Analysis of DCS Signal Interference Causes and Their Suppression

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With the development of China’s industry, the demands for higher levels of automation in industrial control systems are also increasing. To meet these requirements for automated control, DCS systems have been widely used in industrial automation. Therefore, the reliability of the DCS control system has a direct impact on the safe production and economic operation of industrial enterprises, among which the system’s resistance to interference is key to its reliable operation. Currently, most of the control systems used in automated systems operate in a harsh electromagnetic environment created by high-voltage circuits and high-voltage equipment. At construction sites, interference often occurs when electrical equipment, instruments, DCS control systems, and various DCS systems transmit signals to each other, which can lead to system instability or even incorrect operations; this issue must be given proper attention. —— What are the main sources of interference? Resistance coupling interference occurs during operation because many factories pay little attention to the problem of wire aging. When multiple signal lines transmit signals simultaneously, leakage current caused by the aging of the insulating material on the wires can affect other transmitted signals, interfering with them. In addition, not only does the aging of signal wires affect other signal wires, but aging of wires in high-power circuits (such as heat-generating circuits) can also cause significant interference with signals during use. Capacitive and inductive coupling interference: Usually, the control cabinets in the controlled facility are equipped with numerous signal wires; some of these wires are housed in cable trays while others are placed in cable ducts. Especially when multiple signals are wired simultaneously, the presence of capacitive coupling between them can cause interference with other signal wires. In particular, alternating magnetic flux is generated around the signal lines, which in turn creates a potential difference between the parallel conductors, resulting in potential interference signals. Lightning interference: When a lightning strike occurs, significant electromagnetic interference may arise around the desired signal; of course, the pathways through which this interference is introduced can also involve various grounding wires to some extent. There are mainly 2 types of lightning interference: one is overhead power lines, which cause the signal lines to be struck by lightning and thus experience interference ； Another scenario is when a lightning strike hits around the signal cable, resulting in capacitive and inductive coupling along the signal line; this in turn creates significant interference, which can damage equipment in severe cases and even lead to accidents involving people. Power supply line interference: Due to the frequent start-up and shutdown of large-scale equipment in power plants, as well as the frequent switching of large components, instantaneous large alternating magnetic fields are generated when these large motor devices are switched on and off. Alternating magnetic fields can couple to the signal lines, thereby causing interference in the system; these alternating magnetic fields also generate high-frequency interference in the power supply lines. If the level of such interference exceeds the specified limits, it can have an impact on the system as well. To protect the system when switching large motors, a transformer can be incorporated into the circuit to ensure that excessive voltage changes do not generate an alternating magnetic field during motor switching. —— The main cause of interference is material and equipment-related issues. Control signals fed into a DCS system are typically transmitted to the system using cables as the transmission medium. When the insulation material of these signal cables ages or the shielding material is damaged, they can be affected by other electromagnetic interference sources. Such interference can lead to measurement errors, and in severe cases, it can cause damage to the equipment. Irregular construction: In DCS control systems, many signals often enter the DCS at the same time. These signal wires run either in cable trays or in cable ducts, resulting in many different types of cables being installed along the same path on site. There is distributed capacitance between these signals, and interference is introduced onto other signal lines through this distributed capacitance. At the same time, alternating magnetic flux is generated in the environment surrounding the transmitted signal; devices such as power lines, motors, generators, power transformers, and relays all produce such magnetic fields. These magnetic fields often cause significant interference. Improper grounding: A common grounding issue is when both ends of the signal line are grounded, which can cause significant interference due to differences in ground potential. If both ends of the signal line are grounded at the same time, a large potential difference may arise, and this difference can generate a significant circulating current in the signal line between the two ends. It generates electromagnetic waves, which interfere with signals. There are also issues such as the total grounding resistance of the DCS system not meeting the requirements for its proper operation. —— How to suppress interference – ensuring proper use of signal wires: During the use of signal wires, it is necessary to avoid having multiple wires tangled together. Additionally, regular inspections of these wires should be carried out, and any wires with damaged insulation should be repaired or replaced, in order to ensure their proper functioning and prevent leakage that could interfere with the transmission of signals. Lay the cables in layers; separate the cables that transmit different signals from each other, and arrange them in layers in order to ensure that there is no interaction between them. For cables that transmit the same signal, it is also necessary to avoid dense entanglement of the wires as much as possible. Since computer ports can perform a certain degree of precise analysis on signals when processing them, if similar signals interfere with each other, it will affect the computer’s ability to make judgments, leading to analytical errors during the computer’s operation. Signal line isolation: During the operation of the equipment, cable isolation can be used to separate the signals, thereby ensuring that they do not affect each other. However, this isolation method can not only isolate the transmission signal lines but also categorize and isolate signals at the computer end; by using this approach, the damage caused by common-mode interference to the computer can be significantly reduced. Isolation amplifiers can also be used to completely isolate the signal input from the computer; by this method, it is possible to prevent the formation of loops among interference signals, thereby reducing the impact of such interference and ensuring the proper operation of the system. Isolation of the power supply system: Since high-power motors are often switched on and off in some installations, high-frequency signals are generated during this switching process. To prevent these high-frequency electromagnetic waves from interfering with the system, transformers can be installed in the power supply lines; these transformers help to contain the high-frequency electromagnetic emissions resulting from the switching of high-power motors, thereby avoiding any impact on the system. Twisting and shielding: The twisting of signal wires, that is, the use of twisted pairs instead of parallel wires, can help to suppress magnetic field interference. Shielding involves using metal conductors to isolate components, assemblies, and signal lines. Shielding can effectively suppress capacitive coupling noise. The common method is to connect analog signals using shielded twisted pair. Normally, not only electrical noise interferes with the signal, but strong alternating magnetic fields also interfere with it. Therefore, not only electrical shielding but also magnetic shielding needs to be considered, and shielding measures can be implemented using conductors with excellent magnetic conductivity (such as iron, nickel, etc.). Proper grounding is essential for the DCS grounding system, which serves two main functions: firstly, it can handle overload currents when signals and power supplies enter the DCS system or when faults occur within the system itself, and it is able to channel such currents into the ground ； On the other hand, the grounding system can provide a shielding layer for the DCS, thereby isolating it from electronic noise interference, and it can also provide a reference zero potential for the entire system. Therefore, a DCS system usually includes two types of grounding: operational and protective.