Interference resistance analysis of PLC control systems

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Interference Resistance Analysis of PLC Control Systems I. Overview With the development of science and technology, PLCs are being used more and more widely in industrial control. The reliability of PLC control systems has a direct impact on the safe production and economic operation of industrial enterprises, and the system’s ability to resist interference is key to its reliable operation. The various types of PLCs used in automation systems are either installed centrally in control rooms or at the production sites and on various motorized equipment; most of them operate in a harsh electromagnetic environment created by high-voltage circuits and high-voltage equipment. To improve the reliability of PLC control systems, it is necessary, on the one hand, for PLC manufacturers to enhance the interference resistance of their equipment ； On the other hand, it requires high attention during engineering design, installation, operation, and maintenance; only through coordinated efforts can issues be properly resolved, thereby effectively enhancing the system’s resistance to interference. II. Electromagnetic interference sources and their impact on systems 1. Sources of interference and general classification of interference The sources of interference that affect PLC control systems are, similar to those that affect industrial control devices, mostly found in areas where current or voltage changes sharply. These areas of rapid charge movement are the sources of noise, that is, the sources of interference. Interference types are usually classified according to the causes of the interference, the patterns of noise interference, and the waveform characteristics of the noise. Among them: depending on the cause of the noise, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc ； Based on the waveform and nature of the noise, it is classified into continuous noise, sporadic noise, etc ； Depending on the noise interference pattern, it is divided into common-mode interference and differential-mode interference. Common-mode interference and differential-mode interference are relatively common classification methods. Common-mode interference is the potential difference of a signal with respect to ground, which is primarily caused by the injection from the power grid, ground potential differences, and the common-mode (in the same direction) voltages induced on the signal lines by spatial electromagnetic radiation. The common-mode voltage can sometimes be quite high, especially in supply rooms equipped with power distribution units that have poor isolation capabilities. The common-mode voltage of the transmitter’s output signal is generally high, and in some cases it can exceed 130 V. The common-mode voltage can be converted into differential-mode voltage through asymmetric circuits, directly affecting the measurement and control signals and causing damage to components (this is the main reason for the high failure rate of I/O modules in some systems). Such common-mode interference can be either DC or AC. Differential mode interference refers to the interference voltage that acts between the two poles of a signal; it is primarily caused by the coupling and induction of spatial electromagnetic fields between signals, as well as by voltages generated by unbalanced circuits that convert common mode interference. This type of interference is added directly to the signal, affecting the accuracy of measurement and control. 2. Main sources of electromagnetic interference in PLC control systems: (1) Radiative interference from space. The radiative electromagnetic fields (EMI) in space are primarily generated by power grids, transient processes in electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc. This type of interference is commonly referred to as radiative interference, and its distribution is extremely complex. If a PLC system is placed within an RF field, it will experience radiation interference, the effects of which occur through two pathways: one is direct radiation inside the PLC, where interference is generated by electrical circuits ； Rather, it is the radiation from the internal network of PLC communication that causes interference due to induction from the communication lines. Radiation interference is related to the layout of on-site equipment as well as the magnitude of the electromagnetic fields generated by the equipment, particularly the frequency; protection is generally achieved by using shielded cables, local shielding for PLCs, and high-voltage discharge components. (2) Interference from leads outside the system is mainly introduced through power and signal lines, and is commonly referred to as conductive interference. This kind of interference is quite severe in industrial sites in our country. Interference from the power supply: Experience has shown that many failures in PLC control systems are caused by interference introduced by the power supply. The author encountered such a problem during the commissioning of a project; the issue was resolved only after replacing the PLC power supply with one that offered better isolation capabilities. The normal power supply for PLC systems comes from the electrical grid. Due to its wide coverage, the power grid is subject to all spatial electromagnetic interference, which induces voltages and currents in the lines. In particular, changes within the power grid—such as surge currents caused by switch operations, startup and shutdown of large electrical equipment, harmonics generated by AC/DC drive systems, and transient impacts from grid short circuits—all are transmitted to the primary side of the power source through the transmission lines. PLC power supplies typically use isolated power supplies, but due to their design and manufacturing processes, their isolation performance is not ideal. In fact, absolute isolation is impossible due to the presence of distributed parameters, especially distributed capacitance. Interference introduced from signal lines: Various signal transmission lines connected to PLC control systems, in addition to transmitting valid information, are always subject to the intrusion of external interference signals. There are mainly two ways such interference occurs: one is grid interference that enters through the power supply of the transmitter or the power supply shared by signal instruments, and this is often overlooked ； Second is the interference on the signal lines caused by spatial electromagnetic radiation, that is, external induced interference