Power surge is a populist name for various kinds of dips and voltage spikes in the supply network. The term is not found in scientific and professional literature.
Power outages are usually not dangerous, ascents are considered in the literature. The term jump rather means quick change. Visually, this is reflected in the blinking of the bulbs. LED and gas discharge lighting devices are powered by drivers and do not show changes in performance following voltage variations. Or at least, this is expressed much weaker.
Consequently, in the current era, the expression that tension jumps would not have arisen. A strong effect is noted in garage cooperatives where individual welding machines are used in violation of the norms. They consume considerable power from the network. Local transformer capacity is limited, voltage failure occurs. As a rule, the unfortunate consequences do not happen,
put up with the phenomenon. The rise in voltage is caused by various factors. The potential difference grows so rapidly that it sometimes breaks through electrical insulation. The phenomenon is not called a surge, but an overstrain. According to the factors that caused the situation, the following factors are distinguished:
- Internal overvoltages that occur when the equipment is turned on and off. This is especially true of inductive loads capable of storing a significant amount of energy: motors, transformers. The capacitor compensation blocks also cause overvoltage or sinking. When regulating the reactance, a sharp absorption or return of energy is noticeable.
- Atmospheric overvoltages are caused on lines by lightning, arc discharges, reaching millions of volts. True, in a short time - tens of microseconds. Internal overvoltages are much longer - 50-100 ms.
Internal overvoltages do not exceed the nominal more than 2.5 - 3.5 times.
Even today there is no single theory of what is happening in thunderstorm clouds. The study involved more Benjamin Franklin and Lomonosov. The intensity of the earth's atmosphere is 100 V / m. A feature inherent in the Earth is the increase in the number of free charge carriers with height. This is explained by cosmic radiation, even emanating from the sun. At an altitude of 80 km, the air conductivity, despite its low density, is 3 billion times higher than at the surface of the planet. This is comparable to fresh water.
Physicists represent the Earth as a large spherical capacitor. One lining becomes the surface of the soil, at the very least conducting electric current, and the second - the ionosphere. The dielectric is an atmospheric layer of air. Charging the forces of nature with this gigantic capacity, the universe provokes numerous processes that take place at different heights.
Between the ground and the height of 80 km, the voltage reaches 200,000 V, which is a trifle compared to what is achieved in a thundercloud by electrification. Constantly there is a current of 1400 A between heaven and earth, but the density is low due to the great area of the planet. Multiplying two quantities, find the power component of 300 MW.
During friction, free charges accumulate in thunderclouds. Under the influence of the field of the Earth they are stratified. This happens in electrophore. If air conducts little by little, pure evaporated water is considered an insulator with a permeability coefficient of 81. A loose cloud is formed, similar to a conductor in an electric field. The charges on its surface are distributed in order to balance the applied external exposure.
The wind starts to blow, the moisture rises from the ground, a lot of small droplets are formed. On their surface, the large curvature of the form creates increased tension, which causes the positive charges to flow to the surface of the planet, and the negative ones to rise in the direction of the ionosphere. The capacitor is charged, and its energy is multiplied by the use of a dielectric in the form of a cloud. As a result, the tension on the cloud surface reaches 30 kV / cm. This is tens of thousands of times the normal value.
The cloud is too heavy to climb up and in contact with the ionosphere, the main shock is received by the Earth. Ionization begins on the surface of the cloud, then the arc moves along a random trajectory in the direction of less resistance. As long as the cloud is a dielectric, the path is mostly closed, lightning falls to the ground.
The tops of grounded objects have zero potential, often becoming a target. A wet tree conducts a good charge, and therefore serves as the point of the most likely hit. The resulting arc and step voltage kill everything that is nearby. Often the target becomes a pole or lightning conductor. Electromagnetic fields of incredible strength create strong pickups in the line, causing overvoltage. Therefore, it is required to turn off the electronics during thunderstorms.
Qualitative Description of Lightning Strike
The shape of a thunderstorm current pulse has the shape of a triangle with a sharply rising front and a relatively gentle decline. The whole process takes dozens of microseconds. The current pulse can have an amplitude of 200 kA, which causes a regular voltage surge across the load in proportion to the size of the resistances of these sections.
The line and the consumer form a resistive divider. Depending on the ratio of their resistances, the total effect is calculated. For example, with a negative voltage transformer current will flow in his direction, because the potential of the sky is higher than any of the voltage classes used by mankind. The lightning discharge consists of a series of fast pulses, including three parts:
- A relatively small, long, smoothly increasing current of the leader.
