by Anatoly PERUNOV, Cand. Sc. (Tech.), department Chief, High-Tension Research Center, V.I. Lenin Institute of Electrical Engineering,
Valentin FILIPPOV, Cand. Sc. (Tech.), Deputy Director of the Research Center,
and Alexander KHARCHENKO, Cand. Sc. (Tech.), branch Director, High-Tension Research Center
Thunderstorms have been a blight for humans at all times. Lightnings kill individuals and set buildings and ships on fire. We have evidence of ancients looking for ways to protect themselves from lightning. Excavations in Egypt, for example, have yielded elements of devices researchers have identified as lightning conductors that were used thousands of years ago.
Lightning is an electrical discharge in the atmosphere. This natural phenomenon of an enormous scale is, in essence, identical to what we watch in static electricity experiments at laboratory. This obvious fact for people living today was only established in the mid-18th century, independently by Mikhail Lomonosov and Georg Richman, of the St. Petersburg Academy in Russia, Benjamin Franklin in America, and Prokop Divis in Bohemia.
Lightnings are generated by storm (tumulus) clouds, in which positive and negative charges are separated. As this happens, we see, within the cloud or between it and the ground, jagged lines with numberless forkings. This is a linear lightning. It does not always need storm clouds, striking literally out of the blue. This type of lightning is particularly hazardous for flying aircraft.
The way a lightning develops over time was first recorded by Benjamin Shawnland, a British scientist, in the 1930s. In that recorded episode, as the leader (the name the scientist gave to the moving lightning channel) touched the earth, a fulgent wave of light streaked along it up to the cloud at almost the speed of light. It was found decades later (in 1980s, when the International Electrochemical Commission published the measurement results) that the maximum current intensity of such phenomena could reach 200 kA.
The negatively charged parts of a storm cloud are the principal source of lightning. As they are concentrated, the electric field intensity rises, and, on reaching a maximum, it ionizes the air, causing the charge to dart toward the ground. Initially, its channel develops stepwise, elongated by 5 to 100 m/usec. As it proceeds downward, each of its new legs has a very bright luminosity, the preceding one fading. Simultaneously, the electrical intensity in the atmosphere becomes sufficiently high to produce a return leader, leaping up from objects rising above the ground, be it a tree, a high-rise building or just a hillock. When 25 m to 100 m remains between the two leaders moving at a frightful speed (0.05 to 0.5 the speed of light) toward each other, an electric discharge flashes, accompanied by dazzling light (lightning) and compression waves (thunder).
Most frequently, a lightning consists of two discharges (components), but occasionally their number may be greater, as large as 30. Besides, more than a single charge concentration area may frequently develop within a single storm cloud, and they are discharged successively, one after another, from the lowest to the highest. Since the leader of a successive discharge travels along an ionized path made for it by the preceding discharge, it has a higher speed. Besides, it is continuous, rather than stepped. It is then called a dart variety. At the modern stage of scientific knowledge, this confirms the ancients' descriptions of fiery darts throws at them from the sky.
A long spark, a discharge in an air gap over 1 m long, is in nature akin to lightning. This affinity allows the lightning resistance of various objects to be simulated in laboratory. The idea is focused not so much on ground objects (they are taken care of by man's longstanding experience), as on attempts to protect aircraft, helicopters, and space rockets.
It is common world practice to have flying vehicles tested in two stages. First on models in laboratory to determine possible risk points and spark discharge outlets (lightning vulnerability), in an effort to obtain the widest possible spread of both. In laboratory experiments, the voltage pulse front time may reach 100 to 5,000 usec at positive polarity. Meanwhile, the scale model of a test vehicle, designed to certain specifications, is placed successively in 35 different positions. In each position, the model is hit with 30 rupturing discharges, and rupture points are recorded on a spark channel photograph.
At the next stage, full-size, real vehicles or any of their components are exposed to currents simulating lightning bolts to find out their resistance to stroke. The test results are then framed into recommendations on ways to improve the design and reliability of protective devices. The kind of reliability required is splendidly illustrated by the launch of the American Apollo 12 moon probe in 1969, when it was hit twice after takeoff, at 2 km and 4 km, without suffering the slightest damage.
