With the development of science and technology, DCS is more and more widely used in the automatic control of the power industry. The reliability of the DCS control system directly affects the safe production and economic operation of the unit. The anti-interference ability of the system is the key to the reliable operation of the whole system. Various types of DCS used in the automation system are installed centrally in the control room, and some are installed in the production site and on various electrical equipment. Most of them are in the electromagnetic environment formed by strong electric circuits and high-voltage equipment. To improve the reliability of the DCS control system, DCS manufacturers are required to improve the anti-interference ability of the equipment; on the other hand, it is required to pay great attention to engineering design, installation, construction and maintenance, and multi-party cooperation can solve the problem effectively. Ground enhancement system's anti-jamming performance.
1 Electromagnetic interference source and interference to the system
1.1 General classification of interference sources and interference
The interference sources that affect the DCS control system are the same as the interference sources that generally affect the power control equipment. Most of them generate parts where the current or voltage changes drastically. The part where these charges move violently is the noise source, that is, the interference source. Interference types are usually classified into common mode interference and differential mode interference according to different noise interference modes. Common mode interference and differential mode interference are a common classification method. Common mode interference is the potential difference between the signal and the ground, which is mainly formed by the power grid stringing, the ground potential difference and the common state (same direction) voltage induced by the space electromagnetic radiation on the signal line. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, which directly affects the measurement and control signals, causing component damage (this is the main reason for the high damage rate of some system I/O modules). This common mode interference can be DC. It can also be an exchange. Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling of the space electromagnetic field between the signals and the common mode interference caused by the unbalanced circuit. They are directly superimposed on the signal, which directly affects the measurement and control accuracy. .
1.2 Main sources of electromagnetic interference in DCS control systems
1.2.1 Radiated electromagnetic fields (EMI) from space radiated interference spaces are mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasting, radar, high-frequency induction heating equipment, etc., commonly referred to as radiated interference. Its distribution is extremely complicated. If the DCS system is placed in the RF field, it will receive radiated interference, and its influence mainly passes through two paths: one is direct radiation to the DCS, and the circuit induces interference; the other is radiation to the network within the DCS communication. Interference is introduced by the induction of the communication line. Radiated interference is related to the size of the electromagnetic field generated by the arrangement of the field devices and the equipment, especially the frequency.
1.2.2 Interference from the external leads of the system is mainly introduced through the power supply and signal lines, usually called conducted interference, which is more serious in the field.
(1) Interference from power supply: It has been proved that the DCS control system is faulty due to the interference introduced by the power supply. The normal power supply of the DCS system is powered by the working power supply. Due to its wide coverage, it will be subject to all space electromagnetic interference. Voltage and current are induced on the line. Especially the internal changes, the start and stop of large-scale power equipment, harmonics caused by AC and DC transmissions, etc., are transmitted to the primary side of the power supply through the transmission line.
(2) Interference from signal line introduction: In addition to transmitting various types of information, there are always external interference signals in addition to various types of signal transmission lines connected to the DCS control system. There are two main ways of such interference: The interference caused by the power supply of the transmitter or the power supply that is shared with the signal system of the electrical system is often ignored. Second, the signal line is interfered by the space electromagnetic radiation, that is, the external inductive interference on the signal line. This kind of interference is very serious. The interference introduced by the signal will cause the I/O signal to work abnormally and the measurement accuracy is greatly reduced. In severe cases, the component will be damaged. For the system with poor isolation performance, the signals will interfere with each other. Causes the common system bus to reflow, causing logical data changes, misoperations and crashes. The number of I/O module damage caused by signal introduction interference in the DCS control system is quite serious, and there are many cases of system failure.
(3) Interference from the disorder of the grounding system: Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can both suppress the influence of electromagnetic interference and suppress the external interference of the equipment; Incorrect grounding will introduce serious interference signals, making the DCS system unable to work properly. The ground wire of the DCS control system includes: system ground, shield ground, AC ground, and protection ground. The disturbance of the grounding system to the DCS system is mainly due to the uneven distribution of potential at each grounding point. There is a ground potential difference between different grounding points, causing grounding loop current, which 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 such as a lightning strike occurs, the ground current will be larger. In addition, the shielding layer, the grounding wire and the earth may constitute a closed loop. Under the action of the changing magnetic field, an induced current may appear in the shielding layer, and the signal loop is interfered by the coupling between the shielding layer and the core wire. If the system ground and other grounding treatments are confusing, the generated ground loop may generate unequal potential distribution on the ground line, affecting the normal operation of the logic circuit and the analog circuit in the DCS. The logic voltage interference tolerance of DCS operation is low, and the distributed interference of logic ground potential easily affects the logic operation and data storage of DCS, resulting in data confusion, program running or crash. The distribution of the analog ground potential will result in a decrease in measurement accuracy, causing severe distortion and malfunction of the signal measurement and control.
1.2.3 Interference from the DCS system is mainly caused by mutual electromagnetic radiation between components and circuits inside the system.
Such as: the mutual interference of logic circuits and their influence on analog circuits, the interaction between analog ground and logic ground and the mismatch between components. This is part of the DCS manufacturer's electromagnetic compatibility design inside the system. It is more complicated and cannot be changed as an application department. It does not need to be considered too much, but it is necessary to select a system with more application performance or test.