- The main impulse, short, powerful.
- Afterglow Segment. Represents a gradual decrease in current to zero, becomes a reflection of the leading part along the time axis.
Pulses in a bundle can be up to 20, but more often - two or three, the amplitude gradually decreases. Once a cloud is a dielectric, the discharge of a lightning appears as a flow of electrons to the earth. After the first peak, their surface density decreases sharply, carriers rush from other parts of the cloud. The potential is growing again, along the fresh route of ionized air again rushes down. This happens until the voltage of the cloud falls to the limit where the arc discharge is impossible.
Lightning occurs simultaneously in two places. When the flow of electrons begins to move downward, it electrifies the earth with influence, and the resulting potential difference ionizes the air near the ground. At the same time, two leaders are moving towards each other:
- is down - negative;
- up - positive.
As a rule, the spark gap is minimal relative to a certain height: wood, mast, mountain peak. The discharge flows exactly here. The potential is poorly distributed by dielectrics, lightning hits the well-protected objects that are under the potential of the soil. This explains the fact that the discharge rarely hits strategically important objects like oil lakes. Being a dielectric, natural fuel is able to accumulate a charge, but poorly conducts it.
Lightning often hits the ocean. Sea water is considered an excellent electrolyte; water bodies cannot be considered in the context of oil. Now readers can easily imagine what the oil stain on the water will lead to. They say that oil is even worse: its layer has sank along the trajectory of the Gulf Stream and is now in the midst of the ocean.
The pulses shown in the figure are not symmetrical. Their front is steeper than a recession. The parameters of the pulses are shown in the table, often observed and maximum deviations in both directions. According to statistics, only 2% of lightning currents reach a value of 100 kA, half of which is in the area up to 18 kA.
Consequences of lightning strikes
It has been established that the discharge can melt the wires of communication lines or small fuses. Despite the short duration, the pulses carry considerable energy. Overvoltages induced by lightning are divided into two categories:
- Direct strike.
- Induced Current.
The magnitude of the overvoltage is affected by the lightning impulse current and the steepness of the front. With a direct impact on Ohm’s law, it is possible to find a voltage. Suppose the line impedance is 10 ohms and the input impedance of the television is 500 ohms. With a current pulse of 20 kA, we obtain the voltage at the load U = 500 x 20.000 / 510 = 19.6 kV.It is clear that such a threat is not neglected, on the power lines the wires are protected by lightning conductors. Depending on the voltage class of the event are different.
In addition to direct impact, the potential skidding is caused by the phenomenon of step voltage. The wire is usually connected to the ground through a neutral, each pillar of the power line is grounded. As a result, bridges are formed through which current flows into the metal parts of the equipment. That is why the pole is supplied with a string of insulators. However, precaution does not save, and the Arago-Foucault currents are induced in the line, leading to power surges. The value is calculated according to the formula shown in the figure( in the numerator, the lightning current and the line suspension height, in the denominator, the distance from the point of impact to the path of the power line laying).
In order to further reduce damage, it is recommended to take the wave impedance of a line from one wire equal to 400 Ohms, and doubled( phase splitting) - 250. Then, with the observed lightning discharge characteristics, its attenuation on reactive resistances looks the greatest, industrial frequency 50 Hzpasses with small losses. The characteristic impedance is calculated as the square root of the ratio of the inductive part of the impedance to the capacitance.
At the discontinuity of the line, a wave generated by lightning radiates into space. If at the end there is a cable with an impedance of 50 ohms, some of the energy will be reflected. The remaining wave will undergo refraction on the way to the consumer. The laws of reflection and refraction are described in terms of impedances( wave impedances of the lines).To ensure the frequency of 50 Hz( or other range), matching devices are used.
Overvoltage caused by lightning, the most dangerous and significant in amplitude. Consideration of other voltage surges in practice is not performed. Provided that the line has the proper insulation of the wires to protect against lightning strikes, other mandatory measures have been taken. Dielectric strength of the dielectric is determined by the maximum field strength.
There is a huge paradox here: the danger is higher on low-current lines. The small curvature of the wires greatly increases the electric field. Insulators of different materials used in multilayer structures are designed to have the same capacity, if possible. Otherwise, there will be a significant bias( see the serial connection of capacitors).What decreases the total voltage, withstand a multilayer insulator without breakdown.