US3531690A

US3531690A - Voltage surge diverter

Info

Publication number: US3531690A
Application number: US759844A
Authority: US
Inventors: Eugene C Sakshaug; James S Kresge
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-09-16
Filing date: 1968-09-16
Publication date: 1970-09-29
Anticipated expiration: 1987-09-29
Also published as: CH512845A; GB1274887A; SE348895B; DE1944437A1

Description

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United States Patent O U.S. Cl. 317-68 16 Claims ABSTRACT F THE DISCLOSURE A current limiting gap lightning arrester with extra valve resistance for raising the discharge voltage at the beginning of the discharge and time delayed acting means for effectively shunting out the extra valve resistance to lower the discharge voltage at an appropriate time after the start of the discharge. The time delayed acting means may be a magnetic blow out series gap with auxiliary electrodes shunting the extra valve resistance which electrodes are bridged after a time delay by the magnetically blown out arc. The current limiting gaps may be disposed in parallel columns for flip-flop operation and fast acting means may be employed for shunting out the extra series valve resistance in response to impulse discharge current of large value.

This invention relates to voltage surge diverters and more particularly to improvements in valve type lightning arresters with current limiting gaps or gap units, especially such arresters for protecting high voltage direct current power systems.

A valve type lightning arrester comprises one or more valve resistors in series with one or more spark gaps. A valve resistor has an instantaneous negative resistancecurrent characteristic. A current limiting gap has a noninstantaneous or delayed rising voltage characteristic after sparkover. As arrester voltage during discharge, i.e., after sparkover and before seal off or interruption of power follow current, is the sum of the valve and gap voltages it is customary to use less valve resistance with current limiting gaps than with ordinary gaps in order to limit the arrester voltage during discharge. Current limiting gaps are particularly useful in arresters for direct current circuits because of their ability to seal 01T against the the non-cyclic circuit voltage.

However, in certain applications of valve type current limiting gap arresters the low discharge voltage immediately after sparkover is undesirable; for example, when such arresters are used to protect electronic or ionic valves in the rectifying and inverting converters at the ends of a high voltage direct current power transmission line. In such converters it is customary to bridge connect the electronic valves and series-parallel connect the bridges on their direct current sides. The paralleling is done by bus conductors and the series connection permits operation of the line at a voltage which is a multiple of the individual valve voltages. The arresters are connected between the busses and ground. In such an arrangement, if an arrester connected to an intermediate bus is sparked over by a voltage surge its low discharge voltage pulls the bus voltage way down but the line voltage cannot change that fast due to stray capacitance in the circuit. Consequently the electronic valves between the line and the lowered voltage bus are subjected to excessive voltage causing them to arc back and fail.

Merely increasing the valve resistance of the arrester is no solution of the problem because that will increase the arrester voltage during the discharge period thus impairing the protective characteristics of the lightning arrester.

In accordance with this invention, novel time delayed acting means is provided for effectively shunting out extra valve resistance. In this manner, the arrester is prevented from having too low a discharge voltage before the current limiting gap has time to build up substantial voltage and afterthat time the arrester is prevented from having too high a discharge voltage by the shunting out of the extra valve resistance.

An object of the invention is to provide a new and irnproved voltage surge diverter.

Another object of the invention is to provide a new and improved valve type lightning arrester with current limiting gaps.

A further object of the invention is to control the discharge Voltage of a valve type arrester with current limiting gaps at both ends of its conduction period.

An additional object of the invention is to raise the discharge voltage immediately after sparkover and lower the discharge Voltage before seal off or clearing of a valve type lightning arrester having a current limiting series gap or gaps.

A still further object of the invention is to provide novel slow shunting means for extra valve resistance in a valvetype lightning arrester.

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

In the drawings:

FIG. 1 is a schematic circuit diagram of an embodiment of the invention,

FIG. 2 is an exploded detailed view of a portion of the time delayed acting means,

FIG. 3 is a modification in which the current limiting gaps are in parallel columns for producing a ip-flop mode of operation with some of the valve resistance in the columns and with the rest of the valve resistance and the extra resistance in the common circuit of the arrester,

FIG. 4 is a diagram of a modified form of time delayed acting means, and

FIG. 5 is a still further modification of the time delayed acting means.

Referring now to the drawings and more particularly to FIG. 1, there is shown therein in schematic diagrammatic form a valve type lightning arrester 1 comprising a current limiting gap unit or units 2 in series with a normal valve resistor 3. In addition, there is an extra series valve resistor 4 and a special slow acting shunting means 5 for the extra resistor 4. The terminals of the arrester are indicated at 6 and 7 either one of which may be connected solidly to ground and the other connected to a power circuit element to be protected against overvoltage.

