Abstract: Lightning has always been an important hazard affecting the safe operation of substations. With the digital transformation and construction of substation, it is more important to do a good job of lightning protection for the secondary system of substation. This paper expounds the characteristics of substation secondary system, and then reveals the relationship between lightning damage and secondary system. On this basis, the main forms of lightning hazards are summarized, and finally the specific technical measures for lightning protection of secondary system in substation are put forward.
Paper Keywords: substation lightning protection; Secondary system; Lightning protection; Technical measures
I. Introduction
With the advancement of power system reform, the digital transformation and construction of substations are also developing in depth. The number of integrated automation substations is increasing, and the harm of lightning to weak current equipment is becoming increasingly prominent. From the domestic reports and the actual operation of substations, it is common that the secondary equipment of substations is struck by lightning, resulting in equipment damage, communication interruption and system exit. This not only seriously threatens the safe operation of the power grid, but also brings a lot of inconvenience to people's lives. According to the working practice, according to the characteristics of the secondary system of the substation, this paper analyzes the harm ways of the lightning wave, and connecting with some current lightning protection technologies and the situation of the substation of the power supply bureau, probes into the lightning protection measures of the secondary system of the substation.
Second, the structural characteristics of substation secondary system
Substation secondary system refers to internal protection equipment, automation equipment, communication system, computer network equipment and monitoring system, AC and DC power supply system and other secondary equipment. The secondary system concentrates the important equipment of substation automation monitoring and management, and has the functions of microcomputer monitoring, monitoring, protection, small current grounding line selection, fault wave recording, low frequency load shedding and "four remote" remote transmission, which plays an important role in the field of power dispatching automation.
Due to the crisscross of connecting wires in the secondary system, the surge overvoltage directly caused by the discharge of the earth, overhead lines and thunderstorm clouds near the lightning strike, or caused by electrostatic and electromagnetic induction, can easily invade the automation system in the form of conduction, coupling and radiation through various interfaces, which may cause lightning accidents that endanger the normal work of the system or even destroy the system.
Third, the main harm forms of lightning discharge to the secondary system of substation
Lightning is a powerful pulse discharge process in nature. There are many ways for lightning to invade buildings or equipment on the ground and cause disasters. Generally speaking, there are direct lightning, induced lightning, electromagnetic pulse radiation, lightning overvoltage intrusion and counterattack.
(1) Direct lightning: The main destructive power lies in the current characteristics rather than the high potential generated by discharge, and the powerful lightning generated by it is converted into heat energy, causing damage to objects.
(2) Induced Lightning Strike: During the whole process from dense Lei Yun to lightning discharge, two physical phenomena-electrostatic induction and electromagnetic induction-appear in the lightning activity area almost at the same time, which may cause a harmful form called induced lightning strike.
(3) Electromagnetic pulse radiation: When lightning is discharged, its current changes with time rather than uniformly, and the pulse current radiates electromagnetic waves. Although the electromagnetic pulse radiation decreases with the increase of distance, it is relatively slow. The electromagnetic pulse radiation of lightning is coupled to modern electronic equipment which is extremely sensitive to instantaneous electromagnetic pulse through space in the form of electromagnetic waves, causing equipment damage.
(4) Invasion of lightning overvoltage: Direct lightning strike or induced lightning can cause overvoltage on wires or metal pipes, and these overvoltage will invade buildings or equipment from distant minefields or lightning protection areas along wires or metal pipes.
(5) Counterattack: In the thunderstorm activity area, when lightning strikes the lightning receiving device of the building, although the grounding system of the lightning receiving device is very good and the grounding resistance is very small, the potential of the grounding down lead and the grounding device will suddenly rise to hundreds of volts due to the large lightning current amplitude and high wave head steepness.
Fourthly, analysis of lightning protection technical measures for secondary system of substation.
Weak current equipment has low overvoltage resistance, and is vulnerable to lightning waves in thunderstorm season, resulting in equipment damage and misoperation. The power supply system of weak current equipment may be damaged by intrusion overvoltage and induced overvoltage. In practical application, SPD should be installed for multilevel protection to reduce overvoltage to harmless level. Signal lightning protectors shall be installed on signal wires, network wires and microwave feeders introduced into the control room to ensure the normal work of automation systems, telecontrol equipment and communication. As a whole, the lightning protection of weak current equipment is a comprehensive problem. Long-term lightning protection practice tells us that in lightning protection, we should seriously treat all links from direct lightning protection to grounding, voltage sharing, shielding, restriction, shunt and isolation to ensure the safety of equipment.
(1) Grounding and voltage sharing
Grounding is the most direct and effective measure to improve the lightning protection level of secondary equipment. All lightning current can be introduced into the earth through the grounding grid, which is reliable. Grounding can effectively avoid the harm of surge voltage to secondary equipment. The lightning protection code stipulates that different grounding grids have different resistance values. On the premise of economy and rationality, reducing the grounding resistance as much as possible can effectively limit the rise of ground potential.
Grounding and voltage equalization complement each other. The so-called voltage sharing means that a closed-loop grounding busbar should be set around the same level and room. All instruments, equipment casings, power cables, signal cables and metal pipes in the same room shall be directly connected to the grounding bus and firmly connected to ensure the equipotential of each grounding point. The magnitude and steepness of lightning current are very large, and the place where it flows immediately rises to a high potential relative to the earth with zero potential, and objects around the earth with zero potential will produce lateral flashover discharge. This kind of lateral flashover will not only lead to fire and explosion in buildings filled with flammable and explosive substances, but also cause induced potential to indoor electronic equipment and damage it. Perfect equipotential can effectively prevent accidents caused by potential difference between non-equipotential bodies.
(2) shielding
Shielding refers to the use of shielded cables, various artificial shielding boxes, boxes, Faraday shielding cages and various available natural shields to block and attenuate electromagnetic pulse interference applied to electronic equipment. It should be emphasized that the shielding shell must be effectively grounded, and all kinds of power lines and signal lines entering the shielding room must be effectively isolated by electromagnetic pulses and high-frequency electromagnetic wave filtering devices, otherwise a wire from the interference source environment that has not been filtered or isolated will make the shielding cage lose its shielding effect.
Generally speaking, in order to reduce the external lightning electromagnetic interference, the building steel bars and metal floor frames of the communication machine room and the communication dispatching complex should be welded with each other to form an equipotential faraday cage. When the equipment has high requirements for shielding, a metal shielding net should be laid on six sides of the machine room, and the shielding net should be connected to the annular grounding bus in the machine room at even points.
(3) Diversion and isolation
The main function of the shunt is to directly connect the lightning receptor of direct lightning to the grounding device through a plurality of scattered grounding downlead, and shunt the lightning current to the ground, so as to avoid great interference caused by excessive lightning current flowing on each grounding downlead and the strong electromagnetic field generated around it. The lightning current of direct lightning is effectively introduced into the ground through the grounding downlead, instead of jumping into the working area of weak current equipment. It should be emphasized that the housings of various equipment (such as microwave receivers) at the top of the building should be connected with the main grounding down lead or grounding belt in a radioactive way, and the housings of the equipment should not be connected in series. The former is to avoid the strong induced potential generated by lightning current on the ungrounded down conductor, and the latter is to avoid the equipment damage caused by lightning current connected in series to the equipment. In order to ensure the effect of dispersing current, the grounding down conductor should have enough area, especially to prevent the middle of the grounding down conductor from being corroded and broken or connected with equipment. Grounding down lead and grounding grid must be measured and checked regularly.