Misunderstandings about Lightning and Lightning Protection

    Abstract: This article explains the reasons for the formation of lightning, its generation process and lightning protection methods, so as to correctly select lightning protection devices.

    Keywords: lightning formation, lightning protection methods

    With the development of electronic technology, electronic devices have entered the era of large-scale integrated circuits. The functions of electronic equipment have been improved, and the reliability of operation has been continuously improved. However, the ability to protect against lightning has been greatly reduced. Nowadays, hundreds of millions of dollars of damage are caused by lightning strikes every year, so research on protecting microelectronic equipment from lightning hazards has become an important issue. Although many lightning protection methods and many lightning protection devices have emerged in the past two centuries, the expected results are often not achieved due to incomplete understanding of lightning or biases against device performance. A lot of money was wasted due to improper methods. This article explains the causes of lightning and points out current misunderstandings about lightning protection, trying to break the seemingly frozen norms of lightning protection methods in order to make progress in lightning protection research.

1 The formation of lightning

1.1 Free charges in nature

    In electronics, when people study the phenomenon of electricity, they find that in the atoms that make up the microunits of matter, the outer electrons rotating at high speed around the nucleus are easily affected by external conditions and escape, causing the atoms to lack electrons or free electrons to exist alone. The charging phenomenon that forms an electric field to the outside.

    The difference between the internal structures of metallic conductors and insulators is that the internal gravitational force of free electrons in metallic conductors is weaker, while the internal gravitational force of insulators is stronger. Therefore, in a metal conductor loop, if a force is added to cause free electrons to escape (this force we call voltage), due to the existence of voltage in the loop, the electrons in the metal will flow in a displacement manner, but the electrons in the metal will flow. The absolute values ​​of the positive and negative charges are equal. Once the voltage applied to the loop is removed, the loop is immediately neutral, there is no flow of electrons, and no electric field is generated.

    For non-circuit metals, such as two parallel metal plates with air as the medium between them, if a voltage is applied to the two plates, the electrons in the metal conductor will repel each other according to the law of same sex and attract opposite sex. The flow of electrons in one direction creates an electric field, a phenomenon called electrostatic phenomenon. At this time, for a certain piece of metal, the absolute values ​​of the positive and negative charges of their charges are not equal. If the voltage applied to it is removed, it will not be electrically neutral like a loop, but it will still maintain its charged nature. , there is still an electric field, but as time goes by, this electric field will naturally disappear. The orthodox theoretical explanation is that the electrons of sheet A are gradually released to sheet B of metal through the dielectric layer. This is an argument based on the loop current theory. However, if two pieces of charged metal are instantly pulled apart to a distance where it is impossible to release electrons from A to B, will the two pieces of metal be permanently charged? Facts tell us that the electrification phenomenon disappears as time goes by. What is the reason for this? The frictional electrification phenomenon mentioned in textbooks means that after insulators rub against each other, the insulators become charged. In this case, do two objects need to be in contact again to make the insulators neutral? This is not the case. Regardless of the nature of the charging of these charged objects suspended in space, as long as they come into contact with the earth, the charging phenomenon will disappear immediately. Therefore, this phenomenon tells us that in nature, the charge given by A to B does not have to be withdrawn from B, and the excess charge from B does not necessarily export to A. This is different from the metal loop current theory. At the same time, it can be inferred that the absolute values ​​of the electric charges of various objects in natural space (including the earth) are not equal, which means that nature has a huge amount of free electric charges.

    There are a large number of free charges in nature. From the perspective of the concept of potential formation, there are various reasons such as electromagnetic effects, chemical effects, frictional electrification and rays. Modern science can measure the movement of human brain currents to judge brain activities. The cause of free charge in nature is regulated by the law of conservation of energy. It can be said that wherever there is movement of matter (including cosmic rays), electron movement will occur and free charge will be formed. This is the conversion of one energy into another. Energy conversion process, so the movement of natural materials is the source of free charges in nature.

    The so-called nature includes vast spaces such as the sky and the earth. There is no neutral charge in this space. As far as the earth is concerned, we call it zero potential. However, the potential of the earth itself is not zero due to the movement of matter. It has a large amount of For free charge, we can do a simple little experiment: use a pair of headphones, or a millivolt meter, and two metal rods with the same metal properties. Insert the metal rods into the ground at a certain distance, with a You can hear the noise of the ground charge in the earphones. If you connect a millivolt meter and find a voltage indication, this indication will not disappear due to the extension of the discharge time. In single-line transmission telephone lines, the noise in the earphones of the telephone is also continuous. These All indicate the existence of free charges on the earth. Of course, the free charge in the sky cannot be measured with the above method, but when we listen to the radio station with long-wave and medium-wave radios, the noise interference is also continuous, which proves that there is a continuous discharge phenomenon in the sky, indicating that there are abundant free charges in the sky. It can also form an electric field discharge of a certain intensity.

