Lightning is a natural discharge phenomenon with immense destructive power, posing a significant threat to human life and property. Thermal control instruments and distributed control systems (DCS) based on large-scale microelectronic devices, in particular, have low withstand voltage and are especially sensitive to electromagnetic pulses. The electromagnetic pulses generated by lightning, through electromagnetic induction and current waves, can cause overvoltages of hundreds, thousands, or even tens of thousands of volts, increasing the likelihood of lightning-induced interference and damage to instruments and control systems. In recent years, several lightning strikes in Anhui Province have led to malfunctions in thermal control protection units, causing significant economic losses to power plants. [b]1 Two Cases of DCS Lightning Strike Accidents[/b] 1.1 Unit #2 of a Power Plant Tripped Due to Lightning Strike On August 4, 2004, Unit #1 (300MW) of a power plant tripped at 0:59, triggering the A induced draft fan to activate the auxiliary equipment fault load reduction function (RB), resulting in a forced load reduction of the unit; Unit #2 tripped at 1:02 due to the boiler MFT activation caused by "high furnace pressure". Analysis determined that the accident was caused by strong lightning activity in the area surrounding the chimney around 1:00 AM on August 4th, with a large lightning current (the maximum measured by the lightning location system was 197kA). Such a powerful lightning current, when introduced to the ground through the chimney, generated a significant induced electromotive force, interfering with the thermal control systems of the 1A, 1B, 2A, and 2B induced draft fans. This resulted in damage to the temperature elements of the fan bearings, motor bearings, and inlet stator actuator feedback boards of these fans, including varying degrees of damage to the corresponding DCS input cards. Consequently, the induced draft fans and the entire unit tripped. According to design requirements, thermal control signal cables must be grounded at one end. This grounding method provides good shielding against low-frequency signal interference, but its shielding effect against high-frequency signal interference such as lightning is limited. Especially given the intensity of this lightning strike, existing grounding protection measures were insufficient to withstand it. 1.2 A power plant's #7 unit experienced a erroneous MFT (Mechanical Toll Failure) triggered by an electrical oil switch signal. On June 18, 2005, at 23:00, the load of the #7 unit at a power plant was 180MW. AGC and primary frequency regulation were operating normally, all three pulverizing systems were running, and 16 pulverizers were in operation. The weather was stormy. At 23:15:19, the MFT signal was issued. One second later, the main power supply to the pulverizer tripped, and the boiler was shut down. Historical curves and operation records revealed that the cause of the MFT was a 2-second pulse in the electrical oil switch signal (BDI043) at 23:15:17. This 2-second delay directly triggered the MFT (electrical oil switch tripping is one of the conditions for MFT activation). The initial cause indicated a tripped main electrical switch. After electrical inspection, the auxiliary contacts provided by the 2707 switch to the DCS are three normally closed contacts of phases A, B, and C connected in series. Only when all three phases of the switch are disconnected can the "oil switch trip" signal be issued. In fact, it did not occur, and there were no abnormal phenomena. Because the signal cable is long (600-700 meters), there is a coupling capacitance. At 23:15, a strong lightning strike occurred, which induced the signal and caused the false signal (other signals of the same DI card were normal). 2 Pathways of Lightning Intrusion into the Instrumentation and Control System The harm of lightning to the instrumentation and control system of thermal power plants is mainly in the form of direct lightning strikes and lightning electromagnetic pulse interference (also known as lightning waves). The harm to the instrumentation and control system is mainly caused by the following coupling pathways: (1) Lightning backflash caused by ground potential fluctuations due to direct lightning strikes. When the lightning protection device of the control system building is connected to lightning, the strong instantaneous lightning current flows into the grounding device through the down conductor, which will cause the local ground potential to fluctuate and generate step voltage. If the grounding device of the lightning protection is independent and there is not enough insulation distance between it and the grounding body of the control system, a discharge will occur between them. This phenomenon is called lightning backflash, which will interfere with or even damage the DCS system in the control room. (2) When the lightning protection device of the control system building is connected to lightning, a strong instantaneous lightning current will pass through the down conductor. If there are cables (including power, communication and I/O cables) connecting the DCS system within a certain distance around the down conductor, the lightning current in the down conductor will generate electromagnetic radiation on the DCS cable, introducing lightning waves (high potential) into the DCS system, interfering with or damaging the DCS system. (3) When lightning discharge occurs around the control system, the electromagnetic field radiated in the space will generate induced voltage (including electromagnetic induction and electrostatic induction) on various metal pipes and cable lines, thereby causing the instrumentation and control system to fail or be damaged. 