US2135352A

US2135352A - Multiple arcing gap

Info

Publication number: US2135352A
Application number: US46753A
Authority: US
Inventors: Harold L Rorden
Original assignee: Ohio Brass Co
Current assignee: Ohio Brass Co
Priority date: 1935-10-25
Filing date: 1935-10-25
Publication date: 1938-11-01
Anticipated expiration: 1955-11-01

Description

Nov. 1, 1938. H. 1.. RORDEN 2,135,352

MULTIPLE ARC ING GAP Filed 001;. 25, 1935 INVENTOR ham/d L. fiar'den BY Patented Nov. 1, 1938 UNITED STATES PATENT OFFICE MULTIPLE ABCING GAP Application October 25, 1335, Serial No. 48,753

Claims.

This invention relates to protective gaps for electrical apparatus, and has for one of its ob- Jects the provision of a gap having substantially constant characteristics and a short time lag,

especially for high transient voltages.

A further object of the invention is to provide a multiple gap in which the division of voltage between a plurality of gaps in series shall remain substantially constant notwithstanding exposure to weather and air laden with foreign material.

A further object of the invention is to provide protective gap apparatus for high voltages which shall have a short time lag and substantially constant characteristics.

A further object of the invention is to provide a device of the class named which shall be of improved construction and operation.

Other objects and advantages will appear from the following description.

The invention is exemplified by the combination and arrangement of parts shown in the accompanying drawing and described in the following specification, and it is more particularly pointed out in the appended claims.

In the drawing:

Fig. 1 is an elevation of a multiple gap showing one embodiment of the present invention.

Fig. 2 is a circut diagram showing the relation of the various electrical quantities of the device shown in Fig. 1.

In the protection of electrical apparatus from lightning, it has heretofore been the practice to provide an arcing gap in parallel with the apparatus for the purpose of diverting the surge due to voltage induced by lightning, or to a direct stroke, so as to protect the apparatus from injury due to the surge. Heretofore, it has generally been assumed that if the gap is set so as to flash over at a lower voltage than the associated apparatus, the apparatus would be protected. There is, however, another factor besides voltage which must be taken into consideration and that is the flashover time lag of the gap and the apparatus. By time lag in this connection is meant the time which elapses between the impression of a voltage of sufilcient magnitude to produce fiashover, and the discharge. This time lag varies with the nature of the gap or of the apparatus. A needle or rod gap, ior instance, although it will fiashover at a lower voltage for a given spacing than a sphere gap'of proper dimensions, has a much longer time lag than does the sphere gap. This is due to the fact that in the case of a needle or rod gap ionization between the electrodes proceeds progressively from each electrode toward (Cl- PIS-30 the other, and fiashover occurs when the corona, built up from the two electrodes, spans the space between the two. In the sphere gap, on the other hand, ionization takes place across the entire gap almost simultaneously with the impression of a voltage sufficient to cause fiashover. Thus, if a needle gap and a sphere gap of the same spacing were subjected to a slowly increasing voltage, the needle gap would fiashover at a much lower voltage than the sphere gap, but if the two were subjected to a wave of steep front of sufficient volt age to fiashover the sphere gap, the sphere gap would go ahead of the needle gap because of the difference in time lag characteristics of the two gaps.

It is apparent, therefore, that for the protection of electrical apparatus, the quick action of the sphere gap is to be preferred to the slow action of the needle or rod gap, but there are other disadvantages in the use of a sphere gap. It must be set closer for a given voltage than a rod gap, and if the surfaces of the spheres for any reason do not remain substantially true and smooth, they may take on the characteristics of a needle gap and consequently fiashover at a much lower voltage than that for which they were originally set and thus cause unnecessary and troublesome interruptions at much lower voltages than that for which the apparatus requires protection. A drop of water on the surface of one of the spheres, or an insect or any other foreign matter, may change the nature of the sphere gap so as to interfere with its true operation.

In order to avoid the defects of both the sphere gap and the rod gap, it has been proposed to use a gap having modified terminals approaching those of a rod gap and to control the electrostatic field about the discharge terminals by insulated flux controls so as to produce a field similar to that of the sphere gap, thus giving the gap the short time lag characteristics of a sphere gap while avoiding the effects of slight variations in the surface of the discharge terminals. Such a gap is shown in Patent No. 2,011,136, granted Aug. 13, 1935 to Arthur 0. Austin and assigned to The Ohio Brass Company, of Mansfield, Ohio. This gap has been found to work efllciently for voltages up to approximately 100 kilovolts but above that voltage it is necessary, in order to make the device maintain constant characteristics, to use porcelain parts of such large proportions that it becomes expensive and consequently not well suited for commercial operation.

