Calculate the duration of lightning if. We count the frequency of lightning strikes into a building

Lightning current parameters

Lightning parameter

Protection level

Peak current value, kA

Full charge, C

Charge per pulse, C

Specific energy kJ/Ohm

Average slope kA/μs

3.1.3. Lightning and atmospheric electricity

Lightning is one of the common causes of unwanted overvoltages, interruptions and failures in automation systems. The charge accumulated in clouds has a potential of about several million volts relative to the Earth's surface and is often negative. The direction of the lightning current can be from the ground to the cloud, with negative charge clouds (in 90% of cases), and from cloud to ground (in 10% of cases). The duration of a lightning discharge is on average 0.2 s, rarely up to 1...1.5 s, the duration of the leading edge of the pulse is from 3 to 20 μs, the current is several thousand amperes, up to 100 kA, the temperature in the channel reaches 20,000 ˚C, a powerful magnetic field and radio waves appear [Vijayaraghavan]. Lightning can also form during dust storms, blizzards, and volcanic eruptions. During a lightning discharge, several pulses appear (Fig. 3.64). The steepness of the front in subsequent pulses is much greater than in the first (Fig. 3.65).

The frequency of lightning strikes on buildings with a height of 20 m and dimensions of 100x100 m is 1 time in 5 years, and for buildings with dimensions of about 10x10 m - 1 hit in 50 years [RD]. The number of direct lightning strikes into the 540 m high Ostankino TV tower is 30 strikes per year.

,

where is the maximum current; - correction factor; - time; - front time constant; - decay time constant.

The parameters included in this formula are given in table. 3.23. They correspond to the most powerful lightning discharges, which are rare (less than 5% of cases [Vijayaraghavan]. Currents of 200 kA occur in 0.7...1% of cases, 20 kA in 50% of cases [Kuznetsov]).

The dependences of the first lightning current pulse and its derivative on time, constructed according to formula (3.2), are shown in Fig. 3.65. Please note that the time scales on the graphs differ by a factor of 10 and that the scale is logarithmic. The maximum rise rate (first derivative) of the first pulse is 25 kA/µs, subsequent pulses - 280 kA/µs.

The rate of current rise is used to calculate the magnitude of the induced pulse in automation cables.

Buildings and structures or parts thereof, depending on their purpose, the intensity of lightning activity in the area of ​​location, and the expected number of lightning strikes per year, must be protected in accordance with the categories of lightning protection device and the type of protection zone. Protection against direct lightning strikes is carried out using lightning rods various types: rod, cable, mesh, combined (for example, cable-rod). Rod lightning rods are most often used; cable lightning rods are used mainly for protecting long and narrow structures. The protective effect of a lightning rod in the form of a mesh applied to the structure being protected is similar to the action of a conventional lightning rod.

The protective effect of a lightning rod is based on the ability of lightning to strike the highest and well-grounded metal structures. Thanks to this, the protected building, which is lower in height compared to the lightning rod, will practically not be struck by lightning if all its parts are included in the lightning rod’s protection zone. The protection zone of a lightning rod is considered to be the part of the space around the lightning rod that provides protection of buildings and structures from direct lightning strikes to a certain extent

reliability. The surface of the protection zone has the least and constant degree of reliability; As you move deeper into the zone, the reliability of the protection increases. Type A protection zone has a reliability level of 99.5% or higher, and type B has a reliability level of 95% or higher.

General scheme for solving the problem: produced quantification the probability of lightning striking a protected object located on a flat area with fairly uniform soil conditions on the site occupied by the object, i.e., the expected number of lightning strikes per year of the protected object is determined. Depending on the category of the lightning protection device and the obtained value of the expected number of lightning strikes per year of the protected object, the type of protection zone is determined. The mutual distances between lightning rods taken in pairs are calculated and the parameters of protection zones at a given height from the ground are calculated.

Depending on the type, number and relative position of lightning rods, protection zones can have a wide variety of geometric shapes. The reliability of lightning protection at various heights is assessed by the designer, who, if necessary, clarifies the parameters of the lightning protection device and decides on the need for further calculations.

Industrial, residential and public buildings and structures, depending on their design characteristics, purpose and significance, the likelihood of an explosion or fire, technological features, as well as the intensity of lightning activity in the area of ​​their location, are divided into three categories according to lightning protection: I - industrial buildings and structures with explosive premises of classes B-1 and B-2 according to the PUE; it also includes buildings of power plants and substations; II - other buildings and structures with explosive premises not classified as category I; III - all other buildings and structures, including fire hazardous premises.

To assess thunderstorm activity in different areas of the country, a map of the distribution of the average number of thunderstorm hours per year is used, on which lines of equal duration of thunderstorms or data from the corresponding local meteorological station are plotted.

