The reliability of instrumentation and control systems directly affects the safe and stable operation of modern industrial production plants, and the anti-interference capability is the core factor ensuring stable system operation. With the wide application of DCS and fieldbus technologies, controlled objects and measured signals are often scattered across various locations, with long distances separating them from control stations, resulting in long signal and control cables on site.
In addition, numerous high-power electrical equipment on-site generate severe interference to measurement and control systems during startup and operation. Meanwhile, space radiation interference and external lead wire interference are particularly prominent. Apart from valid signals, irrelevant currents and voltages inevitably exist due to various factors, which are collectively referred to as interference (noise). If such interference is not properly eliminated during measurement, it will distort test results and even render instruments or computers completely inoperable in severe cases.
Practice has proven that anti-interference performance is critical for all electronic measuring devices. Especially with the rapid development and popularization of DCS and fieldbus technologies, effective elimination and suppression of various interferences have become an urgent research topic. Interference can trigger logic disorder, cause malfunction of system measurement and control, reduce product quality, damage production equipment and even lead to safety accidents. Therefore, sufficient attention must be paid to anti-interference technologies in the design, manufacturing, installation and daily maintenance of instrument measurement and control systems.
Common Interference Sources and Their Impacts on Systems
Most measurement and control signals are weak DC signals or slowly varying signals. Space radiation interference affects systems mainly in two ways: direct internal radiation to computers inducing circuit interference; and radiation to peripheral equipment and communication networks introducing interference via induction of peripheral devices and communication lines.
Transmission interference mainly includes two categories: power grid interference coupled through sensor power supplies or shared instrument power supplies; and interference caused by electromagnetic radiation induction on signal lines, which may lead to component damage, logic errors and major system failures.
Grounding system interference primarily stems from chaotic grounding design, including noise coupling interference generated by shielding ground wires, chassis ground wires, signal ground wires, power ground wires and AC power ground wires of measurement and control systems.
In summary, three essential conditions are required for interference sources to generate interfering currents or voltages in measuring and detection systems: noise sources, noise-sensitive receiving circuits, and transmission paths between noise sources and receiving circuits.
General Anti-Interference Technologies
Targeting the three essential conditions of noise interference, corresponding solutions are adopted: eliminating or restraining noise sources, cutting off interference transmission paths, and reducing the noise sensitivity of receiving circuits. The above measures belong to hardware anti-interference methods.
With the widespread application of microcomputers, intelligent sensors and smart instruments, software-based interference suppression methods such as digital filtering and digital signal processing have been widely adopted, greatly improving the operational safety of instrument measurement and control systems.
Common anti-interference means include isolation, shielding, suppression, grounding protection and software technologies, which are elaborated as follows:
Isolation covers two aspects: reliable insulation to prevent leakage current between wires by complying with specified voltage resistance grade and insulation resistance standards; and reasonable wiring routing to keep signal lines away from interference sources. For instance, maintain safe spacing between power lines and signal lines during parallel laying, arrange cross wiring vertically as much as possible, separate power cables and signal cables into different conduits, avoid laying signal lines with different amplitude levels in the same pipeline, isolate different-level cables from power lines with metal partitions in trunking, and refrain from mixing different-amplitude signal wires in one multi-core cable.
Shielding and suppression enclose sensitive components, assemblies, circuits and signal lines with metal conductors to restrain current noise coupling and achieve magnetic shielding effect. Using twisted-pair cables instead of parallel wires is also an effective way to suppress magnetic field interference.
Grounding protection ensures equipment and personal safety as well as interference suppression, classified into shielding grounding, intrinsic safety grounding, protective grounding and signal loop grounding.
Protective grounding: reliably connect non-live metal parts of electrical equipment and instruments to grounding bodies to release short-circuit current safely.
Working grounding: guarantee accurate and stable operation of instruments, including signal loop grounding, shielding grounding and intrinsic safety instrument grounding.
Software Anti-Interference Technologies: Hardware measures fail to cope with complex on-site industrial environments such as industrial control computer crashes and control errors, which may cause severe production losses. Hence software anti-interference methods are essential, mainly including self-monitoring, mutual monitoring of real-time control systems and key data backup technologies.
Practical Application of Anti-Interference Technologies
1. Technical Renovation to Eliminate System Interference and Restore Unit Interlock Protection
The designed 33t/h granulator is a large-scale domestic extrusion unit equipped with 294 interlock and alarm monitoring points for temperature, pressure, flow and vibration to ensure safe shutdown under abnormal conditions. Due to inadequate design experience, frequent false temperature interlock trips occurred after commissioning, forcing temporary cancellation of temperature interlocks while retaining alarms. This brought huge production pressure and potential safety hazards.
After systematic analysis, the root causes were confirmed: excessive control cabinet temperature, poor shielding of control cables, plastic-shell secondary instruments powered by 220V AC, leading to severe interference on weak thermocouple signal circuits.
Optimization solutions implemented:
Replace thermocouples with thermal resistors to enhance signal anti-interference capability.
Adopt shielded control cables to reduce signal interference.
Change power supply of secondary indicating instruments from 220V AC to 24V DC to lower cabinet temperature and signal disturbance.
Install exhaust fans inside control cabinets for heat dissipation.
Standardize construction procedures to ensure installation quality.
After renovation, all interlock functions of the unit were fully restored after one-year suspension, ensuring stable operation of core large-scale production units.
2. Isolation & Shielding Technologies Ensure Stable DCS Operation and Eliminate Hidden Dangers
The 200,000-ton annual output polypropylene plant adopts advanced imported gas-phase bulk polymerization technology with numerous advanced control loops in its instrument control system. Frequent instrument false trips and DCS card damage occurred in the initial commissioning stage.
Investigation showed insufficient shielding and isolation measures for electric-control cables caused DI cards of DCS to induce 170~200V stray voltage, triggering logic misoperation and plant shutdown. By installing isolation relay cabinets for over 30 key signal circuits and renewing shielded control cables, the problem was thoroughly solved.
3. Reduce Remote Communication Control to Eliminate Signal Disturbance
A key compressor control system consists of local and central control panels, with anti-surge control valves supporting remote automatic/manual mode switching via long-distance communication. Stable during commissioning, the valves frequently switched to manual mode automatically after formal production due to severe on-site signal interference, resulting in repeated interlock shutdowns.
Cable laying irregularities in cable trays were confirmed as the main interference source. To solve the problem, remote communication control functions were relocated from local panels to central control cabinets. The reform achieved remarkable results with no more interference-induced shutdown accidents. This optimized design was also applied in new project design, supporting successful one-time commissioning of the new plant.