The present invention provides means for overcoming this difliculty and consists broadly in the use of two gaps similar to that of the Austin patent, connected in series with means for maintaining a uniform division of voltage between the series gaps. Heretofore it has been found that when gaps were connected in series, the total voltage across the series would be divided between the gaps only so long as the intermediate terminals were insulated from ground by practically infinite impedance. It has been found, however, practically impossible to maintain such impedance under ordinary conditions of operation. Any dirt or moisture on the support for the intermediate terminal causes suflicient conductivity to ground to bring the intermediate terminal to a potential approaching ground potential with the result that the fiashover voltage of the device is not the sum of the two gaps but the flashover voltage of the gap connected to the high potential, since the intermediate terminal approaches ground potential. After fiashover occurs between the high potential terminal and the intermediate terminal, the intermediate terminal at once takes on the voltage of the high potential terminal and flashover readily occurs between the intermediate terminal and the ground terminal. The present invention provides means for maintaining the distribution of voltage between the several gaps, notwithstanding variation in the impedance of the circuit to ground from the intermediate terminal.

In the embodiment of the invention shown in Fig. l, the numerals l0 and II indicate the terminals of an arcing gap having insulated flux control members l2 electrically connected therewith in the manner more fully described in the Austin patent referred to above. The terminal I0 is supported by an adjusting rod 13 provided with means It by which it is connected to the high potential transmission line. The rod i3 is supported by insulators l5 having relatively high capacitance and these insulators 15 are in turn supported by a stack of insulators 16 which rest upon a grounded support IT. The gap terminal i is carried by an adjusting rod I8 which is mounted on a pedestal I8 carried by insulators 20 similar to insulators IS, the insulators 20 also resting upon the grounded support IT. A second gap formed of gap terminals 2| and 22 and having electrostatic control members 23 is connected in series with the first gap, the terminal 22 being substantially at ground potential as will be more fully explained. A conducting tie or jumper 25 connects the upper ends of the insulator stacks I6 and 20 so that these stacks are connected in parallel and provide a capacitance between the intermediate conductor terminal member I! and ground H.

The relation of the parts will be understood more readily from Fig. 2 in which the two gaps are designated by the numerals A and B respectively, and the capacitance provided by the insulators I5 is designated by the letter C, and the combined capacitance of the two stacks of insulators I5 and 20 is designated by the letter D. From this diagram, it will be seen that an impedance path from the high potential line to ground is formed by the two capacitances C and D in series. It will also be apparent that the voltage drop for an alternating or impulse discharge over this impedance path will be divided between the two capacitances C and D in proportion to the impedances of the respective capacitors. This will be true irrespective of the wave front or frequency of the impulse or alternating current. If the capacitance of the members C and D is large enough to pass a current of sumcient energy, the division of voltage between the two parts of the circuit will remain substantially constant, notwithstanding any slight variation in the capacitance of the insulators l5, l6 and 20, due to foreign matter on their surfaces. The potential of the intermediate terminal or the voltage gradient between the terminals Ill and 22 is, in this way, determined by the division of impedance between the condensers C and D so that the ilashover voltage of the two gaps A and B in series may be approximately the sum of the flashover voltages of the two gaps taken separately. In this way, any desired total flashover voltage may be built up by a series of smaller gaps in which the time lag characteristics may be accurately determined without the use of excessively large parts.

Generally, it is desirable to procure a substantially equal division of voltage between the several gaps in series, to secure the maximum benefit of the invention and make possible the use of uniform flux controls of minimum size for all gaps. However, beneficial results may be obtained so long as the fixed relative impedances of the condensers C and D remain the dominant factor in determining the voltage distribution or the discharge circuit. It has been found by test: that for voltages for which the device is intended, namely, all commercial voltages above kv., a capacitance of approximately seventy-five micro-microfarads for the condenser C and thirty micro-microfarads for the condenser D gives very satisfactory results and that the leakage conductivity of the condensers may be varied by moisture or other contamination to values up to 25 percent of their capacitance conductivity without seriously interfering with the operation of the device. If, under severe conditions, it is found that the capacitance values given above are not suiilcient to maintain eflicient distribution of voltage between the gaps, the capacitance of the insulators may be increased but it is believed that the values given above will be found suitable for most conditions.

