US2942152A - Discharge gap protective device - Google Patents

Description

June 21, 1960 H. O. STOELTING 2,942,152

DISCHARGE GAP PROTECTIVE DEVICE Filed May 13, 1954 /a- Capacitor msulatwn Strength I 1| Ir u /0 M 3 INVENTOR.

. lt' Flea haver- Characten'stic 0f g af Protective Gap Device 0/ fig. 4 WAMUO 72mg flttarnsy United States Patent DISCHARGE GAP PROTECTIVE DEVICE Herman 0. Stoelting, Milwaukee, Wis., assiguor to Mc- Graw-Edison Company, a corporation of Delaware Filed May 13, 1954, Ser. No. 429,495

Claims. (Cl. 311- 12 This invention relates to discharge gaps and in particular to discharge gaps for the protection of series connected elements in electrical power circuits;

During fault conditions on electrical power circuits the currents and voltages on series connected elements, e.g., series capacitors, are several times the maximum working value. The volt-ages which capacitors can withstand without damage to the dielectric are conventionally shown inthe form of an insulation strength cur-ve plotting potential versus duration of overvoltage. Voltagelimitingprotective devices such as discharge gaps connected to by-pass the capacitor are often used to limit the voltage rise across the series capacitor. The capacitor should be by-passed'during the first half cycle of fault current. The breakdown voltage of a discharge gap rises more at short time lags than most insulation, which means that arelatively short gap is required to provide protection against surges having steep wave fronts. The geometrical configuration of the arc gap surfaces and thecondition of the surfaces affect substantially the voltage at which the arc gap breaks down, and after the arc gap has functioned several times it frequently loses the precision of flashover characteristics which is necessary in the protection of series capacitors. If the breakdown current is high, the gap electrodes tend to weld together because of their close proximity, and even if-such welding does not occur, beads may form on the electrodes which will change the flashover voltage of the gap.

The flashover characteristic of a discharge gap varies considerably with atmospheric conditions, the flashover voltage varying directly with absolute humidity and inversely with temperature and barometric pressure. Even when a triggering control electrode is provided between the main gap electrodes, the initiation of the breakdown of the gap is dependent upon the formation of corona, making it impossible to predetermine the flashover voltage with accuracy. The control electrode of three electrode discharge gap structures heretofore utilized was disposed between the main electrodes where it was subjected to erosion from themain discharge with the result 2,942,152 Patented June 1960 ICC paratus for the parallel protection of series connecte elements in power circuits.

It is an object of the invention to provide a new and improved voltage limiting device for the parallel protection of series connected elements in an. electrical'power circuit. Q

Another object of the invention is to-provide discharge gap rneans for the parallel protection of series capacitors in which the flashover voltage is inherently consistent. 3 Another object of the invention is to provide a discharge gap device for parallel protection of series capacitors in which breakdown is initiated in an auxiliary control gap and then transferred to the main electrodes, thus permitting the main electrodes to be massive and spaced relatively far apart so that they will resist-prolonged arcing and the tendency to weld togetheras in that flashover characteristics changed as the device was used.

In an application of Graham H. Johnson, Serial No. 378,495, tiled September 4, 1953 and having the same assignee as the present invention, and now Patent No. 2,881,346, a discharge gap is disclosed wherein a control electrode cooperates with one-main electrode to define a discharge initiating auxiliary gap whose flashover voltage is relatively independent of humidity, temperature and pressure. The breakdown of the gap is not dependent upon the formation of corona as an initiating atmosphere, and the gap provides remarkable consistency of flashover voltage incomparison to .prior art devices.

In accordance with the present invention, the inherently uniform and precise flashover voltage characteristics of the discharge gap structure of the aforementioned prior art apparatus. A further object of the invention is to provide such a discharge gap protective device in which the control electrode is not in the .path of the main discharge and will not be burned with resulting change in flashover characteristics as in prior art devices.

Still another object is to provide means in such a discharge gap protective device for limiting the current in the control gap and thus preventing burning of the control electrode and consequent changes'in flashover characteristics.

A still further object of the invention is to provide three electrode, discharge gap means for the parallel protection of series capacitors in which the flashover voltageis relatively independent of humidity, temperature, and pressure variationswhich materially affected flashover characteristics of prior art discharge gaps. An object of one embodiment of the invention is to provide such a discharge gap protective device'whose flashover characteristic is closely correlated with themsulation strength curve of the series capacitor tobe protected.