2 Anti-interference design for field application of DCS control system
In order to ensure that the system is protected from or reduced internal and external electromagnetic interference in the power electromagnetic environment, three aspects of suppression must be taken from the design stage: suppressing the interference source; cutting off or attenuating the propagation path of the electromagnetic interference; improving the anti-interference ability of the device and the system. . These three points are the basic principles for suppressing electromagnetic interference. The anti-interference of DCS control system is a system engineering, which requires the manufacturing unit to design and produce products with strong anti-interference ability, and it depends on the use department to comprehensively consider in engineering design, installation and operation and operation and maintenance, and combine with specific conditions. Comprehensive design can ensure the electromagnetic compatibility and operational reliability of the system. When conducting anti-jamming design for a specific project, it is mainly considered from the following two aspects:
2.1 Equipment selection When selecting equipment, first of all, choose products with high anti-interference ability, including electromagnetic compatibility (EMC), especially anti-interference ability, such as: using floating technology, good isolation performance DCS system; secondly, it should also understand the anti-interference indicators given by the manufacturer, such as: common mode rejection ratio, differential mode suppression ratio, withstand voltage capability, allowing operation in the environment of large electric field strength and high frequency magnetic field strength; According to China's standard (GB/T13926) reasonable choice. In addition, it is to investigate its application in similar projects.
2.2 Comprehensive anti-jamming design
The main considerations are several suppression measures from outside the system. The main contents include: shielding the DCS system and external leads to prevent electromagnetic interference from space radiation; isolating and filtering the external leads, especially the power cables should be layered to prevent conductive electromagnetic interference from being introduced through the external leads; correctly designing the grounding point and Grounding device, perfect grounding system. In addition, software must be used to improve the security and reliability of the system.
3 Main anti-interference measures
3.1 Using a power supply with excellent performance
Suppressing the interference introduced by the power supply In the DCS control system, the power supply occupies a very important position. The power supply interference string DCS control system is mainly coupled through the power supply of the DCS system (such as CPU power, I/O power supply, etc.) and the instrument power supply with direct electrical connection with the DCS system. At present, for the power supply of the DCS system, the power supply with better isolation performance is generally used, and the power supply of the instrument with direct electrical connection to the power supply of the transmitter and the DCS system has not received sufficient attention, although it has been adopted. Certain isolation measures, but generally not enough. Therefore, the on-site uninterruptible power supply (UPS) is used to increase the safety and reliability of the power supply. The UPS should also have strong interference isolation performance. It is an ideal power supply for the DCS control system.
3.2 Cable selection and laying
In order to reduce the radiated electromagnetic interference of the power cable, especially the feeder cable of the inverter device, the copper tape armored shielded power cable should be used, which can achieve satisfactory results, thereby reducing the electromagnetic interference generated by the power line. Different types of signals are transmitted by different cables, and signal cables should be laid in layers according to the type of transmission signals. It is strictly forbidden to use the different wires of the same cable to simultaneously transmit power and signals, so as to avoid the signal lines and power cables being laid close to each other to reduce electromagnetic interference.
3.3 Hardware filtering and software anti-interference measures
Before the signal is connected to the computer, a capacitor is connected between the signal line and the ground to reduce common mode interference; adding 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 eliminate interference effects at all. Therefore, in the software design and configuration of the DCS control system, anti-interference processing should also be carried out in software to further improve the reliability of the system. Commonly used measures: digital filtering and power frequency shaping sampling, can effectively eliminate periodic interference; timing correction reference point potential, and use dynamic zero point, can effectively prevent potential drift; use information redundancy technology, design corresponding software flag; Use indirect jumps, set software traps, etc. to improve the reliability of the software structure.
3.4 Correct selection of grounding points
Perfecting the grounding system The perfect grounding system is one of the important measures for DCS control system to resist electromagnetic interference. The system grounding methods are: floating mode, direct grounding mode and capacitor grounding. For the DCS control system, it is a high-speed low-level control device, and should be directly grounded. Due to the influence of signal cable distribution capacitance and input device filtering, the signal exchange frequency between devices is generally lower than 1 MHz. Therefore, the grounding line of the DCS control system adopts one-point grounding and one-point grounding in series. The centrally arranged DCS system is suitable for parallel grounding, and the center grounding point of each cabinet is led to the grounding pole by a separate grounding wire. If the device spacing is large, a one-point grounding method in series should be used. Use a large cross-section copper busbar (or insulated cable) to connect the cabinet center grounding point of each device, and then connect the grounding busbar directly to the grounding pole. The grounding wire is made of copper wire with a section larger than 22 mm, and the main busbar is a copper bar with a section larger than 60 mm. The grounding resistance of the grounding electrode is less than 2Ω, and the grounding point of the DCS system must be more than 10 m away from the grounding point of the high-voltage equipment. When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the DCS side; when there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multi-point grounding must be avoided; When the shielded twisted pair of the point signal is connected with the multi-core twisted pair main screen cable, the shield layers should be connected to each other and insulated, and a proper grounding point should be selected for single point grounding. It must be grounded in strict accordance with the grounding requirements of DCS manufacturers.
4 Conclusion
The interference in the DCS control system is a very complicated problem. Therefore, in the anti-interference design, all aspects should be considered comprehensively, and the anti-interference should be reasonably and effectively suppressed. For some interference situations, specific analysis is needed and corresponding measures are taken. Enables the DCS control system to work properly.
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