With the rapid development of the power industry and the increase in the level of thermal automation, the Distributed Control System (DCS) has been widely used in domestic power plants. This has played a very important role in ensuring the safety, economical, and civilized operation of the power plant, and has achieved Good results.
DCS reasonable and reliable system grounding is a very important part of the 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 system grounding methods, grounding requirements, signal shielding, cross-section selection of grounding lines, grounding electrode design, and grounding box layout. This paper discusses the DCS system grounding according to the DCS system design specification requirements and briefly introduces it for everyone's reference in DCS system design, installation, and maintenance.
1. Basic Requirements for DCS System Grounding DCS system grounding is to ensure that when entering the DCS system signal, power supply or DCS system equipment itself, CONTROLENGINEERING China, effective grounding system can withstand overload current and can quickly import the overload current Earth. The grounding system can provide shielding for the DCS, eliminate electronic noise interference, and provide a common signal reference point (ie, reference zero potential) for the entire control system. When a problem occurs in the grounding system (large grounding resistance, multiple grounding, disconnection of the grounding wire, or contact of the grounding wire with high-voltage, high-current devices, etc.), it may cause electric shock and damage to the equipment. It is understood that some Power plant DCS systems often "crash" (or unexplained "crash"), mostly due to poor grounding systems or problems. Therefore, the perfect, reliable, and correct grounding of CONTROLENGINEERINGChina is the key to the DCS system being safe, reliable and well-functioning.
1.1 DCS grounding classification Under normal circumstances, the DCS control system requires two types of grounding: protected ground and working ground (logical ground, shielded ground, etc.). For systems equipped with safety barrier explosion protection measures, such as those used in the chemical industry, there is also a requirement for intrinsically safe areas.
1.1.1 The protective ground (CG, CabinetGrounding) is a protection measure taken to prevent static charge accumulation of the device housing and to avoid personal injury. All operator cabinets, field control station cabinets, printers, and terminal cabinets of the DCS system should be connected to a protective ground. The protective ground should be connected to the professional electrical grounding network of the plant, and the grounding resistance should be less than 4Ω.
1.1.2 Logical Ground: Also called the logical ground of the machine, the host power ground, the internal ground of the negative logic level of the internal logic of the computer, and also the power output ground of +5V. For example, the CPU is plus or minus 5 volts and plus or minus 12 volts. Need to access the common ground electrode.
1.1.3 The analog ground (AG) is also called analog ground. It can shield the interference from the on-site 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 shield must be grounded at one end to prevent closed loop interference. Metal armor of armoured cables should not be used as a shielded protective earth. It must be a copper wire mesh or aluminized shielded earth. Access to the common ground electrode.
1.1.4 Intrinsically safe earthing system shall be set independently, with grounding resistance ≤ 4Ω. The grounding system of the local safety should be kept independent, and the distance from the grounding network of the plant's electrical ground network or other instrumentation system should be more than 5m.
1.2 DCS system grounding method DCS system general grounding method 1.2.1 use of electrical grounding nets as DCS grounding net CONTROLENGINEERINGChinaCopyright, that is, common ground with electrical grounding net;
1.2.2 Set up an independent grounding network for the DCS system;
1.2.3 set up a DCS-specific grounding network, and then connect it to the electrical grounding network through the grounding wire;
Because the third grounding method and the second grounding method are more similar, in the past, computers or DCS systems have used dedicated grounding nets. However, the shortcomings of this grounding method are: large area, high investment, large consumption of steel wire and grounding network, a considerable distance from the plant (because it is not easy to find a suitable location in the plant), management, maintenance, It is inconvenient to measure and find the grounding electrode and grounding wire, and the result is not very good. According to the actual operation, it is difficult and insecure to set a dedicated DCS grounding network. For example, a power plant once caused the unit to trip dozens of times due to grounding problems. According to the survey, many power plants DCS later switched to grounding with electrical grounding networks and achieved good results.
1.3 Requirements for the common ground electrode (network) 1.3.1 When the electrical resistance of the plant's electrical grounding network to the ground is ≤ 4Ω, the electrical grounding network of the plant 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 disordered, the grounding system should be set independently, which is the common grounding pole (network) of the DCS system.
1.3.3 The common ground electrode (net) with no intrinsically safe access to the earth has a resistance of less than 4 ohms; there is less than 1 ohm of the intrinsically safe earth. The line impedance of the grounded mains is less than 0.1 ohms.
