The sudden short-term voltage rises to a value dangerous for the insulation of an electrical installation are called overvoltage. By their origin, there are two types: external (atmospheric) and internal (switching). Atmospheric overvoltage arises from direct lightning strikes into an electrical installation or from lightning strikes in the immediate vicinity of it. Atmospheric overvoltage poses the greatest danger to an electrical installation since with direct lightning strikes they can reach 1000000 V, with a lightning current - up to 200 kA. They do not depend on the value of the rated voltage of the electrical installation. They are especially dangerous for installations with lower voltages since in these installations the distances between live parts and the insulation level are lower than at high voltages. Atmospheric overvoltages are subdivided into induced and from a direct lightning strike. The former occurs during a lightning discharge near an electrical installation, for example, a substation or power line. Overvoltage is generated by the inductive effect of a thundercloud charged to a very high potential (several million volts). In case of a direct lightning strike, in addition to the electromagnetic action causing overvoltage, mechanical damage is also noted, for example, the splitting of wooden posts or traverses of overhead power transmission lines. Induced overvoltages are of the order of 100 kV, which is significantly less than the overvoltage caused by a direct lightning strike.
They propagate along the wires of the overhead line after the discharge in the form of damped waves. A lightning strike in most cases consists of a series of individual impulses following one after the other. The entire discharge lasts tenths of a second, and individual impulses have a duration of tens of microseconds each. The number of individual impulses during a lightning strike can be from 1 to 40.
Protection of electrical installations against atmospheric overvoltage.
It was noted above that atmospheric overvoltage can reach several million volts. The insulation of electrical installations cannot withstand such voltage levels, so it needs additional protection against breakdown. These means prevent damage to electrical equipment and should be used in electrical installations both to increase the uninterrupted power supply to consumers and to protect people and animals. Particular attention should be paid to the overvoltage protection of overhead lines with a voltage of 10 and 0.4 kV, as well as substations of consumers located in rural areas. Fires can be a serious consequence of overvoltage, in particular, due to direct lightning strikes. Therefore, the organization of correct and reliably working protection against atmospheric overvoltage (or lightning protection) is given the most serious attention. The problem of lightning protection includes measures to protect individual elements of electrical installations from direct lightning strikes, isolate electrical machines and devices from breakdowns, from impulses running from the line of overvoltage waves. These measures are reduced to the installation of protective devices and devices that divert an impulse (wave) of overvoltage into the ground before the wave reaches any critical element of the installation and disables it. Therefore, the main part of all protective devices is earthing switches. They must be made in accordance with the EIR (electrical installation rules) and provide a reliable discharge of the charge to the ground. Lightning rods, arresters, and spark gaps are used as the main protective equipment against atmospheric overvoltage. Lightning rods orient the atmospheric discharge towards themselves, taking it away from the current-carrying parts of the installation. To protect concentrated objects (for example, substations or other structures), rod lightning rods are used, and to protect extended ones (for example, overhead line wires), catenary wire lightning rods are used. Arresters and spark gaps are installed to drain the charge into the ground. For lightning protection of station generators and transformers, a set of means is provided both for protection against direct lightning strikes and overvoltage waves incident from the line. Protection against direct lightning strikes is performed by rod lightning rods and catenary wire lightning rods at the approaches of the overhead line to the station or substation. Generators are protected from waves incident from the line with arresters that limit the wave amplitude to a value that is not dangerous for the insulation of an electrical machine. Large generators are not recommended to be directly connected to outgoing power lines. For small stations supplying electricity to consumers at generator voltage, such a connection is possible if special arresters with improved characteristics are additionally installed at the generator. If the generators are connected directly to the step-up transformers, that is, according to the "generator-transformer" block diagram, then they do not require special measures of protection against poly overvoltage. Overhead lines with a voltage of 6 - 35 kV, made on wooden poles, do not require special overvoltage protection. Their lightning-proof insulation is provided by the insulating properties of wood. Here it is only important to maintain the following minimum insulation distances between the wires (in wood): 0.75 m for voltages of 6-10, 1.5 m for a voltage of 20, and 3 m for a voltage of 35 kV. Individual sections of overhead lines with weakened insulation (for example, using metal or reinforced concrete supports, connecting the overhead line with a cable, etc.) are protected by arresters or spark gaps (at low currents) (see - Tubular arresters and valve arresters). The resistance of the grounding devices of these devices should be no more than 10 ohms. Arresters and spark gaps are installed on the supports of two overhead lines intersecting with each other, or at the intersection of an overhead power line with a communication line. The resistance of the grounding devices here should be no higher than 15 ohms. Grounding slopes on supports must have a bolted connection, and their cross-section must be at least 25mm2. To restore power over the overhead line after fast-passing lightning damage, automatic reclosing (automatic reclosing) devices of the lines are used. With the successful operation of automatic reclosing devices as a means of lightning protection, consumers will not feel an interruption in the power supply, which will not be more than 0.2 s, and their normal operation will not be disturbed. Cable glands are protected at both ends by arresters. The protection of consumer networks with a voltage of 0.38 / 0.22 kV is carried out especially carefully. These networks are performed, as a rule, by air and their design is most susceptible to atmospheric overvoltage, since they rise above all other structures and pass through open areas. Low-voltage networks are equipped with lightning protection devices that divert impulse discharge currents to the ground. This allows you to protect people and animals, prevent fires that arise as a result of lightning discharges and their penetration into internal electrical wiring. In low voltage networks, connections to lightning protection grounding of hooks or pins of insulators of all phase wires and the neutral wire are provided. On supports with wiretaps to houses or directly at the inputs to buildings, grounding is also provided. The resistance of the protective grounding device must not exceed 30 ohms. At substations of 10 / 0.4 kV consumers, low-voltage windings connected to overhead lines must be protected by arresters. They are installed as close as possible to the transformer and connected to the common grounding circuit of the substations. When the power of the transformer is 630 kVA and above, two additional protective groundings are made on the lines extending from it - 50 and 100 m from the substation with the indicated resistance value.
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