US2095311A - Arc protection means - Google Patents

Description

Uct. l2, w H. BURLESON ARC PROTECTION MEANS Filed Nov. 24, 1934 mvENToR Wade H. Eur/e500.

ATTORNEY Patented Oct. 12, 1937 PATENT OFFICE ABC PROTECTION MEANS Wade H. Burleson,

Mansfield, Ohio, asslgnor to The Ohio Brass Company, Mansfl eld, Ohio, a

corporation or New Jersey Application November 24, 1934, Serial No. 154,609

40laims.

a. path of discharge parallel to the insulator which will serve as a safety gap to prevent the e from following the insulator itself.

A further object of the invention is to provide a parallel time lag than that of the the insulator and yet having a sufilclent time lag to prevent discharge into the string.

Other objects and advantages will appear from the following description.

The invention is exemplified by the combination andarrangement of parts shown in the accompanying drawing and described'in the iollowing specification. and it is more particularly so pointed out in the appended claims.

In the drawing:

Pig. 1 is an elevation of an insulator string showing one embodiment of the present invention.

85 8. 2 is a top plan horn.

Fig. 3 is a plan view of the lower arcing horn.

In the construction of high potential transmission lines it is of course necessary to insulate 40 the line against flashover at normal voltages, but,

generally, it is not abnormal voltages such as are produced by lightning. It is therefore desirable to provide a discharge path for such abnormal voltages such that the discharge will not injure the insulators nor the supported conductor.

Heretofore, various flux control devices have been attached to insulators for the alleged purpose of uniform distribution of the dielectric field. surrounding the insulator or for producing a uniform voltage gradient in the insulator string or for other purposes related to the electrostatic field. It is doubtful if any of these devices produce the results that they are al- 56 leged to produce because of the time necessary,

view of the upper arcing A further object of the invention is to provide path of discharge having a shorter discharge path over feasible to insulate against after the voltage is applied, to set up the conditions which they are said to establish. A grading ring, for instance, may materially affect the distribution of potential in the insulator string for normal operating voltages and frequency, but unequal distribution at normal voltage is not of serious consequence because it is not a very difficult matter to provide an insulator which will withstand the voltage to which it is subjected when the string is under normal voltage. When the string is subjected to abnormal voltage, time is required for charging the series of insulator units constituting the string, as each unit is a small condenser, and until they are charged, the grading ring or other device is ineffective to establish equal distribution among the various units. It is very probable that if the voltage is high enough to cause flashover, this will occur before equalization of voltage among the units with the usual grading ring.

Attempts to establish uniform flux distribution in the field to control flashover are probably of little or no avail under conditions where danger of fiashover is greatest. Lightning of course occurs during stormy weather when the atmosphere is full of moisture so that the fiux distribution is affected more by the drops of water in the air than it is by the flux control devices. The effect of a few drops of water on an electrostatic field is a well-known phenomenon. The operation of a sphere gap is, for instance, entirely changed by very little moisture in the air.

The present invention is directed to the protection of the insulator and the conductor in case of flashover, and no special attempt has been made to produce uniform grading in the string or to produce uniform fiux distribution for the reason that these matters are regarded as of very-minor consequence as compared with the protection of the insulators and the supported conductor.

In order to protect the conductor and the supporting string, there are three things that must be accomplished. A parallel path must be provided which will have a shorter time lag, i. e., elapsed time of application of fiashover voltage before fiashover occurs, than the path of discharge over the insulator. This of course is to prevent the arc irom cascading over the insulator rather than from following the parallel path. Although an insulator string may stand one or more cascades without serious damage, there is always danger that the heat from the are or the concussion caused by the discharge will injure can be established where the string is equipped 2 some of the insulator units. It is of course also important to prevent the are from striking the conductor.- The third condition which it is desirable to avoid is a partial cascade; that is, a condition in which the arc starts from the arcing horn andstrlkes 'into'the string of insulator All of these results can of course be produced by providing an arcing gap in parallel with the insulator string which is short enough that discharge over the gap, rather than over the string, is insured for all over-voltages. However, it is desirable that this parallel gap shall not be any shorter than is necessary for safety, because the insulation afiorded by the string is expensive, not only for the insulators themselves but for the supporting structures necessary to secure clearance space for the insulators. It is therefore inexpedient to provide insulation and then cut out a material portion short arcing gap.