1.3.4 There is no access point for lightning protection within 15 meters around the grounding pole, and there is no access point for the shell of high and low voltage power equipment above 30KW within 8 meters. When the site cannot meet this condition, the lightning protection zone is connected to the main ground of the common grounding pole through the arrester/shock suppressor. Do not connect the grounding electrode to the common grounding electrode and its grounding grid. The distance between the two should be more than 10 meters.
2. Grounding principle of DCS system 2.1 Grounding device set in DCS system 2.2.1 Operation table, printing table, server cabinet: There is a protective floor screw.
2.2.2 Relay Cabinets, UPS Cabinets, and Power Distribution Cabinets: Protective screws are provided.
2.2.3 DCS I/O Cabinet: shielded ground busbars and ground screws. Systemically (+24V ground) is suspended.
2.2.4 Instrument cabinets and manual operation panel: It is equipped with a shielded grounding busbar and a protective ground screw.
2.2.5 Safety Fence Cabinet: There is a shielded grounding busbar, and the local safety grounding busbar is provided by CONTROLENGINEERING China.
2.2 Signal shielding and grounding 2.2.1 According to the relevant technical requirements, the shielding layer of the signal cable of the computer or DCS system must not be floating and must be grounded. The grounding method of the signal cable must meet the following requirements:
2.2.1.1 When the signal source is floating, the shield should be grounded on the computer side;
2.2.1.2 When the signal source is grounded, the shield should be grounded at the 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 is connected to the common mode ground (When the signal source is grounded, the signal ground is connected. When the signal source is floating, it is connected to the field ground).
2.2.1.4 When the shielded cables are routed through the junction box or combined, the shields at both ends of the cables shall be connected within the terminal box.
2.2.2 DCS system signal cable selection and laying, should be implemented in strict accordance with the relevant provisions. The shielded cable shield should be grounded as described above. In order to improve the DCS system's anti-interference ability, DCS system switch input / output signal, the use of flame-retardant twisted pair copper screen computer cable is quite appropriate.
3. Grounding method of DCS system 3.1 DCS equipment grounding method of centralized arrangement 3.2 DCS equipment grounding method of decentralized arrangement Distributed DCS system The connection between the equipment is generally a network (communication) line, for example: the field control station is dispersed to the site, and The operator stations are located in different control rooms with a dispersed diameter in the range of 500 meters. Multi-mode optical fibers or Category 5 twisted pair or DP shielded twisted pair cables are used between the stations.
3.2.1 Sites using fiber optic connections: The grounding methods in each station are the same as the centralized DCS equipment.
3.2.2 Sites using Category 5 twisted pair or DP shielded twisted pair:
3.2.2.1 The ground wires of the control room are first connected to the common connection board, and the common connection board is connected to the common grounding electrode through the grounding trunk line. Seen from the common grounding pole, the entire grounding network is a star structure.
3.2.2.2 Use Category 5 twisted pair or DP shielded twisted pair The two ends are connected to the DCS SWITCH, HUB, REPEAT, or other network equipment through a network surge protection device (signal lightning arrester, flow rate not less than 5KA). The two sides of the site have their own common grounding poles, and they do not need to have metal connections. The grounding method of each site is the same as the centralized DCS equipment. Category 5 twisted pair or DP shielded twisted pair must be laid on galvanized steel pipe or metal bridge, and the steel pipe or bridge must be reliably grounded. When lightning strikes, or electrical accidents cause the potential difference between the two sides is too large, the signal lightning arrester can protect the equipment on both sides.
3.3 DCS equipment grounding installation 3.3.1 grounding body: to be ground into a good conductor, the current sent from the grounding main trunk through the grounding body into the earth. The brazed joint between the grounding body and the grounding trunk line shall be treated with anti-corrosion after welding. The grounding grid can be used to connect multiple grounding bodies into a network. The grounding grid should meet the DCS system grounding resistance requirements. When the grounding wire and the grounding body are lap welded, the overlap length must be 2 times the width of the flat steel or 6 times the diameter of the round steel. Figure 3-2 shows a typical multi-grounding installation.
3.4 DCS system grounding method to reduce the soil resistivity 3.4.1 Change the soil structure around the grounding body. In the range of 2~3m around the grounding body, do not incorporate water-absorbent substances with good water absorption, such as charcoal, coke cinder, or slag. This method can reduce the soil resistivity to 1/5 of the original. ~1/10.
