Main factors of DCS signal disturbance

Resistance coupled disturbance In the process of operation, many factories neglect the problem of wire aging. When multiple signal lines transmit signals together, other transmission signals will be affected and disturbed due to aging and leakage of insulating materials on the wires. In addition, not only the aging of signal lines will affect other signal lines, but also the aging of some high-power circuits (such as heat generating circuits) will greatly disturb the signals in case of line aging.   Capacitance inductance coupling disturbance Generally, many signal lines are connected to the control cabinet at the controlled site. Some of these signal lines use cable trays, others use cable tubes, especially many signals are wired together. Capacitance dispersion between signals will disturb other signal lines. In particular, the alternating magnetic flux is formed around the AC signal line, so as to form an electric potential between parallel conductors and form an electric potential disturbing signal.   Lightning disturbance When lightning strikes, large electromagnetic disturbances may be formed around the required signal. Of course, the introduction of disturbances will also be caused by various grounding wires to a certain extent. There are two main methods of lightning disturbance: one is overhead power line, which causes the signal line to be disturbed by lightning; The other is that the signal cable is struck by lightning, which results in capacitive and inductive coupling dispersion on the signal line, thus forming a large disturbance, which may damage the equipment and cause personal accidents in serious cases.   Disturbance of power supply line Because the large equipment in the power plant starts and closes frequently, and the switching operation of large components is also frequent, the instantaneous large alternating magnetic field will occur when these large electrical equipment are switched. The alternating magnetic field will couple on the signal line, and then disturb the system. These alternating magnetic fields will also disturb the power line at high frequencies. If the signal level exceeds the regular range, it will also affect the system. In order to ensure the maintenance of the system when switching large motors, transformers can be used in the circuit to ensure that the alternating magnetic field will not occur due to excessive voltage changes when switching motors.   What are the main causes of disturbance? Material and equipment problems The control signals connected to the DCS system generally use cables as the transmission medium to send the signals to the DCS system. When the insulation materials of the signal cables are aging and the shielding materials are damaged, they will be disturbed by other electromagnetic disturbance sources, which will constitute measurement errors, and serious disturbances will constitute equipment damage.   Non standard construction In DCS control system, many signals are often connected to DCS together. These signal lines either run through cable trays or cable ducts, forming many different types of cables attached to the site in the same way. There are dispersion capacitors between these signals, through which the disturbance will be added to other signal lines. Together, the alternating magnetic flux occurs in the surrounding environment of the transmission signal, such as power line, motor, generator, power transformer and relay. These magnetic fields tend to form large disturbances. Unreasonable grounding The common grounding problem is that both ends of the signal line are grounded, which will cause great disturbance due to the ground potential difference. If both ends of the signal line are grounded together, there may be a large potential difference, which may cause a large circulation on the signal line between the two ends. Electromagnetic wave occurs to disturb the signal. In addition, the total grounding resistance of the DCS system does not meet the normal operation requirements of the DCS system.   How can I restrain the disturbance? Ensure the normal operation of signal line During the use of signal lines, prevent multiple signal lines from being tied together, regularly check the signal lines together, and correct and replace the signal lines with damaged insulating colloids to ensure the normal use of signal lines and prevent electric leakage from disturbing the transmission signals. Lay cables in layers The cables that transmit different signals shall be separated, and the layered laying method shall be used to regulate the cables to a certain extent, and then ensure that the cables will not interact with each other. As far as cables transmitting the same signal are concerned, it is also necessary to avoid the restriction of dense lines as much as possible. Because the computer port can conduct a certain degree of fine analysis of the signal when processing the signal, so if there are similar signals disturbing each other, it will affect the computer's judgment, and then cause analysis errors in the computer operation process.   Signal line blocking During the operation of the equipment, the cable blocking method can be selected to block the signals, and then ensure that the signals will not affect each other. However, the blocking method can not only block the transmission signal lines, but also classify the signals at the computer terminal. This blocking method can greatly reduce the damage of common mode interference to the computer. The blocking amplifier can also be selected to completely block the input end of the signal from the computer. This method can be used to form a loop between the blocking disturbing signals, suppress the damage of the disturbance, and ensure the normal operation of the system.   Power supply system isolation Because some sites often switch high-power motors, and high-frequency signals will occur during the switching process. To prevent high-frequency electromagnetism from disturbing the system, a transformer can be installed on the power supply line to restrain the high-frequency electromagnetism generated by high-power motor switches and prevent the system from being affected.   Twisting and shielding Twisting of signal conductor means that the twisted pair replaces the parallel conductor, which can suppress the disturbance of magnetic field. Shielding refers to the adoption of metal conductor insulation elements, assemblies and signal wires. Shielding can suppress capacitive coupling noise. The common method is to connect the analog signal through shielded twisted pair. In general, the signal is disturbed by non photoelectric noise, and strong alternating magnetic field will also disturb the signal. Therefore, it is not only necessary to consider electric shielding, but also need to consider magnetic shielding. The shielding measures can be completed by adopting conductors with superior magnetic conductivity (such as iron, nickel, etc.).   Grounding With regard to DCS grounding system, it mainly has two functions: on the one hand, when the signal and power supply enter the DCS system or their own faults occur, they can bear the overload current and transmit it to the ground; On the other hand, the grounding system can provide shielding layer to DCS to isolate electronic noise disturbance, and together provide reference zero potential to all systems. Therefore, the DCS system generally includes two grounding points for operation and maintenance.