1. What is overvoltage? How are overvoltages classified?
Due to lightning discharges, system operations, faults or other reasons, the voltage in some parts of the power system may rise abnormally, and sometimes may greatly exceed the rated voltage of the normal operation of the electrical equipment, causing damage to the insulation of the equipment. This voltage increase in the power system that endangers the insulation is called overvoltage.
Overvoltage can be divided into atmospheric overvoltage and internal overvoltage according to its energy source.
Atmospheric overvoltage is caused by lightning strikes to power system facilities, lightning induction, or lightning overvoltage invasion along lines to transformers. It is called direct lightning overvoltage, induced lightning overvoltage, and high potential intrusion. Because its energy comes from outside the power system, it is also called external overvoltage. The amplitude of atmospheric overvoltage depends on lightning parameters and lightning protection measures, and has no direct relationship with the rated voltage of the power grid. This type of overvoltage has a pulse nature, and its duration is generally only about ten microseconds. However, the harm it causes is great.
Internal overvoltage can be divided into power frequency overvoltage, operating overvoltage, accident overvoltage and resonance overvoltage according to the origin, nature and form of electromagnetic oscillation. The classification of overvoltage is now detailed as follows:
American cable connector
2． What is operating overvoltage? What are the causes of operating overvoltage?
With the normal operation or switching fault operation of the circuit breaker or isolating switch in the power system, the overvoltage generated between the phases and ground, between phases and phases of the system, and between the two contacts of the circuit breaker or isolating switch is called operating overvoltage.
The reason for operating overvoltage is that many devices in the power system are energy storage components. During the opening process of the circuit breaker or isolating switch, the magnetic energy stored in the inductor and the electrostatic field energy (electrical energy) stored in the capacitor undergo a conversion and transition oscillation process. The oscillation causes overvoltage.
The characteristic of operating overvoltage is that its duration is usually longer than lightning overvoltage, but shorter than transient overvoltage. Generally between hundreds of microseconds and 100ms, and decays very quickly. This can usually be simulated using standard operating shock waves.
3. How is thunder and lightning formed? What are the hazards?
Lightning is an electrical discharge phenomenon in the atmosphere. During the thunderstorm season, thunderclouds have strong friction with the air during their formation and accumulate charges. When the charged clouds approach the ground, they have an electrostatic induction effect on the earth. At this time, the earth under the clouds induces an opposite charge to the thunderclouds, and the two form a huge "capacitor." The charge distribution in the thundercloud is uneven, and the electric field intensity of the thundercloud to the ground is also different. When the electric field intensity at densely charged areas in the cloud reaches 25-30kV/cm, it will break down the insulation strength of the nearby air, and a leader discharge will occur from the cloud to the ground. When the path of the pilot discharge reaches the earth, the earth and clouds will produce a strong "neutralization", and a huge current will appear, which is called the main discharge. The temperature of the main discharge is 20,000°C, which produces a dazzling flash, rapidly expands the air, and emits an ear-splitting roar (i.e., thunder). The main discharge time is extremely short, about 30~50μs, the current ranges from thousands to hundreds of kiloamps, its wavefront time is only 1~4μs, and the steepness is 7. About kA/μs.
Cold shrink cable head
Lightning is very harmful. Its harmful methods include:
(l) Direct lightning strike: When lightning passes directly through houses, trees, towers, antennas and other objects, the powerful lightning current will burn these objects. The voltage drop produced when lightning current passes through these objects and soil is lightning high voltage. It can cause serious damage to surrounding objects, people and animals. Direct lightning strikes on transmission conductors will produce direct lightning overvoltage, which may cause interphase insulation flashover. When lightning directly strikes a lightning protection wire or tower, when the voltage drop generated by the lightning current's wave impedance and grounding resistance is too high, it will break down the insulation between it and the conductor, causing a counterattack overvoltage on the conductor and causing damage.
(2) Induction lightning: In the leading stage of a lightning strike, opposite bound charges will be induced on nearby towers, lightning protection wires and transmission conductors grounded through the neutral point of the transformer. If the ground near a transmission line is struck by lightning, the bound charge suddenly becomes a free charge during the main discharge stage of the lightning strike. It moves to both sides along the wire at the speed of light to form an overvoltage, which is called induced overvoltage. Its amplitude may reach 300-500kV, which can cause insulation flashover of equipment below 110kV. In addition, the magnetic field changes of the lightning main discharge current will also induce overvoltage on the transmission conductors, which is generally very small.
High potential intrusion refers to the high potential (lightning wave) of lightning overvoltage that invades substations or users along overhead lines. This high potential can occur due to direct lightning strikes or induced lightning on the line. According to statistics, lightning accidents caused by high potential intrusion in power systems account for more than half of lightning accidents, which is a considerable proportion. Therefore, considerable attention should be paid to protection against high potential intrusion.