In order to insure a short time lag for the arcing gap, it would seem at first that the gap should be provided with a, terminal of extensive area, such as a sphere gap or a broad plane, because a gap of this kind has less time lag than a needle gap or a gap having small terminals. Such a gap could be set to withstand the maximum voltage that the insulator string would withstand without flashover and could be relied upon to flashover ahead of the insulator string when the two were subjected to such voltage. For this reason, it

might seem that arcing rings are preferable to arcing horns because the rings have a larger extent of discharge surface than the horns and consequently a shorter time lag. However, too short a conditions will be time lag for protective devices of this kind has been found, to produce another result which must be avoided, namely, arcing into the string. As previously stated, when the conductor is subjected to an impulse voltage, time is required to build up the potential gradient in the string and to permit streamers to build out from the arcing terminals and especially from the upper terminal, such that they will guide the discharge are away from the string. This latter is particularly true when the conductor is positively charged, which is the condition of greatest severity.

If the time lag of the arcing device connected to the conductor is too short, discharge may occur before streamers have been built out from the upper terminal of suflicient length to guide the arc to that terminal and also before the ultimate potential gradient in the string, due to the abnormal voltage, has'been established. Both of these conditions combine to cause the arc to strike into the string but if an arcing terminal of greater time lag characteristics is used, these remedied before discharge occurs and. the arc will strike free. In the case of a lightning stroke, there is usually a great excess of voltage over that required to flashover eitherthe insulator or the gap so that the selection of the path is entirely controlled by the relative time lag. It is therefore of greatest importance that the time lag of the parallel path shall not be too great on the one hand so as to cause cascading of, the insulator, as to cause the arc to strike into the string.

, It has been found that a discharge ring at the bottom of the string, unlessit is too small to be effective or too large to be economical or set so high as to reduce materially the insulation, produces arcing into the string, while a discharge horn is apt to have too long a time lag, giving rise of that provided by means of a or too small on the other hand so having the same to cascading over the therefore-employs a discharge member adjacent.

found that when a device of this kind is employed in conjunction with a horn at the top of the string,

the parts may be properly proportioned and related without materially reducing the insulation oi the string so that discharge will invariably take place over the path between the outer ends of the two discharge members and will never cascade over the string or strike into the string.

Devices of this kind have been subjected to impulses of approximately 3,000,000 volts time after time by means of an impulse generator, with the I discharge device of greater extent at its lower end, the arc struck into the string and when equipped with discharge horns spaced for a similar voltage, the insulators were iound to cascade. By the use of horns with compa'ratively restricted terminals at the top, in conjunction with horns at the bottom having proper time lag characteristics, streamers are caused to form at the discharge points which assist in guiding the are clear of the insulator.

In the embodiment of the invention shown in the drawing, the numeral l0 designates the units of an insulator string carried by a grounded structure II and supporting a conductor i2. Attached to the support for the conductor I2 is a pair of oblong loop shaped members l3, the axes of which extend in the direction of the conductor i2 above the conductor, the members being slightly curved ranged and proportioned relative to the insulator string and the upper horn I that they form an arcin gap parallel to the insulator string having a time lag for lightning surges less than that of the insulator string but sufllcient to permit a surge to establish a potential gradient in the string and also to permit the formation, of corona streamers from the ends of the horn l4 prior to discharge from the lower member, so as to prevent the arc from striking into the string. The upper membet I, as shown in the drawing, is secured to the grounded support II and extends outwardly in the vertical plane of the conductor and is provided with downwardly extending discharge points IS. The extremities of the arcing horn [4 may be turned upwardly and pointed,-as shown at l6, and the horizontal portion of the horn may be provided with upwardly extending points I! to prevent birds from perching on this member.