An object of another embodiment of the invention'is to provide discharge gap means for the parallel protection of a seriescapacitor wherein the flashover voltage varies with the rate'of rise of current through the 'capacitor.

These and other objects of the invention will become apparent upon consideration of the following detailed description when taken in consideration with the accompanying drawing wherein:

Fig. 1 is a schematic circuit diagram of one embodiment of the invention;

Fig. 2 is a schematic circuit diagram of an alternative embodiment wherein the flashover volt-age varies with steepness of the surge impulse wave front and including means for limiting the current in the maingap;

Fig. 3 is a schematic. circuit diagram of an alternative embodiment wherein it is unnecessary to neutralize the inductive reactance of the means for limiting the current in the main gap;

Fig. 4 is a schematic circuit diagram of an embodiment of the invention wherein "the flashover characteristic of the parallel discharge gap is closely correlated with the insulation strength curve oi the capacitor it protects; and

Fig. 5 is a, graph showing how the flashover characteristic of the discharge gap device of Fig. 4 closely follows 3 15, as disclosed in the aforementioned application, Serial No. 378,495, is utilized to ,by-pass the capacitor and limit its voltage rise to a desired percentage of its rated value. One terminal of the capacitor 10 is connected by a lead 17 to the hemispherical gap electrode 14 and the other terminal of the capacitor 10 is connected by a lead 18 to the plate electrode 15. The plate gap electrode is provided with a transverse opening 19 adapted to receive an auxiliary control electrode 20 centrally thereof. One terminal of a resistor 22 is connected to the control electrode 20 and the other terminal thereof is electrically connected to one terminal of the series capacitor 10.

,Under normal conditions, the voltage across the series capacitor 10 is applied across the tripping control gap between the control electrode 20 and the plate electrode 15. This gap can be made sufiiciently small so that its flashover voltage is low enough to protect the series capacitor 10, for example, a flashover voltage equal to ap proximately twice the rated voltage of the series capaci tor 10. Because of the close spacing of control electrode 20 and plate electrode 15, the flashover characteristics are inherently consistent and relatively independent of humidity, temperature, and barometric pressure. The resistor 22 in series with the control electrode 20 limits the current flow in the control gap and thus prevents burning of the control electrode 20, which in prior art discharge devices caused change of flashover characteristics with use.

The are in the control gap ionizes the main gap be The heavy currents that flow under fault conditions are thus carried between the main gap electrodes 14 and 15 which are relatively massive and spaced relatively far apart so that they will resist prolonged arcing. The relatively large gap spacing for the main electrodes is possible only because of the initiating gap between clectrodes 15 and 20. It will be noted that any erosion of the main electrodes has no effect on flashover characteristics which are determined by the spacing of the electrodes of the auxiliary gap. The auxiliary tripping gap carries limited current because of resistor 22, and is out of the path of the main gap and thus not subject to appreciable arcing which would alter its flashover charac' teristics.

tel

In the usual manner, means such as a magnetic contactor or an automatic circuit breaker is connected to close and by-pass the gap and capacitor and to open some time after the fault has clearedto transfer current back through the capacitor.

The flashover voltage of the embodiment of the discharge gap protective means shown in Fig. 2 varies with the rate of rise of current through the series capacitor. Greater protection is thus provided against steep wavefront fault currents having a high percentage of high of the series capacitor '10 by lead 17, and the lead 18 from the plate electrode 15 is connected to the junction of the capacitor 10 and reactor 24. A'resistor 22 in series with the control electrode 20 and connected to the power line 11 eyond the inductive reactance 24 the current through the auxiliary tripping gap. The

voltage developed across the inductive reactance 24 is thus applied to the auxiliary control gap between control electrode 20 and plate electrode 15. Inasmuch as the voltage across the inductive reactance 24 required to flashover the control gap is small in comparison to the voltage developed across the series capacitor 10, the inductance of the current impedance 24 may be kept small so that it does not add appreciably to the inductive reactance of the power line 11 nor prevent the capacitor 10 from neutralizing the voltage drop due to this power line inductive reactance. The impedance of the inductive reactance 24 increases directly with frequency, thus causing the control gap to flashover and initiate the main gap discharge at a lower voltage magnitude for a steep wave front pulse such as a lightning surge having a high percentage of high frequency harmonies than for a sine wave voltage. The resistor 22 limits the current in the control electrode gap; thereby causing the are to transfer to the main gap electrodes. The inductive reactance 24 limits the current that may flow in the main gap. The device of Fig. 2 thus uses less of the available capacitor insulation strength than the embodiment of Fig. 1 and is less susceptible to tripping on low frequency inrush currents where the capacitance is most useful in voltage regulation.

