US3474287A - High power,nonlinear,gas tube resistor - Google Patents

Description

Oct. 21, 1969 c, CHURCH ETAL 3,474,237

HIGH POWER, NONLINEAR, GAS TUBE RESISTOR Filed July 15, 1966 REFRACTORY Y R o T c A R F E R Ell-25 WWIHH 3 22 -REFRACTO RY REFRACTO T N E R R U C mozfirmamm Y m 5 m m 0 mm 4 F J r E H w M H R Q 2 4 d w 0 A c 5 M a H R United States Patent Ofiice US. Cl. 315-36 2 Claims ABSTRACT OF THE DISCLOSURE A nonlinear gas tube resistor having a shock resistant housing capable of absorbing and rapidly dissipating large amounts of electrical energy in a gaseous discharge within said housing.

The subject matter of the present application relates to a passive, nonlinear resistor capable of handling large amounts of electrical energy and has particular but not exclusive utility in the lightning arrester art.

At the present time, high power surge protective devices, such as valve-type lightning arresters, generally use a plurality of valve blocks which are essentially voltage-dependent, nonlinear resistors usually made of silicon carbide crystals either bonded together or held in compression. Such resistors are substantially nonconducting under normal voltage conditions but become capable of conducting large surge currents when a sufiiciently high voltage is applied across them. However, silicon carbide resistors have a negative temperature coefficient which tends to localize the area of the high current path through the body of the resistor. This problem is enhanced by the fact that the resistance is generally not uniform throughout the resistor body due to the uneven distribution of the particulate matter in the body. Consequently as a particular area of the body becomes heated due to the current flow it conducts more current in the hot area thus concentrating current in the one (heated) area without further distribution of current in the resistor body. As a result of this high current concentration, the resistor block tends to crack and disintegrate.

The present disclosure describes a unique and novel flash tube structure designed to provide a variable resistance across a discharge arc and capable of absorbing large amounts of electrical energy in the discharge area. The structure shown and described herein uses a constricted channel are discharge that has a nonlinear but reproducible variation of resistivity with current in a gas tube having Wall structures that are thermally and mechanically shock resistant. The inherent disadvantages and deficiencies of resistor blocks as explained above are not found in the present structure. Eflicient heat transfer from the high current discharge to the shock resistant walls throughout the tube cools the arc and results in an increase in discharge voltage. Thus, much of the high electrical energy in say a rapidly rising surge of current caused by a lightning stroke is transferred to a heat resistant wall structure surrounding a high current discharge. Normal alternating current energy on say a power line would not follow the surge discharge path after the first swing through zero of the alternating current cycle, such energy being blocked by either a spark gap device in series with the novel discharge device disclosed herein or by the device itself. However, applicants novel flash tube device is not limited to AC circuit applications only. Applicants device is capable of handling and dissipating large amounts of DC energy such as encountered in DC circuit interruption.

Patented Oct. 21, 1969 Therefore, the principal object of the present invention is to provide a nonlinear resistor capable of handling and dissipating large amounts of electrical energy.

Another object of the invention is to provide a nonlinear flash tube resistor having a tube wall structure capable of withstanding large amounts of rapidly produced heat in close proximity thereto without shattering or becoming otherwise weakened.

Yet another object of the present invention is to provide a passive nonlinear gas tube resistor in which the shape of the resistance-current characteristic curve remains constant and with a predetermined shape.

Still another object of the invention is to provide a passive nonlinear resistor having a resistance value that can be easily varied.

A further object of the invention is to provide a flash tube nonlinear resistor in which the shape of the resistance-current characteristics can be changed during operation of the tube.

Numerous other advantages and objectives of the invention will become apparent to those skilled in the art from the following description of preferred embodiments of the invention taken together with the accompanying drawing in which:

FIGURE 1 shows a longitudinal cross sectional view of a nonlinear resistance gas discharge tube constructed in accordance 'with the principles of the present invention;

FIG. 2 shows a longitudinal cross sectional view of a second embodiment of the invention having two concentric tube structures as a discharge tube envelope;

FIG. 3 shows a longitudinal cross sectional view of a third embodiment of the invention employing a movable electrode means;

FIG. 4 shows a fourth embodiment of the invention in which a U-shaped envelope structure is employed;

FIG. 5 shows a fifth embodiment of the invention in which a reentrant envelope structure is employed;

FIG. 6 shows the shape of the resistance-current characteristic curve of the devices generally shown in FIGS. 1-3, and

FIG. 7 is a schematic diagram of an arrester circuit employing the present invention.

FIGURE 1 shows a first embodiment of the invention which includes a housing structure 10 comprising an elongated insulating tubular structure 11 sealed at both ends by electrode inserts 12 and 14 serving also as end caps for sealing purposes. Tubular structure 11 is made from thermally and mechanically shock resistant refractory materials such as alumina, zirconia or quartz with a suitable thickness dimension dependent upon the refractory material employed and the amount of energy to be dissipated. Electrodes 12 and 14 are chosen from a group of refractory metals having low arc erosion characteristics such as tungsten, though other suitable heat resistant electrode materials may be used such as carbon or molybdenum. Electrodes 12 and 14 are sealed to refractory structure 11 for the purpose of sealing in a gas under a desired pressure. The physical dimensions, the type of gas and gas pressure are chosen so as to give a resistance characteristic approximating the one desired. Diatomic gases, such as nitrogen and oxygen, have been found to be particularly adaptable for the purposes of this invention though rare gases are usable depending again on the type of operating characteristic desired.

