US3489949A

US3489949A - Lightning arrester with main and preionizing gaps

Info

Publication number: US3489949A
Application number: US681991A
Authority: US
Inventors: Thomas J Carpenter
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1967-11-13
Filing date: 1967-11-13
Publication date: 1970-01-13
Anticipated expiration: 1987-01-13
Also published as: CH474881A; BR6803977D0; SE336831B; JPS55866B1; DE1808374B2; DE1808374C3; GB1172489A; DE1808374A1

Description

Jan. 13, 1970 I T. J. CARPENTER 3,489,949

LIGHTNING ARRESTER WITH MAIN AND PREIONIZING GAPS Filed Nov. 13, 1967 United States Patent 3,489,949 LIGHTNING ARRESTER WITH MAIN AND PREIONIZING GAPS Thomas J. Carpenter, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Filed Nov. 13, 1967, Ser. No. 681,991 Int. Cl. H01j 7/44, 13/46' U.S. Cl. 315-36 14 Claims ABSTRACT. OF THE DISCLOSURE A trigger gap is connected in shunt circuit relation to at least one of at least two series connected main gaps whose voltage distribution under increased applied voltage is upset by a shunt grading impedance network. The trigger gap is externally adjustable and preionized and the grading network comprises linear and nonlinear resistors. Coupling and decoupling impedances are connected between the trigger gap and the rest of the circuit.

3,489,949 Patented Jan. 13, 1970 improved means for producing cascaded sparkover of a lightning arrester multigap unit.

A further object of the invention is to provide a multigap unit whose 60 cycle sparkover voltage can be adjusted to narrow tolerances after gap assembly.

An additional object is to provide a multigap unit whose main gap spacings are not critical.

Still another object is to provide a multigap unit having a constant sparkover, that is, the sparkover is not affected by the main gap electrode surface conditions.

An added object is to provide a multigap unit which will interrupt with an applied 60 cycle voltage in the order of 90% of its sparkover 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 combination schematic representation and circuit diagram of a preferred embodiment of the in- I vention, and

FIG. 2 is a partially disassembled perspective view of the embodiment of the invention shown schematically and diagrammatically in FIG. 1.

The multigap unit comprises gaps 1, 2, 3, 4, and 5 in L the form of horn gaps connected in series between termiceed the insulation strength of the connected apparatus.

After the surge has passed, the follow current produced by the normal circuit voltage is limited by the resistor to a value which can be interrupted by the gap.

Modern lightning arresters for high voltage circuits have a sophisticated multigap unit in the form of a plurality of gaps in series provided with means for producing cascaded sparkover. This brings the sparkover voltage of the gap unit as a whole closer to its interrupting voltage which is generally desirable.

Heretofore there have been two means for producing cascade sparkover both of which require an impedance networkshunting the gaps for grading the total voltage distribution among them prior to sparkover of any one of them. One means has comprised a linear impedance grading network and a fixed trigger gap shunting one of the main gaps and having a lower sparkover voltage than the main gaps. The other means has comprised a nonlinear impedance grading network for upsetting the voltage distribution in response to increased voltages. The latter requires accurate gap spacing and small tolerances in the impedance values of the network.

In accordance with this invention, there is provided a new and improved multigap unit whose sparkover voltage can be adjusted to narrow tolerances after assembly, whose main gap spacings are not as critical, whose sparkover voltage is independent of main gap electrode conditions, and which will interrupt with an applied voltage of the order of 90% of its sparkover voltage.

Briefly, the invention comprises a preionized externally adjustable trigger gap shunting one of a plurality of series connected main gaps which in turn are shunted by a voltage grading network preferably comprising nonlinear resistors. In addition, one or more coupling and decoupling impedances may be connected between the trigger gap and the rest of the circuit.

An object of the invention is to provide a new and improved lightning arrester multigap.

Another object of the invention is to provide new and nals 6 and 7. Actually these gaps are not in the same plane as shown in FIG. 1 but are in parallel planes in stacked or superposed relation as shown in FIG. 2 wherein they are sandwiched between insulating plates or discs 8 having registering edges and forming therebetween arcing chambers. The gap 5 is shunted by a magnet coil 9 for producing a magnetic flux normal to the gap spacings and of such direction as to propel arcs in the gaps 15 out to the extremities of their horn electrodes, i.e., the coil 9 reinforces the arc propelling action of the horn gap electrodes themselves.