    It is repeatedly demonstrated here that free charges exist in nature, and the purpose is to explain the origin of lightning, because the loop theory in the textbook cannot explain the cause of lightning.

1.2 Generation of lightning field

    The energy of thunder and lightning is huge. In human activities, the electrical energy emitted by any single power station cannot produce the energy released by a thunder and lightning. So how is such a large energy accumulation formed?

    As mentioned above, due to the movement of matter, huge free charges are naturally generated. Of course, these free charges are the source of thunder and lightning. We know from electronics that to form a strong electric field, there must be an accumulation of charges of the same nature on one side. However, the air in the sky is insulated, and charges of the same nature repel each other. They cannot accumulate together. It may form a concentration of energy. The matter in the sky is affected by air currents and cosmic rays to generate free charges, which continue to increase and move toward the upper levels of space under the squeeze of the atmosphere, forming an ionized layer. This ionized layer contains single charges. The energy of its electric field is immeasurable.

    When moist air appears in the atmosphere and encounters cold air during the ascent stage to form water-like clouds, the clouds can be regarded as a whole conductor. Under the action of the ionospheric electric field force, the electrons in the clouds are pushed toward On the end facing the ground, although the absolute values ​​of the positive and negative charges of the clouds are equal, an electrostatic field is actually formed. On a sunny day, the clouds are far away from the ground and the moist air between the clouds and the ground is thinner. The degree of dielectric insulation between them is It is relatively high and breakdown discharge is less likely to occur. However, on rainy days, especially in hot rainy seasons, due to falling clouds and humid air, charged clouds break through the air and discharge to the earth to form lightning.

    Lightning is not simply a space-to-ground discharge, but also often forms in space. This is because the position of charged clouds is high in space. When the moist air on the ground rises rapidly, the electric field formed by it and the charged clouds discharges in space, forming high-altitude lightning.

    As mentioned above, clouds are affected by the electric field force of the ionosphere to produce static electricity. After these clouds are discharged to the ground, they themselves are ionized, that is, the absolute values ​​of the positive and negative charges of the clouds are not equal, forming a charged phenomenon. The charged clouds follow The movement of the airflow forms an electric field with another cloud. When they gradually approach, the discharge phenomenon is the cause of the formation of airborne thunder. When we observe lightning discharge in space, we often have a continuous feeling one after another.

1.3 Lightning process

    The lightning process is also the dielectric breakdown process in the electric field explained in the electrostatic theory. As mentioned above, the cause of lightning is that lightning is a phenomenon in which charged clouds discharge during movement. The discharge position is not fixed, but there are certain fixed conditions. For example, the thickness of the medium in the electric field, insulation coefficient, gas temperature and surface conductivity all affect the lightning strike location. What we often call a minefield should mean that the area has several of the above factors. But some people think that thunder and lightning are generated at this location. This is a misunderstanding. The reason is simple: What force can accumulate so much energy in this region? It should be the discharge of charged clouds during movement to form lightning. Of course, under the action of charged clouds, where the discharge occurs is related to the aforementioned conditions on the ground. In terms of landforms, the higher the relative height, the more likely it is to be struck by lightning. This refers to The most common ones are high buildings, mountains and protrusions on the ground, but lightning strikes do not necessarily occur in these places, because the dielectric parameter in the electric field does not simply refer to the thickness, but also depends on the insulation coefficient, that is, the temperature of the environment and the temperature of the gas. We have found that the lightning strike point is often not on the top of the mountain but on the plains. This is because the humid air and temperature there make the insulation of the electric field medium lower than that of the mountains, causing lightning strikes. In addition, the conductivity of the earth's surface also has an impact. The lightning field generated by good conductive geology is much larger than that of difficult-to-conduct geology. Therefore, easily conductive geology is easy to trigger lightning.

