Created by: Glen Zhu | Updated Date: May 27th, 2024
The surge voltage that low-voltage power supply systems may encounter is mainly caused by lightning strikes or large-capacity electrical equipment. In the increasingly complex situation of current power supply systems, how to protect against lightning and surges has become an important issue of concern.
Lightning discharges can occur between cloud layers, within cloud layers, or between clouds and the ground; the impact of thundercloud discharges on power supply systems (in China, AC 50Hz 220/380V) and electrical equipment is becoming more apparent. Lightning strikes between clouds and the ground consist of one or several individual lightning flashes, each carrying high amplitudes and short durations of current. A typical lightning discharge will include two or three flashes, with approximately one-twentieth of a second between each flash. Most lightning currents fall within the range of 10kA to 100kA, with a duration generally less than 100 microseconds. Lightning flashes may act on low-voltage power supply systems through two pathways:
(1)Direct lightning strike: Lightning discharge directly hits components of the power system, injecting a large pulse current.
(2)Indirect lightning strike: Lightning discharge hits the ground near equipment, inducing moderate intensity current and voltage on power lines.
Due to the use of high-capacity equipment and variable frequency devices within the power supply system, internal surge problems are becoming increasingly severe. We attribute this to the influence of transient overvoltage (TVS). Any electrical device has an allowable range for supply voltage. Sometimes even narrow overvoltage impulses can cause damage to the power supply or all equipment. This is where transient overvoltage (TVS) comes into play. Especially for some sensitive microelectronic devices, even small surge impacts can sometimes cause fatal damage. The sources of surge voltages in the power supply system come from both external and internal factors:
The occurrence of internal surges is related to the start-stop of equipment and faults in the power supply network; Internal surges in the power supply system can have adverse effects on electrical equipment due to factors such as the start-stop of high-power equipment, line faults, switching actions, and operation of variable frequency devices. In particular, it can cause fatal impacts on microelectronic devices such as computers and communication systems. Even if it does not result in permanent damage to equipment, abnormal operation and interruptions in the system can lead to serious consequences. For example, nuclear power plants, medical systems, large-scale factory automation systems, securities trading systems, telecom exchange switches, network hubs etc.
Direct lightning strikes can cause the most severe consequences especially when overhead transmission lines near user incoming lines are struck by lightning. The voltage on overhead transmission lines will rise to several hundred thousand volts which may cause insulation flashover. Lightning currents can travel distances up to one kilometer or more along power lines with peak currents exceeding 100kA near strike points. The current on low-voltage lines at user incoming points per phase can reach 5kA to 10kA during thunderstorm activity.
In areas where lightning activity is frequent, power facilities may experience severe lightning currents from direct strikes multiple times a year while such events are rare for underground cable-fed or less frequently hit areas by lightning activities. Indirect lightning strikes and internal surges have a higher probability of occurrence with most cases leading to damages in electrical equipment. Therefore surge protection focuses on absorbing and suppressing this surge energy.
Surge protection for power supply systems should adopt a staged approach for transient overvoltage (TVS) protection caused by surges particularly for low-voltage supply systems starting from the entry point gradually absorbing surge energy and suppressing transient over-voltages in stages.
Level 1 protection: A high-capacity surge protector for the incoming lines of each phase and ground at the user’s power system entry point. Generally, this level of power protector is required to have a maximum impulse capacity of over 100kA per phase, with the specified limiting voltage being less than 1500V. We refer to it as a Class I power surge protector. These power surge protectors are designed to withstand large currents and high-energy surges caused by lightning strikes and inductive lightning, diverting a large amount of surge current to the ground. They only provide protection at a moderate level of limiting voltage (the maximum voltage on the line when the impulse current flows through SPD becomes the limiting voltage), as Class I protectors are mainly for absorbing large surge currents. Relying solely on them cannot fully protect sensitive electrical equipment inside the power supply system.
Level 2 protection: It should be installed at distribution points supplying power to important or sensitive electrical equipment. These SPDs provide a more comprehensive absorption of residual surge energy passing through user-supplied entrance surge arresters and have excellent suppression effects on transient over-voltages. The required maximum impulse capacity for the power surge protector used in these locations should be over 45kA per phase, with a specified limiting voltage below 1200V. We refer to it as a Class II power surge protector. Achieving second-level protection in general user-supplied systems can meet operational requirements for electrical equipment (see relevant provisions in UL1449-C2).
