Electromagnetic interference sources and their interference with the system
1. Interference sources and general classification of interference
The interference sources that affect the DCS control system, like those that generally affect industrial control equipment, are mostly generated in areas with drastic changes in current or voltage. These areas where charges move violently are noise sources, namely interference sources.
The types of interference are usually divided according to the causes of interference, noise interference modes, and different waveform properties of noise. Among them, according to the different causes of noise generation, it can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc; According to the different waveforms and properties of noise, it can be divided into continuous noise, occasional noise, etc; According to different noise interference modes, it can be divided into common mode interference and differential mode interference. Common mode interference and differential mode interference are commonly used classification methods. Common mode interference is the potential difference between a signal and ground, mainly formed by the superposition of common state (in the same direction) voltages induced by grid connection, ground potential difference, and spatial electromagnetic radiation on the signal line. The common mode voltage is sometimes high, especially when using power distribution rooms with poor isolation performance. The common mode voltage of the transmitter output signal is generally high, and some can reach over 130V. The common mode voltage can be converted into differential mode voltage through asymmetric circuits, directly affecting the measurement and control signal, causing component damage (which is the main reason for the high damage rate of some system I/O modules). This common mode interference can be DC or AC. Differential mode interference refers to the interference voltage acting between the two poles of a signal, mainly formed by the coupling induction of spatial electromagnetic fields between signals and the conversion of common mode interference by unbalanced circuits. This directly superimposes on the signal and directly affects the measurement and control accuracy.
2. The main sources of electromagnetic interference in DCS control systems
2.1 Radiation interference from space
The radiation electromagnetic field (EMI) in space is mainly generated by transient processes of power networks, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., commonly known as radiation interference, and its distribution is extremely complex. If the DCS system is placed in the RF field, it will recover radiation interference, which mainly affects through two paths: first, it directly affects the radiation inside the DCS, causing interference through circuit induction; But rather the radiation of the network within DCS communication, which is introduced by the induction of communication lines to interfere. Radiation interference is related to the layout of on-site equipment and the size of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting shielded cables, DCS local shielding, and high-voltage relief components.
2.2 Interference from external system leads
Mainly introduced through power and signal lines, commonly referred to as conducted interference. This kind of interference is more severe in industrial sites in our country.
(1) Interference from power supply
Practice has proven that there are many cases of DCS control system failures caused by interference introduced by power sources. The author encountered this issue during a certain project debugging, and it was only after replacing the DCS power source with a higher isolation performance that the problem was resolved.
The normal power supply of the DCS system is supplied by the power grid.
Due to the wide coverage of the power grid, it will be subject to all spatial electromagnetic interference and induce voltage and circuits on the lines. Especially the internal changes in the power grid, such as surges during switch operation, starting and stopping of large power equipment, harmonics caused by AC/DC transmission devices, and transient impacts of power grid short circuits, are all transmitted to the power source through transmission lines. DCS power supplies usually use isolated power sources, but their isolation performance is not ideal due to their structure and manufacturing process factors. In fact, due to the presence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.
(2) Interference introduced by signal lines
The various signal transmission lines connected to the DCS control system, in addition to transmitting effective information, will always have external interference signals invading. There are two main ways for this interference: one is through the power supply of the transmitter or the power supply of the shared signal instrument connected in series to the power grid, which is often overlooked; The second is the interference caused by spatial electromagnetic radiation induction on the signal line, that is, external induction interference on the signal line, which is very serious. The introduction of interference from signals can cause abnormal operation of I/O signals and greatly reduce measurement accuracy, and in severe cases, it can cause damage to components. For systems with poor isolation performance, it will also lead to mutual interference between signals, causing common ground system bus backflow, causing logical data changes, false alarms, and crashes. The DCS control system has suffered significant damage to I/O modules due to signal interference, leading to numerous system failures.
(3) Interference from chaotic grounding systems
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic devices. Proper grounding can suppress the impact of electromagnetic interference and also suppress external interference from equipment; And incorrect grounding can actually introduce serious interference signals, making the DCS system unable to function properly.
The ground wire of the DCS control system includes system ground, shielding ground, AC ground, and protection ground. The interference of chaotic grounding systems on DCS systems is mainly due to uneven potential distribution at various grounding points, and the existence of ground potential differences between different grounding points, which causes ground loop currents and affects the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there is a ground potential difference, and current flows through the shielding layer. When an abnormal state occurs, such as a lightning strike, the ground wire current will be greater.
In addition, the shielding layer, grounding wire, and ground may form a closed loop. Under the action of changing magnetic fields, induced currents may appear inside the shielding layer, which can interfere with the signal loop through coupling between the shielding layer and the core wire. If the system grounding and other grounding treatments are chaotic, the generated ground current may generate unequal potential distribution on the ground wire, affecting the normal operation of logic circuits and analog circuits in DCS. The logic voltage interference tolerance of DCS operation is relatively low, and the distribution interference of logic ground potential can easily affect the logic operation and data storage of DCS, causing data confusion, program runaway or crash. Simulating the distribution of ground potential will lead to a decrease in measurement accuracy, causing serious distortion and misoperation of signal measurement and control.