    The lightning field is a huge electrostatic field that cannot be constructed by humans. The huge electric field area and the huge energy accumulated are immeasurable and immeasurable. People often estimate the size of the object struck by lightning from the degree of damage to it after lightning. It is not excessive to describe the lightning current with words of hundreds of millions of amperes. The lightning field has similarities with the electrostatic field discharge during the discharge process, but there are also differences. The energy storage of the artificially formed electrostatic field is extremely limited, so During the discharge process, the discharge current gradually weakens from the maximum value, but the lightning field is different. Due to the huge energy storage, it is impossible to weaken the electric field at the beginning of the discharge due to the resistance of passing through the space. Instead, the air is heated during the discharge and then discharged. The current reaches a maximum value, and then the discharge current decreases as the electric field weakens. Therefore, during a lightning strike, the lightning current increases from small to large and then weakens. In terms of the nature of electricity, since it is a discharge of an electrostatic field, the direction of the current is unchanged, and what is formed is a pulsating DC current with a huge amplitude.

    Therefore, the main component of the lightning current is the DC component, but the electromagnetic energy of the harmonics and high-order harmonics formed by the pulsating part and the thermal disturbance generated when the lightning current contacts the air and the ground is also quite large, so the AC component in the lightning process is also It should not be underestimated. During a lightning strike, the wide spectrum from low frequency to meter waveband is interfered with to varying degrees. From harmonic theory, it is known that the interference in the low frequency band is more serious.

    If we place an object on the ground at a certain location, the interference caused by lightning to the object can be divided into induced interference and direct interference. An object is not in the lightning field, but due to the discharge process of lightning, the strong electromagnetic waves generated by it cause the object to be impacted by the electromagnetic waves. We call such lightning "induction lightning". When an object is placed in the lightning field, Moreover, the object acts as a conductor of lightning current. The huge current passes through the object and causes serious damage to the object. This kind of lightning is directly placed in the lightning field and is impacted by lightning. We call this kind of lightning "direct lightning". In the case of modern microelectronics, whether induced lightning or direct lightning will cause permanent damage to microelectronic devices.

2 Misunderstandings about Lightning Protection

2.1 Lightning rod and arrester

    In the late 19th century, people discovered the phenomenon of tip discharge of metal conductors. Lightning rod is a typical lightning protection device made using the principle of tip discharge. A metal needle is set up on the object to be protected and connected to the ground. How does it protect against lightning? The explanation is this: When the lightning rod is placed in the lightning field between the air and the ground, because the lightning rod has good contact with the earth, the electric field energy is discharged through the lightning rod, and the lightning field disappears, preventing the discharge of large currents, thereby eliminating the problem. The role of thunder. However, this explanation is also unclear, that is, can a lightning rod located under a strong lightning field slowly discharge according to people's wishes to make the lightning field disappear? It doesn't make sense from electrical principles either. Because a powerful lightning field is like an explosive that lacks a fuse, the space pointed by the lightning rod is like a fuse, and it will explode due to the guidance of the lightning rod. Because its height and good grounding conditions are better than other locations, and the electric field formed at the tip is larger than other places, the powerful lightning field takes the lightning rod as the central discharge area. If the lightning rod itself has no reactance, the grounding resistance reaches zero. , hundreds of millions of amperes of lightning current can pass through it smoothly without forming thermal effects and lightning potential, thus achieving the purpose of lightning protection. However, there is reactance between the lightning rod itself and the lead, and the grounding resistance cannot be zero. Therefore, during the lightning strike, it has no lightning protection ability and only plays a guiding role in the lightning strike location. People realize this, but they have a preference for lightning rods or do not understand the cause of lightning. They explain that lightning is generated at this location. That is to say, for reasons that are unclear, an electric field is formed where the lightning rod is set up and in the relative space. , due to the gradual discharge of the lightning rod, the electric field cannot be established, so the lightning rod plays the role of lightning elimination. In fact, people have openly questioned the lightning protection effect of lightning rods since the 20th century, because it is not uncommon for lightning rods to become lightning rods.

    However, lightning rods can play a certain role in the following situations. When the charge of the charged cloud is very small and it is far away from the ground to form a not too strong electric field, the lightning rod gradually discharges its electric field to eliminate the electric field. Some objects on the ground are insulated from the earth, such as ancient buildings with wooden structures. Under the action of induction lightning and direct lightning, they may be charged with static electricity. The presence of static electricity may cause fires. If lightning rods are set up on these objects, it will It can make buildings and the earth form equal potentials to prevent these objects from being charged with static electricity under the action of lightning fields.

