Basic principles
There are many reasons for interference on-site. To solve the interference problem, the cause must first be identified, and then the problem can be addressed accordingly. Some interference can be remedied afterward, while others are very difficult to resolve later. For example, cabling is usually specified during construction; otherwise, rewiring on-site is very difficult, and sometimes simply not allowed.
The interference is primarily caused by changes in current generating a magnetic field, which in turn produces electromagnetic radiation on the equipment; changes in the magnetic field generate current, and electromagnetic high-speed waves are produced. There are four situations in the field that can cause such changes:
1. Strong electrical interference:
Instrument signals and PLC control signals are low-voltage signals and are susceptible to interference from high-voltage signals. Therefore, when wiring outside the cabinet (using cable trenches, cable trays, conduits, etc.), communication lines, signal lines, control lines, and other low-voltage signals must be kept away from high-voltage signals, with a distance of at least 20 cm. When there are multiple layers in the cable trench, low-voltage cables must be laid below high-voltage cables.
2. Interference within the cabinet:
PLCs cannot be installed in the same switch cabinet as high-voltage electrical appliances. PLC outputs are isolated from external switching signals using intermediate relays. If site conditions limit the isolation of input signals from high-voltage cables, small relays can be used to isolate the input switching signals. Of course, it is generally unnecessary to use relays to isolate input signals from within the control cabinet and input signals located nearby.
The control cabinet contains numerous signal cables. Disorganized wiring can cause equipment malfunctions, making troubleshooting extremely difficult. Therefore, this situation should be considered during control cabinet design, with equipment arranged in layers and wiring clearly laid out. When assembling a complete system, PLC I/O lines and high-power lines should be routed separately. If they must be in the same cable tray, AC and DC lines should be bundled separately. If conditions permit, separate cable trays are best, ensuring maximum spacing between them to minimize interference.
Different signal lines should not be connected to the same connector. If the same connector must be used, they should be separated by a spare terminal or a ground terminal to reduce mutual interference.
PLCs should not be installed in the same switch cabinet as high-voltage electrical appliances. Inside the cabinet, the PLC should be kept away from power lines (the distance between them should be greater than 200mm). Inductive loads such as relays and contactor coils installed in the same cabinet as the PLC should be connected in parallel with an RC arc suppression circuit.
3. Signal line anti-interference
Signal lines are responsible for transmitting detection and control signals, and their transmission quality directly affects the accuracy, stability, and reliability of the entire control system. Interference with signal lines mainly comes from electromagnetic radiation in space, and includes both differential-mode interference and common-mode interference.
Differential-mode interference refers to interference signals superimposed on the measurement signal line. This interference is mostly high-frequency alternating signals, and its source is generally coupling interference. Methods to suppress normal-mode interference include:
Connect an RC filter or a dual-T filter to the input circuit; try to use a dual-slope integrating A/D converter, as this type of integrator has a certain effect in eliminating high-frequency interference due to its operating characteristics.
The voltage signal is converted into a current signal before transmission.
Common-mode interference refers to interference signals shared by signal lines. It is generally caused by a potential difference between the grounding terminal of the signal being measured and the grounding terminal of the control system. When the period and amplitude of this interference are approximately equal on both signal lines, the methods described above cannot eliminate or suppress it. The method is as follows:
A differential amplifier employing dual differential inputs exhibits a very high common-mode rejection ratio.
The input line uses twisted wire, which reduces common-mode interference, and the inductance cancels each other out.
Opto-isolation can be used to eliminate common-mode interference;
Use shielded cable and ground it on one side only;
To avoid signal distortion, impedance matching is important for signals transmitted over long distances.
4. Inverter interference
First, the harmonics generated during the startup and operation of the frequency converter cause conducted interference to the power grid, resulting in voltage distortion and affecting the power supply quality. Second, the output of the frequency converter generates strong electromagnetic radiation interference, affecting the normal operation of surrounding equipment.
Interference handling for frequency converters is relatively complicated and generally involves the following methods:
A. Add an isolation transformer. This is mainly to address conducted interference from the power supply. It can block most conducted interference before it reaches the power supply. It also serves as a voltage converter.
B. Using a filter
Filters are divided into active and passive types, and passive filtering is generally effective. These filters have strong anti-interference capabilities and can prevent interference from the device itself from being conducted to the power supply. Some also have peak voltage absorption functions.
C. Output reactor
Adding an AC reactor between the frequency converter and the motor primarily reduces electromagnetic radiation generated during energy transmission from the frequency converter output, which could affect the normal operation of other equipment. The reactor must be installed as close as possible to the frequency converter. This method is not necessary if armored cables are used to connect the frequency converter and the motor. However, the cable armor must be reliably grounded at the frequency converter end. The grounded armor must remain intact; it cannot be twisted into a rope or braid, nor can it be extended with other wires. The cable must be connected to the frequency converter's ground terminal before grounding the frequency converter itself.
