DE2110144A1 - Lightning protection - Google Patents

Description

DR. ING. E. HOEFMÄNS - HIFI ,. IBEG. W. EITLE - De. KER. JSTAT. K. HOFFMAXX

PATEKTAHWlLTK

- ARABEtLSSTRASSE 4 - TEIlEFOl! * PMtJ, 9TOST ¹

2110H4

Mitsubishi Denki Kabushiki Kaisha, Tokyo, Japan.

LIGHTNING DEVICE

The present invention relates to a lightning protection, consisting of a pair of V-shaped oppositely disposed discharge electrodes and a spark extinguishing medium m ,, forming electrically negative gas.

Since electrically negative gases - for example sulfur hexafluoride CSFg) - have excellent properties with regard to the extinguishing of electric arcs, they are used to a very wide extent as a medium for extinguishing arcs in the event of a power failure. It has therefore already been attempts to use such gases in connection with lightning protection elements. These attempts: however, were not very successful-

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rich. In contrast to power interruption, with lightning protection © elements the relationship between the size of the electric field strength before the interruption of the discharge current and the size of the discharge voltage after the interruption of the discharge current and not so much the size the dielectric strength alone. In the case of lightning protection elements that use SF, gas as an arc crushing medium, it was derrick

According to the excellent difficult Stromtmterbrechungseigenschaften _r such gas fully to bring used.

In Blitzschutzelementeii it is known to form the discharge electrodes of a plurality of elongated, rounded at both ends of elements _r which for the formation of the discharge gap that is divergent at one end are arranged closer to each other, in addition magnetic means are provided, which essentially a perpendicular generate a magnetic field extending to the main plane of the electrodes, as a result of which the electric arc formed at the narrow end of the gap is driven in the direction of the wider end of the gap. If the high dielectric _{strength SF C} gas is used as the arc crushing condition in a discharge gap arrangement as described above, then the minimum discharge gap at which the electric arc occurs between the electrodes is necessarily relatively small. This is particularly the case with a discharge gap in which the SF, gas is under high

Pressure is present. The use of a narrow gap width, however, creates increased problems for magnetic drive of the electric arc. When the electric arc is driven out magnetically, the Usually a certain time interval before the electric arc starts moving after the occurrence of the magnetic field

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is brought. This time interval is relatively long, particularly in the case of SF _ß gas, with the result that the current interruption properties are not satisfactory. In the case of higher discharge currents, it also proves to be difficult to drive away the forming electric arc magnetically. Accordingly, _{lightning protection elements filled with SF g} gas with an electrode arrangement already described above are disadvantageous insofar as their mode of operation is unsatisfactory under heavy loads.

Accordingly, an object of the present invention to provide a container filled with an electrically negative gas ä lightning protection, which is suitable due to an improved electrode geometry for heavy loads.

According to the invention, this is achieved in that the discharge electrodes have a discharge trigger part, the smallest of which The radius of curvature is smaller than the size of the part formed by the discharge triggering parts of the electrodes Discharge gap.

An advantageous embodiment of the lightning protection according to the invention is characterized in that the smallest radius of curvature is not greater than times the magnitude of the discharge trigger parts of the electrodes formed part of the discharge gap, and that the spark-quenching space forming parts of the discharge electrodes have 0.3 of the by% have a radius of curvature is greater than the minimum radius of curvature of the discharge triggering parts.

Further details of the invention are to be explained and described in more detail below with reference to exemplary embodiments referring to the attached drawing.