3.4.2 Use salt, charcoal to reduce the soil resistivity Layered with salt, charcoal compaction. Charcoal and fine doping are one layer, about 10~15cm thick, and then 2~3cm salt, total 5~8 layers. After being laid into the grounding body. This method can reduce the resistivity to 1/3 to 1/5. However, the salt will be lost with the water for a long time, and it will normally be added once more than two years.
3.4.3 Use a long-lasting chemical resistance reducer. The long-acting chemical resistance-reducing agent method can reduce the soil resistivity to 40%.
3.5 DCS system grounding materials and requirements 3.5.1 Material requirements for grounding body and grounding network trunk The steel specifications used for the grounding body and grounding network trunk can be selected according to the following table. If the grounding resistance cannot meet the requirements, copper can also be used. If the grounding body and grounding network cable are installed in places with high corrosiveness, corrosion protection measures such as hot-dip galvanizing, hot-dip tinning, or appropriate cross-sections should be taken according to the nature of the corrosion.
3.5.2 Ground connection requirements The DCS system's protective ground and shielding ground connection should use copper core insulated wire or cable to connect to the factory electrical special grounding net or grounding body. The small table lists the available specifications for various types of grounding cables. When the distance of the grounding connection is long, the DCS system requires high grounding resistance, or the number of branching lines for grounding the mains is relatively large, the wires and cables with larger sections in the table should be selected.
4. Common grounding precautions for on-site grounding 4.1 Site control station: The grounding screw is insulated by the rubber between the cabinet body and the base, and the shielded ground busbar is insulated from the base. The site control station must be properly grounded according to the regulations. That is, they are connected to the ground busbars of the field control station. The power ground of the I/O cabinet and the power ground of the UPS must be connected to the same ground to ensure equal potential.
4.2 Field Control Station: The operator station, engineer station, network switch, server host, system monitor, etc. are grounded or directly connected to the electrical grounding network.
4.3 I/O Module: The 40-terminal of the analog module is the negative terminal of 24 V DC connected to the logic bus. The logical busbar is connected to the shield ground and then to the total ground bus.
4.4. The protective earth of the site control station shall be connected from the grounding screw at the bottom of the cabinet to the earthing branch line, and the shielded ground of the field control station shall 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.
DCS reasonable and reliable system grounding is a very important part of the 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 system grounding methods, grounding requirements, signal shielding, cross-section selection of grounding lines, grounding electrode design, and grounding box layout. This paper discusses the DCS system grounding according to the DCS system design specification requirements and briefly introduces it for everyone's reference in DCS system design, installation, and maintenance.
1. Basic Requirements for DCS System Grounding DCS system grounding is to ensure that when entering the DCS system signal, power supply or DCS system equipment itself, CONTROLENGINEERING China, effective grounding system can withstand overload current and can quickly import the overload current Earth. The grounding system can provide shielding for the DCS, eliminate electronic noise interference, and provide a common signal reference point (ie, reference zero potential) for the entire control system. When a problem occurs in the grounding system (large grounding resistance, multiple grounding, disconnection of the grounding wire, or contact of the grounding wire with high-voltage, high-current devices, etc.), it may cause electric shock and damage to the equipment. It is understood that some Power plant DCS systems often "crash" (or unexplained "crash"), mostly due to poor grounding systems or problems. Therefore, the perfect, reliable, and correct grounding of CONTROLENGINEERINGChina is the key to the DCS system being safe, reliable and well-functioning.
1.1 DCS grounding classification Under normal circumstances, the DCS control system requires two types of grounding: protected ground and working ground (logical ground, shielded ground, etc.). For systems equipped with safety barrier explosion protection measures, such as those used in the chemical industry, there is also a requirement for intrinsically safe areas.
1.1.1 The protective ground (CG, CabinetGrounding) is a protection measure taken to prevent static charge accumulation of the device housing and to avoid personal injury. All operator cabinets, field control station cabinets, printers, and terminal cabinets of the DCS system should be connected to a protective ground. The protective ground should be connected to the professional electrical grounding network of the plant, and the grounding resistance should be less than 4Ω.
1.1.2 Logical Ground: Also called the logical ground of the machine, the host power ground, the internal ground of the negative logic level of the internal logic of the computer, and also the power output ground of +5V. For example, the CPU is plus or minus 5 volts and plus or minus 12 volts. Need to access the common ground electrode.
