First, lightning damage ways
Thunder and lightning hazards are five ways, one is the direct lightning: thunder and lightning directly hit the building, architecture, trees and other objects, due to the hybrid effect of pyroelectric effects directly on the object damage; Second, thundercloud electrostatic induction: the general target In terms of the lines, an overvoltage of 300-400kV can be induced on a high-voltage overhead line under a certain intensity of thundercloud, an overvoltage of 100kV can be induced on a low-voltage overhead line, and 40-60kV can also be induced on a telecommunication line Of the over-voltage; Third, electromagnetic induction lightning: Thunder and lightning flow through the lead into the ground, the magnetic field generated around the lead-off line, lead wire around the various metal lines produced by the transient over-voltage sensor; Fourth ground potential Counterattack: direct lightning strike flashback lightning rod, such as lightning, lightning and so straight into the ground, leading to the ground potential rise, high voltage through the equipment ground wire into the electronic equipment caused by counterattack; thunder and lightning waves intrusion: power lines and communication lines Suffered direct lightning or lightning induced lightning load over-voltage and lightning current induced coupling to the line, and then invade the device.
Second, the transient over-voltage on the hazards of self-control equipment
Normally, the design of the pumping station for the lightning protection has been straightforward measures. Most of the automation system is placed in the pumping station structure, network lines, power lines laid in the cable trench, which suffered direct lightning is unlikely. According to lightning electromagnetic pulse (LEMP) theory and practical experience shows that:
Computer and other automation systems equipment damage is mainly caused by lightning surge voltage induced. Lightning Induced Surge Voltage is a spike voltage that occurs between microseconds to milliseconds, ie transient overvoltage. It can introduce inductive surge voltage wave into the device through the power cable, antenna feeder, communication cable and signal cable, respectively damaging the power supply module, communication module and I / O module, causing the device to malfunction, temporarily paralyzing or burning components immediately . As of March this year, I was attacked by lightning at the Huangpu Sewage Treatment Station. Transient overvoltages caused damage to the PLC interface, UPS insurance, the ports and power supplies of the IPC and telephone lines.
Third, drainage pump station control system lightning protection measures
Thus, the existing lightning protection system has been unable to meet the current automation system security requirements, should be from a simple one-dimensional passive protection into anti-lightning strike, anti-static induction, electromagnetic induction lightning, anti-ground potential counter-defense Induction lightning wave intrusion and the operation of transient over-voltage effects such as the combination of active and passive protection of the three-dimensional protection system. We should start from several aspects of the entire distribution system, signal system, antenna system, and computer network of the automation system of the pumping station, and adopt lightning protection, shunting, equalizing, shielding and grounding to carry out a full range of lightning protection Voltage protection.
(A) self-control distribution system lightning
When a lightning strike occurs in the transmission line or in the vicinity of the transmission line, a lightning shock wave will be formed on the transmission line. The lightning shock wave is easily coupled with the power frequency loop to enter the power module of the automatic control device. Therefore, the lightning protection of the distribution line is controlled System lightning protection is an important part. The use of three surge voltage protector (also known as transient over-voltage protection SPD) is the ideal control system lightning protection measures.
Using this measure, like other lightning protection measures, pay special attention to strictly separating the power lines of the self-controlled equipment from other power lines such as lighting . The distribution of three surge voltage protectors is as follows: The first stage is on the three phases and the neutral line after the secondary side of the transformer and the circuit breaker of the incoming line cabinet, and they are connected in parallel to the ground respectively, and the main discharges outside the line are strong Over-voltage, the Thunder flux, but these arresters start voltage is high and has a larger dispersion capacitance, and the load into a shunt relationship, so that added to the next device on the residual voltage is high, usually arrester start voltage Of 2 ~ 2.5 times. The second level in the PLC or UPS and other special distribution bus at the three phase and neutral, respectively, in parallel to the ground, the main release of the first residual voltage, split the distribution line in the transmission process of induction or coupling Voltage and other electrical equipment over-voltage operation, effectively suppress various electromagnetic interference. The third stage PLC, UPS or other self-controlled device wiring board fuse after the phase and neutral, respectively, in parallel to the main relief in front of the residual pressure to further protect the device from over-voltage interference.