The probability of an object being struck by lightning depends on the intensity of thunderstorm activity in the area of ​​its location, the height and area of ​​the object and some other factors and is quantified by the expected number of lightning strikes per year. For buildings and structures not equipped with lightning protection, the number of damage is determined by the formula

Where S And L - respectively, the width and length of the protected building (structure), which has a rectangular shape in plan, m; h - greatest

height of the protected object, m; P- average annual number of lightning strikes per 1 km 2 earth's surface at the location of the object, value P with equal intensity of thunderstorm activity are determined from tables. For buildings complex configuration when calculated as S And L the latitude and length of the smallest rectangle into which the building can be inscribed in the plan are considered.

The category of lightning protection device and the expected number of lightning strikes per year of the protected object determine the type of protection zone: buildings and structures belonging to category I are subject to mandatory lightning protection. The protection zone must have a reliability degree of 99.5% or higher (type A protection zone); protection zones for buildings and structures belonging to category II are calculated according to type A, if N> 1, and type B otherwise; zones belonging to category III are calculated according to type A, if N > 2, and type B otherwise. This applies only to buildings and structures that are classified as explosive and fire hazardous. For all other objects of this category, regardless of the value N protection zone type is accepted B.

Calculation of lightning protection of buildings and structures involves determining the boundaries of the lightning rod protection zone, which is the space protected from direct lightning strikes. Protection zone of a single lightning rod height h  150 m represents circular cone, which, depending on the type of protection zone, is characterized by the following dimensions:

h
she

h
she

(12.16)

Where h 0 - apex of the protection zone cone, m; r 0 - radius of the base of the cone at ground level, m; r x - radius of the horizontal section of the protection zone at height h x from ground level, m; h x - height of the protected structure, m.

The protection zone of a single rod lightning rod in plan is graphically depicted by a circle of the corresponding radius. The center of the circle is at the point where the lightning rod is installed.

Protection zone of a double rod lightning rod up to 150 m high with a distance between lightning rods equal to L, shown in Fig. 12.1. The figure shows that the protection zone between two lightning rods has significantly big sizes, than the sum of the protection zones of two single lightning rods. Part of the protection zone

between the rod lightning rods in the section passing through the axes of the lightning rods is joint (Fig. 12.1), and its remaining parts are called end parts.

Determination of the outlines of the end parts of the protection zone is carried out according to the calculation formulas used to construct the protection zone of single lightning rods, i.e. dimensions h 0 , r 0 , r x 1, r x2, are determined depending on the type of protection zone using formulas (12.15) or (12.16). In plan, the end parts are semicircles with a radius r 0 or r x, which are limited by planes passing through the axes of lightning rods perpendicular to the line connecting their bases.

The joint part of the protection zone is limited from above by a broken line, which can be constructed using three points: two of them lie on lightning rods at a height h 0, and the third is located in the middle between them at a height h c. Cross-sectional outline of the protection zone A-A(Fig. 12.1) are determined according to the rules and formulas adopted for single rod lightning rods.

The protection zones of the double rod lightning rod have the following dimensions:

(12.17)

Zone A exists when L  3 h , otherwise, lightning rods are considered as single;

(12.18)

Zone B exists when L  5h, otherwise lightning rods are considered as single. In formulas (12.17), (12.18) L - distance between lightning rods, m; h c - height of the protection zone in the middle between lightning rods, m; r With - cross-sectional width of the joint protection zone A-A(Fig. 12.1) at ground level, m; d - width of the horizontal section of the joint protection zone in section A-A on high h x from ground level, m.

The main condition for the presence of a joint protection zone of a double rod lightning rod is the fulfillment of the inequality r cx > 0. In this case, the configuration of the joint protection zone in the plan consists of two isosceles trapezoids having a common base of length 2 r cx, which lies in the middle between the lightning rods. The other base of the trapezoid has length 2 r X. The line connecting the installation points of lightning rods is perpendicular to the bases trapezoid and divides them in half. If r cx = 0, the joint protection zone in plan represents two isosceles triangles, the bases of which are parallel to each other, and the vertices lie at one point, located in the middle between the lightning rods. If the construction of a protection zone is not carried out.

Objects located over a fairly large area are protected by several lightning rods (multiple lightning rod). To determine the external boundaries of the protection zone of multiple lightning rods, the same techniques are used as for single or double lightning rods. In this case, to calculate and construct the external outlines of the zone, lightning rods are taken in pairs in a certain sequence. The main condition for the protection of one or a group of structures with a height h x with reliability corresponding to the protection zones A And B, is the fulfillment of the inequality r cx > 0 for all lightning rods taken in pairs.

To protect long and narrow structures, as well as in some other cases, single cable lightning rods are used.

The protection zone formed by the interaction of cable and rod (single or double) lightning rods is determined in the same way as the protection zone of a multiple rod lightning rod. At

In this case, the supports of the catenary lightning rod are equal to rod lightning rods of height A and the radius of the base of the protection zone r, depending on the type of protection zone.

Self-test questions

1. Give a classification of electrical installations regarding electrical safety measures.

List the types of grounding used.

Describe the grounding arrangement and the design of the grounding conductors.