The flu)! control members I2 and 23 provide a distribution of the electrostatic field about the gap terminals which will give a short time lag to the gap which is maintained even with a plurality of gaps in series. This is important in the protection of various forms of electrical apparatus and especially in the protection of transformers or other apparatus in which the parts are immersed in oil. Apparatus in which the discharge terminals are covered with oil has a short time lag discharge characteristic similar to that of sphere gaps because of the suppression of the corona due to the oil about the electrode surfaces. In the protection of this kind of apparatus, it is therefore of utmost importance to have a gap with a short time lag characteristic as otherwise the apparatus is apt to fail ahead of the gap flashover.

The invention thus far described has been found to be well adapted for the protection of all kinds of electrical apparatus. In installations, however, where a lightning discharge is apt to be followed by a power arc and where the overload cut-outs used in connection with the system are not sufliciently rapid to clear the line without causing interruptions, it may be desirable to provide means in connection with the gap for breaking the power arc. To provide for this operation the terminal 22, instead of being directly grounded, is supported by condensers 28.

These condensers 26 have sufficiently high capacitances so that when a steep wave impulse is impressed upon the conductor, the terminal 22 will be substantially at ground potential so far as the steep wave impulse is concerned and the two gaps will flashover under impulse substantially v the same as if the electrode were directly connected to ground through a conductor. A capacity of 300 micro-microfarads has been found suitable for the condenser 26. As soon as flashover occurs, the electrode 22 will be raised substantially to linepotential and arcing will then take place through an arc interrupter 21 to ground. The interrupter 21 may be a tubular member of insulating material, such as hard fibre, having terminals at the opposite ends thereof for directing a discharge through the interior of the tube. Such a device will clear the power are and leave the apparatus ready for further operation. Of course a fuse or other disconnecting device could be connected between terminal 22 and ground and would clear the line 01 the power arc, but a tubular interrupter of the nature described has the advantage that it does not require replacement after an operation and will therefore clear the are for multiple discharges which are very common in impulses produced by lightning. A tubular clearing device has the further advantage that it does not require replacement after an interruption.

I claim:

1. The combination with a high potential transmission line, of protective means for apparatus connected to said line, said protective means comprising a plurality of arcing gaps connected in series in a circuit between said line and ground, each of said gaps having flux controls associated with the terminals thereof for imparting a short time lag characteristic to said gaps, normally open circuit means in series with said gaps for interrupting a power arc following an impulse discharge over said gaps, a condenser in parallel with said power are interrupting means to facilitate discharge across said gaps and a plurality of condensers in series with each other in a circuit in parallel with said first named circuit, each of said condensers being in shunt with a portion of said first named circuit including one of said gaps.

2. The combination with a high potential transmission line, of means for protecting electrical apparatus connected with said line, said means comprising a pair of arcing gaps and a condenser connected in series between said line and ground, means for controlling the division of voltage between said gaps, and normally open circuit means in parallel with said condenser for interrupting a power arc following a discharge over said gaps, said open circuit means being adapted to flash over when said gaps have flashed over to impress on said normally open circuit means the excess voltage on said line.

3. The combination with a high potential transmission line, of means for protecting electrical apparatus, connected with said line, from high potential surges, said protecting means comprising a pair of arcing gaps and a condenser connected in series between said line and ground, a tubular member in parallel with said condenser for interrupting power arcs following impulse discharges over said arcing gaps, and an impedance path comprising a plurality of condensers connected between said line and ground, the intermediate electrode between said arcing gaps being connected at an intermediate point in said impedance path to control the division of voltage between said arcing gaps.

4. In combination a high potential electrode having a discharge terminal thereon, a low potential electrode having a discharge terminal thereon, an intermediate electrode having a pair of discharge terminals thereon cooperating respectively with the terminals on said high potential and said low potential electrodes, capacitance means for supporting each of saidelectrodes and for insulating said electrodes from ground, a portion of the capacitance of said means being connected in parallel with the gap between said high potential and said intermediate electrodes, a less portion of said capacitance means being connected in parallel with the gap between said intermediate electrode and said low potential electrode, the capacitance between said low potential electrode and ground being several times the capacitance in parallel with either of said gaps, an arc extinguishing tube in parallel with the capacitance between said low potential capacitance and ground, and means for directing a discharge arc through said are extinguishing tube from said low potential electrode to ground.

HAROLD L. RORDEN.