(B) signal system and antenna system lightning protection
Control system signal lines generally use special shielded twisted pair (such as DH +, MB +), the general wear pipe laying, lightning here in the induction voltage lkV-2 kV range, but it directly into the PLC or computer communication port (voltage 5V-48V between), may cause greater damage. Computer data exchange or communication frequency is from DC to tens of megahertz (depending on the system), in the selection of arrester to use a dedicated signal SPD. When the communication cable induces lightning to generate transient over-voltage, the lightning current is discharged to the ground through the SPD branch, and the output of the SPD is limited to the allowable voltage of the device.
Low power radio communication between pumping stations, the cables are used coaxial cable. Lightning on the antenna is the main choice of antenna feeder line SPD, used to suppress the introduction of lightning from the antenna wave, which uses channel diversion technology, the lightning current and useful signals separate. When lightning strikes quickly and effectively induced lightning current through the lightning branch to the earth.
(C) shielding, equal potential processing
Pump room control room power cables (lines), communication cables (lines) should try to use shielded cable. The control room can also be arranged along the ground copper cloth row to form a closed-loop grounding busbar, the power distribution cabinet (box) metal shell, power ground, SPD ground, cabinet enclosure, doors and windows and other potential grounding close to the busbar and 4-10 square mm copper wire as the equipotential bonding wire.
(D) a reasonable ground
The first principle of lightning protection is to directly connect the lightning current to the ground to discharge it, so the "grounding" must be taken seriously. The grounding of the general station mainly has the structure grounding, the distribution system and the strong electrical equipment ground, the computer controlled system grounding.
1, the structure commonly used grounding lightning rod, lightning protection or lightning arrester, their grounding in addition to the use of steel structures within the ground, it should also be laid down alone lead to the structure of the grounding network. Because if only using the structure of steel structure grounding, the construction of the structure of the steel welding quality is not always guaranteed, the average voltage requirements of lightning can not be guaranteed, prone to strong magnetic field within the structure. And these lead-off device should be set as far as possible a few more, so that lightning current diversion more ways to reduce the amount of each line of discharge current to reduce the induced energy. Indoor computers, self-control equipment should be placed far away from the lightning conductor metal body.
2, the distribution system and the grounding of strong electrical equipment has been quite mature measures, and the use of results is relatively satisfactory, there have been no major problems, this is no longer described.
3, the computer controlled system is a special electricity system, which includes the following types of grounding: system work grounding (grounding resistance is not greater than 4 ohms), safety ground (grounding resistance is not greater than 4 ohms), DC ground (signal shielding , Logically no more than 2 ohms). Under the existing pump station difficult to separate, you can use the joint ground, but to ensure that the grounding resistance must be less than 2 ohms.
4, the three major grounding network should be set separately, mainly taking into account the following three reasons: First, most of the pumping station structures in the construction did not consider the computer and other weak equipment, and then take it to the ground and the equipment itself has been set separately. Second, a pump station, for the common power supply equipment, high and low voltage power distribution systems, all use a grounding system, due to the complexity of electricity, often in operation and lightning strike zero line (ground) current is not Zero (Id). Such as the use of joint grounding (Rd), will inevitably make the computer ground potential raised to Id 〓 Rd, which may cause counterattack. Huangpu Station lightning accident by the city mine test center, concluded that one of the reasons is the potential counter-attack caused. The third is the new computer, PLC system, to the ground with the structure, power distribution systems and equipment grounding grounded, the grounding resistance to ensure that less than 0.5 ohms, so generally difficult to achieve and costly. When setting up the grid separately, avoid flashover between the grids. When struck by lightning, a very high potential will be generated in the ground network and nearby conductors. When the ground network is separated, it may cause the flashover grounding body to flash over to other grounding bodies. Therefore, when it comes to automatic control system grounding the distance between the ground should be greater than 10M. When the ground wire is introduced into the room, if it is too close to other ground networks, measures of both insulation and shielding may be taken locally.
references:
1, "Lightning Protection Design Code" GB50057-94
2, "Computer Room Lightning Design Code" GB-50174-93
3, "Prevention of Lightning Accident of Computer Control System" Tao Hanxiong "Electric World" 2003 年 02 期
4, "Risk Assessment Level of Lightning Electromagnetic Pulse Protection in Information System" Fan Dayong "Electricity World" 2003 年 02 期
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