Study on Anti-interference of Power Plant Control System
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The type of interference is usually divided according to the cause of the interference, the noise interference pattern, and the waveform nature of the noise. Among them: according to the different causes of noise, divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the noise waveform, different nature, divided into continuous noise, occasional noise, etc.; according to different noise interference modes, divided into Mode interference and differential mode interference. Common-mode interference and differential-mode interference are more commonly used classification methods. Common-mode interference is the signal-to-ground potential difference, which is mainly caused by the superposition of common-mode (in the same direction) voltage induced on the signal line by the grid string in, ground potential difference, and space electromagnetic radiation. Common-mode voltage is sometimes large. Especially in power distribution rooms with poor isolation performance, the common-mode voltage of the output signal of the transmitter is generally high, and some may reach more than 130V. The common-mode voltage can be converted to differential-mode voltage through an asymmetrical circuit, directly affecting the measurement and control signals and causing damage to the components (this is the main reason for the high breakdown rate of some system I/O modules). This common-mode interference can be DC. Can also be exchanges. Differential mode interference refers to the interference voltage acting between the two poles of the signal, which is mainly caused by the coupling of the space electromagnetic field in the signal and the voltage formed by unbalanced circuit conversion common mode interference. This direct superimposition on the signal directly affects the measurement and control. Accuracy, now to solve the common mode, differential mode interference is more common and easy way is to add the signal isolator on the original circuit, can be very effective solution.
2.2 Main sources of electromagnetic interference in the control system 2.2.1 Radiated interference from space Radiated electromagnetic fields (EMI) in space are mainly caused by transient processes in the power network and electrical equipment, lightning, radio broadcasting, television, radar, and high-frequency induction heating. The equipment, etc., is usually called radiated interference and its distribution is extremely complicated. If the system is placed in the RF field, radiated interference is recovered. The effect is mainly through two paths: one is the direct radiation to the control device, and the other is induced by the circuit; it is the radiation to the communication network of the control device. The induction of the communication line introduces interference. The radiated interference is related to the field equipment layout and the electromagnetic field generated by the equipment, especially the frequency. Generally, shielded cables and PLC partial shields and high-pressure discharge components are used for protection.
2.2.2 Interference from the external wiring of the system is mainly introduced through the power supply and signal lines and is usually referred to as conducted interference. This kind of interference is more serious in the field of power generation. Because the power plant is a dense place with strong electric fields and strong electromagnetic fields.
2.2.2.1 Interference from Power Sources Practice has proved that there are many cases of control system failure caused by interference introduced by power supply. Most of the power supply of the control system comes from the power supply network of the power plant. It covers the entire plant and changes within the power grid, such as switching surges. Large-scale power equipment starts and stops, harmonics caused by AC/DC transmission devices, and transient short-circuit of the power grid are all transmitted to the power supply through the distribution line. The control power supply is usually an isolated power supply, but its organization and manufacturing process factors make its isolation unsatisfactory. In fact, absolute isolation is not possible due to the presence of distributed parameters, especially distributed capacitance.
2.2.2.2 Interference from the signal line The transmission lines of various types of signals (signal lines and control command lines) connected to the control system, in addition to transmitting various types of valid information, always have external interference signals intruded. This interference mainly has two ways: First, through the power supply of the transmitter or the power supply of the common signal instrument, the power grid is disturbed by the power supply, which is often overlooked; second, the signal line is interfered by space electromagnetic radiation induction, ie, the signal line. The external inductive interference is very serious. The introduction of interference by the signal will cause abnormal I/O signal operation and greatly reduce the measurement accuracy. In severe cases, it will cause component damage. For systems with poor isolation performance, they will also cause interference between the signals, causing common-system bus backflows, causing logic data changes, misoperations, and crashes. The number of I/O module damages caused by the control system's introduction of interference is quite serious, and there are many cases of system failure. Therefore, it is very important to isolate the signal from the detection end (eg, Beijing Pinghe PH series isolators), which can ensure the signal transmission accuracy and protect the system I/O modules from being damaged.
2.2.2.3 Interference from grounded system disturbance Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can not only suppress the influence of electromagnetic interference, but also inhibit the equipment to send out interference; but the wrong grounding, it will introduce serious interference signals, so that the system will not work properly.
The ground wire of the control system includes system ground, shielding ground, exchange ground and protection ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven distribution of the potential of each grounding point. There is a difference in ground potential between different grounding points, which causes the ground loop current and affects the normal operation of the system.
For example, the cable shield must be grounded at one point. If both ends of the cable shields A and B are grounded, there is a difference in ground potential. Current flows through the shield. When an abnormal state such as a lightning strike occurs, the ground current will be greater.
