How to protect yourself from lightning

Lightning always awakened a person’s imagination and desire to know the world. She brought fire to the earth, having tamed which, people became more powerful. We do not yet count on the conquest of this formidable natural phenomenon, but would like "peaceful coexistence." After all, the more perfect the equipment we create, the more dangerous atmospheric electricity is for it. One of the methods of protection is to preliminarily, using a special simulator, assess the vulnerability of industrial facilities for the current and electromagnetic field of lightning.

Loving the storm in early May is easy for poets and artists. The power engineer, signalman or astronaut will not be delighted from the beginning of the thunderstorm season: he promises too much trouble. On average, each square kilometer of Russia annually accounts for about three lightning strikes. Their electric current reaches 30,000 A, and for the most powerful discharges it can exceed 200,000 A. The temperature in a well-ionized plasma channel of even moderate lightning can reach 30,000 ° C, which is several times higher than in the electric arc of the welding machine. And of course, this does not bode well for many technical facilities. Fires and explosions from direct lightning are well known to specialists. But ordinary people are clearly exaggerating the risk of such an event.

The tip of the flagpole of the Ostankino television tower. Traces of reflow are visible. In reality, the “celestial electric lighter” is not so effective. Imagine: you are trying to make a fire during a hurricane, when due to the strong wind it is difficult to light even dry straw. The air stream from the lightning channel is even more powerful: its discharge gives rise to a shock wave, the thunderous rumble of which breaks and extinguishes the flame. A paradox, but a weak lightning is fire hazard, especially if a current of about 100 A flows through its channel for tenths of a second (for ages in the world of spark discharges!), The latter is not much different from an arc, and an electric arc will ignite everything that can burn.

However, for a building of normal height, lightning strikes are not a frequent occurrence. Experience and theory show: it is “attracted” to a ground structure from a distance close to its three heights. The ten-story tower will collect about 0.08 lightning annually, i.e. an average of 1 hit in 12.5 years of operation. A cottage with an attic is about 25 times smaller: on average, the owner will have to “wait” for about 300 years.

But let’s not downplay the danger. Indeed, if lightning strikes at least one of 300-400 village houses, local residents are unlikely to consider this event insignificant. But there are objects of much greater length - say, power lines (NEP). Their length may well exceed 100 km, their height is 30 m. This means that each of them will collect blows from the right and left, with strips 90 m wide. The total area of ​​lightning “pulling” will exceed 18 km2, their number is 50 per year. Of course, the steel supports of the line will not burn out, the wires will not melt. Lightning strikes about 30 times a year at the tip of the flagpole of the Ostankino TV tower (Moscow), but nothing terrible happens. And in order to understand why they are dangerous for power lines, you need to know the nature of electrical, not thermal, effects.

THE MAIN POWER OF LIGHTNING

When struck in the support of the electric line, the current flows into the ground through the ground resistance, which, as a rule, is 10-30 Ohms. Wherein Ohm's law even the "medium" lightning, with a current of 30,000 A, creates a voltage of 300-900 kV, and powerful - several times more. So there are thunderstorm overvoltages. If they reach the megavolt level, the insulation of the power transmission line does not withstand and breaks through. A short circuit occurs. The line is disconnected. Even worse, when a channel of lightning breaks directly to the wires.Then the overvoltage is an order of magnitude higher than with damage to the support. The fight against this phenomenon today remains a difficult task for the electric power industry. Moreover, with the improvement of technology, its complexity only grows.

The Ostankino TV tower acted as a lightning rod, having missed a lightning strike 200 meters below the peak.To satisfy the rapidly growing energy needs of mankind, modern power plants must be combined into powerful systems. A unified energy system is now operating in Russia: all of its facilities operate interconnected. Therefore, the accidental failure of even one power transmission line or power plant can lead to serious consequences similar to what happened in Moscow in May 2005. A lot of system accidents caused by lightning have been noted in the world. One of them - in the USA in 1968, caused multimillion-dollar damage. Then a lightning discharge turned off one power line, and the power system could not cope with the energy deficit that arose.

It is not surprising that specialists pay due attention to the protection of power lines from lightning. Special metal cables are suspended along the entire length of overhead lines with a voltage of 110 kV and more, trying to protect the wires from direct contact from above. Their insulation is maximized, the grounding resistance of the supports is reduced to the utmost, and semiconductor devices, such as those that protect the input circuits of computers or high-quality TVs, are used to further limit overvoltages. True, their similarity is only in principle of operation, but the operating voltage for linear limiters is estimated in millions of volts - evaluate the scale of the cost of protection against lightning!