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Show it

FIG. 1 shows a side view of discharge electrodes in a conventional lightning protection device;

FIG. 2 is a sectional view of one of the electrodes along the line H-II of FIG. 1;

FIG. 3 is a graph showing the sticking time of an electric arc as a function of the pressure of sulfur hexafluoride

FIG. 4 shows a graph showing the sticking time of an electric arc as a function of the size of the discharge gap when using sulfur hexafluoride

FIG. 5 shows a partial view of the discharge electrodes of a lightning protection device constructed in accordance with the present invention;

FIG. 6 shows an enlarged sectional view along the line VI-VI of FIG. 5;

Fig. 7 is a graph showing the relationship between the discharge voltage and the radius of curvature of the discharge initiating part of electrodes placed _{in SF g gas;}

FIGS. 8-10 partial sectional views of modified embodiments of discharge electrodes according to the invention;

FIG. HA shows a side view of discharge electrodes of a lightning protection device according to the invention;

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Figures HB and C are sectional views along the lines bb and cc of Figure HA;

FIG. 12 shows a sectional view of a further embodiment according to the invention; and

FIG. 13 is a graph to explain the mode of operation of the embodiment shown in FIG.

In the following, the drawing, in particular Fig. 1 and ^ Reference should be made to 2, in which an arc protection of known construction is shown. This arc protection exists of a pair of elongated flat electrodes 1, which are rounded at their ends and along their edges. These electrodes 1 are arranged in a V-shape to one another, see above as can be seen from FIG. 1. This known electrode arrangement is in a gas, for example sulfur hexafluoride - Arranged, which has a high electrically negative conductivity coefficient. When an electrical Voltage spikes - for example due to a lightning strike - a discharge is triggered in the area where in which the electrodes 1 are arranged close to one another. After the voltage peak has dropped, it continues to flow (| a current caused by the voltage source, which is only made to go out at the first zero crossing. To prevent the electrodes 1 from being damaged by the occurrence of an arc that lasts too long, and in order to improve the current interruption properties, additional magnetic devices, not shown, are included connected to the discharge electrodes, which generate a magnetic field substantially perpendicular to the plane of Fig. 1 runs, whereby the electric arc in the

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Arc quenching space 3 is pushed, in which the electrodes 1 have a relatively large distance from each other.

Due to its high dielectric strength, the use of sulfur hexafluoride inevitably requires a very small gap distance in the discharge trigger area 2 of the electrodes 1. The minimum gap size is therefore relatively small compared to conventional lightning protection elements which are filled with nitrogen gas. In particular, when using high pressures in conjunction with SF _g , the gap widths are very small. As a result, in the case of _{lightning protection elements filled with SF g} gas, it was difficult to make both the discharge release gap wide and the gas pressure high.

With a very small arc gap, however, problems arise in order to drive the electric arc away magnetically. This is because a certain time interval elapses before the arc is set in motion by the magnetic field after the arc occurs. This sticking phenomenon occurs in particular with SF _g gas, the sticking time being greater, the greater the gas pressure and the smaller the gap width. In this connection it should be mentioned that the size of the magnetic field only affects the speed of the movement of the electric arc, so that by increasing the magnetic field strength no reduction of the sticking time can be achieved. In order to achieve good interruption properties, it is necessary to very quickly To trigger movement of the arc in the direction of the arc extinguishing chamber 3, it has been found that a hold time should be equal to or less than 0.8 milliseconds.

Figure 3 shows the mean stick time of an electrical light

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arc in milliseconds as a function of the pressure of SF ^ gas

with a gap width of 1 mm and an arc current of 2 kiloamperes. This curve shows that at Use of SF, gas with a pressure of 4 at absolute pressure the sticking time is on the order of 0.4 milliseconds with a gap width of 1 mm.

FIG. 4 shows the mean sticking time in milliseconds as a function of the 3 gap width when using SF _g gas at 2 at absolute pressure and an arc current of 2 kA. On the basis of this curve it can be seen, for example, that at 2 at pressure f the sticking time cannot be made short enough unless the gap width is chosen to be relatively large.

From the foregoing it follows that with sulfur hexafluoride filled lightning protection elements, which have an electrode arrangement according to FIG. 1, it is impossible both to make the gas pressure as well as the electrode spacing large enough with regard to the required discharge voltage. As a result, the sticking time of the electric arc cannot be sufficiently reduced, and that the interruption properties of such lightning protection elements are good. Because of the small distance between the electrodes, _ a high current through the discharge electrodes 1 to occur ™ the phenomenon that magnetic expulsion of the arc does not take place due to the high discharge current, so that such lightning protection elements are not suitable for large currents.