The flashover characteristic of the circuit of Fig. 3 is similar to that of Fig. l, but the device is analogous to the embodiment of Fig. 2 in that means are provided to limit the current in the main gap. The reactive impedance 24 is connected between the plate electrode 15 and one terminal of the series capacitor 10. This embodiment has the advantage over the device of Fig. 2 in that the reactance 24 for limiting the current in the main gap is not in series with the power 11, and thus it is not necessary for the series capacitor 10 to neutrahze the inductive reactance of the reactor 24. Inasmuch as the voltage drop across the reactor 24 appears across the scries'capacitor 10, the inductive reactance of the reactor 24 must not be made sufiiciently large to cause greater than rated voltage to appear across series capacitor It).

The flashover characteristic of the discharge gap device shown in schematic circuit diagram in Fig. 4 is correlated closely with the insulation strength curve of the series capacitor 10 to be protected. It will be noted in Fig. 5 that at all points on the fiashover characteristic curve the discharge gap device breaks down to shunt the series capacitor before the overvoltage is impressed on the series capacitor for a sufricient duration to damage the capacitor dielectric. The desired fiashover characteristic curve is obtained by charging a control condenser connected in series with the auxiliary control gap to trigger the control gap, and by the selection of the proper magnitude of components in the charging circuit to vary the charging curve of the control condenser as a function of the duration of the overvoltage impulse on the series capacitor.

The main gap between hemispherical electrode 14 and plate electrode 15 is connected in shunt to the series capacitor 10 by conductors 17 and '18 as in the other can bodiments of the invention. The auxiliary control gap between plate electrode 15 and control electrode 20 is connected in series with a control condenser 30, and the control gap flashes over when the control condenser 30 is charged to a predetermined voltage. A transformer 31 having its primary winding in shunt with the series capacitor 10 provides the voltages for a full wave rectifier circuit for charging the control condenser 30 through a resistance 34. Two rectifiers, 36 and 37, are connected in series with the secondary winding 38 of the transformer 31, and the control condenser 30 is connected in series with the resistance 34 between the midpoint of the transformer secondary winding 38 and the junction of the rectifiers 36 and 37. During one half cycle of current the control condenser '30 is charged through the rectifier nitude of voltage as ordinates.

3 6 and resistor 34 in series; upon reversal of polarity during the following half cycle the condenser 30 is charged through rectifier 37 and resistor 34 in series.

A bleeder resistor 41 is connected in shunt to the control condenser 30. The condenser 30 must becharged to a predetermined potential before the auxiliary control gap between control electrode 20 and plate electrode 15- breaks down, and the charging rate of condenser 30 is determined by the magnitude of resistance 34 and 41. At zero time the rectified potential derived from an overvoltage surge impulse appears mainly across the resistance 34 and'builds up across the control condenser 30 at a rate dependent upon the time constant.

In Fig. 5 both the sparkover characteristics of the dis charge gap device of Fig. 4- and an approximate curve of insulation strength for a series capacitor are plotted with duration of impressed voltage as abscissae and mag- It will be noted that a power capacitor can withstand high overvoltages for relatively short intervals without damage to the dielectric but that the capacitor life curve bends rapidly downward with increase in duration of the overvoltage. In order to result in maximum life of the series capacitor .10, the gap discharge device is constructed to have a flashover characteristic which follows closely the insulation strength curve, but which remains below it at all points on the curve. This assures that the discharge gap breaks down before the overvoltage is impressed for sufficient duration to damage the series capacitor dielectric. Because of the time required to charge the control condenser 30, a relatively high magnitude of short duration impulse, such as is characteristic of a lightning stroke, is required to trigger the auxiliary discharge gap", whereas a relatively low magnitude sixty cycle inrush current will charge the control condenser 30 sufliciently over its relatively longer duration to break down the auxiliary control gap between plate electrode 15 and control electrode 20.