Level 3 protection: An integrated surge protector can be used in the internal power supply section of electrical equipment to completely eliminate small transient over-voltages. The maximum impulse capacity required for the surge protector used in this area should be 20KA/phase or lower, and the required limiting voltage should be less than 1000V. Level 3 protection is necessary for some particularly important or sensitive electronic devices. At the same time, it can also protect electrical equipment from transient over-voltages generated internally by the system.
China’s power system has a high level of informatization, and computer intelligent management has been fully implemented in power production, command, dispatching, etc. Paperless offices have also been promoted in power system management many years ago. However, due to the extensive use of these microelectronic devices, lightning disasters have increased their damage to power command and management systems compared to before.
Lightning is a common source of strong electromagnetic interference in the atmosphere. In order to better defend against lightning electromagnetic pulses, not only should the location of computer rooms be correctly selected but also effective measures such as equipotential connection, shielding, and overvoltage protection should be taken.
First, the selection of the location of the computer room in the building. According to the “skin effect” of lightning current, the lightning current is almost concentrated on the outer walls with reinforced steel bars, and the magnetic field intensity indoors is highest near the columns through which the current flows. Therefore, computer rooms should be placed in the middle of buildings, and avoid columns on the outside of buildings that serve as down conductors.
Secondly, regarding the placement of equipment inside the computer room. Due to large peak values and the steepness of lightning currents, strong transient electromagnetic fields will occur in their surrounding space. Conductors within these electromagnetic fields will induce significant electromotive forces. Therefore, when arranging equipment inside a computer room, a certain distance should also be maintained from exterior wall pillars.
Thirdly, equipotential bonding techniques are used to connect lightning protection devices with metal structures of buildings, external conductors, electrical devices etc., using connecting wires or surge protectors (surge arresters), so as to reduce potential differences between various metal components when lightning currents pass through them.
Fourthly, shielding measures involve welding (connecting) together metal frames, doors and windows, floors, etc, forming a “Faraday cage”, which is then well grounded by connecting it with an earth grid. Shielding pipelines generally require underground cables for entry into households; their metal shielding layers need to be properly grounded at both ends.
Fifthly, protection against overvoltage caused by lightning strikes: When lightning strikes hit power grids or occur near power grids they can generate over-voltages along transmission lines. These over-voltages propagate into computer rooms causing damage to computers and related equipment. Power supply systems should have multi-level protections with step-by-step leakage discharge so that residual voltages are limited to 2 times the rated voltage value U. The transient electromagnetic fields generated by lightning can induce over-voltages on signal lines and their loops damaging corresponding interface circuits. Therefore during actual installation requirements include placing protective devices close to protected equipment; using twisted pair cables for connections at both ends of protective elements; and reducing the total area covered by coupling loops thereby weakening magnetic field coupling effects.
Computer damage caused by thunderstorms is multifaceted; only comprehensive measures for thunderstorm protection can effectively prevent damage to computer equipment.
(1) Introduction
With the increase in power system capacity and the continuous improvement of automation level, many county-level power bureaus in agricultural power systems have used a considerable number of computers, RTUs, and other microelectronic devices. Some microelectronic devices operate at voltages as low as a few volts and transmit information currents as small as microamps. They are extremely sensitive to external interference, and the interference and damage caused by transient electromagnetic fields generated by lightning currents are even more serious for microelectronic devices. During the thunderstorm season, some county power bureaus often experience damage to dispatch buildings, automated display systems owned by power bureaus, communication liaison systems (modems, carrier machines, programmable switches), etc., resulting in significant direct and indirect economic losses. Although certain lightning protection measures have been taken for some power dispatch automation systems, due to their imperfections, lightning accidents still occur.
(2) Microelectronic Device Impulse Withstand Level
The impulse withstand capability of TTL digital circuits is the weakest among microelectronic devices. A pulse voltage with a width of 10V and 30ns can cause damage to TTL circuits; the magnetic field generated by lightning currents can cause the misoperation of microelectronic devices. Even if the channel for lightning current is far away at 1km without electromagnetic shielding when there is no electromagnetic shielding present.