The slow shunting means 5 is shown as comprising a horn gap 8 having characteristic curved electrodes with an intermediate sparkover spacing and diverging are runners. In addition, the horn gap 8 is provided with a pair of auxiliary electrodes 9 insulated from each other and from the main electrodes of the horn gap 8 but in the path of the magnetically blown out arc between those main electrodes. The auxiliary electrodes are connected in shunt circuit relation with both the extra valve resistor 4 and the horn gap electrodes 8. In addition, there is provided a resistor 10 of typically a few thousand ohms for carrying the grading current of the arrester, the voltage grading network of which has not been shown as it is not a part of the present invention.

A suitable structural form of the slow shunting gap is shown in FIG. 2 in which the electrodes of the horn gap 8 are mounted on a ceramic disc 11 which is provided with an auxiliary insulated electrode 9 and another auxiliary nsulated electrode 9 mounted on a face of another ceramic disc 12. These two faces of the discs 11 and 12 when the discs are placed together form an arc chamber for the arc of the horn gap 8. The resistor is shown connected across the gap 8.

The current limiting gap unit 2 may be of any suitable type such, for example, as is fully described in Pat. 3,320,482 granted May 16, 1967 on a joint application of Eugene C. Sakshaug and Earl W. Stetson filed June 2, 1964 and assigned to the present assignee.

The operation of the invention shown in FIG.1 is as follows. When a sufficiently high voltage surge is impressed across the terminals of the arrester all of its gaps spark over and discharge current flows through the arrester. Because of the extra valve resistor 4, the arrester is prevented from having too low a discharge voltage. As current continues to fiow through the horn gap 8, the curvature or loop in the current path feeding its arc produces an electromagnetic force on the arc, moving or blowing it out along the arc runners to the right as viewed in the drawing. Successive positions of the blown out arc at the ends of the arc runners are indicated by the wavy lines at 13 and 14. Finally the blo'wn out arc will bridge the electrodes 9, whose spacing is somewhat greater than the sparkover distance of the electrodes of the horn gap 8. When this happens, the arc 15 forms between the auxiliary electrodes 9 and the horn gap arc is extinguished. The arc 15 shunts or effectively short circuits the extra valve resistor 4 as well as the horn gap 8. Consequently, toward the end of the arresters discharge period or at about the time that the discharge current of a switching surge begins to subside, the extra valve resistor is removed from the arrester circuit because at that time the current limiting gap 2 has built up suicient voltage for interrupting follow current and sealing off the arrester.

In the modification shown in FIG. 3 there is shown a more complicated arrester 16 having a so-called flip-Hop portion 17 and a common portion 18. The flip-flop portion consists of two parallel columns each comprising at least one current limiting gap 2 and at least one valve resistor 19. The common portion 18 which, of course, is in series with the flip-Hop portion 17 comprises a non-current limiting gap 20, a common valve resistor 21 and an extra valve resistor 4 corresponding to the extra valve 4 of FIG. l. The valve resistor 4 is arranged to be shunted by a slow shunting gap unit 5 similar to the element 5 of FIG. 1. In addition, all or part of the common resistance is shunted by a current switching gap unit 22. The details of the non-current limiting gap and the current switching gap 22 are not shown as they form no part of the present invention. Suitable examples of such gaps are disclosed in the above identified Sakshaug et al. patent.

The concept of parallel columns of current limiting gaps having a flip-flop mode of operation is the subject matter of another invention and is per se disclaimed herein. Briefly, the operation is such that the columns do not conduct simultaneously but alternately because upon the occurrence of an overvoltage one current limiting gap will naturally spark over before the other. However, as its voltage builds up after breakdown or sparkover, the other gap in the parallel column will be subjected to the full voltage of its parallel column, i.e., the built up voltage of the current limiting gap and the voltage drop in the valve resistor in series with the latter so that when that voltage exceeds the sparkover voltage of the non-sparked over gap in the parallel column, the latter will sparkover thus-clearing the first column and initiating the second half of a cycle of flip-flop operaton.

The operation of the extra valve resistor 4' and the slow shunting gap unit 5 is essentially the same in FIG. 3 as in FIG. 1 in that the extra valve resistor 4 prevents the discharge voltage of the arrester particularly in its initial stages, from being too low but after a certain length of time which is long in comparison with the duration of an ordinary impulse but which is short in comparison with the duration of an ordinariy switching surge, the unit 5 will shunt out the extra valve resistance.