People often ask whether it is feasible to design an absolutely lightning-resistant line? The answer is yes. But here two new questions are inevitable: who needs it and how much will it cost? Indeed, if it is impossible to damage a reliably protected power transmission line, then it is possible, for example, to form a false command to disconnect the line or simply destroy the low-voltage automation circuits, which in modern design are built on microprocessor technology. The operating voltage of the chips decreases every year. Today it is calculated in units of volts. That's where there is room for lightning! And there is no need for a direct strike, because it is able to act remotely and immediately over large areas. Its main weapon is the electromagnetic field. It was mentioned above about the lightning current, although both the current and its growth rate are important for assessing the electromotive force of magnetic induction. In lightning, the latter can exceed 2 • 1011 A / s. In any circuit with an area of ​​1 m2 at a distance of 100 m from the lightning channel, such a current will induce a voltage of approximately twice as high as in sockets of a residential building. It does not take a lot of imagination to imagine the fate of microchips designed for a voltage of the order of one volt.

In world practice, there are many serious accidents due to the destruction of lightning control circuits. This list includes damage to the on-board equipment of airliners and spaceships, false shutdowns of entire “packages” of high-voltage power lines, and failure of the equipment of antenna mobile communication systems. Unfortunately, a noticeable place here is occupied by the “damage” to ordinary citizens' pocket for damage to household appliances, which more and more is filling our homes.

WAYS OF PROTECTION

We are used to relying on lightning protection. Remember the ode to the great scientist of the XVIII century, academician Mikhail Lomonosov on their invention? Our famous compatriot was delighted with the victory, said that heavenly fire has ceased to be dangerous. Of course, this device on the roof of a residential building will not allow lightning to set fire to a wooden flooring or other combustible building materials. With regard to electromagnetic effects, he is powerless. It makes no difference whether the lightning current flows in its channel or through the metal rod of the lightning rod, it nevertheless excites a magnetic field and induces a dangerous voltage due to magnetic induction in internal electrical circuits. To combat this effectively, a lightning rod is required to intercept the discharge channel at remote approaches to the protected object, i.e. become very high, because the induced voltage is inversely proportional to the distance to the current conductor.

Today, great experience has been gained in using such structures of different heights.However, the statistics are not very comforting. The protection zone of a rod lightning rod is usually presented in the form of a cone, the axis of which it is, but with an apex located slightly lower than its upper end. Usually a 30-meter “core” provides 99% reliability of building protection if it rises about 6 meters above it. To achieve this is not a problem. But with an increase in the height of the lightning rod, the distance from its top to the "covered" object, the minimum necessary for satisfactory protection, is rapidly growing. For a 200-meter structure of the same degree of reliability, this parameter already exceeds 60 m, and for a 500-meter structure - 200 m.

The aforementioned Ostankino TV tower also plays a similar role: it is not able to protect itself, it misses lightning strikes at a distance of 200 m below the peak. The radius of the protection zone at ground level for high lightning rods also increases sharply: for a 30-meter one, it is comparable to its height, for the same TV tower - 1/5 of its height.

In other words, one cannot hope that lightning rods of a traditional design will be able to intercept lightning at distant approaches to the object, especially if the latter occupies a large area on the surface of the earth. This means that we must reckon with the real probability of a lightning discharge into the territory of power plants and substations, airfields, warehouses of liquid and gaseous fuels, and extended antenna fields. Spreading in the ground, the lightning current partially enters the numerous underground communications of modern technical facilities. As a rule, there are electrical circuits of automation, control and information processing systems - the very microelectronic devices mentioned above. By the way, the calculation of currents in the earth is complicated even in the simplest formulation. Difficulties are exacerbated due to strong changes in the resistance of most soils, depending on the strength of the kiloampere currents spreading in them, which are just characteristic of atmospheric electricity discharges. Ohm's law does not apply to the calculation of circuits with such nonlinear resistances.

To the "nonlinearity" of the soil is added the probability of the formation of extended spark channels in it. Repair crews of cable lines are well acquainted with such a picture. A furrow stretches along the ground from a tall tree on a forest edge, as if from a plow or an old plow, and breaks off just above the track of an underground telephone cable that is damaged in this place - the metal sheath is crumpled, the insulation of the cores is destroyed. So the effect of lightning manifested itself. She struck a tree, and its current, spreading along the roots, created a strong electric field in the ground, formed a plasma spark channel in it. In fact, lightning continued its development, as it were, not only through the air, but in the ground. And so it can pass dozens, and in especially poorly conducting currents soils (rocky or permafrost) and hundreds of meters. The breakthrough to the object is not carried out in the traditional way - from above, but bypassing any lightning rods from below. Sliding discharges along the soil surface are well reproduced in the laboratory. All these complex and highly nonlinear phenomena need experimental research, modeling.

The current for generating a discharge can be generated by an artificial pulsed source. Energy is accumulated in the capacitor bank for about a minute, and then “spilled” into the pool with soil in a dozen microseconds. Such capacitive drives are in many high-voltage research centers. Their dimensions reach tens of meters, mass - tens of tons. You cannot deliver such to the territory of an electrical substation or other industrial facility in order to fully reproduce the conditions for the spreading of lightning currents. This is possible only by accident, when the object is adjacent to a high-voltage stand - for example, in an open installation of the Siberian Research Institute of Energy, a pulsed high-voltage generator is placed next to a 110 kV transmission line. But this, of course, is an exception.

