3 and R4 having substantially equal resistance, R5, R and R1 having substantially equal resistance about half that of R1, R2 having a resistance about half that of R5, C2 Ihaving a capacitance about four times that of C1.

9. A gap unit as in claim 8 in which said spark gaps G have a minimum electrode spacing of about 44 mils and a sparkover voltage of about 5.25 kv., said preionizer gaps have a minimum electrode spacing of about -20 mils and a sparkover voltage of about 2 or 3 kv., R1 being about 10,000 ohms, R2 being about 2,200 ohms, R5 being about 5,600 ohms, C1 being about 200 picofarads and C2 being about 750 picofarads.

10. In a lightning arrester current switching shunt gap unit of the type having an even number of similar main (horn) gaps, and a common arc elongating coil for said main gaps all connected in series (with said coil electrically and physically in the middle of said series connection), and with another (horn) gap connected across said coil, all said gaps having substantially the same sparkover voltage, the combination of an impedance network connected to said gap unit for lowering its over-all sparkover voltage, ilattening its volt-time sparkover characteristic (and raising its reseal voltage to prevent restrike when switching long duration sWitc-hing surge currents), said network comprising as many resistors as there are main gaps of different resistance (in substantialiy a geometric progression having a common ratio of about three), a preionizing spark gap having a substantially lower voltage sparkover than said main gaps mounted for ionizing one end of said main gaps, Said preionizing gap and the highest resistance one of said resistors being serially connected across said end one of said main gaps, Said other resistors being respectively connected across the remaining main gaps in order of decreasing resistance with the next highest resistance resistor being connected across the main gap which is adjacent said end one of said main gaps, and with the lowest resistance resistor being connected across the other end main gap, separate (corona type) preionizers for respectively preionizing said remaining main gaps connected respectively in parallel with the other resistors, and as many as there are main gaps less one capacitors of different capacitance (in substantially geometric progression having a common ratio of about four), the highest capacitance capacitor being connected across said coil, the remaining capacitors being connected respectively across the lowest resistance resistors with the highest capacitance remaining capacitor being connected across the lowest resistance resistor, and with the next lower capacitance remaining capacitor being connected across the next highest resistance resistor.

1l. In an extra high voltage lightning arrester current switching shunt gap of the type having four main horn gaps and a common arc elongating coil for said main gaps all connected in series with said coil electrically and physically in the middle of said seri-es connection and with another horn gap connected across said coil, all of said gaps having substantially the same electrode spacing, the combination of an impedance network -connected to said gap unit for lowering its overall sparkover voltage, flattening its volt-time sparkover voltage characteristic (and raising its reseal voltage to prevent restrike when switching long duration switching surge currents), said network comprising four resistors of diierent resistance (in substantially a geometric progression having a common ratio of about three), a preionizing spark gap having a substantially lower voltage spark over than said main gaps mounted for ionizing the remaining end one of said main gaps, said preionizing gap and the highest resistance one of said resistors being serially connected across said end one of said main gaps, said other resistors being repectively connected across the remaining main gaps in order of decreasing resistance with the next highest resistance resistor being connected across the main gap which is adjacent said end one of said main gaps, and with the lowest resistance resistor being connected across the other end main gap, separate (corona type) preionizers for respectively preionizing said remaining main gaps connected respectively in parallel with the other resistors, and three capacitors of different capacitance (in substantially geometric progression having a common ratio of about four), the highest capacitance capacitor being connected across said coil, the remaining capacitors being connected respectively across t-he lowest resistance resistors with the highest capacitance remaining capacitor bein-g connected across the lowest resistance resistor, and with the next lower capacitance remaining capacitor being connected across the next higher resistance resistor.

l2. In a lightning arrester, a gap unit comprising a pair of equal accurately calibrated main gaps connected in series, a resistor, an auxiliary roughly calibrated spark gap having any sparkover voltage lower than the individual sparkover voltage of the main gaps, and means including a common coupling element for effectively connecting the roughly calibrated Spark gap in shunt circuit relation with one main gap and the resistor in shunt circuit relation with the other main gap, whereby upon applying an increasing voltage to the gap unit substantially all of the applied voltage is .across the auxiliary spark gap which consequently is rst sparked over at a value of applied voltage less than the sparkover voltage of a main gap so 'as then to limit the voltage across said one main gap to the arc voltage of the auxiliary spark gap thus causing substantially all of the applied voltage to appear across said other main gap which will next spark over when the applied voltage attains that gaps accurately calibrated sparkover voltage whereupon substantially all of the applied voltage is transferred to said one main gap for next sparking it over thus resulting in sequential cascaded sparkover of the serially connected main gaps at an applied voltage substantially equal to their individual sparkover voltage.