on the signal lines, which is quite serious. Interference introduced by signals can cause abnormal operation of I/O signals and a **reduction in measurement accuracy; in severe cases, it can lead to damage to components. In systems with poor isolation performance, this also leads to interference between signals, causing backflow in the common-ground system bus, which results in changes in logical data, erroneous operations, and system crashes. PLC control systems suffer from quite serious damage to their I/O modules due to interference introduced by signals, which in turn leads to numerous system failures. Interference from chaotic grounding systems. Grounding is one of the effective methods for improving the electromagnetic compatibility (EMC) of electronic devices. Proper grounding can both suppress the effects of electromagnetic interference and prevent the equipment from emitting interference ； Incorrect grounding, on the other hand, can introduce severe interference signals, preventing the PLC system from functioning properly. The ground wires of a PLC control system include system ground, shielding ground, AC ground, and protection ground, etc. The interference caused by a chaotic grounding system on PLC systems stems mainly from the uneven distribution of potential at various grounding points; there are voltage differences between these points, which generate ground loop currents and affect the proper operation of the system. For example, the cable shield must be grounded at one point only; if both ends A and B of the cable shield are grounded, a potential difference across the ground exists, causing current to flow through the shield. In the event of an abnormal condition such as a lightning strike, the current in the ground wire will be even greater. Furthermore, the shielding layer, ground wire, and earth may form a closed loop; under the influence of a changing magnetic field, induced currents can arise within the shielding layer, and through the coupling between the shielding layer and the core wire, these currents can interfere with the signal circuit. If it is systematically mixed up with other grounding treatments, the resulting ground loop currents can cause uneven potential distributions along the ground wires, affecting the proper operation of the logical and analog circuits within the PLC. PLCs have a low tolerance to logical voltage interference; disturbances in the distribution of logical voltages can easily affect their logical operations and data storage, leading to data corruption, abnormal program execution, or system crashes. The distribution of simulated ground potentials will lead to a decrease in measurement accuracy, causing severe distortion and erroneous operations in signal monitoring and control. (3) Interference from within the PLC system is primarily caused by the mutual electromagnetic radiation between components and circuits within the system, such as radiation between logic circuits and its impact on analog circuits, the interaction between analog grounds and logic grounds, as well as mismatches in the use of various components. All of these are part of the electromagnetic compatibility design carried out by PLC manufacturers within their systems; it is a complex process, and the application departments cannot make any changes to it, so there is no need to worry too much about it. However, it is important to choose systems that have a track record of successful use or have been proven reliable. III. Anti-interference design for the engineering applications of PLC control systems. To ensure that the system is protected from or experiences less internal and external electromagnetic interference in industrial electromagnetic environments, it is necessary to adopt suppression measures in three aspects starting from the design phase: suppressing the sources of interference ； Cut off or attenuate the pathways through which electromagnetic interference propagates ； Improve the anti-interference capability of devices and systems. These three points are the basic principles for suppressing electromagnetic interference. Interference resistance in PLC control systems is a systematic endeavor that requires manufacturers to design and produce products with strong interference resistance. It also depends on the users’ thorough consideration during engineering design, installation, construction, and operation and maintenance, as well as comprehensive design tailored to specific conditions, in order to ensure the electromagnetic compatibility and operational reliability of the system. When carrying out anti-interference design for specific projects, focus should be placed on the following two aspects. 1. Equipment selection: When choosing equipment, it is necessary to opt for products with high interference resistance, which includes electromagnetic compatibility (EMC), and in particular the ability to resist external interference – such as PLC systems that utilize floating ground technology and have good isolation properties ； Secondly, it is also necessary to understand the interference resistance specifications provided by the manufacturer, such as the common-mode-to-summing-mode ratio and differential-mode-to-summing-mode ratio, as well as the voltage tolerance, and the range of electric field strengths and magnetic field strengths under which operation is permitted ； Another aspect is to assess its performance in similar tasks. When choosing imported products from abroad, it is important to note that China uses a power grid system with 220V and high internal resistance, while Europe and the United States use a power grid system with 110V and low internal resistance. Due to the high internal resistance of China’s power grids, significant drift in zero potential, and large variations in ground potential, electromagnetic interference at industrial sites is at least 4 times higher than in Europe and the United States. This requires higher standards for the system’s resistance to interference; PLC products that function properly abroad may not necessarily operate reliably in Chinese industrial environments. Therefore, when using foreign products, it is necessary to select them appropriately in accordance with China’s standards (GB/T13926). 2. Comprehensive anti-interference design mainly focuses on several suppression measures against interference from outside the system. The main contents include: shielding the PLC system and its external leads to prevent electromagnetic interference from spatial radiation ； Isolate and filter the external leads, especially those related to the power supply cables; arrange them in layers to prevent the introduction of conductive electromagnetic interference through these external leads ； Properly design the grounding points and grounding devices to improve the grounding system. In addition, software solutions must also be utilized to further enhance the security and reliability of the system. IV. Main anti-interference measures 1. Use power supplies with excellent performance to suppress interference introduced from the power grid. In PLC control systems, the power supply plays a crucial role. Grid interference enters PLC control systems mainly through coupling via the power supplies of the PLC system (such as CPU power supplies, I/O power supplies, etc.), the power supplies of transmitters, and the power supplies of instruments that are in direct electrical connection with the PLC system. Currently, for the power supplies that supply energy to PLC systems, power supplies with good isolation performance are generally used. However, insufficient attention is paid to the power supplies that supply energy to transmitters and to the instruments that are electrically connected directly to the PLC system. Although certain isolation measures have been taken, they are often not sufficient; this is mainly because the isolation transformers used have large distributed parameters, resulting in poor ability to suppress interference, allowing common-mode and differential-mode interference to be introduced through power coupling. Therefore, for the power supply of transmitters and instruments that share signals, it is necessary to use power distribution units with low distributed capacitance and a wide suppression band (such as those that employ multiple isolation and shielding methods as well as leakage inductance techniques) in order to reduce interference in the PLC system. Furthermore, to ensure uninterrupted power supply to the grid feed points, an online uninterruptible power supply (UPS) can be used to enhance the safety and reliability of the power supply. Furthermore, UPS also has strong interference isolation capabilities, making it an ideal power supply for PLC control systems. 2. Installation considerations for cable selection: To reduce electromagnetic interference generated by power cables, especially those used for feeding frequency conversion devices. In a certain project, the author used copper tape-armed shielded power cables, which reduced the electromagnetic interference generated by the power lines; satisfactory results were achieved after the project was put into operation. Different types of signals are transmitted via separate cables. Signal cables should be arranged in layers according to the type of signal they transmit. It is strictly prohibited to use different wires within the same cable to transmit both power supply and signals simultaneously. Signal wires should not be laid close to power cables in order to reduce electromagnetic interference. 3. Hardware filtering and software anti-interference measures: Before the signal is fed into 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 completely eliminate its effects. Therefore, when designing and configuring the software for PLC control systems, anti-interference measures must also be implemented at the software level in order to further improve the reliability of the system. Some commonly used measures include digital filtering and power-frequency shaping sampling, which can effectively eliminate periodic interference ； Timely adjustment of the reference point potential along with the use of a dynamic zero point can effectively prevent potential drift ； Utilize information redundancy techniques to design corresponding software flag bits ； Use indirect jumps and set up software traps to improve the reliability of the software structure. 4. Select the grounding point properly and improve the grounding system. There are usually two purposes for grounding: one is for safety, and the other is to suppress interference. A proper grounding system is one of the important measures to protect PLC control systems from electromagnetic interference. The system grounding methods include floating ground, direct grounding, and capacitive grounding. For PLC control systems, which are high-speed low-level control devices, direct grounding should be employed. Due to factors such as the distributed capacitance of signal cables and the filtering by input devices, the signal exchange frequency between devices is generally below 1 MHz; therefore, PLC control systems use either single-point grounding or series single-point grounding for their grounding wires. A centrally arranged PLC system is suitable for a parallel single-point grounding scheme, with the grounding points at the center of each device’s enclosure being connected to the ground electrode via separate grounding wires. If the distance between devices is large, a series single-point grounding method should be adopted. Connect the central grounding points of the cabinets of each device using a large-cross-section copper busbar (or insulated cable), and then connect the grounding busbar directly to the grounding electrode. The grounding wire is made of copper wire with a cross-sectional area greater than 22 mm2, while the main busbar uses copper bars with a cross-sectional area greater than 60 mm2. The grounding resistance of the grounding electrode should be less than 2Ω. It is advisable to bury the grounding electrode at a distance of 10 to 15 meters from the building, and the grounding point of the PLC system must be at least 10 meters away from the grounding points of high-voltage equipment. When the signal source is grounded, the shielding layer should be grounded on the signal side ； When not grounded, grounding should be done on the PLC side ； When there are connectors in the middle of the signal cable, the shielding layer must be securely connected and insulated; multiple grounding points must be avoided at all costs ； When the shielded twisted-pair wires of multiple measurement points are connected to a multi-core twisted shielded cable, the respective shielding layers should be properly connected to one another and insulated. Select an appropriate grounding point as a single contact point. V. Conclusion Interference in PLC control systems is a highly complex issue; therefore, comprehensive consideration of various factors is necessary in anti-interference design to effectively suppress such interference. For certain types of interference, specific analysis is required, and targeted solutions must be adopted to ensure the proper functioning of the PLC control system.