DCS reasonable and reliable system grounding is a very important content of DCS system. In order to ensure the monitoring and control accuracy and safe and reliable operation of the DCS system, careful design and overall consideration must be given to the system grounding method, grounding requirements, signal shielding, ground wire cross-section selection, ground electrode design, and ground box layout. This article discusses and briefly introduces the grounding of the DCS system according to the design specifications of the DCS system for everyone to refer to in the design, installation, and maintenance of the DCS system.
1. Basic requirements for DCS system grounding
The DCS system grounding is to ensure that when there is a problem with the signal, power supply or DCS system equipment entering the DCS system, the effective grounding system can withstand the overload current and can quickly lead the overload current to the ground. The grounding system can provide a shielding layer for DCS, eliminate electronic noise interference, and provide a common signal reference point (that is, reference zero potential) for the entire control system. When there is a problem with the grounding system (the grounding resistance is too large, multiple points of grounding, the grounding wire is broken or the grounding wire is in contact with high-voltage, high-current equipment, etc.), it will cause electric shock injury to the personnel and damage to the equipment. It is understood that some Power plant DCS systems often "crash" (or unexplained "crash"), mostly caused by poor or existing grounding systems. Therefore, perfect, reliable and correct grounding is the key to the safe, reliable and good operation of the DCS system.
1.1 DCS grounding classification
In general, DCS control systems require two types of grounds: protective ground and working ground (logic ground, shield ground, etc.). For systems with safety barrier explosion protection measures, such as those used in the chemical industry, intrinsic safety is also required.
1.1.1 Protection Ground (CG, Cabinet Grounding) is a protective measure taken to prevent the accumulation of static charge on the device casing and avoid personal injury. All operator cabinets, field control station cabinets, printers, terminal cabinets, etc. of the DCS system should be connected to the protective ground. The protective ground should be connected to the electrical grounding grid in the plant area, and the grounding resistance should be less than 4Ω.
1.1.2 Logic ground: It is also called the machine logic ground and the host power ground. It is the common ground of the negative end of the logic level inside the computer, and also the power output ground of + 5V and so on. For example, the negative end of the positive and negative 5 volts of the CPU and the positive and negative 12 volts. Need to connect to the common ground electrode.
1.1.3 Analog Grounding (AG, Analog Grounding) is also called analog ground, it can shield the interference received by the field signal transmission to improve the signal accuracy. The shielding layer of the signal cable in the DCS system should be shielded and grounded. The cable shielding layer must be grounded at one end to prevent the formation of closed loop interference. The metal armor of the armored cable should not be used as shielding protection ground, but must be grounded with copper wire mesh or aluminum-plated shield. Connect to the common ground electrode.
1.1.4 Intrinsically safe ground should be set independently with a grounding system with a grounding resistance of ≤4Ω. The grounding system of the intrinsically safe ground should be kept independent, and the distance to the grounding grid of the plant electrical ground or other instrument systems should be more than 5m.
1.2 DCS system grounding method DCS system general grounding method
1.2.1 Use the electrical grounding grid as the DCS grounding grid, that is, share the ground with the electrical grounding grid;
1.2.2 Set up a dedicated independent grounding network for the DCS system;
1.2.3 Set up a dedicated DCS grounding grid, and then connect it to the electrical grounding grid through the grounding wire;
Since the third grounding method has many similarities with the second grounding method, in the past, the computer or DCS system used to use a dedicated grounding network. However, the disadvantages of this grounding method are: too large floor area, high investment, large cable and ground grid steel consumption, a considerable distance from the plant (because it is not easy to find a suitable location in the plant) It is inconvenient to measure and find the ground electrode and ground wire, and the effect is not very good. According to the actual operation, it is difficult and unsafe to set up a dedicated DCS grounding grid. For example, a power plant had tripped dozens of times due to grounding problems. According to the survey, many power plants DCS later switched to electrical grounding grids and achieved good results.
1.3 Requirements for the common ground electrode (grid)
1.3.1 When the resistance of the electrical grounding grid in the plant area to the ground is less than or equal to 4Ω, the electrical grounding grid in the plant area can be regarded as the common grounding pole (net) of the DCS system.
1.3.2 When the grounding resistance of the electrical grounding grid in the plant area is large or messy, the grounding system should be set independently, which is the common grounding pole (net) of the DCS system.
1.3.3 The distribution resistance to ground of the public ground electrode (net) without intrinsically safe ground connection is less than 4 ohms; with intrinsically safe ground is less than 1 ohm. The line impedance of the grounding main line is less than 0.1 ohm.
1.3.4 There is no access point for lightning protection within 15 meters around the grounding electrode, and there is no access point for the housing of high and low voltage electrical equipment of more than 30KW within 8 meters. When this condition cannot be met on site, the lightning protection ground is connected to the main line of the common grounding pole through the surge arrester / shock wave suppressor. Welding ground must not overlap with the common grounding electrode and its grounding grid, the distance between the two should be more than 10 meters.
2. The grounding principle of DCS system
2.1 Grounding device set in DCS system
2.2.1 Operation desk, printing desk, server cabinet: with protective ground screws.
2.2.2 Relay cabinet, UPS cabinet, power distribution cabinet: with protective ground screws.
2.2.3 DCS I / O cabinet: It is equipped with shielded ground bus bar and protective ground screw. Systematic (+ 24V ground) suspension.
2.2.4 Instrument cabinet, hand-operated panel: with shielded grounding busbar and protective grounding screw.
2.2.5 Safety grid cabinet: equipped with shielded grounding busbar, intrinsically safe grounding busbar, and protective ground screw.
2.2 Signal shielding and grounding
2.2.1 According to the requirements of relevant technical regulations, the shielding layer of the signal cable of the computer or DCS system must not float, and must be grounded, and the grounding method should meet the following requirements:
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 is connected to the shield, and the other end should be connected to the common mode ground (when the signal source is grounded, then to the signal ground. When the signal source is floating, then to the ground)
2.2.1.4 When the shielded cable is broken or merged through the junction box, the shield layers of the two ends of the cable should be connected in the junction box.
2.2.2 The selection and laying of signal cables for the DCS system shall be carried out in strict accordance with the relevant regulations. The shielding layer of the shielded cable should be grounded according to the above requirements. In order to improve the anti-interference ability of the DCS system, it is more appropriate to select the flame-retardant twisted copper network shielded computer cable for the digital input / output signal of the DCS system.
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