4. What is an air-termination device? What are the commonly used air terminals?
The so-called air terminal is a metal object specially designed to directly receive lightning strikes (thunder flashes). The metal rod that connects to lightning is called a lightning rod; the metal wire that connects to lightning is called lightning protection wire; the metal strips and metal mesh that connects to lightning are called lightning protection belts and lightning protection nets; all air terminals should be connected to the ground body through grounding down conductors.
(1) Lightning rod uses the principle of tip discharge to prevent thunderclouds from direct lightning strikes on buildings and structures.
(2) Lightning protection wires are usually set at the top of overhead lines, which are both overhead and grounded, so they are also called overhead ground wires.
(3) Lightning protection belts and lightning protection nets are similar to lightning protection wires and are installed on and around the top of buildings.
(4) Lightning arrester and discharge gap are discharge paths set up to prevent direct lightning and induced lightning from invading along the lines and causing high potential damage to the substation equipment. 5. What is the function of tubular arrester? What is its basic structure?
Tubular arresters are generally used on lines. Valve-type arresters are generally used in substations.
The tubular arrester consists of three parts: gas tube, internal gap and external gap, as shown in the figure.
Trachea tubes are made of fiber, Plexiglas or plastic. The internal gap is installed inside the gas production tube. One electrode is rod-shaped and the other electrode is ring-shaped as shown in the figure. S1 in the picture is the internal gap of the tubular arrester.
The external gap is installed between the tubular arrester and the live line, as shown in Figure 67. When the line is struck by direct lightning or induced lightning, the atmospheric overvoltage causes the internal and external gaps of the tubular arrester to breakdown, and a strong lightning current flows into the earth through the grounding device. However, what follows is the power frequency freewheeling current in the power grid, whose value is also very large. The lightning current and power frequency follow-up current generate a strong arc in the internal gap of the tube, burning the material on the inner wall of the tube and producing a large amount of arc-extinguishing gas. Since the volume of the tube is small, the pressure of these gases is very high. The high-pressure gas is rapidly ejected from the open end of the tube, blowing the arc strongly. When the power frequency current crosses zero, the arc is extinguished. At this time, the air in the external gap S2 restores the insulation, isolates the tubular arrester from the system, and restores the normal operation of the system.
In order to ensure that the tubular arrester works reliably, when selecting a tubular arrester, the upper limit of the breaking current should not be less than the maximum effective value of the short-circuit current at the installation location (regardless of the non-periodic component); the lower limit of the breaking current should not be greater than the possible minimum value of the short-circuit current at the installation location (regardless of the non-periodic component).
The minimum external clearance of tubular arresters is: 3kV, 8mm; 6kV; 10 mm; 10kV, 15mm.
Cold shrink cable joint
6． What is a magnetic blow valve arrester? What are its characteristics?
The magnetic blown valve type arrester is mainly composed of a magnetic blown spark gap and a high temperature valve piece.
The magnetic blowing gap is divided into two types: elongated arc type and rotating arc type. The basic principle is to use the magnetic field of the magnet to interact with the power frequency freewheeling current to elongate or rotate the arc, so as to divide and cool the arc and produce a strong deionization effect to extinguish the arc.
The magnetic blow valve type arrester has low discharge voltage, low residual voltage and large inrush current capacity, which can achieve good insulation coordination. It can be used for internal overvoltage protection of some equipment with large energy storage, and can also reduce the insulation requirements for the protected equipment. Magnetic blow valve arresters are often used for overvoltage protection of rotating electrical machines.
7． What are the lightning protection measures for power substation (distribution)?
Lightning protection measures for power substations (distribution) should usually consider the following aspects:
(l) Install lightning rods
Lightning rods can be used as direct lightning protection for the entire building or structure.
The lightning rod can be a single independent pole, or it can use the structure of an outdoor power distribution device or a lighthouse with floodlights. However, the portal structure of the transformer cannot be used to install lightning rods to prevent the overvoltage generated by lightning strikes from tip discharge on the transformer.
(2) Install valve-type arrester and protective gap on the high-voltage side
This is mainly used to protect the main transformer from high potential invading the most important equipment of the substation along the high-voltage lines. For this reason, it is required that the arrester and protection gap should be installed as close as possible to the transformer, and its grounding wire should be jointly grounded with the low-voltage inverted neutral point of the transformer and the metal shell of the transformer, as shown in Figure 6-12.
The protective wiring of the 10kV power distribution device against high potential intrusion is shown in Figure 6-13. Usually, valve type surge arresters are installed on each incoming line terminal and busbar. If the incoming line is an overhead line with a section of cable, the valve type arrester or tube type arrester should be installed at the terminal head of the overhead line.
(3) Install a valve-type arrester and protective gap on the low-voltage side
This method is mainly used in areas with multiple minefields to prevent lightning waves from intruding from the low-voltage side and breaking down the insulation of the transformer. When the low-voltage neutral point of the transformer operates in an ungrounded mode, the neutral point should also be equipped with a lightning arrester and protective gap.
High voltage arrester, arrester manufacturer, metal oxide arrester