4. Communication interference: It is best to use isolated communication methods or Juten's serial port to fiber optic ring module.
II. Lines (Innate and Acquired Favorable)
1. The cable must be compatible.
Switching signals (such as signals provided by buttons, limit switches, proximity switches, etc.) generally have no special requirements for cables and ordinary cables can be used. When the signal transmission distance is long, shielded cables can be used.
Shielded cables should be selected for analog signals and high-speed signal lines (such as signals provided by pulse sensors, counters, etc.).
Communication cables require high reliability, and some communication cables have very high signal frequencies. Generally, dedicated cables provided by PLC manufacturers should be selected. When the requirements are not high or the signal frequency is low, shielded twisted-pair cables can also be used, but the quality must be good.
2. Pipeline wiring must be correct.
Keep low-voltage signal cables such as communication lines, signal lines, and control lines away from high-voltage cables, with a minimum distance of 20 cm. The distance between power cables with a voltage of 220V or higher and a current of 10A or higher and signal cables should be greater than 60 cm.
Isolate from high-voltage electricity or keep away from high-frequency interference sources (such as high-power thyristor devices, frequency converters, high-frequency welding machines, and large power lines).
If the interference still cannot be resolved after the above treatment on site, cover the pipeline with metal pipes or metal mesh.
III. Land
1. Safe ground or power supply ground;
Connecting the power cord's grounding terminal to the cabinet's grounding point constitutes a safety ground. In the event of a power leak or the cabinet becoming energized, the current can be conducted to the ground through this safety ground, preventing harm to people.
2. System grounding or main ground
As shown in the figure, the PLC controller is grounded to ensure it is at the same potential as all the controlled devices; this is called system grounding. The grounding resistance value must not exceed 4Ω.
As shown in the figure, the PLC system ground and the negative terminal of the switching power supply in the control cabinet are generally connected together to form the control system ground. ANCO's OPEN_PLC system ground is the GND terminal on the power module.
The diagram above shows the grounding schematic of the control cabinet and signal lines. Generally, signal lines must have a unique reference ground. Shielded cables, when encountering situations where conducted interference may occur, must also be uniquely grounded locally or in the control room to prevent the formation of "ground loops."
3. Signal and shielding grounding
1) Signal line grounding
Digital signals do not require grounding. Modular signals, however, must be grounded. The diagram below illustrates the grounding requirements for various wiring configurations.
a. 2-wire transmitter signal, using power supply grounding
b. For 3-wire transmitters, it is best to add isolation or use an isolated input module.
c. For 4-wire transmitters, it is best to ground the transmitting end. If grounding at the receiving end is necessary, remember to leave the transmitting end floating.
2) Shielding ground can only be grounded at a single point (if it is high frequency, then both ends should be grounded; generally, for analog signal transmission, the main purpose is to prevent interference, so it is not advisable to ground both ends).
3) Communication grounding
If RS-485 communication is non-isolated, the GND of the power supply (5V, GND) of each node must be grounded. RS-485 communication uses isolation, as shown in the diagram below; single-point grounding ensures more stable communication. Shielded twisted-pair cables are used for communication, so the shield must be grounded – single-point grounding. Field conditions are complex and can cause various interferences to signal lines. How can simple testing equipment be used to determine if interference exists? The steps are as follows:
1) Use a multimeter in AC mode to test the receiver.
㊀. Interference will generate an AC signal. If this signal is small, it will have little to no impact on signal acquisition. If the AC signal is large, it will affect the numerical values, and a solution needs to be found.
2) Check if the terminal is grounded. If so, check for any floating or poor grounding issues. Use a multimeter to measure the voltage difference between the terminal and ground (either system ground or signal ground).
3) The presence of AC voltage indicates the presence of interference;
4) If there is no AC voltage, there is a DC voltage difference. A large voltage difference will affect the system; a small difference will have a negligible impact.
5) Next, check if the shielding layer is grounded, whether it is single-point grounded or double-point grounded. Generally, it is single-point grounded.
4.2 Is there any interference with the grounding wire?
1) Bend the signal cable and use a multimeter to test the signal cable in the corresponding range. If the signal is normal, then it's OK!
2) Determine if the negative terminal is grounded. If it is, then OK! If it is not grounded, it is best to ground it at the sensor end.
3) If the problem persists, add an isolator to the signal line at the receiving end.
To summarize the above three points, the following aspects should be considered when handling grounding in a project:
1. Connect the machine body to the AC power supply Gnd and ground it.
2. Connect the ground terminal of the DC power supply used inside the cabinet to the system ground.
3. Ground the shielding cable for transmitting analog signals at a single point. To discharge high-frequency interference, the shielding layer of digital signal lines should be connected in parallel with a potential equalization line whose resistance should be less than 1/10 of the shielding layer's resistance, and the two ends of the shielding layer should be grounded. If the interference still cannot be resolved, add an isolator.
