by Academician Vladimir FORTOV, Joint Institute for High Temperature Physics, Russian Academy of Sciences;
Edouard BAZELIAN, Dr. Sc. (Tech.), G. Krzhizhanovsky Energy Institute
The phenomenon of lightning has always fired human imagination and a striving to learn more about the world we live in.
By taming lightning man learned to use fire and added to his might. Being unable yet to harness this awesome natural phenomenon, we opt for "peaceful co-existence" with it.
In fact, the better technologies we make, the greater the hazard from atmospheric electricity.
One way of protection is to test-using a special simulator - the vulnerability of industrial facilities to the strokes and electromagnetic field of lightning.
Lightning is easily admired by poets and artists. But a power and communications engineer or a cosmonaut would not be as delighted, for a thunderstorm is a promise of great trouble. Annually each square kilometer of Russian territory is hit by three lightning strokes on the average*. Their electric current reaches 30,000 A. and the most powerful electrical discharges exceed 200,000 A. The temperature of a well-ionized plasma channel in moderate lightning may reach 30,000°C - that is several times higher than in an electric welding arc. Indeed, this does not bode well to any technological facility. Fires and explosions caused by a direct lightning stroke are well known to experts. But a layman would obviously exaggerate the risk of such an event.
* See: A. Perunov, V. Filippov, A. Kharchenko, "Taming the Lightning", Science in Russia, No. 2, 2000. - Ed.
The tip of the Ostankino television tower, the target of about 30 strokes of lightning a year. Visible traces of melting.
In reality "the heavenly brand" is not that effective. Imagine you try to make a fire in a hurricane, though in a strong wind you can hardly light even dry straw. A lightning channel produces even stronger compression waves- a lightning stroke generates a Shockwave whose thunderous peal blows and extinguishes the flame. It's a paradox, but a weak lightning is more dangerous, especially if its channel carries a current of about 100 A in one tenth of a second (it's an eternity in the world of spark discharges!). This current is like a welding arc, and an electric arc would burn everything capable of burning.
However, a building of an average height is hit but rarely. Experience and theory show that lightning "is attracted" to a ground structure from a distance three times as large as its height. A ten-story tower would attract about 0.08 lightnings annually, or 1 impact in 12.5 years. The risk for a country house with a penthouse is 25 times as low: its owner should "wait" for about 300 years.
But we should not underestimate the danger. In fact, if lightning hits just one in 300 - 400 houses of a community, residents would hardly find this event insignificant. Indeed, there are objects of much greater extension, for instance, the power transmission lines (PTL). Their length may well exceed 100 km and height - 30 m. This means that to the right and to the left each would attract strokes from 90 m-wide bands. The total area of attraction would be over 18 km2, and the number of bolts, 50 per year. Certainly, steel supports of a line would not burn down, and wires would not melt. The tip of the Ostankino television tower flagstaff in Moscow is hit about 30 times annually, and nothing happens to it. To understand why lightning is dangerous to PTLs we must know the nature of electrical effects.
THE MAIN FORCE OF LIGHTNING
When lightning strikes an electric power line support the current streams toward the ground through the resistance of grounding which, as a rule, makes up 10 - 30 ohm. Thus according to Ohm's law even an "average" lightning of 30,000 A generates 300 - 900 kV, and powerful lightning will generate much more voltage. This way lightning surges arise. If such surges reach a megavolt level, PTL insulation will not be able to resist and is broken down. A short circuit occurs. The line is disconnected. It is even worse when a lightning channel breaks through directly to wiring. This generates overvoltage an order higher than when a PTL support is struck. It is still a challenge to power engineers of today. Upgraded technologies just compound the problem.
To promptly meet the growing energy needs of mankind, modern power stations should be integrated in large power systems. Russia has this kind of integrated power grid: all its facilities work interdependently. Therefore, an accidental failure in one PTL or a power station may lead to grave consequences similar to the power failure in Moscow in May 2005. System failures caused by lightning occur regularly worldwide. One that took place in the USA in 1968 inflicted multimillion-dollar damages. At that time a lightning discharge disconnected one PTL, and the power supply system could not cope with the energy shortage.
Small wonder that experts are paying this much attention to protecting PTL against lightning. All the way along the air lines of 110 kV and higher protective metal cables are suspended against direct bolts from above. Their insu-
Ostankino TV tower as a lightning diverter not capable of protecting itself if struck 200 m below its top.
lation is reinforced as much as possible, the resistance of support grounding is minimized, and for additional protection against overloads semi-conductor devices are used, similar to ones protecting input computer circuits or high-quality TVs. However, this is only conceptual similarity as the working voltage for line limiters is counted in millions of volts-consider the scale of expenses for protection against lightning!
