In the operation of the power plant, the application of the DCS control system plays an important role in the operation of the power plant. Therefore, it is necessary to improve the reliability of the DCS control system, avoid various interference factors of the system, optimize the operation of the system, improve the efficiency of the power plant operation, and ensure the stability of the power plant work. This paper first analyzes the significance of the power plant to improve the reliability of the DCS control system, and then proposes measures to improve the reliability of the DCS control system.
With the development of science and technology, DCS has become more and more widely used in industrial control. The reliability of the DCS control system directly affects the safe production and economic operation of industrial enterprises. The anti-interference ability of the system is the key to the reliable operation of the whole system. The various types of DCS used in the automation system are concentrated in the control room, and some are installed in the production site and on various electrical equipment. Most of them are in the harsh electromagnetic environment formed by strong electric circuits and strong electric 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 requires high attention in engineering design, installation and maintenance, and multi-party cooperation can solve the problem and effectively enhance the system. Anti-jamming performance.
Electromagnetic interference sources and interference to the systemGeneral 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 industrial 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.
The type of interference is usually divided according to the cause of the interference, the noise interference mode, and the waveform properties of the noise. Among them: according to the causes of noise, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the waveform and nature of noise, it is divided into continuous noise, sporadic noise, etc.; according to different noise interference modes, it is divided into Mode interference and differential mode interference. Common mode interference and differential mode interference are a common classification method. Common mode interference is the potential difference of the signal to the ground, which is mainly formed by the grid-in, ground potential difference and the common-state (same-direction) voltage induced by the space electromagnetic radiation on the signal line. The common mode voltage is sometimes large, especially in the power supply room of the distributor with poor isolation performance. The common mode voltage of the transmitter output signal is generally higher, and some can be as high as 130V or more. 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. This is directly superimposed on the signal and directly affects the measurement and control. Precision.
The main source of electromagnetic interference in DCS control systems comes from radiated interference in space.Radiation electromagnetic fields (EMI) in space are mainly generated by power networks, transient processes of electrical equipment, lightning, radio, television, radar, high-frequency induction heating equipment, etc., commonly referred to as radiated interference, and their distribution is extremely complicated. If the DCS system is placed in the RF field, the radiation interference is recovered, and the influence mainly passes through two paths: one is direct radiation to the DCS, and the circuit induces interference; instead, the radiation of the network within the DCS communication is caused by The induction of the communication line introduces interference. Radiation interference is related to the size of the electromagnetic field generated by the arrangement of the field equipment and the equipment, especially the frequency. It is generally protected by providing shielded cables and DCS partial shielding and high-voltage bleeder components.
Interference from external leadsPrimarily introduced through power and signal lines, often referred to as conducted interference. This kind of interference is more serious in China's industrial scene.
(1) Interference from power supply
Practice has proved that there are many situations in which the DCS control system is faulty due to the interference introduced by the power supply. The author has encountered in the debugging of a certain project, and then replaces the DCS power supply with higher isolation performance, and the problem is solved.
The normal power supply of the DCS system is powered by the grid.
Due to the wide coverage of the grid, it will be subject to electromagnetic interference from all spaces and induce voltages and circuits on the line. In particular, internal changes in the power grid, surges in switching operations, start and stop of large-scale power equipment, harmonics caused by AC and DC transmissions, and short-circuit transients of power grids are transmitted to the primary side of the power supply through transmission lines. DCS power supplies typically use isolated power supplies, but their mechanical and manufacturing process factors make them less than ideal. In fact, absolute isolation is not possible due to the presence of distributed parameters, especially distributed capacitance.
(2) Interference from signal line introduction
In addition to transmitting various types of information, various types of signal transmission lines connected to the DCS control system always have external interference signals. There are two main ways of this interference: one is the power grid interference through the power supply of the transmitter or the power supply of the shared signal meter, which is often neglected; the second is that the signal line is interfered by the space electromagnetic radiation, that is, the signal line The external inductive interference, which is very serious. The introduction of interference from the signal will cause the I/O signal to work abnormally and the measurement accuracy will be greatly reduced. In severe cases, the component will be damaged. For systems with poor isolation performance, it will also cause mutual interference between signals, causing 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 chaotic grounding system
Grounding is one of the effective means to improve electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can not only suppress the influence of electromagnetic interference, but also suppress the external interference of the equipment; and the wrong grounding will introduce serious interference signals, so that the DCS system will not work normally.
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 the potential of each grounding point. There is a ground potential difference between different grounding points, which causes the ground loop current 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 a 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 systematically and other grounding processing is 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.
Interference from within the DCS system It is mainly generated by mutual electromagnetic radiation between internal components and circuits of the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the interaction between analog ground and logic ground, and the mismatch between components. This is the content of the DCS manufacturer's electromagnetic compatibility design inside the system. It is more complicated. As an application department, it cannot be changed. It does not need to be considered too much, but it is necessary to select a system with more application performance or test.
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