13. In a lightning arrester, a gap unit comprising a pair of equal accurately vcalibrated main gaps connected in series, a resistor, an auxiliary roughly calibrated spark gap having any sparkover voltage lower than the individual sparkover voltage of the main gaps, and means including a common coupling element for effectively connecting the roughly calibrated spark gap in shunt circuit relation with one main gap and the resistor in shunt circuit relation with the other main gap, whereby upon applying an increasing voltage to the gap unit the auxiliary spark gap is rst sparked over a value of applied voltage less than the sparkover voltage of a main gap so as to limit the voltage across said one main gap to the arc voltage of the auxiliary spark gap thus causing substantially all of the applied voltage to appear across said other main gap which will next spark over when the applied voltage attains that gaps accurately calibrated sparkover voltage whereupon substantially all of the applied voltage is trans ferred to Said one main gap for next sparking it over resulting in sequential cascaded sparkover `of the serially connected main gaps at an applied voltage substantially equal to their individual sparkover voltage.

14. In a lightning arrester, a gap unit comprising a pair of equal accurately calibrated main gaps connected in series, a resistor, an auxiliary roughly calibrated spark gap, and means including a common coupling resistor for elfectively connecting the resistor in shunt circuit relation with one main gap and the roughly calibrate-d spark gap in shunt circuit relation with the other main gap, said auxiliary spark gap having a lower sparkover voltage than the individual sparkover voltage of the main gaps whereby upon applying an increasing voltage to the gap unit the auxiliary spark gap first sparks over so as t0 limit the voltage across its shunt connected main gap to the arc voltage 0f the auxiliary spark gap thus causing substantially all of the applied voltage to appear across the remaining gap which will then spark over when the applied voltage reaches that gaps accurately calibrated sparkover voltage whereupon substantially all of the applied voltage is transferred to the remaining main gap for sparking it over resulting in sequential cascaded sparkover of the series connected main gaps at an applied voltage substantially equal to their individual sparkover voltage.

15. In a lightning arrester, a gap unit comprising a pair of similar accurately calibrated main gaps in series, a resistor in shunt with one main gap, an auxiliary roughly calibrated spark gap in shunt wit-h the other main gap, and a coupling resistor connected between the junction of the main gaps and the junction of the resistor and the auxiliary spark gap, said auxiliary spark gap having a lower sparkover voltage than the sparkover voltage of the main gaps, whereby upon applying an increasing voltage to t-he gap unit the auxiliary gap rst sparks over so as to limit the voltage across the main gap which is shunted by the auxiliary gap to the arc volta-ge of the auxiliary gap thus causing substantially all the applied voltage to appearY across the remaining main gap which will then spark over when the applied voltage reaches the accurately calibrated sparkover voltage of said main gap whereupon substantially all the applied voltage is transferred to the remaining main gap for sparking it over resulting in sequential cascaded sparkover of the series connected main gaps at an applied voltage equal to the sparkover voltage of one of them.

References Cited UNITED STATES PATENTS 730,601 6/1903 Ben -317-70 3,259,792 7/19661 Jensen 315-36 FOREIGN PATENTS 1,268,953 6/1961 France.

MILTON O. HIRSHFIELD, Primary Examiner.

J. D. TRAMMELL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,348,100 October 17, 1967 James S. Kresge It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line Z3, for "seen" read seem line 45, for "preioning" read preionizing column 4, line 74, after "terminal" insert 7 so that it sparks over. Finally with gaps Gl through column 5, line 38, for "pocfarads" read picofarads line 6l, after "Coil" insert 8 column 8, line 13, after "end" insert one Column 9, line 30, after "over" insert at Signed and sealed this 26th day of November 1968.

(SEAL) Attest:

EDUHU)J.BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

Claims

1. A VALVE RESISTOR SHUNTING CURRENT SWITCHING GAP UNIT FOR A LIGHTNING ARRESTER COMPRISING, IN COMBINATION, AT LEAST TWO MAIN SPARK GAPS, A MAGNETIC COIL, SAID MAIN GAPS AND COIL BEING SERIALLY CONNECTED WITH THE COIL IN THE MIDDLE, A SPARK GAP CONNECTED ACROSS SAID COIL, SAID SPARK GAPS ALL HAVING ABOUT THE SAME INDIVIDUAL SPARKOVER VOLTAGE, AND MEANS FOR SEQUENTIALLY SPARKING OVER SAID GAPS AT AN APPLIED VOLTAGE ACROSS THE SERIES CIRCUIT WHICH IS BETWEEN THE INDIVIDUAL SPARKOVER VOLTAGE OF SAID GAPS AND THE SUM OF THE SPARKOVER VOLTAGE OF ALL SAID GAPS, THE SEQUENCE BEING SUCH THAT ONE MAIN GAP SPARKS OVER BEFORE THE COIL SHUNTING GAP SPARKS OVER.