Is it really possible to design an absolutely lightning-proof line? The answer is certainly "yes". But here two new questions crop up: who will need it and how much will it cost? While it is impossible to damage reliably protected PTL, it is possible to issue a false command to switch off a line or just destroy low-voltage circuits of automatic machinery now based on microprocessor technologies. The working tension of microcircuits is decreased every year and is today counted in several volts only. That's where lightning may feel free! It will not need a direct strike-for it is capable of working its way at a distance and over large areas at once. It uses an electromagnetic field as weapon. Now to estimate the electromotive force of magnetic induction we should take account of the current and its buildup rate alike. This buildup rate may exceed 2*1011A/s. Such a current can induce in any 1 m circuit at a distance of 100 m from the lightning channel a voltage about twice as high as in AC sockets of an apartment house. No stretch of imagination is needed to see what is going to happen to microcircuits designed for one V current.
There have been many grave incidents of control circuit destruction by lightning discharges. The toll includes damages of air- and spacecraft onboard equipment, false simultaneous shutoffs of high-voltage PTLs, and failures of mobile antenna equipment systems. We know of many instances of damages caused to our home appliances, all that hitting at our pocketbooks, too.
MEANS OF PROTECTION
We are used to rely on lightning rods (diverters). Remember the ode of Academician Mikhail Lomonosov, the great Russian savant of the 18th century, dedicated to the invention of lightning rods? Our well-known compatriot was proud that the heavenly brand was not dangerous any more. Certainly, this contemption on an apartment house roof will not let lightning burn a wooden flooring or other combustible building materials. But it is quite useless against electromagnetic effects. No matter whether the current flows in a lightning channel or in the metal core of a rod, it will generate a magnetic field and through magnetic induction produce a dangerous voltage in internal electric circuits. For effective handling of this problem a diverter (rod) should allow early interception of the discharge channel in order to protect a facility, i.e. it should be mounted very high, because induced voltage is inversely proportional to the distance of the conductor.
We have accumulated extensive experience of using such structures of various height. However, the statistics is not much reassuring. The protection area of a diverter rod is usually represented as a cone with an axis made by the diverter itself, but with the vertex located somewhat lower than its tip. Usually a 30-meter rod ensures 99 percent protection of a building if it is located about 6 m higher. This is no problem. But with an increase in the height of a diverter the distance from its top up to the protected facility soars. For a 200-meter structure of the same safety level this parameter exceeds 60 m, and for a 500-meter structure - 200 m.
The Ostankino TV tower furnishes a good example: it is not capable of self-protection and is hit by lightning 200 m below its top. The protection area radius for tall diverters at ground level likewise expands: for a 30-meter building it is comparable to its height, and for the Ostankino tower it makes up 1/5.
In other words, there is no hope that conventional lightning rods can intercept lightning at a safe distance-especially if a facility occupies a large area on the ground. Which means we should weigh the real probability of a lightning stroke into the area of power plants and substations, air fields, warehouses of liquid and gas fuel, and extended antenna fields. Spreading on and into the ground, some of the lightning current creeps into numerous underground utilities. Located there are the electric circuits of automation, control and data-processing sys-
A 5m-sliding spark channel is simulated in laboratory. Electric current needed for its formation can be generated by an artificial pulse source.
A happy (from an experimenter's point of view) neighborhood: a 110 kVPTL and an open-air high-voltage pulse generator at the Siberian Research Institute of Energy.
tems - the microelectronic devices we have talked about. By the way, electric current measurement in the ground is most difficult even in the simplest formulation. Problems are further aggravated because of the great changes of resistance in most soils depending on the force of kA level currents, which are typical of atmospheric electricity discharges. Ohm's law does not apply for circuits with such kind of nonlinear resistances.
Extended spark channels add to its formed in the ground "nonlinearity". Repair crews of cable communication lines are well familiar with that. As though a furrow was made by a plough and stretching on the ground from a tall tree on the wood edge and breaking precisely above the line of an underground telephone cable, which is damaged exactly there-the metal coating is crumpled and the insulation of wires is destroyed. That's the doings of lightning. Its bolt hit the tree, and its current, spreading down the roots, created a strong electric field in the soil, and generated a plasma spark channel in it. Actually lightning kind of continued its progress, now in the ground. And this way it can travel dozens and-even hundreds of meters-especially in poorly conducting soils (rock or permafrost). It breaks through to the object most unusually-from below, bypassing any diverters. Discharges sliding along the soil surface are well reproduced in laboratory conditions. Such complicated and expressly nonlinear phenomena should be studied experimentally and modeled.