In order to prevent the arrester from having too high a discharge voltage during the flow of large impulse discharge currents which are of short duration, the fast shunting current switching gap 2.2 will sparkover lwhenever the voltage drop across the resistors 4' and 21 exceeds a predetermined value, but this gap like a current limiting gap will in a time 'which is short in comparison with the duration of the switching surge build up substantial voltage thus switching the lower magnitude but longer duration switching surge discharge current into the resistors 4' and 21.

FIG. 4 is a modified form of slow shunting gap unit 5 in which the extra valve resistor is in the form of three

separate resistors

4a, 4b, and 4c, 4a and 4c being connected on one side of the

horn gap

8 and 4b being connected on the other side of the horn gap 8, the auxiliary electrodes 9 shunting the entire assembly 5 and there being a magnetic coil 23 for augmenting the magnetic blowout action of the gap 8, the coil 23 being connected across the horn gap 8 and the resistor 4c. The purpose of the latter is to augment the action of the coil 23 by increasing its voltage and hence its flux. It will be understood that the polarity of the coil 23 is such that its magnetic flux drives the

arcs

13 and 14 of the horn gap 8 out along the horns, i.e. in the same direction that they are propelled by the inherent action of the horn gap itself.

In the modification 5" shown in FIG. 5 only one extra valve resistor 4 is employed, the extra valve resistors 4b and 4c being omitted and the coil 23 being connected directly across the horn gap 8. 4In this figure the horn gap action is relied on entirely for starting the blow out action of the arc whereas in FIG. 4 the extra resistor 4c tends to overcome the iductive time lag effect of the coil 23 and cause faster coil produced starting of the arc motion.

In both FIGS. 4 and 5, however, the arc motion is more positive because of the presence of the coil than in FIGS. 1 and 2 which rely entirely on the horn gap effect.

It will, of course, be obvious that either the modification 5 or the modification 5" can be .substituted for the slow shunting gap unit 5 in FIGS. 1 and 3.

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

1. In a Valve type lightning arrester with at least one current limiting series gap, and at least one valve resistance, an extra valve resistance, and means for effectively shunting out the extra valve resistance after a time delay following sparkover ofthe current limiting gap and before the current limiting gap clears and seals ofl".

2. An arrester as defined in claim 1 in which said means is a magnetic blow out series gap with auxiliary electrodes insulated from each other and from the main electrodes but in the path of the blown out arc between the main electrodes shunting said extra valve resistance and magnetic blow out series gap.

3. An arrester as defined in claim 2 in which said magnetic blow out series gap is a horn gap.

4. An arrester as defined in claim 2 in which the magnetic blow out series gap is shunted by conductive means for passing grading current.

5. An arrester as defined in claim 4 in which said conductive means is a resistor. p

6. An arrester as defined in claim 4 in which said conductive means is a magnetic coil for also blowing out the arc.

7. An arrester as defined in claim 2 in which said extra valve resistance is in the form of two resistors respectively connected on opposite sides of said magnetic blow out series gap.

8. An arrester as defined in claim 6 in which said coil shunts both said magnetic blow out gap and at least a part of said extra valve resistance.

9. An arrester as defined in claim 6 in which said extra valve resistance is in the form of three resistors, two of which are connected on one side of said magnetic blow out gap and the other of which is connected on the other side of said magnetic blow out gap, said coil shunting both said magnetic blow out gap and the nearest of the two extra valve resistors connected on one side of said magnetic blow out gap.

10. An arrester as delined in claim 1 with a current switching gap also in shunt circuit relation to said extra resistance.

11. In a valve type lightning arrester, a pair of parallel columns each comprising at lea-st one current limiting gap and one valve resistor, said columns having a flip-Hop mode of operation during arrester discharge, common series valve resistance and means for effectively shunting out at least a portion of the common valve resistance after a predetermined time delay following initiation of discharge through the arrester but before the arrester seals off and terminates the discharge.

12. An arrester as defined in claim 11 in which said common series valve resistance is in the form of a normal resistor and an extra resistor and said means shunts out only the extra resistor.

13. An arrester as defined in claim 11 having a common series noncurrent limiting gap.

14. An arrester as defined in claim 12 having a currentl References Cited UNITED STATES PATENTS 1,902,510 3/1933 McEachron et al. 317-70 3,019,367 l/l962 Kalb 315-36 3,320,482 5/1967 Sakshaug et al 317-69 X 3,348,100 10/1967 Kresge 317-68 X JAMES D. TRAMMELL, Primary Examiner U.S. Cl. X.R.