All these studies have emphasized the need for large-scale simulation and, therefore, a considerable lengthening of spark gaps for models to the placed in, and enhancement of the parameters of voltage pulse generators. These requirements are fully met by the outdoor high-tension test setup (Russian acronym OVIS) developed at our Center. It is employed to simulate lightnings (including head-on leaders) and produce electric discharges in long air gaps (normally, over 20 m, and occasionally from 150 to 200 m).
The OVIS is built around a pulse voltage generator (GIN-9 MV), comprising an Arkadiev-Marx voltage multiplier circuit in which capacitors are charged in parallel and discharged consecutively through pulse-shaping resistors.
The circuit is switched over by multiple-gap discharges controlled from helical generators. This truly unique setup has the following parameters: total (maximum) charge voltage - 9 MV; maximum stored energy - 1.33 MJ; pulse front time - 2-600 usec; pulse time itself-50-7,500 usec; and height of the stack- deck assembly (15 decks) resting on glass-epoxy cylindrical supports - 43 m.
In addition to its core units, the OVIS has control panel rooms, and testing and auxiliary equipment. In particular, an Istra photo-opto-electronic recorder, FER-14, developed at the Russian Research Institute of Optophysical Measurements, is used to register optical phenomena accompanying discharge. Its specifications are unique as well. For example, it registers radiation at wavelengths of 0.38 u, to 0.75 u in day- and night-time. It has a dynamic resolution of at least 1~1 mm on an electron-optical intensifier (EOI) screen, with image brightness being reduced automatically on exceeding a preset threshold. The device is started by either light or electric pulses and begins normal operation within a maximum of 0.5 usec. Errors in linear scan time do not exceed 10 percent. The operator himself can select the scan time manually within the range of 1, 3, 10, 30, 100, 300, 1,000, 3,000, and 10,000 usec. Scan nonlinearity
within the 35 mm operating length on the EOI screen does not exceed 10 percent. Moreover, scans are initiated relative to the gate opening time at strictly fixed time intervals of 1, 5, 10, 30 or 60 usec. Finally, the input optical system of the device provides a visual angle of 10 and 30 (with interchangeable lenses). Another important contributing factor is that the FER-14 is housed separately 80 m from the test site itself, to make staff work completely safe.
Noise resistance of the measurement system as a whole is achieved by its diagnostic devices being powered from an independent circuit through insulated transformers, by circuit shielding, use of waveguides, etc.
The OVIS auxiliary equipment includes an electrode suspension system, resting at one point on the GIN-9 MV generator tower. At the other point, it is supported by a 45 m-high welded structure standing on a concrete base 115m away and having three guys. It carries a hoist to vary the electrode suspension height. A metal platform 18-18 m2 is mounted on insulators 45 m away from the first support point, with a shielded measuring cabin placed underneath.
Very high voltages must be registered to obtain new data on leader discharge parameters. Previous techniques no longer meet today's requirements.
Recent breakthroughs in fiber optics allow measurements to be made directly within the discharge. In particular, the OVIS is provided with a waveguide transmitter to measure the intensity of the electric field distorted by the leader volume discharge in long air gaps. This system allows electronic equipment to be moved to a considerable distance (up to several kilometers) from the test object, so the recording equipment stays out of direct contact with the field source and is reliably protected from noise. The measuring complex used for this purpose comprises three optoelectronic transducers with a passband of 350 MHz and transient performance buildup time of 10.2 nsec; several modifications of armored fiber cables; coupling boxes with sockets, and three radiation modules.
The complex measures current (charge) intensity at voltages of up to 4 MV, converting the electrical parameter to a modulated light flux. This is done by means of a fiber waveguide running from the high-potential zone to the ground potential zone, the original signal being converted to an analog signal that is sent to the computer database. This system for registering and processing experiment results is fast, cost-saving, and much more accurate than anything before it.
Its important advantage is that the armored cable between the panel and the generator with grounded surface cannot be shifted alone. The system itself can easily be transferred to where the experimenters want it to be installed.