1.1.3 The analog ground (AG) is also called analog ground. It can shield the interference from the on-site 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 shield must be grounded at one end to prevent closed loop interference. Metal armor of armoured cables should not be used as a shielded protective earth. It must be a copper wire mesh or aluminized shielded earth. Access to the common ground electrode.
1.1.4 Intrinsically safe earthing system shall be set independently, with grounding resistance ≤ 4Ω. The grounding system of the local safety should be kept independent, and the distance from the grounding network of the plant's electrical ground network or other instrumentation system should be more than 5m.
1.2 DCS system grounding method DCS system general grounding method 1.2.1 use of electrical grounding nets as DCS grounding net CONTROLENGINEERINGChinaCopyright, that is, common ground with electrical grounding net;
1.2.2 Set up an independent grounding network for the DCS system;
1.2.3 set up a DCS-specific grounding network, and then connect it to the electrical grounding network through the grounding wire;
Because the third grounding method and the second grounding method are more similar, in the past, computers or DCS systems have used dedicated grounding nets. However, the shortcomings of this grounding method are: large area, high investment, large consumption of steel wire and grounding network, a considerable distance from the plant (because it is not easy to find a suitable location in the plant), management, maintenance, It is inconvenient to measure and find the grounding electrode and grounding wire, and the result is not very good. According to the actual operation, it is difficult and insecure to set a dedicated DCS grounding network. For example, a power plant once caused the unit to trip dozens of times due to grounding problems. According to the survey, many power plants DCS later switched to grounding with electrical grounding networks and achieved good results.
1.3 Requirements for the common ground electrode (network) 1.3.1 When the electrical resistance of the plant's electrical grounding network to the ground is ≤ 4Ω, the electrical grounding network of the plant 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 disordered, the grounding system should be set independently, which is the common grounding pole (network) of the DCS system.
1.3.3 The common ground electrode (net) with no intrinsically safe access to the earth has a resistance of less than 4 ohms; there is less than 1 ohm of the intrinsically safe earth. The line impedance of the grounded mains is less than 0.1 ohms.
1.3.4 There is no access point for lightning protection within 15 meters around the grounding pole, and there is no access point for the shell of high and low voltage power equipment above 30KW within 8 meters. When the site cannot meet this condition, the lightning protection zone is connected to the main ground of the common grounding pole through the arrester/shock suppressor. Do not connect the grounding electrode to the common grounding electrode and its grounding grid. The distance between the two should be more than 10 meters.
2. Grounding principle of DCS system 2.1 Grounding device set in DCS system 2.2.1 Operation table, printing table, server cabinet: There is a protective floor screw.
2.2.2 Relay Cabinets, UPS Cabinets, and Power Distribution Cabinets: Protective screws are provided.
2.2.3 DCS I/O Cabinet: shielded ground busbars and ground screws. Systemically (+24V ground) is suspended.
2.2.4 Instrument cabinets and manual operation panel: It is equipped with a shielded grounding busbar and a protective ground screw.
2.2.5 Safety Fence Cabinet: There is a shielded grounding busbar, and the local safety grounding busbar is provided by CONTROLENGINEERING China.
2.2 Signal shielding and grounding 2.2.1 According to the relevant technical requirements, the shielding layer of the signal cable of the computer or DCS system must not be floating and must be grounded. The grounding method of the signal cable must meet the following requirements:
2.2.1.1 When the signal source is floating, the shield should be grounded on the computer side;
2.2.1.2 When the signal source is grounded, the shield should be grounded at the 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 is connected to the common mode ground (When the signal source is grounded, the signal ground is connected. When the signal source is floating, it is connected to the field ground).
2.2.1.4 When the shielded cables are routed through the junction box or combined, the shields at both ends of the cables shall be connected within the terminal box.
2.2.2 DCS system signal cable selection and laying, should be implemented in strict accordance with the relevant provisions. The shielded cable shield should be grounded as described above. In order to improve the DCS system's anti-interference ability, DCS system switch input / output signal, the use of flame-retardant twisted pair copper screen computer cable is quite appropriate.
