At present, various interferences exist in various industrial sites, so the reliability of instruments and control systems directly affects the safe and stable operation of modern industrial production equipment. The anti-interference ability of the system is the key to the reliable operation of the entire system. With the application of DCS and fieldbus technology, the controlled objects and measured signals are often distributed in different places, and there is also a considerable distance between them and the control station. Therefore, both signal lines and control lines may be long lines. Secondly, there are often many high-voltage devices on site, and their start-up and operation will have a strong impact on the measurement and control system. At the same time, issues such as radiation interference from space and interference from external system leads are particularly prominent. Therefore, in addition to useful signals, there will inevitably be some currents or voltages that are unrelated to the measured signal due to various reasons. These unrelated currents or voltages are commonly referred to as? quot; Interference (noise). During the measurement process, if these interferences are not handled properly, they will distort the measurement results and, in severe cases, even render the instrument or computer inoperable. A large amount of practice has shown that anti-interference performance is a very important issue for various electronic measurement devices, especially with the widespread application and rapid development of DCS and fieldbus technology. Effectively eliminating and suppressing various interferences has become an urgent problem that must be explored and solved, because interference can not only cause logical confusion, make system measurement and control failures, reduce product quality, but also damage production equipment and cause accidents. Therefore, anti-interference technology must be given sufficient attention in the design, manufacturing, installation, and daily maintenance of instrument measurement and control systems.
　　
Common sources of interference and interference to the system:
　　
Due to the fact that measurement and control signals are often weak DC or slowly changing radiation interference from space, their impact on measurement and control systems mainly occurs through two paths: one is direct radiation to the computer's internal environment, which is induced by circuits and causes interference; The second is the radiation to computer peripheral devices and communication networks, which introduces interference through the induction of peripheral devices and communication lines.
　　
There are two main ways of interference from transmission: one is interference from the power supply of sensors or the power supply of public signal instruments, that is, interference from the power distribution equipment in series with the power grid; The second is the interference caused by electromagnetic radiation induction in space on signal lines, which can lead to component damage, logic errors, and major system failures in severe cases.
　　
The interference of the self grounding system is mainly caused by the disorder of the grounding system: noise coupling interference caused by the shielding grounding wire, chassis grounding wire, signal grounding wire, power grounding wire, AC power grounding wire, etc. of the measurement and control system.
　　
In summary, we can conclude that various interference sources (noise sources) can cause interference currents (voltages) to measuring devices and detection systems, and three elements need to be present simultaneously: (1) noise sources; ⑵ Noise sensitive receiving circuits; The transmission path between the noise source and the receiving circuit.
　　
Universal anti-interference technology:
　　
Since the formation of noise interference on measuring devices and detection systems requires the 'three elements', methods for eliminating and reducing noise interference should also take measures targeting the three factors, namely: (1) eliminating or suppressing noise sources; ⑵ Block the transmission path of interference; ⑶ Weaken the sensitivity of the receiving circuit to noise interference. The above three measures are all hardware measures. With the widespread application of microcomputers in industrial production, intelligent sensors, and smart instruments, more measures and methods to suppress interference such as digital filtering and digital processing have been applied in software, greatly improving the safety level of instrument measurement and control systems. The above measures are usually implemented through isolation, shielding, suppression, grounding protection, and software technology. The following is an introduction to these technologies.
　　
Isolation has two meanings: one is reliable insulation, which ensures that there is no leakage current between wires, so the withstand voltage level and insulation resistance of the wire insulation material must comply with regulations; Another is reasonable wiring, which requires signal lines to avoid interference sources as much as possible. For example, when power lines and signal lines are laid in parallel, they must maintain a certain distance, and when they intersect, they should be as perpendicular as possible. When wires are laid in conduits, power lines and signal lines should be in different conduits. Signal wires with different radiation values should not be threaded in the same conduit. When using metal cable trays for laying, wires, cables, and power lines with different radiation values need to be separated by metal partitions. It is not advisable to have signal lines with different radiation values within the same multi-core cable.
　　
Shielding and suppression are the use of metal conductors to surround shielded components, assemblies, circuits, and signal lines, mainly used to suppress current induced noise coupling and provide a certain magnetic shielding effect. Another effective method to suppress magnetic field interference is to use twisted pair cables instead of two parallel wires.
　　
Grounding protection refers to protecting equipment and personal safety and suppressing interference through grounding. It is usually divided into shielding grounding, intrinsic safety grounding, protective grounding, and signal circuit grounding. The following introduces them respectively: ⑴ Protective grounding is a good metal connection between the metal parts of electrical equipment and electrical instruments that are not charged under normal circumstances and the grounding body. If the instrument panel is accidentally charged, the grounding short-circuit current mostly passes through the grounding resistor; ⑵ Working grounding is to ensure the reliable and normal operation of instruments, including signal circuit grounding, shielding grounding, and intrinsic safety instrument grounding.