3. Main Measures for Lightning Protection of Instruments and DCS Systems The next few years will be the peak period for the construction and commissioning of thermal power units. If lightning protection measures for instruments and DCS are considered in the design stage of thermal control engineering, it will be an effective way to improve the lightning resistance of the instrumentation and control system of thermal power plants. The following points should be fully considered: (1) Grounding The most unclear but necessary problem in DCS application is the grounding problem. Not only are many users unclear, but even some DCS manufacturers may not be clear about it. At present, there are two main grounding measures for thermal instrumentation control systems in thermal power plants: floating ground and multi-point grounding. Floating ground means that the working ground of the instrument is kept insulated from the grounding system of the building. In this way, electromagnetic interference in the grounding system of the building will not be conducted to the instrument system, and changes in ground potential will not affect the instrument system. However, since the outer casing of the instrument needs to be protected by grounding, when the lightning is strong, a very high voltage may appear between the outer casing of the instrument and its internal electronic circuits, which will break down the insulation gap between the two and cause damage to the electronic circuits. Multi-point grounding refers to separating the working ground and protective grounding of equipment such as DCS, instruments (transmitters, actuators, etc.), and PLC. The main advantage of this grounding method is that it allows for proximity grounding and has low parasitic inductance in the grounding wire. However, if a strong lightning surge enters the system through the protective ground, the electronic circuits can still be damaged due to the high voltage. Therefore, neither of the above two grounding methods can meet the needs of lightning protection. Therefore, it is necessary to consider connecting the protective ground to the working ground. That is, the DCS system and its connected transmitters, actuators, etc., must be equipotentially grounded, and the DCS system should be single-point grounded with the common grounding system. In this way, after the DCS system and the lightning protection system are equipotentially connected to the lightning protection grounding system, even if subjected to lightning backflash, because there is no potential difference between them, it is impossible for lightning backflash to pose a threat to electronic components. Equipotential bonding is an important measure to protect the DCS system from lightning strikes. If the DCS system cannot be equipotentially connected to the grounding system of the lightning protection system, according to the "Code for Electrical Design of Civil Buildings" (JGJ/T 16-92) [1], the distance between the two grounding systems should not be less than 20 meters. When designing the project, when a cable is laid close to the down conductor, it is necessary to consider maintaining a distance of more than 2 meters between the cable and the down conductor. The "Code for Lightning Protection of Electronic Information Systems in Buildings" [2] specifies the calculation formula for this distance. The grounding system of the thermal instrumentation control system of the thermal power plant can be designed with reference to Figure 1. [align=center]Figure 1 Grounding of the thermal instrumentation control system of the thermal power plant[/align] (2) Shielding The instrumentation control system of the thermal power plant uses a large number of semiconductor devices, integrated circuits and cables for transmitting signals. The transient electromagnetic pulse generated by lightning can directly radiate to these components, or induce transient overvoltage waves on the power supply or signal lines, which can intrude into the electronic equipment along the lines, causing the electronic equipment to malfunction or be damaged. Using a shield to block or attenuate the energy propagation of electromagnetic pulses is an effective protective measure. Lightning protection shielding for instrumentation and control systems mainly includes three aspects: control room shielding, field instrument shielding, and signal and power line shielding. * **Control Room Shielding:** The control room (electronics room) is the heart of the DCS system and is highly sensitive to electromagnetic pulses generated by lightning, requiring special attention to its shielding. The control room should be a windowless, enclosed structure. Electrical connections should be made at the intersections of the structural steel reinforcement bars in the walls, welded to a metal door frame to form a shielding cage with a door opening. A protective grounding ring (connected to lightning protection ground) should be installed around the perimeter of the walls inside the room, and the grounding ring should be effectively electrically connected to the shielding cage. * **Field Instrument Shielding:** Field instruments (transmitters, actuators, etc.) can be shielded against lightning using metal instrument boxes (covers). The instrument boxes (covers) must be equipotentially connected to other field metal facilities and connected to the lightning protection grounding system. * **Signal and Power Line Shielding:** To prevent transient overvoltage waves induced by lightning electromagnetic pulses on signal or power lines, all signal lines and low-voltage power lines should use cables with a metallic shielding layer. For the shielding grounding of signal cables, the principle is to ground one end and leave the other end suspended. However, single-end grounding can only prevent electrostatic induction (i.e. capacitive coupling) and cannot prevent voltage induced by changes in magnetic field strength (i.e. inductive coupling), and does not help to prevent the intrusion of lightning waves. In order to reduce the induced voltage of the shielding core wire, if only one end of the shielding layer is equipotentially connected, a double-layer shielding with insulation should be used, and the outer shielding should be equipotentially connected at least at both ends (i.e., two-point grounding). In this case, the outer shielding layer and other conductors that are also equipotentially connected form a loop, inducing a current, thus generating a magnetic flux that reduces the source magnetic field strength, which can basically cancel the voltage induced when there is no outer shielding layer (as shown in Figure 2). For this purpose, metal cable trays or metal pipes can also be used as the second shielding layer and grounded at both ends. For control cables that transmit