    However, modern buildings are almost all made of reinforced concrete structures, which have formed an equal potential with the earth, so it is obviously redundant to erect lightning rods. But today's buildings still follow the old rules of erecting lightning rods. The reasons are obvious, mainly due to liability and regulatory issues. To be honest, without a lightning rod, who can guarantee that the building will not be struck by lightning? If you install a lightning rod and get struck by lightning, it is God's fault and no one is responsible.

    Almost at the same time as the lightning rod appeared, people used the tip discharge phenomenon to invent the tip discharge arrester on power transmission lines. The electric field formed by the two tips discharges within a certain distance. The size of this distance can be set to discharge at a certain voltage, so the electric field formed by the two tips discharges at a certain voltage. It is installed on the transmission line, so that the overvoltage value of lightning is guided into the ground through the discharger to achieve the purpose of lightning protection. At the beginning of the 20th century, sheep's horn lightning arresters that were shaped like sheep's horns were commonly installed on transmission lines. However, during the process of discharging lightning, the air was heated by the sheep's horn arresters, causing arcs to continue. Although the arc was guided to rise, the circuit could not supply power normally after the lightning. . Therefore, a voltage-sensitive resistive element is added to the tip discharge. When the rated voltage is exceeded, the resistance of this element is small. On the contrary, the resistance increases and plays a switching role in the current caused by overvoltage. This kind of arrester is called " Valve type arrester". Gas-sensitive and zinc oxide devices are derived from the pressure-sensitive principle.

    Regardless of the claw type, valve type, gas-sensitive or pressure-sensitive arresters, their structures attempt to achieve one purpose: to clamp the overvoltage value on the transmission line at an artificial setting value through these devices, thereby reducing the terminal voltage of the user equipment. Do not exceed the rated voltage to ensure the safety of user equipment.

2.2 Response of lightning protection devices when used in different circuits

    Nowadays, there are all kinds of lightning arresters. It is incomplete to judge whether they have lightning protection function simply based on their own structure. It also depends on what circuit these devices are used in. The following introduces the reactions of several circuits during lightning:

    (1) Distribution and transition of lightning potential in high-voltage transmission lines

    High-voltage transmission lines are three-phase and three-wire systems, and the lines are insulated from the ground. Regardless of whether the transmission line is affected by induced lightning or direct lightning, the potential and phase of the lightning potential in the three lines are the same, and the potential difference between the lines is equal to zero. Therefore, when lightning strikes a high-voltage transmission line, it mainly endangers the ground insulation of the transmission line and the transformers running on the line. In a three-wire transmission line, the insulation coefficients of the three wires to the ground are different due to various reasons. In particular, it is more difficult to obtain consistent insulation performance of the arrester on the high-voltage side. Therefore, during a lightning strike, the first wire discharges to the ground. Due to the discharge of one line, the lightning potential of the line drops rapidly. At this time, the lightning potential of the other two lines is higher than the discharge line, and a lightning potential difference appears between the lines. This voltage passes through the high-voltage side winding of the transformer, and the low-voltage side (i.e. the transformer secondary Side) Lightning voltage occurs due to electromagnetic induction. When this voltage is very high, it endangers the safety of user equipment.

    (2) Distribution and transition of lightning potential in low-voltage transmission lines

    The low voltage is a three-phase four-wire system, and the neutral wire is connected to the earth. When lightning occurs on the low-voltage wire, due to the reactance of the neutral wire itself, the ground resistance cannot reach zero. The lightning on the four wires all discharges to the ground. At this time, The zero potential of low-voltage transmission lines rises sharply first. Of course, the phase lines rise accordingly due to the rise of zero potential. Moreover, when each phase discharges to the zero line, it is done through user equipment. Due to different loads, the corresponding lightning potentials are also different. In the same way, lightning currents appear between relative neutral lines and between phases. Therefore, when lightning strikes a low-voltage line, the first cause of damage to user equipment is the ground insulation, and the second is overvoltage and overload. The most obvious failure is often the increase in the neutral line potential that destroys the user's insulation.

    (3) Small current circuit

    The so-called small current circuit refers to a circuit network with small power capacity and high internal resistance of the power supply. Commonly seen such circuits include telephone external lines and electronic circuits themselves.