Discharge-generating electric current can be produced by an artificial pulse source. The energy accumulated in a condenser battery for about one minute, "spills over" into a pool filled with soil just in a mere ten microseconds. Similar capacitor storages can be found in many high-voltage research centers. Their dimensions may reach tens of meters and weight, tens of tons. Such facilities cannot be brought to electric power substations or other industrial facilities for full-scale reproduction of lightning current flows. This can be done only occasionally, when the facility is next to a high-voltage stand-for example, an open-air high-voltage pulse generator at the Siberian Research Institute of Energy is located next to a PTL of 110 kV. But this is rather an exception.
LIGHTNING STROKE SIMULATOR
In practice we should talk about an ordinary situation instead of a unique experiment. Experts need badly such full-scale imitation of lightning current, for only this way
The prototype model of a compact mobile source of pulse currents with lightning current parameters before first field tests.
An explosion in the high-resistance chamber of a lightning simulator destroys a 0.5 m long coil cum liner.
it is possible to obtain an authentic picture of the distribution of currents in underground channels, to measure the effects of the electromagnetic field on the microprocessor equipment, and to determine the pattern of distribution of sliding spark channels. Proper tests should become routine and should be performed prior to a commissioning of every new technological facility as it has been a routine in aircraft and astronautics, or in computer engineering. Today there is no alternative to creating a compact and mobile source of pulse currents with lightning current parameters. Its prototype model already exists and it has been tested for the "Donino" substation (110 kV) in September, 2005. All equipment fits in a factory trailer of the production Volga car.
The mobile test complex is based on a generator transforming the mechanical energy of explosion into electric power. This process takes place in any electric machine, where a mechanical force drives the rotor counteracting the stator magnetic field. One essential difference here is in the very high explosive speed of energy generation quickly accelerating the metal piston (liner) inside the coil. Within microseconds it displaces a magnetic field by providing high-voltage excitation in the pulse transformer. After additional reinforcing by the pulse transformer the voltage forms a current in the test object. The idea of this device was offered by Acad. Andrei Sakharov, the "father" of the Russian hydrogen bomb.
An explosion in a special high-resistance chamber destroys only the 0.5 m coil and the liner inside of it. Other elements of the generator are used repeatedly. The circuit can be so adjusted that the acceleration rate and time of the pulse would correspond to the analogous parameters of the lightning current. Besides, the generator may be fitted into a longer object, for example, into a wire between PTL supports, into the grounding circuit of a power substation or into an air liner fuselage.
The prototype model of the generator was tested with 250 g of explosives. It was enough to form a pulse of up to 20,000 A amplitude. However, for the time being no dramatic impact like that was used-the current was limited artificially-only a slight clap of the explosion extinguished by the chamber was heard. The records of digital oscillographs showed: the pulse with desired parameters was entered into the substation diverter. Gauges indicated a voltage jump at various points of the grounding circuit.
The regular system will be attuned to full-scale imitation of lightning currents and placed in a body of a production truck. The explosive chamber is adjusted to 2 kg of explosives. We are certain it will be a universal system. Using it we shall test resistance to strokes of lightning, its electric current and electromagnetic field for power utilities and other facilities like atomic power stations, telecommunication devices, rocket complexes, etc.
Lightning can miss, too
Tests of the prototype model of the lightning stroke simulator.
We wish to conclude on a positive note. True, a regular test system will allow to assess the efficiency of most advanced protective facilities in real terms. And yet we cannot but feel some dissatisfaction. In fact, people are still tied to the chariots of lightning, they have to be reconciled to its willfulness and waste a lot of money. Protective devices call for ever larger dimensions and weight of facilities, and for higher expenses on scarce materials. Paradoxical situations, when the sizes of protective devices exceed those of protected ones. The engineering folklore cites the answer of a well-known aircraft designer when he was asked to build an absolutely safe plane: such work can be performed if the customer could take just one flaw-the plane would never take off. Protection against lightning is a case like that. Experts on lightning are entrenched in all-round defense instead of standing up and going ahead. To break free from the vicious circle, we must understand the mechanism of lightning streamers (leaders) and find facilities of controlling this process via low-level external influences. Although complicated, the problem is not hopeless at all.
Moving from cloud to ground, lightning will never strike a ground object: a spark channel-or return leader-leaps from the top of this object towards the bolt. Depending on the height of an object, the leader extends to dozens and occasionally even to hundreds of meters - and encounters the lightning. Certainly, this "rendezvous" may not take place at all-there can be a miss. But one thing is quite obvious: the earlier the return leader is formed, the farther it will travel toward lightning and have better chances for a reunion. Hence we should learn to "slow down" spark channels from protected objects and activate those from a diverter. Our optimism is based on those rather weak external electric fields where lightning is formed. In storm conditions a field above the ground is about 100 - 200 V/cm-about the same as on the surface of a flat iron or electric razor cord. If lightning can do with as little as that, it means that the controlling effects can be just as small. It is just important to understand when and how they should be proffered. Difficult and interesting research work lies ahead before us.
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