4. All communication lines should be grounded; otherwise, they should be fully isolated or converted to fiber optic communication to avoid any interference.
5. The distance between the shielded grounding electrode and the grounding electrode of other high-voltage equipment such as the transformer neutral line shall be greater than 15m.
6. Signal lines must have a unique reference ground. Shielded cables must also be uniquely grounded locally or in the control room in situations where conducted interference may occur, to prevent the formation of a "ground loop".
4P (Four Professional Skills)
The 4P framework comprises four parts: design, on-site construction, commissioning, and on-site testing. Each part requires technically skilled and experienced engineers to complete.
1. Design
Quality is designed in; in the initial design phase of an engineering project, engineers consider all possible interference phenomena. This generally includes the following aspects:
A. Grounding system design (refer to the three-ground treatment principle)
B. The design of the pipeline should select appropriate signal lines and communication lines, and adopt a conservative pipeline design. In particular, the communication pipeline should preferably use all-metal conduits.
C. Power supply design, especially in applications involving frequency converters, requires particular attention to power supply isolation.
D. The system's communication is the most critical part of the system, and it must be 100% stable and reliable.
E. Matching of signal and module types
The selection of digital input modules, analog input modules, sensors, communication lines, etc., must be made by experienced engineers. For example, analog input modules do not necessarily need to be isolated modules, but knowing when to use isolated modules requires extensive engineering experience. Generally, the following applies:
The digital input uses passive contact inputs; active inputs are not used.
Analog signals are isolated.
Communication lines should be isolated, or preferably fiber optic cables should be used.
Analog signals can only be grounded on one side.
2. On-site construction
The construction site conditions are complex and varied, requiring on-site construction supervision by engineers with considerable experience.
To minimize interference, the engineering designers must explain relevant matters to the site engineers before commencing work, including precautions, special construction requirements, etc. Site construction personnel must receive appropriate training. Construction personnel must follow the drawings precisely, clearly marking all wiring, especially the grounding wire. Sometimes, a single incorrectly connected wire can cause problems for the entire system.
3. On-site testing
Testing requires methodology and should be conducted step by step. When problems arise, it's crucial to analyze them systematically and methodically. Conduct testing step-by-step to identify the root cause of the problem, and then address it accordingly.
4. Test run
The test run requires the cooperation of all units. A detailed plan must be developed in advance, and unified command and division of labor on site are necessary to ensure the test run is effective and safe.
In practice, test drive plans are often overlooked, and without proper prior coordination, the situation can become chaotic. Therefore, it is essential to have experienced personnel coordinate all aspects.
V. Five technologies
1. Instrumentation Technology
The selection of instruments is crucial; incorrect selection will render them unusable and cause significant inconvenience to subsequent work. Before use, instruments must be calibrated to ensure accurate testing. Therefore, two requirements apply to instruments: correct selection and accurate measurement.
2. PLC Design
This includes system design, cabinet design, construction drawing design, program design, wiring design, etc. During the design process, the aforementioned three aspects (one main road, two secondary roads, and three territories) must be taken into account.
3. On-site technology
This refers to technologies such as grounding, power supply, pipeline layout, cable laying, and construction safety. Proper construction procedures include a series of reporting guidelines. For example, for cables, there are requirements ranging from the wiring method to the crimped terminals.
4. Communication technology
This refers to system communication design, communication cable selection, wiring, construction, and testing. Communication options are generally selected based on project requirements.
5. SCADA technology
This refers to the system design, host computer configuration screen, database design, and print report design, all aimed at providing a user-friendly design for the operation and management of the automatic control system.
Provide relevant case studies to illustrate the situation, based on practical applications.
Case 1:
A power plant used ANCO's EIO_2000 series EUI_08 modules to collect signals. Six modules were placed on the same communication line. The module communication would be interrupted. After powering on, the communication would be restored, but it would immediately disconnect again.
1. From the site observation, the 24VDC power supply of the module is connected from PS307. This power supply has stable performance, ruling out interference caused by the power supply.
2. Most EUI_08 modules connect to PT100, K-type and T-type thermocouple signals. Observing the incoming line in the control cabinet, the cables are all shielded twisted-pair cables, and the shield ends are braided and connected to the grounding busbar, which is grounded by the field grounding network. Field interference should be shielded. To ensure this, the module terminals were disconnected, and the module communication status remained unchanged, ruling out the possibility that interference was caused by the signal terminals.
3. Upon opening the cable tray cover inside the control cabinet, it was found that the communication lines between modules used two single wires without grounding, and the wiring in the cable tray was messy, which could potentially cause interference. The modules were connected to the controller using Category 5 cable, which had a short distance and simple routing within the tray, suggesting minimal interference in this area.
The above analysis revealed that the problem likely lay in the communication cable between modules. The solution was to replace the communication cable with a shielded twisted-pair cable, with the shield grounded. After the replacement was performed on-site, communication returned to normal.