The exact shape and proportion of the lower member I3 is of course controlled by the conditions of each installation. For a string of twentyfour insulators having a height of 6 inches each and a maximum diameter of 10% inches designed for a 220 kv. line, it has been found that lower discharge members of the shape sh0wn, having,

a total spreadof 3 feet 4 inches and a maximum breadth of 10 inches, can be used with a horn l4 length as the lower member l3 and will prevent cascading or striking into the string with a spacing that will not materially reduce the insulation afiorded by the string. In

the particular installation referred to, the tips of a the lower members l3 were spaced approximately the lower insulator and the tips of the upper members H were spaced approximately 3 inches below the bottom face of the upper insulator, giving a discharge voltage to the arcing gap approximately that of the discharge voltage of the insulator string but providing a time lag for the gap less than that of the string but sufllcient to permit the establishment of a potential gradient in the string produced by a surge impulse and the formation of streamers on the tips i5 before discharge occurs, so as to prevent the are from striking into the string.

It will of course be necessary to proportion the parts of each installation to suit the particular conditions but if it'is kept in mind that the time in Patent Number 1,997,064, dated April 9, 1935.

lag of the arcing path is the controlling factor and that this can be adjusted by varying the size of the loops I3, the proper proportions for any installation can readily be ascertained by using the above relative proportions as a guide and making a few tests to facilitate adiustments. Fiashover tests can be madeto determine when proper relations have been established by use of an impulse generator set similar to that described Care should be taken to employ a set having sufficient power capacity as alset in which the capacity is too limited to produce conditions approximating those produced by actual lightning conditions is apt to be misleading. It is believed that the use in the past of inadequate im'- pulse generator sets has given rise to a great many erroneous conceptions relative to the behavior of arcing devices under lightning conditions.

I claim: 1 4

1. The combination with an insulator comprising a plurality of capacitance members connected together in series, of a, conductor supported by said insulator and a pair of discharge members connected to the terminals of said insulator respectively, the discharge member adjacent said conductor being of greater lateral extent than the other discharge member while the discharge member away from said conductor has a more restricted area adjacent its discharge point, said discharge members being disposed and related to g each other and to said conductor and insulator so that the time lag of discharge between them, when subjected to a lightning charge, is less than the time lag of discharge across said insulator but as great as the time required to establish the potential gradient in said insulators due to said charge and to produce a streamer from the initial flashover point of said discharge member away from said conductor so that the are between said discharge members will not strike into the insulator.

2. The combination with electrical apparatus to be protected, of an arcing gap connected in parallel with said apparatus and having substantially the same discharge voltage rating as said apparatus but having a shorter time lag of discharge than that of said apparatus, the time lag of said gap being sufliciently great, however, to permit the building up, prior to discharge,'of a streamer from at least one terminal of said gap suflicient to direct the discharge are between said terminals away from said apparatus.

3. The combination with electrical apparatus to be protected, of an arcing gap connected in parallel with said apparatus and comprising two discharge ,terminals, one of said terminals being shaped to produce, when subjected to over-potential, a concentration first mentioned terminal of suiiicient length to guide the flashover are away from said apparatus.

4. The combination with electrical apparatus to be protected, of an arcing gap connected in parallel with said apparatus and comprising two discharge terminals, one of which is grounded, said grounded terminal having a more restricted surface area than the other terminal to cause sufiicient concentration of electrostatic stress to produce a discharge streamer from said terminal prior to arc-over of said gap, the ungrounded terminal having suflicient surface area to impart to said gap a time lag of discharge after discharge voltage has been impressed on said gap and apparatu's which is less than that of said apparatus but great enough to permit building up of a streamer from said grounded terminal suiiicient to guide the flashover are between said terminals.

WADE H. BURLISON.

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