    As mentioned above, the current lightning protection devices are derived from the principle of tip discharge and pressure sensitivity. When these devices are used for line overvoltage protection, the wiring method is generally parallel connection between lines and parallel connection between lines and ground. This kind of device is used in small current circuits. The above can effectively clamp overvoltage current, because the power capacity of small current circuits is small and the internal resistance of the power supply is high. For example: when lightning strikes a telephone user, the lightning current is transmitted back to the terminal of the switch through the subscriber line. If the switch terminal is equipped with a pressure-sensitive device and the pressure-sensitive device discharges the lightning current, the telephone line will transmit the lightning current due to its large resistance. limit so that the pressure sensitive device can be clamped at its threshold. In electronic circuits, we often connect a resistor in series in front of the Zener diode. This resistor is a current-limiting resistor. It can also be regarded as being set to increase the internal resistance of the power supply. Due to the current limiting of this resistor, the voltage stabilization The diode can clamp the voltage at its threshold, but the load current cannot be large, otherwise the regulated voltage value will be lower than the threshold. Therefore, in small current circuits, using varistor devices to clamp the voltage can effectively prevent lightning strikes. , that is to say, the lightning protection effect is significant.

    (4) Large current circuit

    High current circuits generally refer to power supply circuits, which are characterized by large power capacity and small internal resistance of the power supply. If a pressure-sensitive device is used in parallel on such a circuit and is tried to use the overvoltage discharge characteristics of the pressure-sensitive device, it is obviously impossible to clamp the overvoltage value at the threshold of the pressure-sensitive device. For lightning to form an overvoltage state in the power circuit, its power energy must be greater than the energy of the power circuit. It is impossible for such a huge energy to be discharged from the pressure-sensitive device without damaging the device itself. This is one; second, Since the internal resistance of the power supply is small, even during the discharge process of the pressure-sensitive device, the voltage at both ends of the pressure-sensitive device will not be lower than the overvoltage value of the line. In this way, user equipment will also be affected by lightning overvoltage.

    Some devices on the market now claim to have lightning protection functions. They simply connect the lightning protection device and the whole machine to the power supply in parallel, and connect a fuse in series to the power supply circuit. Manufacturers believe that during a lightning strike, the voltage-sensitive device discharges, causing the circuit to overcurrent and blowing the fuse, thereby achieving the purpose of lightning protection. Such wiring has a certain lightning protection effect on power devices, namely motors and power transformers, but has no protective effect on microelectronic equipment. As mentioned earlier, the overvoltage value added to the pressure-sensitive device is also added to the user equipment, and because the internal resistance of the power supply is small, the voltage will not drop much. In addition, the fuse is a thermal element and has a fusing time, so use Arrester devices that combine fuses and pressure-sensitive devices are not advisable for microelectronic equipment.

    In order for the varistor to play a lightning protection role on the power supply circuit, the only way is to increase the internal resistance of the power supply, that is, to connect a reactive component in series with the circuit. However, due to this reactive component, the circuit reduces the operating voltage under normal operating conditions and at the same time changes with the load. The fluctuation makes this power supply unusable, so the focus of today's lightning protection issues is almost the problem of lightning caused by power lines.

    Since reactive components cannot be connected in series on the power line, but voltage-sensitive devices must be used to discharge lightning current, some people started from the lightning spectrum and proposed the lightning current phenomenon. What is wandering? Thunder and lightning are as fierce as water waves and fall rapidly. It is believed that the main component of such an impact current is at high frequency, so using a millihenry-level inductor on the circuit can prevent the current from flowing. Of course, the millihenry-level inductance hardly forms an influential reactance for the power supply frequency of 50Hz. But as mentioned before, lightning is a discharge phenomenon of electrostatic field, the main component is DC, and the harmonic frequency is wide. This AC component is very small, so it is wrong to set the lightning spectrum at high frequency. Therefore, it is impossible to obtain better lightning protection effects by using high-frequency inductors.

    The current lightning protection method for microelectronic equipment uses a 1:1 transformer, which is generally believed to have a better lightning protection effect. Why can such an effect be obtained? Think: it can stop the flow and act as an isolation. But this explanation misses the point. It should be a 1:1 isolation transformer that turns a power supply with a large power capacity into a power supply with a constant power capacity. Since the transformer has a magnetic saturation effect, if a varistor is connected in parallel to its secondary side, the varistor can clamp the voltage due to the limited power capacity. Because the instantaneous current of zinc oxide varistor devices produced today can reach thousands of amperes.

3 Conclusion

    This article briefly expresses the causes and processes of lightning and analyzes current lightning protection methods. The purpose is to put forward a thinking in order to have a rational judgment on the lightning protection effect of various lightning protection devices on the market, so as to achieve the correct selection. Lightning protection devices protect microelectronic equipment.