The present invention aims to eliminate the disadvantages of known lightning protection elements by the geometry of the Electrodes is chosen in a suitable manner, whereby the elec-

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electrode spacing is increased without affecting the discharge voltage at the gap in the same way .will be enlarged.

In the following, reference should be made to FIGS. 5 and 6, in which the same reference symbols as in the embodiment according to FIGS. 1 and 2 are used. These Figures 5 and 6 show a pair of discharge electrodes 1 constructed in accordance with the principles of the present invention. The arrangement shown in FIGS. 5 and 6 differs from the arrangement shown in FIGS. 1 and 2 only in that the discharge triggering part 2 of the electrodes 1 has a pair of opposite crown surfaces which merge into a pair of short edges, the radius of curvature so as small as possible and not greater than a certain value is chosen »

According to FIG. 6, the edge has a circular cross-section Cross-section with a radius R, with one of these edges of the electrodes 1 opposite to the corresponding one Edge of the other electrode 1 to the formation of a V-shaped Arrangement is arranged. This measure means that the electric field in the area of the discharge triggering part 2 is uneven is distributed, whereby the discharge voltage at the gap is reduced with otherwise unchanged geometry will.

As a result, in the arrangement shown in FIGS. 5 and 6, the discharge voltage in comparison to an arrangement according to Figures 1 and 2 decreases. As a result, the electric current sticking time becomes a predetermined one Spatt distance shorter, which has a beneficial effect on the interruption properties. Due to the short term of the elec-

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Trical arc damage to the discharge trigger part 2 are also reduced. At the same time, the upper limit of the current, wherein a driving off the electric arc can still takes place magnetically to be raised.

Similar to the arrangement of FIGS. 1 and 2, the arc occurring at the discharge triggering part 2 is driven very quickly in the direction of the wider gap with the aid of a magnetic field which runs essentially perpendicular to the plane of FIG. 5 and is more magnetic with the aid of Λ not shown Facilities is generated.

FIGS. 8 to 10 show modifications of electrodes according to the invention. As shown in Fig. 8, the discharge trigger part 2 has a pair of opposing flat surfaces and a pair of side wedge-shaped surfaces with an inscribed circle R touching these three surfaces. In this embodiment of FIG. 8 the radius of the circle 4 is equal to or smaller than a predetermined small value R. FIG. 9 shows an arrangement similar to that in FIG. 8 with the exception that the lateral edge is formed by a rectangular extension of the main part of the discharge triggering part this approach being thinner than the main body. The thickness of this % approach should not be greater than a predetermined small value R. The embodiment shown in FIG. 10 differs from the embodiment shown in FIG. 8 in that the lateral surfaces are led up to a front tip whose radius of curvature is zero can be viewed.

The discharge electrodes according to the invention were used

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Various experiments were _{carried out in an SF fi} atmosphere, the gas pressure P of the gas, the minimum electrode spacing 1 and the minimum radius of curvature R of the discharge triggering part of the electrodes being reduced. The results of these tests have indicated that there is an essentially constant relationship between V / Pl and R / I, where V denotes the discharge voltage at the minimum electrode spacing, which depends slightly on the electrode structure. FIG. 7 shows such a relationship between V / Pl and R / l, the value V / Pl being indicated on the ordinate in kilovolts per atmospheric centimeter and the value R / l being indicated on the abscissa. From this figure it can be seen that when the value R / l is less than 0.5, the value V / Pl suddenly decreases, while when the value of R / l is less than 0.3, the minimum gap distance 1 for the same discharge voltage is relative is great.

In the case of conventional lightning protection elements that use nitrogen gas to extinguish the arc, the value is R / l selected to be large because the discharge voltage hardly differs between large and small values of R / l, although the dielectric strength of nitrogen is lower than that of SF ^ gas.