Shunting the gaps is a gap voltage controlling or grading network comprising

linear resistors

10 and 11 in series with nonlinear valve type resistors 12 and 13. As shown, there are

intermediate cross connections

14, 15, 16 and 17 for connecting the

linear resistors

10 and 11 respectively in shunt circuit relation with the gaps 1 and 3 and for connecting the nonlinear resistors 12 and 13 respectively in shunt circuit relation with the gaps 2 and 4.

Under normal conditions, practically the entire applied voltage between the terminals 6 and 7 will be distributed substantially equally across the gaps 1, 2, 3, 4 inasmuch as under these conditions the inductance of the coil 9 is negligible and its resistance predominates so that the series circuit of the grading resistances is completed through the coil 9. However, under impulse voltage conditions, the coil 9 has a high inductive reactance whose ohmic value materially exceeds its ohmic resistance under these conditions. A by-pass capacitor-.18 is therefore connected in shunt circuit relation with the coil 9 so that under impulse conditions th voltage distribution among the gaps 15 is upset in that the reactance of the capacitor 18 becomes very small and the resistance of resistors 12 and 13 greatly decreases with the result that the voltage across the gaps 1 and 3 becomes substantially higher than the voltage across the gaps 2, 4, and 5.

Gaps 1 and 3 are provided with resistance ballasted preionizer gaps in the form of resistors 19 and 20 respectively in series with

gaps

21 and 22. Likewise gaps 2 and 4 are provided with preionizers 23 and 24 in the form of a dielectric tube 25 having an inner cylindrical electrode 26 and an outer cylindrical electrode 27. There is minimum overlap between the inner and outer elec- 3 trodes so as to produce minimum capacitance but due to the termination of 27 on the surface of the dielectric tube 25 it produces high stress concentrations when gaps 1 and 3 sparkover copious corona indicated at 28 for irradiating and preionizing their associated main gaps. The principal purpose of the preionizers is to reduce the time lag and increase the consistency of the sparkover of their associated main gaps. The resistance ballasted

preionizer gaps

21 and 22 are better for slower long time voltage surges and the preionizers 23 are for the transient conditions resulting from sparkover of gaps 1 and 3.

The gap 1 is provided with a trigger gap 29 located between the gap 1 and the preionizer gap 21. As shown in FIG. 2, one of its electrodes is a screw 30 accessible through a lateral space in the stack of discs 8 so that the spacing of the trigger gap can be adjusted externally.

A decoupling resistor 31 is connected between the main gap 1 and the trigger gap 29 for decoupling these elements so that sparkover of the trigger gap 29 will not keep the voltage of main gap 1 too low for it to sparkover. Also a coupling capacitor 32 is connected in parallel with the resistor 31 for lowering the effective impedance of resistor 31 when the trigger gap 29 sparks over. Capacitor 31 also tends to compensate for the stray capacity of main gap 2 during this transient period.

The operation of the invention is as follows. As the total impressed voltage across the multigap between its terminals 6 and 7 increases, the

preionizer gaps

21 and 22 first spark over irradiating trigger gap 29 and main gap 3 and at the same time the preionizers 23 and 24 also irradiate their main gaps and prepare them for fast consistent sparkover when the voltage attains a somewhat higher value. The ballast resistors 19 and 20 of course limit the current through the

preionizer gaps

20 and 21 to a very low value. The trigger gap 29 being closer to the preionizer gap 21 than the main gap 1 is more extensively preionized than main gap 1.

At the same time, the resistance of the nonlinear grading resistors 12 and 13 decreases due to their inherent characteristic so that the normal substantially equal voltage distribution across the gaps 1-4 is altered and the voltage across gaps 1 and 3 becomes considerably higher than the voltage across gaps 2 and 4. At a voltage somewhere between the sparkover voltage of the gap 1 and half that value, the trigger gap 29 sparks over irradiating main gap 1, short circuiting the linear resistor 11 and practically instantaneously raising the voltage of the main gap 3 to well above its sparkover voltage. Main gap 3 then sparks over. This greatly reduces its voltage so that a still greater proportion of the total applied voltage is rapidly impressed across gaps 2 and 4 which will sparkover next. This then further increases the voltage across gaps 1, and and 1 will sparkover next followed finally by the sparkover of coil shunting gap 5. While trigger gap 29 is sparked over it acts as a preionizer for main gap 1. It should be understood, of course, that all of these cascading actions take place practically instantaneously. After sparkover of main gap 1, the trigger gap 2 will clear as its voltage will then be substantially zero for a time.

