Harm of lightning to DCS system of nuclear power plant and key points of lightning protection design

Lightning is a kind of meteorological disaster that often occurs in most areas of our country. It has the characteristics of strong suddenness and great harm, and often brings very serious threats to buildings and electronic equipment systems. In recent years, with the continuous development and progress of science and technology, the construction of nuclear power plants has begun to develop in the direction of digitization and automation. DCS system is a microprocessor-based system that adopts control functions for decentralized display, centralized operation, and consideration of separate and autonomous and comprehensive coordination. The design principle of the instrument control system can also be called a distributed control system or a distributed control system. The DCS system is currently widely used in nuclear power plants and has become an indispensable and important part of nuclear power plants. It can transmit the data obtained by various sensors to the controller, and transmit the control instructions to the actuator, and can also realize the modification of the control logic. and so on. However, there are many large-scale equipment in the nuclear power plant, and their lightning protection capabilities are weak. Once the DCS system encounters lightning attack, it will inevitably cause instrument failures, which will lead to the inability of the nuclear power plant to operate safely and stably. Therefore, it is very important to strengthen the lightning protection design of the DCS system of nuclear power plants.

1 The Harm of Lightning to DCS System of Nuclear Power Plant

1.1 Ways of Lightning Intrusion

1.1.1 Ground potential counterattack

The ground potential counter voltage can be as high as tens of thousands of volts, which is common in process systems where monitoring and control points are not centralized. When a lightning strike occurs, a strong lightning current will be formed, and these lightning currents can enter the earth by means of the grounding body and down conductors. In other words, the radial potential is distributed around the ground body. When other grounding bodies are connected to electronic equipment nearby, high-voltage ground potential counterattack will be generated, which will cause harm to the DCS system.

1.1.2 Surge voltage

Generally speaking, the DCS system will not be damaged by direct lightning strikes, but the surge voltage induced by the lightning electromagnetic pulse and the high voltage induced by the lightning in the power supply, signal or communication lines will damage the DCS. The main factor is that The voltage waveform is very uneven, which brings shock to the DCS system. Specifically, first, because of the extensive expansion of signal sources, the DCS system is far less powerful in resisting lightning intrusion. Second, the internal integration of the DCS system is too high, which makes the equipment less capable of withstanding high voltages and large currents, and cannot withstand lightning strikes such as induced lightning and voltage surges.

1.2 Specific hazards

Under normal circumstances, most nuclear power plant buildings are equipped with direct lightning protection devices on the roof, and the DCS system of nuclear power plants is mainly located under the reinforced concrete structure. Therefore, the damage of lightning to the DCS system of nuclear power plants is mostly in the aspects of surge intrusion and electromagnetic induction. The specific hazards of lightning to the DCS system of the nuclear power plant: First, if the thundercloud passes through the DCS control room of the nuclear power plant, a large amount of electric charge released between the thunderclouds will form lightning. The electromagnetic radiation of lightning waves will cause direct interference to the circuits and electronic components in the DCS control room, and may cause damage in severe cases. Second, if there is a lightning strike near the DCS control room, the induced voltage will generate induced voltage on the surrounding lines and pipes. If these overloaded lines and pipes pass through the DCS control room, their electronic components will be damaged. Third, if the lightning protection device on the roof of the building where the DCS control room is located is damaged by lightning, the instantaneous lightning current will also affect the electronic instrument information system of the entire building including the DCS control room.2. Main points of lightning protection design of DCS system in nuclear power plant

2.1 Design points of direct lightning protection

In the design of lightning protection projects for nuclear power plants, lightning protection technicians should fully consider the key points of the direct lightning protection design of the DCS system, establish and improve the external lightning protection system, and lay out lightning termination devices in accordance with the "Lightning Protection Design Code for Buildings". Make sure that direct lightning strikes do not harm buildings and DCS systems.

In addition, in order to better prevent lightning from directly attacking the DCS system or the cables related to the building where the DCS system is located, it is necessary to carefully analyze the actual situation, and try to choose to build a DCS system.

The control room is arranged in the nearby high-rise building area, and scientific and reasonable direct lightning protection measures are taken.

2.2 The main points of lightning protection design of signal transmission lines and communication cables

If the signal transmission line and the communication cable are connected outdoors (such as the DCS signal detection circuit), the lightning electrode is likely to generate a strong surge in the circuit and enter the room under thunderstorm weather. Therefore, if we want to protect this part of the signal circuit, we need to fully consider the connection method of the cable and the operating frequency, operating voltage and load current of the circuit signal, and use a series-type signal surge protector (SPD) to protect the circuit, so that It not only ensures the normal and safe operation of the DCS system, but also simplifies the operation of the project. Communication transmission lines are generally installed outdoors at high altitudes, so wide, tall and low-voltage lightning protection devices are required. Signal transmission lines need to be equipped with high-energy, easy-to-install lightning protection devices [2]. Also, run shielded wires at signal points between crossover devices and equipotentially connect them to the DCS system ground.

2.3 Design points for eliminating ground loops

2.3.1 Equipotential bonding

Equipotential bonding is to connect the exposed meta

l and electrified parts of various electrical devices and other devices in the building with artificial or natural grounding bodies and conductors to reduce the potential difference. Nuclear power plant DCS

The parts of the system that need to be equipotentially connected include "three units" and "five cabinets".

"Three" mainly refers to the operating table, the printing table and the manual operating table with the shielded grounding busbar. "Five cabinets" refers to server cabinets, relay cabinets, UPS

Cabinets, power distribution cabinets, and instrument cabinets with shielded grounding busbars.

2.3.2 common ground

The grounding method often used in the nuclear power plant DCS system is to share the grounding grid. This kind of grounding mainly involves lightning protection grounding and safety grounding. Lightning protection grounding mainly includes two points. First, the lightning protection grounding of DCS systems and instruments should be shared with the electrical professional lightning protection grounding system, but not with independent lightning protection equipment. Second, after the DCS system, instruments and signal lines enter the room from the outdoors, if a lightning protection grounding connection needs to be established, the lightning protection grounding connection should be implemented. The grounding specially provided for electrical equipment and personal safety is the safety grounding (ie protective grounding) of DCS system relay cabinets, instrument cabinets, power distribution cabinets, operation consoles and cabinets and other electrical equipment, and live metal parts are extremely vulnerable to lightning strikes ( or insulation damage), and may be accompanied by high voltages, so all parts must be safely grounded.

2.4 Key points of safe space design

The design of the safety space mainly involves the safe spacing of the system cables and the placement space of the instrument and equipment. Reasonable wiring and shielding measures should be used to meet the actual requirements of the safety space. In order to minimize the surge formed by the lightning electromagnetic pulse in the electronic information system, it is necessary to take relevant measures such as reasonable circuit layout, building and equipment shielding. The reasonable layout of cables mainly involves two aspects of work: on the one hand, when the cable is routed towards the layout, it is necessary to reduce the electromagnetic induction loop brought by the cable itself; on the other hand, the cable is best placed in metal Pipes, metal parts close to the equipotential bonding network, or wiring ducts, but should keep a certain distance from the shielding layer of the lightning protection zone.

3 Conclusion

To sum up, as a key part of a nuclear power plant, once the DCS system is attacked by lightning, it will inevitably lead to instrument failures, which will lead to the inability of the nuclear power plant to operate safely and stably. Therefore, this paper focuses on the key points of the lightning protection design of the DCS system of the nuclear power plant, and starts to defend the DCS system of the nuclear power plant from various aspects such as direct lightning strike and induction lightning protection, and builds a perfect lightning protection system to better ensure the safe operation of the nuclear power plant DCS system.