Interference problems and treatment methods in the

2022-08-17
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At present, all kinds of interference exist in all kinds of industrial sites, so the reliability of instrument and control system directly affects the safe and stable operation of modern industrial production devices, and the anti-interference ability of the system is the key to the reliable operation of the whole system. With the application of DCS and fieldbus technology, the controlled objects and measured signals are often distributed in different places, and there is a considerable distance between them and the control station. Therefore, the signal line and control line may be long lines. Secondly, there are often many strong current devices on site, and their startup and work will have a strong impact on the measurement and control system. At the same time, the radiation interference from space and the interference of external leads of the system are particularly prominent. Therefore, in addition to useful signals, for various reasons, there are bound to be some currents or voltages unrelated to the measured signal. This irrelevant current or voltage is commonly known as? quot; Interference "(noise) 。 In the process of measurement, if these interferences are not handled well, it will distort the measurement results, and even make the instrument or computer completely inoperable. A lot of practice shows that anti-interference performance is a very important problem for all kinds of electronic measuring devices, especially the wide application and rapid development of DCS and fieldbus technology. Effectively eliminating and suppressing all kinds of interference has become an urgent problem that must be discussed and solved, because interference can not only cause logical confusion, make system measurement and control failure, so as to reduce the quality of products, even damage production equipment and cause accidents. Therefore, anti-interference technology must be given enough attention in the design, manufacture, installation and daily maintenance of instrument measurement and control system

common interference sources and interference to the system:

because the measurement and control signal is often a weak DC or slowly changing radiation interference from space, the impact on the measurement and control system mainly through two paths: first, direct radiation to the internal computer, resulting in interference by circuit induction; The second is the radiation to the computer peripheral equipment and communication network, and the interference is introduced by the induction of peripheral equipment and communication lines

there are two main ways of interference from transmission: one is through the sensor power supply or the power supply of public signal instruments, that is, the electrical interference connected in series by the distributor; Second, the signal line is interfered by the induction of space electromagnetic radiation, which will cause component damage, logic error and major system failure in serious cases

the interference of the self grounding system is mainly the confusion of the grounding system: the noise coupling interference caused by the shielding grounding wire and chassis grounding wire, signal grounding wire, power grounding wire and AC power grounding wire of the measurement and control system

to sum up, we can conclude that various interference sources (noise sources) produce interference current (voltage) to the measuring device and detection system, which need to have three elements at the same time: ⑴ noise source; ⑵ receiving circuit sensitive to noise; ⑶ transmission path from noise source to receiving circuit

general anti-interference technology:

since the formation of noise interference to measuring devices and detection systems requires "three elements", the method of eliminating and weakening noise interference should also take measures against three factors, namely: ⑴ eliminating or suppressing noise sources; (2) block the interference transmission path; ⑶ weaken the sensitivity of the receiving circuit to noise interference. The above three measures are hardware measures. With the application of Microcomputer in industrial production and the universal application of intelligent sensors and intelligent instruments, more interference suppression measures and methods such as digital filtering and digital processing have been applied in software, and the safety level of instrument measurement and control system has been greatly improved. The above measures are usually completed by isolation, shielding, suppression, grounding protection and software technology. These technologies are introduced below

isolation has two meanings: one is reliable insulation, that is, to ensure that there will be no leakage current between wires, so it is required that the withstand voltage grade and insulation resistance of wire insulation materials must meet the requirements; The other is reasonable wiring, that is, the signal line is required to avoid interference sources as much as possible. For example, when the power line and the signal line are laid in parallel, they must maintain a certain distance, and when they cross, they must be as vertical as possible. When the wire is laid through the pipe, the power line and the signal line should be in different conduits. Signal wires with different signal radiation values should not be threaded in the same conduit. When metal trunking is used for laying, conductors, cables and power lines with different radiation values shall be separated by metal partitions. There should not be signal wires with different radiation values in the same multi-core cable

shielding and suppression is to use metal conductors to surround the shielded components, assemblies, circuits and signal lines. It is mainly used to suppress the coupling of current noise and play a certain role of magnetic shielding. In addition, when aging experiments are carried out, using twisted pair instead of two parallel lines is an effective method to suppress magnetic field interference

grounding protection refers to the protection of equipment and personal safety and suppression of interference through grounding. Generally, it is divided into shielding grounding, intrinsically safe grounding and protective grounding. Everyone is producing wire rod recycling machine grounding and signal circuit grounding. The following are introduced respectively: ⑴ protective grounding is to make a good metal connection between the uncharged metal parts of electrical equipment and electric instruments under normal conditions and the grounding body. If the instrument panel is accidentally charged, most of the grounding short-circuit current passes through the grounding resistance; ⑵ working grounding is to ensure the accurate and reliable normal operation of the instrument, including signal circuit grounding, shielding grounding and intrinsically safe instrument grounding

software anti-interference technology: the hardware anti-interference measures in the complex environment of the industrial site are powerless, such as the industrial computer crashed or the control went wrong. This will bring terrible consequences to production, so it is important to use software anti-interference measures to avoid and mitigate these accidents. Commonly used software anti-interference technologies include: self-monitoring method in the operation of real-time control software, mutual monitoring method of real-time control system and important data backup method

application of anti-interference technology in work practice:

1. Implement technical transformation, remove system interference, and restore key unit interlocking

the design of 33 ton/hour granulator is the first large-scale extrusion unit in China, and 294 interlocking and alarm control points such as temperature, pressure, flow and vibration are designed. Once abnormalities occur, the interlocking control program ensures the safe shutdown of the unit. However, due to the inexperience of designers and poor consideration, after the unit was started, the temperature interlock control malfunction occurred many times, causing the extruder to stop, which brought great pressure to the production of the device. After the technicians' treatment, some misoperation points were removed, but the temperature interlock points had to temporarily cancel the interlock and retain the alarm due to frequent misoperation. Although the unit can be started, the temporary removal of 31 interlocking points brings great pressure and potential safety hazards to production, and temperature false alarms occur frequently, causing great ideological pressure on operators. Therefore, it is in front of the leaders and technicians of the division to find a solution as soon as possible and restore the interlocking. By organizing technicians to analyze the design, construction and operation of the control system. It was found that the design was not well considered, the temperature of the control panel was high, the control cable was not well shielded, and the secondary instrument was plastic shell and 220VAC power supply, so that the MV signal of the thermocouple measurement circuit of the primary detection element with weak signal interfered seriously, causing frequent maloperation of the secondary instrument. After the problem is analyzed clearly, we can solve it by improving the system

⑴ change the detection element from thermocouple to thermal resistance to enhance the signal anti-interference ability

⑵ change the control cable into shielded cable to reduce signal interference

⑶. The secondary control indicator is changed from 220VAC power supply to 24VDC power supply to reduce the control panel temperature and signal interference

⑷. The specific characteristics of the control electronic tensile testing machine are as follows: the disk increases the exhaust fan and reduces the temperature

(5) strictly organize the construction to ensure the quality

through the above work, the interlock control of the unit can be put into use after one year of shutdown, which plays a key role in the safe and stable operation of company level large units

2. Adopt isolation and shielding anti-interference technology to ensure the safe and stable operation of DCS system and eliminate production hidden dangers. The polypropylene plant with an annual output of 200000 tons is the first set of gas-phase bulk production process introduced from the world in China, and the advanced control circuit accounts for a large proportion in the instrument measurement and control system. However, due to the influence of design, construction and other factors, when the device was just started up, the instrument misoperation and shutdown occurred frequently, which brought adverse effects to the production, and the DCS card was also damaged from time to time. After analysis, the shielding and isolation measures of the electrical and instrument control cables of the system were poor, causing the di card of DCS to sense the voltage of 170~200v from time to time, causing DCS logic misoperation, and even causing the device shutdown. In this regard, more than 30 sets of signal lines seriously affecting production were added with isolation relay panels and shielded control cables were replaced, and obvious results were achieved

3. Reduce remote communication transmission control and eliminate signal interference

the key compressor control system of a 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 field start-up operation, the field control panel is also designed with some manual control functions, including the compressor anti surge control valve, which can switch between manual and automatic control modes on site, This part of control realizes the connection between the main control room and the site through remote communication transmission, and everything is normal in the commissioning stage. However, after the device was started, due to serious signal interference, the anti surge control valve of the compressor was switched to manual mode from time to time, and no one operated on site, causing interlocking shutdown for many times. After that, the communication signal line was updated, and the situation improved slightly, but occasionally there was signal interference, resulting in interlocking shutdown. After many times of analysis by technicians, it is believed that the communication signal of the main device cannot work well due to various original national quality supervision departments' finding in the supervision of waterproof materials that the cable tray control cable laying is not standardized and there is a large interference. According to the actual situation, the interference of the compressor control signal mainly affects the switching mode of the anti surge control valve. During normal start-up, we choose to automatically control the opening of the anti surge control valve, and manually control the opening of the anti surge control valve in case of abnormal operation on site. After careful demonstration, it is safer and more reliable to move the communication control part of the field panel to the main control panel. After implementation, the effect is very good, and there is no signal interference causing compressor shutdown accident again. In the compressor control design of the new plant, we suggest the foreign party to adopt this scheme, which has achieved good results. With the joint efforts of everyone, we have achieved a good performance of successful start-up of the plant at one time

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