analog signal loops and coaxial cables whose shielding layer is the signal return loop, the shielding layer should be grounded at one point and should not be grounded at two points [3]. [align=center]Figure 2 Lightning protection principle of double shielding[/align] When the cable shielding layer adopts single-point grounding, its grounding point should be determined according to whether the signal source and the receiving end are grounded; when choosing two-point grounding, it should be considered that the cable shielding layer will not be burned under the action of transient current. From the perspective of lightning protection, the cable tray should be made of metal material and should not be made of epoxy resin; in addition, for external cables, such as I/O cables, power cables, and communication cables, the outdoor laying section should be buried as much as possible to form line shielding and reduce the possibility of lightning strikes. (3) The power system of DCS should adopt the grounding method of TN-S system to ensure that the metal shell of the control system (such as the cabinet) is not charged during normal operation. The TN-S system has five wires, namely three phase wires A, B, C, one neutral wire N and one protective wire PE. Only one point of grounding of the power system is used, and the exposed conductive parts of the electrical equipment are connected to the PE line. The TN-S system is characterized by the fact that the neutral line N and the protective earth line PE are grounded together at the neutral point of the transformer, and there is no electrical connection between the two lines. The neutral line N is energized, while the PE line is not energized. This grounding system has a safe and reliable reference potential. Its advantage is that the PE line does not present current during normal operation, so the exposed conductive parts of the equipment do not present voltage to ground, and it has strong electromagnetic adaptability. (4) When designing the location of cable trays and control cabinets, try to avoid proximity to the down conductors of the building's direct lightning protection device. The control cabinet and operating station should also maintain a certain distance from windows and doors. (5) Based on practicality, the possibility and consequences of lightning strikes, surge absorbers (SPDs) should be reasonably configured in necessary places. An SPD is a device that limits transient overvoltages and diverts surge currents. It releases the surge current generated by induced lightning on the line to the ground grid in the shortest possible time, so that the potential difference between points in the building remains approximately unchanged, thereby protecting the equipment. (6) When selecting a DCS, its electromagnetic compatibility (EMC) indicators must be considered, especially surge immunity and pulse magnetic field immunity. In addition, the grounding system must be regularly maintained in accordance with the relevant regulations and standards of the power industry and other industries, especially before each year's lightning strike: • Inspect the integrity and grounding resistance of the entire plant's grounding network, such as the equipotential bonding and shielding of the DCS system, confirming that the grounding and shielding of the DCS system meet design requirements, and whether the distance between the underground portion of the chimney grounding and the main grid grounding meets design requirements. For power plants that have been in operation for many years, it is necessary to conduct risk assessments of the grounding network's lifespan and DCS lightning damage, and promptly rectify any problems found. • Carefully inspect the grounding status of the housings, shielded cables, and cable trays of outdoor-installed control equipment (transmitters, actuators, level gauges, pressure switches, etc.), and strictly prevent abnormal situations such as loose grounding cables, poor connections, detachment, and excessive grounding resistance. • Improve the protection control logic to minimize the possibility of protection malfunctions. For example, in a power plant's #7 unit, the DCS and DEH signals were taken from different contacts of switch 2707. DEH was not interfered with, but DCS experienced interference and a MFT (Mean Time Trip). Therefore, another oil switch signal could be introduced from DEH, and the two signals could be ANDed before entering the MFT. The delay module in the logic of the electrical oil switch tripping the MFT was changed from 2 seconds to 3 seconds. 4. Conclusion To prevent or reduce the failure and damage of field instruments and DCS systems caused by lightning strikes, and to ensure the reliable and stable operation of DCS and units, the entire instrumentation and control system should be designed according to the principle of equipotential bonding, while adhering to relevant national and industry standards. This requires comprehensive consideration of multiple aspects, including the control room, DCS I/O modules, field instruments, instrument signal cables, and power lines. Various measures such as lightning protection, voltage equalization, grounding, and shielding should be adopted. Collaboration among electrical, architectural, and thermal control professionals is also necessary. In addition to system safety, investment costs and operational economics must also be considered to achieve safety, reliability, advanced technology, and economic rationality. [b]References:[/b] [1] JGJ/T 16-92. Code for Electrical Design of Civil Buildings [S]. [2] GB 50343-2004. Code for Lightning Protection of Electronic Information Systems in Buildings [S]. [3] Tian Haibin. Discussion on Grounding Method of Control Cable Shielding Layer [J]. Shanxi Electric Power, 2004, (2): 66-67. [4] DL/T 5190.5-2004. Technical Specification for Construction and Acceptance of Power Engineering Part 5: Thermal Automation [S]. [5] GB 50093-2002. Code for Construction and Acceptance of Automation Instrumentation Engineering [S]. [6] HG/T 20513-2000. Regulations for Grounding Design of Instrumentation Systems [S]. [7] Zhang Qingchao. Lightning Strike and Protection of Distributed Control Systems [J]. Chemical Automation and Instrumentation, 2004, 31 (3): 67-68. Editor: He Shiping