In the 20 years of the GIN-9 MV in operation, studies have been completed in long air gaps of the rod-surface, sphere-surface and torus-surface types at positive and negative polarities of electrodes spaced up to 30 m apart. Charge formation in an artificially charged cloud has been researched as well. Techniques for exploring the exposure of flying vehicles to lightning have been validated and models of aircraft designed by the Tupolev, Ilyushin, Yakovlev, and Sukhoi teams tested, as have those of some aircraft from the Czech Republic. Tests have shown this method to more accurately mirror a thunderstorm situation. It has been found, for example, that a flying vehicle model over 2 m long is liable to be struck by a long spark in the middle of its wings, as also happens with lightning striking an object as a whole.
The so-called anomalous discharges produced in the experiments were particularly remarkable, with a long spark abruptly changing direction, very much like
real lightning, instead of taking a short cut, streaking parallel to the ground, and, finally, swerving sharply away from it. Such discharges were observed in the GIN-9 MV top shield when rectangular voltage pulses were applied.
The new equipment to transmit power and power substations at encouraging voltage ratings of 1,500 to 1,800 kV required adequate insulation to go with it. In an unnatural situation, lightning can strike a power transmission line conductor, for example. This situation can be simulated in the OVIS
on the GIN-9 MV and the world's only string of three FREO 1200/1200 AB/K transformers from the firm TLJR, Germany. This complex is designed to test high-tension insulation of electrical equipment at industrial frequency AC of up to 3,000 kV and switching voltage of up to 3,900 kV, at frequencies of 100 to 250 Hz.
In addition to the equipment already named, the complex includes a switchboard room fitted out with remote-controlled equipment, warning devices for its elements and components, a system to measure, monitor and record electrical parameters during tests,
and an outdoor 10 kV transformer substation.
Switching adapters form part of the complex to produce voltages at frequencies of 100 to 250 Hz and amplitudes of 3,900, 3,000 and 1,600 kV, to match the number of series-connected transformers. Test objects have maximum capacitances of 10, 13 and 23 nF.
Reliability of many devices depends on reliable operation of monitoring and controlling systems containing electronic and electrical components, microprocessors, and computers. They all are vulnerable to pulsating electromagnetic fields. It is not
always possible, however, to measure the resistance of large objects to such fields by computing, for modern devices, complexes, and instruments have complex designs and entangled webs of connections among their components. Experiment is the best way to deal with this problem. Specialized large-size simulating units are used for this purpose. One such electromagnetic pulse generating test unit has been developed to test the resistance of equipment, devices and instruments to the effect of respective natural and artificial fields. The test unit consists of a fenced-off pulse power source, a field-generating system, and warning, measuring and other auxiliary devices. It can reproduce a fairly wide range of maximum electrical (E) and magnetic (H) components of electromagnetic field strength at the work volume center, with E varying from 20 to
100 RV/m and H from approximately 60 to 250 A/m, and within the additional 5 m high volume, where E and H reach a maximum of 200 kV/m and 470 A/m, respectively. Precision of pulse front time within 0.1 to 0.9 of the maximum is set within 5 to 12 nsec, and the duration of the pulse itself at a level of 0.5 its maximum value is chosen within 25 to 60 nsec. Any measurement has a relative error of+10 percent.
The recording equipment of the test unit is housed in two cabins set up on a platform with specialized equipment. The smaller of the two (a single-layer metal shield) contains devices measuring amplitude-time parameters of an electromagnetic field pulse and a device to connect the register to a computer. The complex is powered from an uninterrupted power supply (a storage battery and a rectifier). The larger cabin holds
equipment to register the response of test objects to electromagnetic radiation. The equipment includes a sound warning system monitoring safety standards during tests and an array of devices to measure the temperature, pressure and humidity of the ambient environment. All cable and communication lines are placed in a duct enclosed in metal sheaths for protection.
The electromagnetic pulse generating unit has unique capabilities and design. Not only because it can test resistance of large-size objects to electromagnetic radiation, but also because its test results can be used to formulate requirements to protective devices.
Prepared by Arkady MALTSEV Photos by Valery Miroshnichenko
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A. PERUNOV, V. FILIPPOV, A. KHARCHENKO, TAMING THE LIGHTNING // Hanoi: Vietnam (BIBLIO.VN). Updated: 14.09.2018. URL: https://biblio.vn/m/articles/view/TAMING-THE-LIGHTNING (date of access: 21.02.2024).
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