In addition, the shielding layer, grounding wire and ground may constitute a closed loop. Under the effect of changing the magnetic field, there will be induced current in the shielding layer, and the signal loop will be disturbed by the coupling between the shielding layer and the core wire. If the system is confused with other grounding processes, the resulting ground loop may produce unequal potential distribution on the ground, affecting the normal operation of the logic circuit and the analog circuit. The logic voltage interference tolerance is low, and the logic ground potential distribution interference easily affects the logic operation and data storage, resulting in data confusion, program runaway or crash. The distribution of the simulated ground potential will lead to a decrease in measurement accuracy, causing serious distortion and malfunction of the signal measurement and control. (Example Beijing Pinghe isolator can also effectively solve the signal common problem)
2.3 Interference from the inside of the system is mainly caused by mutual electromagnetic radiation between the internal components of the system and the circuits, such as mutual radiation of logic circuits and their influence on analog circuits, interaction between analog ground and logic ground, and mutual mismatch between components. Use etc. All of this belongs to the manufacturer's internal electromagnetic compatibility design of the system. It is more complex and cannot be changed as an application department. It is not necessary to consider too much. However, it is necessary to select a system that has more application performance or has been tested. 3. Anti-jamming measures In order to ensure that the system is immune to or reduces electromagnetic interference both inside and outside the industrial electromagnetic environment, three measures must be taken from the design phase: suppression of the source of interference; cut-off or attenuation of the propagation path of electromagnetic interference; improvement of the device and system Anti-jamming capability. These three points are the basic principles for suppressing electromagnetic interference.
The anti-jamming of the control system is a systematic project that requires the manufacturing unit to design and produce a product with strong anti-jamming capability, and it depends on the use department to fully consider it in the engineering design, installation, construction, operation and maintenance, and integrates the situation. Design can ensure the system's electromagnetic compatibility and operational reliability. The main consideration comes from the outside of the system if some of the suppression measures. The main contents include: shielding the system and external leads to prevent electromagnetic radiated from space; installing external isolators for leads, especially the principle of power cables, layered layout, to prevent conduction of electromagnetic interference through the external leads; correct design of grounding points And grounding device, perfect grounding system. In addition, software must be used to further improve the system's safety and reliability.
3.1 Using a power supply with excellent performance to suppress the interference introduced by the power grid In the control system, the power supply occupies a very important position. The grid interference control system is mainly coupled through the power supply (such as CPU power supply, I/O power supply, etc.) of the PLC system, the power supply of the transmitter, and the instrument power supply that has a direct electrical connection with the PLC system. Now, for the power supply of PLC system, power supply with better isolation performance is generally used, and the power supply for the power supply of the transmitter and the power supply for the instrument directly connected to the PLC system have not received enough attention, although it has taken certain The isolation measures, but generally not enough, is mainly used in isolation transformers with large distribution parameters, poor ability to suppress interference, and common mode interference and differential mode interference through power coupling. Therefore, for the power supply of the transmitter and the common signal instrument, a small distributed capacitance and suppression band should be selected (for example, using Beijing Pinghe Corporation isolation distributor), which can reduce the interference of the PLC system. In addition, to ensure that the grid feed points are not interrupted, online uninterruptible power supply (UPS) power supply can be used to improve the safety and reliability of power supply. And UPS also has strong interference isolation performance, which is an ideal power source for PLC control systems.
3.2 The cable is laid to reduce the radiated electromagnetic interference of the power cable, especially the frequency converter feed cable. In the project, the author adopted a copper tape armor shielded power cable, which reduced the electromagnetic interference generated by the power line, and achieved satisfactory results after the project was put into operation. Different types of signals and control cables are transmitted by different cables. The signal cables should be layered according to the types of transmission signals and power cables. It is forbidden to use the same cable with different wires to transmit power and signals at the same time so as to prevent the signal cables from coming close to the power cables. Set up to reduce electromagnetic interference. In addition, the signal cable and control cable should be shielded cable 3.3 hardware filtering and software anti-jamming measures before the signal is connected to the computer, the signal line and ground parallel capacitance, in order to reduce common-mode interference; in the signal between the two pole filter Differential mode interference can be reduced. Due to the complexity of electromagnetic interference, it is impossible to completely eliminate the impact of meeting the interference. Therefore, in the software design and configuration of the PLC control system, the anti-jamming processing should be performed in the software to further improve the reliability of the system. Some commonly used measures: digital filtering and frequency-shaping sampling can effectively eliminate periodic interference; timing correction of reference point potential and dynamic zero point can effectively prevent potential drift; and using information redundancy technology, design corresponding software flag bits; Use indirect jumps, set software traps, etc. to improve the reliability of the software structure.
4. Correctly select the grounding point and improve the grounding system. The grounding of the power plant control system is especially important for the control system to resist interference. The quality of the grounding method will directly affect the performance of the control system. The purpose of grounding is usually two, one is for safety, and the other is to suppress interference. A well-grounded system is one of the important measures against electromagnetic interference in the control system. System grounding methods include: floating ground, direct grounding, and capacitive grounding.
4.1 PLC control system grounding:
PLC control system It is a high-speed low-level control device and should be directly grounded. Because of the influence of signal cable distribution capacitance and input device filtering, the signal exchange frequency between devices is generally lower than 1 MHz, so the ground wire of the PLC control system adopts one-point grounding and one-point grounding. The centrally-arranged PLC system is suitable for parallel grounding at one point. The central ground point of the cabinet of each device is led to the grounding electrode by a separate grounding wire. If the distance between the devices is large, a one-point grounding method should be used.