4. List the features of grounding devices in installations up to and above 1 kV.

5. What is the calculation of simple grounding conductors?

6. Calculate the specific equivalent electrical resistance of the earth.

Describe the protective effect of a lightning rod and categorize buildings and structures known to you.

Calculate the protection zone of a single lightning rod.

Calculate the protection zone of a double rod lightning rod and depict the protection zone for different heights of the protected building.

CHAPTER THIRTEEN

ACCOUNTING AND ENERGY SAVINGS

Average annual duration of thunderstorms. Specific density of lightning strikesn M.. Contraction radius Rst.. Number of direct lightning strikes into an object.. Degree of lightning danger.

The designer’s task is to provide in the project a reliable and appropriate lightning protection system for the facility. To determine the sufficient amount of protective measures that provide effective protection against lightning, it is necessary to understand the predicted number of direct lightning strikes into the protected structure. INFirst of all, the frequency of direct lightning strikes depends on the frequency of thunderstorms at the location of the object.

Thus, there are almost no thunderstorms above the Arctic Circle, but in southern regions North Caucasus, Krasnodar region, in the subtropical zone or in some areas of Siberia and Far East, thunderstorms are a frequent occurrence. To assess thunderstorm activity, there are regional maps of the intensity of thunderstorm activity, which indicate the average duration of thunderstorms in hours per year. Of course, these maps are far from perfect. However, they are suitable for rough estimates. For example, for the central part of Russia we can talk about 30–60 thunderstorm hours per year, which is equivalent to 2–4 lightning strikes per year per 1 km 2 earth's surface.

Specific density of lightning discharges

Average annual number of lightning strikes per 1 km 2 surface of the earth or specific gravity lightning discharges ( n M) is determined from the data meteorological observations at the location of the object. If it is unknown, then it can be calculated using the following formula:

n M = 6.7*T d /100 (1/km 2 year)

Estimating the frequency of lightning strikes through the contraction radius

Having determined the specific density of lightning discharges, the designer needs to estimate what proportion of these lightning strikes will hit the protected object.
An assessment can be made using the contraction radius (Rst). Experience shows that an object with height h, on average, attracts all lightning from a distance up to: Rst ≈ 3h.

This is the contraction radius. In the plan, you need to draw a line that is spaced from the outer perimeter of the object at a distance Rst. The line will limit the contraction area (Sst). It can be calculated by any available methods (even using cells on graph paper).

This assessment is also suitable for objects of complex shape, individual fragments of which have fundamentally different heights. Near each of the fragments, based on their specific height, a curve is constructed that limits its own contraction area. Naturally, they will partially overlap each other. Only the area enclosed by the outer envelope should be taken into account, as shown in Fig. 1. This area will determine the expected number of lightning strikes.
Fig.1

The number of direct lightning strikes into a protected object is determined simply: the value of the contraction area, expressed in square kilometers, is multiplied by the specific density of lightning discharges:

N M = n M*Sst.

Practical conclusions

Several obvious conclusions follow from this technique.
Firstly, the number of lightning strikes into a single concentrated object such as a tower or support, whose height is much greater than others overall dimensions, will be proportional to the square of its height (Sst=π(3h) 2 ), and for extended objects (for example, a power line) – proportional to the height to the first power. Other objects occupy an intermediate position in configuration.

Secondly, with the accumulation of many objects in a limited area, when their contraction areas partially overlap each other (urban development), the number of lightning strikes to each of the objects will be noticeably less than to the same object in an open area.
In conditions of dense buildings, when the free space between objects is significantly less than their height, then each of the objects will practically collect lightning only from the area of ​​its roof, and its height will cease to play any noticeable role. All this is convincingly confirmed by operating experience.

Lightning danger level

When assessing the degree of danger of lightning, there is one nuance that is better explained with an example. Suppose we estimate the number of impacts on an antenna mast 30 m high. With good accuracy we can assume that its contraction area is a circle with a radius Rst ≈ 3h = 90 m and is equal to Sst = 3.14*(90) 2 ≈25,000 m 2 = 0.025 km 2 .

If at the location of the mast the specific density of lightning discharges n M= 2, then the mast should annually on average take on Nm = 0.025 x 2 = 0.05 lightning strikes. This means that on average 1 lightning strike will occur every 1/Nm = 20 years of operation. Naturally, it is impossible to know when this will actually happen: with equal probability it can happen at any time, both in the first year and in the twentieth year of operation.

If we assess the degree of lightning danger for a specific antenna mast from the position of the owners mobile phones, then you can probably put up with a break in communication, which can happen once in 20 years of operation. The telephone company itself may have a completely different approach. If it operates not one, but 100 antenna systems, then the company is unlikely to be satisfied with the prospect of annual repairs on average of 100/20 = 5 antenna units.

It should also be said that assessing the frequency of direct lightning strikes in itself says little. In fact, it is not the frequency of lightning strikes that is important, but the assessment of the likelihood of possible destructive consequences from them, which allows us to determine the feasibility of certain lightning protection measures. Read also blog articles about this: