1. Basic requirements for grounding in DCS systems
DCS system grounding is essential to ensure that when problems arise with signals, power supply, or the DCS equipment itself, an effective grounding system can withstand overload currents and quickly conduct them to the ground. The grounding system provides a shielding layer for the DCS, eliminates electronic noise interference, and provides a common signal reference point (i.e., reference zero potential) for the entire control system. Problems with the grounding system (excessive grounding resistance, multiple grounding points, broken grounding wires, or grounding wires coming into contact with high-voltage, high-current equipment, etc.) can cause electric shock injuries and equipment damage. It is understood that frequent "crashes" (or unexplained "crashes") in some power plant DCS systems are mostly caused by poor or problematic grounding systems. Therefore, a complete, reliable, and correct grounding system is crucial for the safe, reliable, and efficient operation of a DCS system.
1.1 DCS Grounding Classification
Under normal circumstances, DCS control systems require two types of grounding: protective ground and working ground (logic ground, shield ground, etc.). For systems equipped with explosion-proof safety barriers, such as those used in the chemical industry, intrinsically safe ground is also required.
1.1.1 Protective grounding (CG) is a protective measure taken to prevent the accumulation of static charge on equipment casings and avoid personal injury. All operator cabinets, field control station cabinets, printers, terminal cabinets, etc., of the DCS system should be connected to protective grounding. The protective grounding should be connected to the plant's electrical grounding network, and the grounding resistance should be less than 4Ω.
1.1.2 Logic Ground: Also called machine logic ground or host power ground, it is the common ground for the negative terminals of logic levels inside the computer, and also the power output ground for +5V, etc. For example, the negative terminals of the CPU's ±5V and ±12V. It needs to be connected to a common ground terminal.
1.1.3 Analog Ground (AG), also known as analog ground, shields against interference during signal transmission, improving signal accuracy. The shielding layer of signal cables in a DCS system should be grounded. One end of the cable shielding layer must be grounded to prevent closed-loop interference. The metal armor of armored cables should not be used as a protective ground; the copper wire mesh or aluminum-plated shielding layer must be grounded. Connect to the common grounding electrode.
1.1.4 The intrinsically safe grounding system shall be independently equipped with a grounding system, and the grounding resistance shall be ≤4Ω. The grounding system of the intrinsically safe grounding system shall be kept independent and the distance from the plant's electrical grounding network or other instrumentation system grounding network shall be more than 5m.
1.2 DCS System Grounding Methods General grounding methods for DCS systems
1.2.1 Utilize the electrical grounding grid as the DCS grounding grid, i.e., share the same ground with the electrical grounding grid;
1.2.2 A dedicated and independent grounding grid shall be provided for the DCS system;
1.2.3 A dedicated grounding grid for the DCS shall be installed, which shall be connected to the electrical grounding grid via a grounding wire;
Because the third grounding method shares many similarities with the second, dedicated grounding grids were previously widely used in computer or DCS systems. However, this grounding method has several drawbacks: it requires a large area, involves high investment, consumes a large amount of cable and grounding grid steel, is located a considerable distance from the plant (due to the difficulty of finding suitable locations within the plant), and is inconvenient for management, maintenance, measurement, and locating grounding electrodes and wires, with less than ideal results. Actual operation has shown that setting up a dedicated DCS grounding grid is both difficult and unsafe. For example, one power plant experienced dozens of unit trips due to grounding issues. Investigations have shown that many power plants later switched to electrical grounding grids for their DCS systems, achieving positive results.
1.3 Requirements for the common grounding electrode (grid)
1.3.1 When the ground distribution resistance of the plant's electrical grounding network is ≤4Ω, the plant's electrical grounding network can be used as the common grounding electrode (network) of the DCS system.
1.3.2 When the grounding resistance of the electrical grounding network in the plant area is large or messy, an independent grounding system should be set up, which is the common grounding electrode (network) of the DCS system.
1.3.3 The grounding resistance of a common grounding electrode (network) without intrinsic grounding connection is less than 4 ohms; the resistance of a common grounding electrode (network) with intrinsic grounding connection is less than 1 ohm. The line impedance of the main grounding line is less than 0.1 ohms.
1.3.4 There should be no lightning protection grounding connection point within 15 meters of the grounding electrode, and no connection point for the casing of high or low voltage electrical equipment exceeding 30KW within 8 meters. When this condition cannot be met on site, the lightning protection grounding should be connected to the main line of the common grounding electrode through a surge arrester/surge suppressor. Welding grounding must not be connected to the common grounding electrode and its grounding grid; the two should be at least 10 meters apart.
2. Grounding principles of DCS systems
2.1 Grounding device of DCS system
2.2.1 Control panel, printing table, server cabinet: equipped with protective grounding screws.
2.2.2 Relay cabinets, UPS cabinets, and power distribution cabinets: are equipped with protective grounding screws.
2.2.3DCS I/O cabinet: Equipped with a shielded grounding busbar and protective grounding screws. System ground (+24V ground) is floating.
2.2.4 Instrument cabinets and manual control panels: equipped with shielded grounding busbars and protective grounding screws.
2.2.5 Safety barrier cabinet: equipped with shielded grounding busbar, intrinsically safe grounding busbar, and protective grounding screw.
2.2 Signal shielding and grounding
2.2.1 According to relevant technical regulations, the shielding layer of computer or DCS system signal cables must not be floating and must be grounded. The grounding method shall comply with the following regulations:
2.2.1.1 When the signal source is floating, the shielding layer should be grounded on the computer side;
2.2.1.2 When the signal source is grounded, the shielding layer should be grounded on the signal source side;
2.2.1.3 When the amplifier is floating, one end of the shielding layer should be connected to the shielding cover, and the other end should preferably be connected to common mode ground (connect to signal ground when the signal source is grounded, and connect to field ground when the signal source is floating).