2.3 Interference from within the DCS system
Mainly generated by the mutual electromagnetic radiation between internal components and circuits in the system, such as the mutual radiation between logic circuits and their impact on analog circuits, the mutual influence between analog and logic circuits, and the mismatch between components. This belongs to the electromagnetic compatibility design of the DCS manufacturing plant for the internal system, which is relatively complex and cannot be changed as an application department. It is not necessary to consider too much, but it is necessary to choose a system with many application achievements or tested results.
Anti interference design for engineering applications of DCS control systems
In order to ensure that the system is free from or reduces internal and external electromagnetic interference in the industrial electromagnetic environment, three suppression measures must be taken from the design stage: suppressing interference sources; Cut off or attenuate the propagation path of electromagnetic interference; Improve the anti-interference ability of devices and systems. These three points are the basic principles for suppressing electromagnetic interference.
The anti-interference of DCS control system is a system engineering that requires manufacturing units to design and produce products with strong anti-interference ability. It depends on the comprehensive consideration of the user department in engineering design, installation, construction, and operation and maintenance, and combined with the actual situation for comprehensive design, in order to ensure the electromagnetic compatibility and operational reliability of the system.
When carrying out anti-interference design for specific engineering projects, the following two aspects should be mainly considered.
1. Equipment selection
When selecting equipment, the first thing to choose is a product with high anti-interference ability, which includes electromagnetic compatibility (EMC), especially the ability to resist external interference, such as using floating ground technology and a DCS system with good isolation performance; Secondly, it is necessary to understand the anti-interference indicators provided by the production plant, such as the common analog ratio, differential analog ratio, voltage resistance, and the allowable working environment in which the electric field strength and magnetic field strength are high; Additionally, it relies on assessing its practical performance in similar work. When choosing imported products from abroad, it should be noted that China adopts a 220V high internal resistance power grid system, while Europe and America use a 110V low internal resistance power grid. Due to the high internal resistance of China’s power grid, large zero point potential drift, and large ground potential changes, the electromagnetic interference on site in industrial enterprises is at least four times higher than that in Europe and America, and the requirements for system anti-interference performance are higher. DCS products that can work normally in foreign countries may not necessarily operate reliably in domestic industry. Therefore, when using foreign products, it is necessary to choose them reasonably according to China’s standards (GB/T13926).
2. Comprehensive anti-interference design
Mainly considering several external suppression measures from the system. The main content includes: shielding the DCS system and external leads to prevent electromagnetic interference from space radiation; Isolate and filter the external leads, especially the principle power cables, and arrange them in layers to prevent the introduction of conducted electromagnetic interference through the external leads; Correctly design grounding points and grounding devices, and improve the grounding system. In addition, software means must be utilized to further improve the security and reliability of the system.
Main anti-interference measures
1. Using high-performance power sources to suppress interference introduced by the power grid
In DCS control systems, power supply plays an extremely important role. The interference from the power grid is mainly coupled into the DCS control system through the power supply of the DCS system (such as CPU power, I/O power, etc.), the power supply of the transmitter, and the power supply of the instrument with direct electrical connection to the DCS system. Nowadays, for the power supply of DCS systems, power sources with good isolation performance are generally used. However, for the power supply of transmitters and instruments with direct electrical connections to the DCS system, it has not received sufficient attention. Although certain isolation measures have been taken, they are generally not enough. The main reason is that the isolation transformers used have large distribution parameters and poor interference suppression ability, leading to common mode interference and differential mode interference through power coupling. Therefore, for the power supply of transmitters and shared signal instruments, distribution capacitors with large suppression bands (such as using multiple isolation, shielding, and leakage sensing technologies) should be selected to reduce interference in the DCS system.
In addition, to ensure uninterrupted power supply to the power grid, online uninterruptible power supply (UPS) can be used to improve the safety and reliability of power supply. And UPS also has strong interference isolation performance, making it an ideal power source for DCS control systems.
2. Ao Design for Cable Selection
In order to reduce the electromagnetic interference radiated by power cables, especially the feeding cables of variable frequency devices. In a certain project, the author used copper tape armored shielded power cables to reduce electromagnetic interference in power line production. The project achieved satisfactory results after being put into operation.
Different types of signals are transmitted through different cables, and signal cables should be layered according to the type of transmission signal. It is strictly prohibited to use different wires of the same cable to simultaneously transmit power supply and signals, avoiding parallel installation of signal lines and power cables to reduce electromagnetic interference.
3. Hardware filtering and software anti-interference measures
Before connecting the signal to the computer, a capacitor is connected in parallel between the signal line and ground to reduce common mode interference; Installing a filter between the two poles of the signal can reduce differential mode interference.
Due to the complexity of electromagnetic interference, it is impossible to fundamentally eliminate the impact of interference. Therefore, in the software design and configuration of DCS control systems, anti-interference treatment should also be carried out in the software aspect to further improve the reliability of the system.
Common measures include digital filtering and power frequency shaping sampling, which can effectively eliminate periodic interference; Timing correction of reference point potential and adopting dynamic zero point can effectively prevent potential drift; Using information redundancy technology to design corresponding software flag bits; Using indirect redirection and setting software traps to improve software structure reliability.