When a high potential in excess of a preset limit appears across electrodes 12 and 14, the gaseous medium inside housing structure 10 ionizes to provide an arc discharge between the electrodes, the are being restricted in cross section by the confining walls of tube structure 11. When the gas ionizes, the light and heat produced by the discharge is transferred to and through the refractory wall of tube 11 thus dissipating the electrical energy in the discharge in the form of light and heat The electrical resistance across the discharge is non-linear and is dependent upon the amount of current in the surge as shown by the resistance-current curve of FIG. 6. With such a resistance-current curve the voltage drop across the discharge can be made substantially constant and independent of current. When no discharge is present in tube 10, a substantially open circuit exists between electrodes 12 and 14.

With one of the electrodes (12 or 14) connected to ground and the other connected to a conductor carrying a system current such as a power line, device can function as an arrester gap insulating the line and potential thereon from ground. With the event of a surge of high voltage, such as results from a lightning stroke, device 10 instantly flashes thus providing an electrical path to ground via an energy absorbing discharge through device 10.

The structure of device 10 and the nature of the discharge plasma make possible a rugged, gaseous discharge device having a nonlinear but reproducible variation of resistance with current. The structure of discharge device 10 may be strengthened mechanically by winding filament wires (not shown) around the outside surface of tube 11.

FIG. 2 shows a second embodiment of the invention in which a second refractory tubular structure 13 is concentrically disposed inside tube 11 to form a constricted annular discharge chamber and space 15 between electrode structures 12 and 14 which may now take the annular form shown in FIG. 2. The end cap structures 16 and 18 are further provided with openings 17 and 19 respectively to allow the circulation of air or other cooling medium through the hollow tube structure 13. Such a structure is more effective and efficient in dissipating heat energy when an arc is struck in device 10. Electrodes 12 and 14 may be embedded in or otherwise secured to end cap structures 16 and 18. Both tubular structures 11 and 13 are properly sealed to end cap structures 16 and 18 to confine a gas within the annular space 15.

Discharge device 10, as depicted in FIG. 2, functions in the same manner as the device of FIG. 1 except that with the concentric housing structure the arc discharge takes on an annular configuration with a substantial increase in tube wall surface exposed to the discharge. This increases the capability of energy dissipation by virtue of the added cooling surface provided by hollow tube 13 as explained above.

The shape of the resistance-current characteristic curve as shown in FIG. 6 remains substantially constant and independent of the physical dimensions of tube 10, the gas pressure and the nature of the gas as mentioned earlier. Changing any of these quantities changes essentially only the magnitude of the resistance as a function of the current. A variable resistance can therefore be made by changing one or more of the physical dimensions or the gas pressure. One simple way of achieving a variation of the physical dimension is shown in FIG. 3 wherein discharge device 10 contains a movable plunger 20 centrally supported in tube 11 by a heat resistant, annular shaped support means 22 suitably mounted in tube 11 in a manner to support and guide plunger 20 within the tube. The right hand end of the tube 11 may be sealed by end cap means 24.

Plunger 20 and support means 22 may be made of a refractory material such as alumina, and annular support 22 may also secure metal electrode 14 around plunger 20 as shown in FIG. 3. With such a structure plunger 20 can constrict the area of the are between electrodes 12 and 14, the amount of constriction depending, of course, on the extent of its insertion through annular electrode 14 and support means 22. A suitable conducti e lead ot sh w m y be co nec d b twe n t e electrode 14 and end cap 24 to connect the electrode to an outside circuit.

Plunger 20 may be made of a heat and resistant erosive resistant metal in which case the plunger itself could function as an electrode thereby eliminating annular electrode 14.

Plunger 20 may be moved by any suitable actuating means, a magnetic actuating means being preferred since the plunger can then move completely within the tube structure without the necessity of sealing means disposed about mechanical linkage means extending into tube 11. In FIG. 3 the actuating means takes the form of a magnetic disc 23 fixed on the plunger behind annular support means 22, and a magnetic device disposed outside tube 11, such as a bar magnet 25. Other magnetic acutating means, as well as mechanical means, may be used without departing from the scope of the invention.

By moving plunger 20, a change in resistance is easily accomplished. The shape of the curve shown in FIG. 6 may be maintained substantially constant, though it may be displaced in relation to the XY axes, by moving (setting) plunger 20 before or after the operation of the tube. Thus resistance, in this case, is a function of one variable, namely, current. However, when an insulating plunger 20 is moved relatively to annular electrode 14 and support 22 during the time the tube is operating, the resistance-current characteristic of tube 10 changes to a different one from that shown in FIG. 6. The shape of the representative curve is changed, and the resistance of flash tube 10 is now dependent on two variables, namely, current and time. The withstand (arc producing) voltage of the tube is not materially affected since the sparking distance remains constant with fixed electrodes 12 and 14.