(3) Voltage and UPS Over-voltage Protection
Lightning-induced or intruding waves entering indoors along the power supply line will cause a sudden increase in voltage which leads to damage to UPSs and subsequent connected equipment. Although some UPSs are equipped with varistors (surge protectors), it is still difficult to protect themselves and subsequent microelectronic devices effectively. For power supplies, a reliable and effective method of lightning protection is adopting three levels of protection using corresponding surge arresters at each level so that the clamping voltage output meets specified requirements.
(4) Carrier Machine Overvoltage Protection
The parts of the carrier machine that are prone to damage when struck by lightning are usually the power supply unit, user line unit, and high-frequency circuit board. The high-frequency circuit board typically has a discharge tube with a certain level of lightning resistance; the power supply section can adopt the above-mentioned overvoltage protection method; due to the inconsistency between ringing voltage and conversation voltage, careful consideration should be given in designing protection devices for the user line unit so that it can effectively protect both parts under two different voltages. The best way is to place protective devices inside the carrier machine, but considering practical situations, externally mounted protection modules should be designed more comprehensively.
To achieve good results, four-level protection should be used for overvoltage protection of user line units, program-controlled exchange communication lines, modems, and signal lines. It is preferable for overvoltage protectors to have functions such as an automatic alarm for module failure in protection modules simultaneously recording occurrences of overvoltages automatically without losing records after power outages.
(5) Grounding and Shielding
Grounding
Good grounding is crucial in lightning protection. The lower the ground resistance value, the lower the overvoltage value will be. Therefore, under economically reasonable conditions, efforts should be made to reduce ground resistance as much as possible.
Communication stations in communication dispatch comprehensive buildings should share grounding systems with the building’s power equipment and directly connect them to lightning protection grounding networks whenever possible. Voltage equalization bands should be laid inside communication equipment rooms with circular grounding busbars surrounding these rooms.
In electrical dispatch communication, comprehensive building’s special equipment needs separate grounding systems that can be connected between main building grounds through spark gaps or surge arresters ensuring isolation during normal times while balancing potentials during lightning strikes.
All other aspects related to grounding must strictly adhere to relevant regulations.
Shielding
To reduce lightning electromagnetic interference, the building reinforcement and metal floors of the communication equipment room and communication dispatch complex should be welded together to form equipotential Faraday cages. When equipment has high requirements for shielding, a metal shielding net should be laid on all six sides of the equipment room, connecting the shielding net with the evenly distributed grounding busbars inside the room.
Overhead power lines should be replaced with shielded cables after being led down from terminal poles within the station; outdoor communication cables should use shielded cables, with both ends of the shielding layer grounded; for cables with both armor and shielding layers, both armor and shielding layers should be grounded at one end while only the shielding layer is grounded at the other end. Cables entering indoors should be buried horizontally underground for more than 10m before reaching a depth greater than 0.6m; non-shielded cables should pass through galvanized steel pipes buried horizontally underground for more than 10m, with good grounding at both ends of the pipes. Adding surge resistors between power lines and steel pipes outdoors can enhance lightning protection.
(6) Comprehensive Lightning Protection Measures
To prevent damage from lightning strikes to power dispatch automation systems, a policy of “comprehensive defense, integrated management, multiple protections” should be adopted. In addition to using the protective measures mentioned above along with grounding measures, metal oxide surge arresters should be installed on both high and low-voltage sides of distribution transformers connected in three points jointly grounded. Outdoor ports such as those in program-controlled exchange rooms or Modems shall have overvoltage protectors installed; when devices like RTUs are far from display screens signal line surge protectors shall also be installed.
(7) Conclusion
Strictly following lightning protection grounding regulations, and applying new technologies and devices while implementing comprehensive lightning protection measures are crucial means to significantly reduce damage caused by lightning strikes to county-level power dispatch automation systems. Good grounding and shielding combined with overvoltage protectors can greatly improve resistance levels against lightning strikes for protected devices.
LSP’s reliable surge protection devices (SPDs) are designed to meet the protection needs of installations against lightning and surges. Contact our Experts!
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