From FIG. 7, however, it can easily be seen that by reducing the value R / l, the discharge voltage at a given gap distance can be made much smaller than with the same electrode distance with one greater value of R / l. On the other hand, the ability of the gap to retain its original electrical insulating properties to be reached again after a power interruption, defined by the minimum gap distance 1, however the value R / l is not greatly influenced, which is also

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ments has been checked. edema can be the relationship between the dielectric strength after current interruption and Discharge savings can be made;> to significantly improve the properties of lightning protection »

As has already been described, “Enlargement enables the distance afe reduction of the term of detention »at a predetermined discharge voltage can be the electrode spacing can be increased by the cross-sectional profile of the electrodes is made sharper. Accordingly, by sharpening the profile of the electrodes, the sticking time can be reduced. In the- j | In this context it could be established that the relationship between the radius of curvature R of the discharge triggering parts of the electrodes and the electrode spacing 1 or R / l preferably equal to or less than 0.3 is selected to be satisfactory To get results.

When the radius of curvature R is from one end to the other end the electrode remains unchanged, the value R / l becomes very small, because the distance in the area of the arc extinguishing space is at the other end of the electrode becomes very large. This causes the arc length in the arc extinguishing space to be relative is large in order to achieve a sufficiently high dielectric strength

ensure. However, this leads to the following disadvantage: ™

It is assumed that a square-wave current flows through such a lightning protection element. The electrical energy occurring at the gap is equal to the product of the magnitude and duration of the current multiplied by the voltage between the two E 's of the arc or in the area of the gap. Accordingly, if the electrode has a radius of curvature α that unve from one end to the other. changes, and if the distance in the area of the Liehtbogenausloschreumes is relative

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is large, then the arc voltage is also increased accordingly, whereby the inside of the discharge gap energy consumed is increased; this does not improve the interruption properties much.

Figure 11 shows a modified embodiment of the invention, to eliminate the disadvantage described above. The arrangement consists of a pair of discharge electrodes, which are shown in the manner already described are arranged similarly as in Figures 1 and 2 and which essentially the in Figure 5 and 6 have the shape described. The electrodes 1 are connected to corresponding terminals 5 via conductors. So As can best be seen from FIG. HB, the electrode 1 has mutually parallel opposite sides flat surfaces. The discharge trigger part 2 is open one side provided with a wedge-shaped cross-section. If this should be desired, the discharge triggering part have one of the geometries described in connection with FIGS. 6, 8, 9 and 10. The ratio between the radius of curvature R of the edge and the size of the associated gap 1 or R / l is equal to or greater than 0.3. This measure serves to shorten the sticking time before the electric arc is driven away magnetically.

The electric arc is magnetically driven until it reaches the arc extinguishing space 3, which between the other ends of the electrodes 1 is arranged. As can best be seen from Figure HC, the im In the area of the light extinguishing space 3, part of the electrode 1 that is present has a lateral edge with a circular cross-section on, whereby the radius of curvature of this edge is made relatively large. In the area of the arc extinguishing chamber 3,

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with the discharge voltage made very high so that the distance between the electrodes in this area can be chosen to be relatively small without affecting the dielectric strength is reduced within this range; the electrical energy consumed within this space is therefore so small wi.e made possible. The opposite side edges the electrodes 1 thus have a gradually increasing radius of curvature from a minimum value in the area of the discharge triggering part up to a maximum value at the other end.

The arrangement shown in FIG. 11 thus results in a discharge gap arrangement for lightning protection elements, which have excellent interruption properties, because the stick time is sufficiently small and the electrical energy used is kept as small as possible.