If the plunger is made electrically conductive, or if it carries one of the discharge electrodes the withstand voltage can be changed as well as the voltage of the arc itself by longitudinal positioning of plunger 20 in flash tube 10. Such a positioning obviously changes the distance between plunger 20 (acting as an electrode) and fixed electrode 12.

To provide discharge device 10 with a more rapid breakdown characteristic at low voltages, the electrodes may be placed in close proximity to each other as shown in FIGS. 4 and 5. The embodiment shown in FIG. 4 comprises a U-shaped refractory envelope structure 11 providing an internal U-shaped discharge region 30 which in turn provides an elongated, energy absorbing path for a high current discharge when an arc is struck. Discharge electrodes 32 and 34 are provided in the ends of the U- shaped envelope 11, and may serve further as end cap sealing means as explained in reference to the embodiment depicted in FIG. 1.

In FIG. 5, discharge device 10 takes the form of a reentrant discharge tube type structure comprising a refractory envelope generally designated 40 having a reentrant portion 42 supporting a first electrode means 44 in close proximity to a second electrode 46 supported on a top wall portion 48 of the tube envelope 40. The closeness of the electrodes 44 and 46 provides a rapid and low breakdown voltage characteristic as in the U- shaped embodiment of FIG. 4. Inside tube 40 and surrounding the reentrant portion 42 of the tube is disposed an inverted cup-shaped refractory bafile structure 50 Sup ported by a suitable means not shown. Reentrant portion 42 and baffie 50 form the long, energy absorbing arc path which in turn results in high discharge voltages and good current interrupting properties when an arc is struck. Bafile 50 further serves to control and constrict the arc path as desired, thereby providing discharge characteristics and breakdown voltages that are substantially independent of each other.

Rapid, low voltage breakdown is characteristic of the devices shown in FIGS. 4 and 5 because of the plasmas developed in the gas in the immediate area of the electrodes. The plasmas create and maintain an intense field between their extremities because of their closeness. Thus, when an overvoltage condition occurs across discharge tube 10, ionization of the gas is caused to occur rapidly. The embodiments depicted in FIGS. 4 and 5 are given by way of example only. Other tube structures may be used that permit the electrodes to be in close proximity to each other.

In the five embodiments shown, a small valve means or seal (not shown) may be disposed on the outside of one of the electrodes or end caps, and at the end of a hole in the electrode or end cap for purposes of evacuating tube and refilling it with the appropriate gaseous medium.

It is sometimes desirable to keep the gas in tube 10 slightly ionized in order to insure instant conduction of a high current surge. Diiferent methods of providing such pre-ionization may be used, one of which is shown in FIG. 7. In FIG. 7 the gas discharge device 10 of the present disclosure is connected between ground and a spark gap 60 protecting a power line 62. Across gap 60 is an impedance 64, the value of which is chosen to give a small permanent current through the gas discharge tube 10 from the line 62. An overvoltage on the line 62 would cause the gap 60 to spark over and the gas discharge tube 10 to conduct the current to ground.

From the foregoing description is should now be apparent that applicants have provided a unique high power non-linear resistor capable of handling and dissipating large amounts of electrical energy without the disadvantages of resistor valve blocks as explained earlier. Applicants novel structure allows dissipation of the energy developed therein without cracking and breaking taking place. Thus the applicants device is stable and reliable and yet its resistance can be easily changed if so desired. In addition to functioning as a nonlinear resistor for high surge currents, applicants unique device also maintains a substantially open circuit and thus can function as a spark gap when subjected to normal potentials in the system in which it is used. In other Words, applicants novel device can serve the dual purpose of an arrester gap device as well as an arrester valve means.

Though the invention has been described with a certain degree of particularity, it is to be understood that other variations, modifications and embodiments are possible within the spirit and scope of the invention.

We claim as our invention:

1. In combination with a spark gap device connected to a circuit carrying alternating current, an excess voltage protective device connected in electrical series with said spark gap device, said protective device comprising a sealed refractory envelope that is thermally and mechanically shock resistant, said envelope containing a gas, electrodes disposed respectively at opposite ends of the envelope, the envelope defining a confined, relatively narrow are discharge region between said electrodes, said protective device being adapted to produce a discharge in the gas when an overvoltage is applied to the circuit of said device, said envelope being capable of absorbing and rapidly dissipating the electrical energy in the discharge Without adversely affecting said envelope.

2. The combination of claim 1 including a high impedance means connected across the spark gap, the impedance means adapted to provide a triggering ionization potential to said electrodes and gas.

References Cited UNITED STATES PATENTS 2,112,097 3/1938 Johnson 31761 X 2,288,050 6/1942 Vatter 313-452 X 3,119,040 1/1964 Gardiner et al. 313-220 X 3,229,145 1/1966 Jensen 313220 X 3,271,612 9/1966 Keller 313-220 3,337,762 8/1967 Vincent 31322X JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R.

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