Lightning Bolt Simulator

In fact, this should not be a unique experiment, but an ordinary situation.Specialists are in dire need of a full-scale simulation of the lightning current, since this is the only way to obtain a reliable picture of the distribution of currents in underground utilities, measure the effects of the electromagnetic field on microprocessor devices, and determine the nature of the propagation of sliding spark channels. Corresponding tests should become widespread and be carried out before the commissioning of each fundamentally new responsible technical facility, as has long been done in aviation and astronautics. Today there is no alternative but to create a powerful, but small-sized and mobile source of pulse currents with lightning current parameters. Its prototype model already exists and was successfully tested at the Donino substation (110 kV) in September 2005. All equipment was housed in a factory trailer from the serial Volga.

The mobile test complex is based on a generator that converts the mechanical energy of an explosion into electrical energy. This process is generally well known: it takes place in any electric machine, where the mechanical force moves the rotor, counteracting the force of its interaction with the magnetic field of the stator. The fundamental difference is the extremely high rate of energy release during the explosion, which quickly accelerates the metal piston (liner) inside the coil. It displaces the magnetic field in microseconds, providing high-voltage excitation in a pulse transformer. After additional amplification by a pulse transformer, the voltage generates a current in the test object. The idea of ​​this device belongs to our outstanding compatriot, the "father" of the hydrogen bomb, Academician A.D. Sakharov.

An explosion in a special high-strength chamber destroys only a 0.5 m long coil and a liner inside it. The remaining elements of the generator are used repeatedly. The circuit can be adjusted so that the growth rate and duration of the generated pulse correspond to similar lightning current parameters. Moreover, it is possible to “drive” it into an object of large length, for example, into a wire between power transmission line supports, into the ground loop of a modern substation or into the fuselage of an airliner.

When testing a prototype generator sample, only 250 g of explosives was put into the chamber. This is enough to form a current pulse with an amplitude of up to 20,000 A. However, for the first time they did not go for such a radical effect - the current was limited artificially. At the start of the installation, there was only a light popping of the blast-off camera. And then the recordings of digital oscilloscopes checked then showed: a current pulse with the given parameters was successfully introduced into the substation lightning conductor. Sensors noted a power surge at various points in the ground loop.

Now the full-time complex is in the process of preparation. It will be tuned to full-scale simulation of lightning currents and at the same time will be placed in the back of a serial truck. The explosive chamber of the generator is designed to work with 2 kg of explosives. There is every reason to believe that the complex will be universal. With its help, it will be possible to test not only electric power, but also other large-sized objects of new equipment for resistance to the effects of current and electromagnetic field of lightning: nuclear power plants, telecommunications devices, missile systems, etc.

I would like to finish the article on a major note, especially since there are reasons for this. The commissioning of a full-time test facility will make it possible to objectively evaluate the effectiveness of the most advanced protective equipment. However, some dissatisfaction still remains. In fact, the person again follows the lead of lightning and is forced to put up with her willfulness, while losing a lot of money. The use of lightning protection means leads to an increase in the size and weight of the object, the costs of scarce materials are growing.Paradoxical situations are quite real when the sizes of protective equipment exceed those of the protected structural element. Engineering folklore stores the response of a well-known aircraft designer to the proposal to design an absolutely reliable aircraft: this work can be done if the customer reconciles with the only drawback of the project - the aircraft will never come off the ground. Something similar is happening in lightning protection today. Instead of an offensive, experts hold a circular defense. To break out of the vicious circle, you need to understand the mechanism of the formation of the lightning trajectory and find means of controlling this process due to weak external influences. The task is difficult, but far from hopeless. Today it is clear that lightning moving from a cloud to the earth never strikes a ground object: from its top towards an approaching lightning a spark channel grows, the so-called oncoming leader. Depending on the height of the object, it stretches for tens of meters, sometimes several hundred and meets lightning. Of course, this “date” does not always happen - lightning can miss.

But it’s quite obvious: the sooner the oncoming leader arises, the further he will advance to lightning and, therefore, the more chances for them to meet. Therefore, you need to learn how to “slow down” the spark channels from protected objects and, conversely, to stimulate from lightning conductors. The reason for optimism is inspired by those very weak external electric fields in which lightning is formed. In a thunderstorm setting, a field near the earth is about 100-200 V / cm — about the same as on the surface of an electric cord of an iron or electric shaver. Since lightning is content with such smallness, it means that the influences controlling it can be just as weak. It is only important to understand at what point and in what form they should be served. Ahead is a difficult but interesting research work.

Academician Vladimir FORTOV, Joint Institute for High Temperature Physics RAS, Doctor of Technical Sciences Eduard BAZELYAN, Energy Institute named after G.M. Krzhizhanovsky.