In the context of the present invention, means for restoring the dielectric strength of a discharge gap after the interruption of the discharge current can also be provided, as will be described in connection with FIG. 12 below. Is shown in FIG 12 are disposed a pair of discharge electrodes 1 within an arc extinguishing chamber, said chamber opposed by a pair ä walls 6 and 7 and a pair formed by side walls, whereby an arc extinguishing chamber 3 according to FIG formed. 11 As soon as an arc 8 occurs between the electrodes 1, the plates 6 and 7 are melted or evaporated by the thermal energy of the arc. The plates 6 and 7 absorb the heat of fusion or evaporation, as a result of which the electric arc is cooled. A certain amount of the heat generated between the arc extinguishing plates is thus absorbed and conducted to the outside for a sufficiently long time. This will cause the breakdown

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strength of the discharge gap between the electrodes 1 restored as soon as possible.

When a high discharge current flows between the electrodes however, the electric arc generates a large amount of heat. As a result, the arc-arc extinguishing panels δ and 7 do not cool the arc sufficiently, so that the temperature of the space surrounding the arc rises very sharply, resulting in unsatisfactory arc extinction. If the distance d between a common neutral plane of the electrodes 1 and the inner surface of the plates 6 and 7 is made smaller, as in the previously known lightning protection elements was the case, then the arc chamber cannot with a sufficiently large amount be filled by SFg gas. As a result, there is not a sufficiently small time constant for the restoration of the Dielectric strength.

The arrangement shown in Figure 12 is characterized in that that the distance between the central neutral plane of the electrodes 1 and the inside of the arc extinguishing plates 6 and 7 is made sufficiently large, with the arc extinguishing chamber with an electrically negative gas - for example SF ^ gas - is filled and an electrode arrangement is

measured figure 11 was chosen.

FIG. 13 shows the results of experiments carried out at an arrangement of Figure 12 have been made to establish the relationship between the time constant for recovery the dielectric strength and the distance d described above. In Figure 13, the ordinate corresponds to the time

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constant in milliseconds and the abscissa the distance d in millimeters. From the curve it can be seen that as long as the SFg gas is kept at a pressure below 4 at, the absolute time constant drops very quickly if the distance d is less than 15 millimeters. _{The part of the SF g} gas that contributes to the absorption of the thermal energy of the arc and thus to its extinction is thus in a space which is determined by the common neutral plane of the electrodes and a surface which is approximately 15 millimeters from both sides of the middle plane away. This means that an electrode arrangement with a good time constant ™ to restore the dielectric strength can be formed in that the up to 15 millimeters from the neutral plane of the electrodes laterally reaching space with an electrically negative gas - for example SF - GAS - in is filled in sufficient quantity, the amount of heat of the arc occurring between the electrodes must be absorbed. While the above-mentioned distance d depends to a small extent on both the pressure of the electrically negative gas and the speed of the arc movement, it has nonetheless been found that for the distance d about 15 millimeters in the range of the available gas pressures and the speed of the arc is chosen. λ

By 1 g of SF gas from room temperature to the ionization temperature d.n. To heat 3,000 ° K is an energy of

4th
1.6 10 joules necessary. Normally, a discharge current of 4,000 amperes flows through the lightning protection for a duration of 2 milliseconds, with the arc voltage being around 200 volts. From this: ergi ¹ "* -. A discharge energy of 1.6 χ 10 Joules. Accordingly, ^ 0.1 g * of SF _g gas is sufficient,

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to consume the energy of the electric arc, without thermal ionization of the gas taking place.

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Claims (2)

Claims I) J lightning protection, consisting of a pair of opposite each other in a V shape arranged discharge electrodes and a spark extinguishing medium forming, electrically negative gas, characterized in that the discharge electrodes (1) a discharge trigger part (2), the smallest radius of curvature of which is smaller than the size of the discharge trigger part (2) the electrodes (1) formed part of the discharge gap. 2) lightning protection according to claim 1, characterized that the smallest radius of curvature is not larger than 0.3 times the size of the part caused by the discharge (2) of the electrodes (1) formed part of the discharge gap, and that the spark extinguishing space (3) forming parts of the discharge electrodes (1) have a radius of curvature which is greater than the minimum radius of curvature of the discharge discharge. release parts (2). 109838/1224 overtop