2.2.1.4 When a shielded cable is broken or merged at a junction box, the shielding layers of the cables at both ends should be connected inside the junction box.
2.2.2 The selection and installation of signal cables for DCS systems should strictly comply with relevant regulations. The shielding layer of shielded cables should be grounded according to the above requirements. To improve the anti-interference capability of the DCS system, flame-retardant twisted-pair copper mesh shielded computer cables are appropriate for DCS system switch input/output signals.
3. Grounding method of DCS system
3.1 Grounding method for centrally located DCS equipment
3.2 Grounding methods for distributed DCS equipment
The connections between devices in a distributed DCS system are generally network (communication) cables. For example, field control stations are distributed to the field, while operator stations are located in different control rooms, with a distribution diameter of 500 meters. Multimode fiber optic cables, Category 5 twisted-pair cables, or DP shielded twisted-pair cables are used to connect the stations.
3.2.1 Sites using fiber optic connections: The grounding method within each site is the same as that of the centrally located DCS equipment.
3.2.2 Sites connected using Category 5 twisted-pair cable or DP shielded twisted-pair cable:
3.2.2.1 All grounding wires in the control room are first connected to the common connection board, which is then connected to the common grounding electrode via the main grounding line. Viewed from the common grounding electrode, the entire grounding network forms a star topology.
3.2.2.2 Use Category 5 twisted-pair cable or DP shielded twisted-pair cable to connect both ends to the DCS's switch, hub, repeater, or other network equipment via network surge protection devices (signal surge arresters with a current carrying capacity of not less than 5kA). Each site has its own common grounding electrode; a metal connection between the two is not necessary. The grounding method for each site is the same as for centrally located DCS equipment. Category 5 twisted-pair cable or DP shielded twisted-pair cable must be laid in galvanized steel conduit or metal cable trays, and the conduit or cable tray must be reliably grounded. When lightning strikes or electrical accidents cause an excessive potential difference between the grounding points, the signal surge arrester can protect the equipment on both sides.
3.3 DCS Equipment Grounding Installation
3.3.1 Grounding electrode: A good conductor driven into the ground, the current from the main grounding line is conducted to the earth through the grounding electrode. The grounding electrode is brazed to the main grounding line, and anti-corrosion treatment should be applied after welding. Multiple grounding electrodes can be connected into a network using the grounding grid main line. The grounding grid should meet the grounding resistance requirements of the DCS system. When the grounding grid main line and the grounding electrode are lap-welded, the lap length must be twice the width of the flat steel or six times the diameter of the round steel. Figure 3-2 shows a typical installation diagram of multiple grounding electrodes.
3.4 Methods for reducing soil resistivity through grounding of DCS systems
3.4.1 Modify the soil structure around the grounding electrode. Within a 2-3m range around the grounding electrode, mix in a water-insoluble material with good water absorption, such as charcoal, coke, coal ash, or slag. This method can reduce the soil resistivity to 1/5 to 1/10 of the original value.
3.4.2 Reducing Soil Resistivity with Salt and Charcoal: Use salt and charcoal in layers, compacting them together. Mix charcoal with finely ground materials to form a layer about 10-15cm thick, then add 2-3cm of salt, repeating this process 5-8 times. After layering, drive in a grounding electrode. This method can reduce the resistivity to 1/3 to 1/5 of its original value. However, the salt will be washed away by water over time, so it generally needs to be replenished every two years.
3.4.3 Use long-acting chemical drag-reducing agents. Using long-acting chemical drag-reducing agents can reduce soil resistivity to 40% of its original value.
3.5 DCS System Grounding Materials and Requirements
3.5.1 Material requirements for grounding electrodes and grounding grid trunk lines
The specifications of the steel used for grounding electrodes and grounding grid trunk lines can be selected according to the table below. If the grounding resistance cannot meet the requirements, copper can also be used. If the grounding electrodes and grounding grid trunk lines are installed in highly corrosive environments, anti-corrosion measures such as hot-dip galvanizing or hot-dip tinning should be taken according to the nature of the corrosion, or the cross-section should be appropriately increased.
3.5.2 Grounding connection requirements
The protective ground and shield ground connections of the DCS system should use copper-core insulated wires or cables to connect to the plant's dedicated electrical grounding network or grounding electrode. The table lists the specifications of various grounding cables that can be used. When the grounding connection distance is long, the DCS system has high requirements for grounding resistance, or the number of branch lines branching from the grounding main line is large, it is advisable to select wires and cables with larger cross-sections as shown in the table.
4. Common precautions for on-site grounding
4.1 Field Control Station: Because the cabinet body and base are insulated by rubber, and the shielding busbar is insulated from the base, the field control station must be properly grounded according to regulations. Specifically, it should be connected to the field control station's grounding busbar. The power ground of the I/O cabinet and the UPS must be connected to the same ground to ensure equipotentiality.
4.2 Field control station: Operator station, engineer station, network switch, server host, system monitor, etc., are grounded by the shell or the power ground wire is directly connected to the electrical grounding network.
4.3 I/O Modules: Connect the negative terminal of the analog module (pin 40, i.e., the DC 24V) to the logic ground bus. Connect the logic ground bus to the shield ground, and then to the main ground bus.
4.4. The protective ground of the field control station should be connected from the grounding screw at the bottom of the cabinet to the grounding branch line, and the shielding ground of the field control station should be connected from the grounding busbar to the common connection board.
4.5 The resistance of the grounding system must be tested to ensure that the grounding meets the requirements of the control system manufacturer.
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