　　
Software anti-interference technology: Hardware anti-interference measures are powerless in complex industrial environments, such as industrial computer crashes or control errors. This will have terrible consequences for production, so it is still important to use software anti-interference measures to avoid and mitigate these accidents. The commonly used software anti-interference techniques include self-monitoring during real-time control software operation, mutual monitoring of real-time control systems, and backup of important data.
　　
Application of anti-interference technology in practical work:
　　
1. Implement technological transformation, eliminate system interference, and restore key unit interlocking
　　
The design of a 33 ton/hour granulator is the largest extrusion unit in China, with 294 interlocking and alarm control points for temperature, pressure, flow rate, vibration, etc. Once an abnormality occurs, the interlocking control program ensures the safe shutdown of the unit. However, due to the lack of experience and poor consideration of the design personnel, multiple temperature interlock control malfunctions occurred after the unit was started, causing the extruder to stop and putting great pressure on the production of the device. After being handled by technical personnel, some of the misoperation points were released, but the temperature interlock point had to temporarily cancel the interlock and keep the alarm due to frequent misoperation. Although the unit can operate, the temporary removal of 31 interlocking points has brought great pressure and safety hazards to production, and frequent temperature false alarms have caused significant mental pressure on operators. Therefore, finding a solution as soon as possible and restoring interlocking has been presented to the leaders and technical personnel of the business unit. By organizing technical personnel to analyze the design, construction, and operation status of the control system. It was found that the design was not well considered, with the control panel temperature being too high, the control cable shielding being poor, and the secondary instrument being a plastic shell with 220VAC power supply, causing severe interference with the M signal of the thermocouple measurement circuit, which originally had weak signals, resulting in frequent misoperations of the secondary instrument. After analyzing the problem clearly, we can solve it by improving the system.
　　
⑴ Change the detection element from thermocouple to thermistor to enhance the signal anti-interference ability.
　　
⑵ Change the control cable to a shielded cable to reduce signal interference.
　　
⑶ Change the secondary control indicator from 220VAC power supply to 24VDC power supply to reduce control panel temperature and signal interference.
　　
⑷ Add exhaust fans to the control panel to lower the temperature.
　　
⑸ Organize construction strictly and meticulously to ensure quality.
　　
Through the above work, the interlocking control of the unit was fully put into operation after one year of shutdown, playing a key role in the stable operation of the company's large-scale units.
　　
2. Adopting isolation and shielding anti-interference technology to ensure the stable operation of DCS system and eliminate production hazards
　　
The annual production capacity of 200000 tons of polypropylene plant is the most advanced gas-phase bulk production process introduced in China, with advanced instrument measurement and control systems
　　
The control circuit accounts for a large proportion. However, due to factors such as design and construction, frequent instrument malfunctions and shutdowns occur when the device is first started, which has a negative impact on production. DCS cards are also damaged from time to time. Analysis shows that the shielding and isolation measures of the electrical to instrument control cables in the system are poor, causing the DI cards of the DCS to occasionally sense voltages of 170~200V, leading to DCS logic malfunctions and even device shutdowns. In response, isolation relay panels were added to over 30 sets of signal lines that severely affected production, and shielded control cables were replaced, achieving significant results.
　　
3. Reduce remote communication transmission control and eliminate signal interference
　　
The key compressor control system of a certain device is designed with a field control panel and a central control room control panel. The central control room control panel is mainly used for compressor control safety interlock logic. In addition to being used for on-site start-up operations, the field control panel also has some manual control functions. Among them, the compressor anti surge control valve can switch between manual and automatic control modes on site. This part of the control is connected between the main control room and the site through remote communication transmission, and everything is normal during the debugging phase. However, after the device was driven, due to severe signal interference, the anti surge control valve of the compressor occasionally switched to manual mode on its own, and there was no one operating it on site, causing multiple interlocking shutdowns. After updating the communication signal line, the situation slightly improved, but occasionally signal interference occurred, causing interlocking parking. After multiple analyses by technical personnel, it is believed that due to various reasons, the cable tray control cable laying of the main device is not standardized enough, resulting in significant interference and causing the communication signal of the device to not work well. In response to the actual situation, the interference in the control signal of the compressor mainly affects the switching mode of the anti surge control valve. During normal driving, we choose to automatically control the opening of the anti surge control valve. In case of abnormalities, we manually control the opening of the anti surge control valve on site. After careful consideration, it has been proven that moving the communication control part of the on-site panel to the main control panel is safer and more reliable. The implementation has yielded good results, and there have been no more incidents of signal interference causing compressor shutdown accidents. When designing the compressor control for the new installation, we suggested that the foreign party adopt this solution, which has achieved good results. With everyone's joint efforts, we have achieved a good performance of successful start-up of the installation.