From the above description, it will be seen that the adjustment or spacing of the main gaps is not critical and consequently that spacing can be purposely increased substantially beyond what it would have to be if there were no trigger gap and the sparkover of the assembly was to be the same. The adjustability of the trigger gap 29 permits accurate adjustment of the whole assemblys sparkover even when the impedance components have greater variations than would be allowable with a circuit not having a trigger gap. Furthermore, the 60 cycle sparkover can be set accurately over a relatively wide range which remains unchanged by repeated discharges due to the fact that the trigger gap 29 only carries limited current for a brief instant. As a result, tests have shown that the assembly can interrupt an applied voltage l in the order of of its sparkover voltage. The discharge current of the trigger gap 29 is limited by the series impedances 10, 12, 13 and 18. These factors result in the electrode surface of the trigger gap 29 being relatively unaifected by the applied discharge current passing through the main gaps. The air density in the trigger gap 29 is relatively constant as the discharge current of the trigger gap 29 is low in magnitude and duration and the dielectric strength of the trigger gap 29 recovers during the passage of current in the main gap 1 and can withstand approximately sparkover voltage when the main gaps interrupt the current. The effect of current discharge on the main gap sparkover is in turn minimized by the increased gap spacing that is permitted by the use of the trigger gap 29.

In a specific multigap assembly in accordance with this invention which is rated 6 kv., the main gap spacing is 50 milsi3 mils. The preionizers 23 and 24 have approximately 9 picofarads. The

preionizers

21 and 22 have gaps of approximately 24 mils and the ballast resistors 19 and 20 have resistance of about 6.8 megohms. The nonlinear resistors 12 and 13 have a voltage drop of 1650 voltsiSO volts when passing a current of .2 milliampere DC. The linear resistors 10 and 12 have about 12.5 megohms resistance. The trigger gap 29 is adjustable between 26 mils and 35 mils. The resistor 31 has a resistance of 5000 to 20,000 ohms. The capacitor 18 has a capacitance of about l550 picofarads.

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. In a multi-gap unit for a lightning arrester, at least two generally similar serially connected main gaps, an impedance network for controlling the voltage distribution of said main gaps before their sparkover comprising at least two separate impedances connected respectively in shunt circuit relation with said main gaps, a trigger gap connected in shunt circuit relation with one of said main gaps and physically adjacent thereto, said main gaps having a sparkover voltage of between one and two times the sparkover voltage of said trigger gap, and a preionizer connected in shunt circuit relation with said trigger gap and physically adjacent thereto.

2. A gap unit as in claim 1 in which said trigger gap is physically between its main gap and its preionizer.

3. A gap unit as in claim 1 including means for adjusting the spacing of said trigger gap.

4. A gap unit as in claim 3 in which said gaps are sandwiched in a stack of insulating separator discs and said means for adjusting the spacing of the trigger gaps is accurate from outside said stack.

5. A gap unit as in claim 1 including a second preionizer connected in shunt circuit relation with and physically adjacent the other main gap.

6. A gap unit as in claim 1 in which said impedances are respectively linear and nonlinear.

7. A gap unit as in claim 6 in which the impedances are resistors and the nonlinear resistor has a characteristic generally similar to a series valve resistor for a lightning arrester, the linear resistor being connected in shunt circuit relation with the main gap having the preionized trigger gap.

8. A gap unit a in claim 1 in which said preionizer is a series resistor ballasted gap having a lower sparkover voltage than said trigger gap.

9. A gap unit as in claim 5 in which said second preionizer is a highly electrically stressed dielectric type.

10. A gap unit as in claim 5 in which said second preionizer is a gap having a higher sparkover voltage than the trigger gap.

5 6 11. A gap unit as in claim 1 having a decoupling re- 3,356,894 12/1967 Lafferty 315-36 X sistor in the shunting connection between said trigger 3,377,503 4/1968 Osterhout 317-70 X gap and its associated main gap. 3,414,759 12/1968 Connell et al 31770 X 12. A gap unit as in claim 10 having a decoupling 3,418,510 12/1968 Melhart 315-36 X resistor in the shunting connection between said second preionizer and said other main ga 5 JAMES W. LAWRENCE, Primary Examiner 13. A gap unit as in claim 11 having a coupling capaci- C R CAMPBELL Assistant Examiner tor in shunt with said decoupling resistor.

14. A gap unit as in claim 12 having a coupling ca- X- pacitor in shunt with said decoupling resistor. 10 70 References Cited UNITED STATES PATENTS 3,328,632 6/1967 Robinson 31536X 3,348,100 10/1967 Kresge 3177O