Although the various embodiments of the invention have been illustrated and'described in connection with the protection of a series capacitor, the invention is not so limited and encompasses the parallel protection of any circuit element connected in series in a power line. Although the discharge gap device has been illustrated and described with one of the main electrodes surrounding the control electrode, as disclosed in the aforementioned application the inherently consistent flashover characteristics, can be attained with electrodes of other configurations and also when the control electrode is in spaced embracing relation with one of the main gap electrodes. Having described several embodiments of the invention, it will be apparent to persons skilled in the art that the invention is not limited to the particular structures disclosed and that many changes and modifications may be made therein without departing from the broad 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 combination, an alternating current electrical power circuit, an electrical element connected in sa d circuit, an overvoltage protective air gap having a pair of spaced main discharge electrodes one of which is connected directly to one side of said element and the other of which is connected directly to the other side of said element and jointly providing a predetermined main discharge gap therebetween, an auxiliary control electrode forming an auxiliary control discharge gap with one only of said main discharge electrodes, one of the electrodes of said control gap being in spaced embracing relation with the other electrode thereof and said other electrode extending to the surface of said embracing electrode facing the other main discharge electrode, said control electrode being removed from the discharge path of least dimension between said main discharge electrodes, the spacing between said control electrode and the main discharge electrode which forms part of said control gap being only a minor fraction of said least air gap dimension between said main discharge electrodes and the spacing between said control electrode and the other main discharge electrode being of approximately the same magnitude as said least air gap dimension, the sparkover potential of said control gap being considerably lower than the sparkover potential of, the main discharge gap and sparkover of said control discharge gap ionizing said maindischarge gap and lowering the sparkover potential thereof, and means including a unitary impedance connected at one end to said control electrode and at the opposite end to said power circuit for developing a voltage proportional to the current in said power circuit and for impressing said voltage across said control gap, said impedance limiting the current in said control gap after sparkover thereof and facilitating transfer of the arc to said main discharge gap.

2. In combination, an electrical power circuit having an electrical element in series therewith, an overvoltage protective air gap connected in shunt to said element and having a pair of spaced main discharge electrodes providing a predetermined main discharge gap therebetween, an inductance in series with said power circuit, an auxiliary control electrode disposed in radially spaced and coaxial relationship with one of said main discharge electrodes to jointly provide an auxiliary control discharge gap therebetween, one of the electrodes of said auxiliary control gap embracing the other and said other electrode extending to the surface of its embracing electrode facing the other main discharge electrode, said control electrode .being removed from the discharge path of least dimension between said main discharge electrodes and the control gap spacing between said control electrode and said one main discharge electrode being only a minor fraction of said main discharge gap spacing between said main discharge electrodes, the sparkover potential of said control gap being considerably lower than the sparkover potential of said main discharge gap and sparkover of said control gap ionizing said main discharge gap and lowering thesparkover potential thereof, and impedance means connected in series with said control gap for limiting the current in said control gap, the serial arrangement of said control gap and said impedance means being connected in shunt to said inductance.

3. Apparatus for the parallel protection of aseries capacitor, comprising, in combination, a pair of spaced maln discharge electrodes jointly providing a predetermined discharge gap therebetween electrically connected in shunt to said capacitor, an auxiliary control electrode, one of said main electrodes being disposed in spaced embracing relation with said control electrode to jointly provide an auxiliary control gap therebetween, an electrical condenser connected in parallel with said control gap, aresistance, and means for charging said condenser.

through said resistance at a rate proportional to the current through said series capacitor.

4. Apparatus for overvoltage protection of an electrical element connected in series with an electrical power circuit, comprising, in combination, a pair of spaced main discharge electrodes jointly providing a predetermined discharge gap therebetween electrically connected in shunt to said element, an auxiliary control electrode, one of said main electrodes being disposed in spaced embracing relation with said control electrode to jointly provide an auxiliary control gap therebetween, an electrical condenser connected in shunt with said contral gap, a resistance connected to said condenser, and means for deriving a DC. potential proportional to the current through said element and for impressing said potential across the serial arrangement of said condenser and said resistance.

5. Apparatus for overvoltage protection of an electrical element connected in series with an electrical power circuit, comprising, in combination, a pair of spaced main discharge electrodes jointly providing a predetermined discharge gap therebetween electrically connected in shunt r spasm auxiliary control gap therebetween, an electrical transformer having one of its windings connected in shunt with said element, an electrical condenser connected in parallel with said control gap, a resistance connected to said condenser, means for rectifying the current generated in the other winding of said transformer, and means for impressing the output of said rectifying means across the serial arrangement of said condenser and said resistance.

6. Apparatus for overvoltage protection of an electrical element connected in series with an electrical power circuit, comprising, in combination, a pair of spaced main discharge electrodes jointly providing a predetermined discharge gap therebetween electrically connected in shunt to said element, an auxiliary control electrode, one of said main electrodes being disposed in spaced embracing relation with said control electrode to jointly provide an auxiliary control gap therebetween, and means for impressing across said auxiliary control gap a voltage which is proportional both to the magnitude of fault current flowing through said electrical element and to the duration of said fault current.

7. Apparatus for overvoltage protection of an electrical element connected in series with an electrical power circuit, comprising, in combination, a pair of spaced main discharge electrodes jointly providing a predetermined discharge gap therebetween electrically connected in shunt to said element, an auxiliary control electrode, one of said main electrodes being disposed in spaced embracing relation with said control electrode to jointly provide an auxiliary control gap therebetween, means for developing a voltage which is proportional to the current flowing through said electrical element, and means for impressing said voltage across said auxiliary control gap and for delaying the increase of said voltage across said auxiliary control gap, whereby said control gap will sparkover at lower voltages on fault currents of long duration flowing through said element than on fault currents of relatively short duration flowing through said element.

8. In apparatus in accordance with claim 7 wherein said last-named means includes a capacitance connected in shunt with said control gap and a resistance in series with said capacitance.

9. In combination, an alternating current circuit including a series capacitor, an overvoltage protective arc gap connected in shunt to said capacitor including a pair of spaced discharge electrodes providing a main discharge gap therebetween, one of said electrodes being a plate having an opening therein communicating with said main discharge gap, an auxiliary control electrode in said opening extending to the surface of said plate electrode which faces the other discharge electrode and providing with said plate electrode an auxiliary control discharge gap having a sparkover potential considerably lower than the sparkover potential of said main discharge gap, said control electrode being removed from the main discharge path established by said discharge electrodes and the spacing between said control electrode and said plate electrode being only a minor fraction of the least are gap dimension between said discharge electrodes, sparkover of said auxiliary control gap ionizing said main discharge gap and lowering the sparkover potential thereof, and means including an impedance connected at one end to said control electrode and at the other end to said alternating current circuit for impressing the potential across said series capacitor upon said control gap, said impedance limiting the current in said control gap and facilitating transfer of the arc to said main gap between said discharge electrodes.

10. In combination, an electric circuit, an electrical capacitor connected in series with said circuit, a first discharge electrodc, an auxiliary control electrode, one of said electrodes being disposed in spaced embracing relation to the other of said electrodes to jointly provide an auxiliary control discharge gap therebetween, a second discharge electrode disposed in spaced relation to said first discharge electrode and jointly providing a predetermined main discharge gap therebetween, said auxiliary control electrode extending to a plane through the surface of said first discharge electrode providing the shortest discharge path to said second discharge electrode, said main discharge gap formed jointly by said first and second discharge electrodes being connected in shunt to said capacitor, the control gap spacing being only a minor fraction of said spacing of least dimension between said discharge electrodes, the sparkover potential of said control gap being considerably lower than the sparkover potential of said main discharge gap and sparkover of said control gap ionizing said main discharge gap and lowering the sparkover potential thereof, and means including an impedance connected at one end to said control electrode and at the other end to said electric circuit for impressing across said control gap a voltage proportional to the current through said series capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 495,853 Thomson Apr. 18, 1893 1,477,304 Allcutt Dec. 11, 1923 1,477,305 Allcutt Dec. 11, 1923 1,477,306 Allcutt Dec. 11, 1923 1,477,307 Allcutt Dec. 11, 1923 1,612,354 Brackett Dec. 28, 1926 2,207,577 Buell July 9, 1940 2,335,240 Golde Nov. 30, 1943 2,363,898 Partington Nov. 28, 1944 2,508,954 Latour et al May 23, 1950

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