3. Grounding method of DCS system 3.1 DCS equipment grounding method of centralized arrangement 3.2 DCS equipment grounding method of decentralized arrangement Distributed DCS system The connection between the equipment is generally a network (communication) line, for example: the field control station is dispersed to the site, and The operator stations are located in different control rooms with a dispersed diameter in the range of 500 meters. Multi-mode optical fibers or Category 5 twisted pair or DP shielded twisted pair cables are used between the stations.
3.2.1 Sites using fiber optic connections: The grounding methods in each station are the same as the centralized DCS equipment.
3.2.2 Sites using Category 5 twisted pair or DP shielded twisted pair:
3.2.2.1 The ground wires of the control room are first connected to the common connection board, and the common connection board is connected to the common grounding electrode through the grounding trunk line. Seen from the common grounding pole, the entire grounding network is a star structure.
3.2.2.2 Use Category 5 twisted pair or DP shielded twisted pair The two ends are connected to the DCS SWITCH, HUB, REPEAT, or other network equipment through a network surge protection device (signal lightning arrester, flow rate not less than 5KA). The two sides of the site have their own common grounding poles, and they do not need to have metal connections. The grounding method of each site is the same as the centralized DCS equipment. Category 5 twisted pair or DP shielded twisted pair must be laid on galvanized steel pipe or metal bridge, and the steel pipe or bridge must be reliably grounded. When lightning strikes, or electrical accidents cause the potential difference between the two sides is too large, the signal lightning arrester can protect the equipment on both sides.
3.3 DCS equipment grounding installation 3.3.1 grounding body: to be ground into a good conductor, the current sent from the grounding main trunk through the grounding body into the earth. The brazed joint between the grounding body and the grounding trunk line shall be treated with anti-corrosion after welding. The grounding grid can be used to connect multiple grounding bodies into a network. The grounding grid should meet the DCS system grounding resistance requirements. When the grounding wire and the grounding body are lap welded, the overlap length must be 2 times the width of the flat steel or 6 times the diameter of the round steel. Figure 3-2 shows a typical multi-grounding installation.
3.4 DCS system grounding method to reduce the soil resistivity 3.4.1 Change the soil structure around the grounding body. In the range of 2~3m around the grounding body, do not incorporate water-absorbent substances with good water absorption, such as charcoal, coke cinder, or slag. This method can reduce the soil resistivity to 1/5 of the original. ~1/10.
3.4.2 Use salt, charcoal to reduce the soil resistivity Layered with salt, charcoal compaction. Charcoal and fine doping are one layer, about 10~15cm thick, and then 2~3cm salt, total 5~8 layers. After being laid into the grounding body. This method can reduce the resistivity to 1/3 to 1/5. However, the salt will be lost with the water for a long time, and it will normally be added once more than two years.
3.4.3 Use a long-lasting chemical resistance reducer. The long-acting chemical resistance-reducing agent method can reduce the soil resistivity to 40%.
3.5 DCS system grounding materials and requirements 3.5.1 Material requirements for grounding body and grounding network trunk The steel specifications used for the grounding body and grounding network trunk can be selected according to the following table. If the grounding resistance cannot meet the requirements, copper can also be used. If the grounding body and grounding network cable are installed in places with high corrosiveness, corrosion protection measures such as hot-dip galvanizing, hot-dip tinning, or appropriate cross-sections should be taken according to the nature of the corrosion.
3.5.2 Ground connection requirements The DCS system's protective ground and shielding ground connection should use copper core insulated wire or cable to connect to the factory electrical special grounding net or grounding body. The small table lists the available specifications for various types of grounding cables. When the distance of the grounding connection is long, the DCS system requires high grounding resistance, or the number of branching lines for grounding the mains is relatively large, the wires and cables with larger sections in the table should be selected.
4. Common grounding precautions for on-site grounding 4.1 Site control station: The grounding screw is insulated by the rubber between the cabinet body and the base, and the shielded ground busbar is insulated from the base. The site control station must be properly grounded according to the regulations. That is, they are connected to the ground busbars of the field control station. The power ground of the I/O cabinet and the power ground of the UPS must be connected to the same ground to ensure equal potential.
4.2 Field Control Station: The operator station, engineer station, network switch, server host, system monitor, etc. are grounded or directly connected to the electrical grounding network.
4.3 I/O Module: The 40-terminal of the analog module is the negative terminal of 24 V DC connected to the logic bus. The logical busbar is connected to the shield ground and then to the total ground bus.
4.4. The protective earth of the site control station shall be connected from the grounding screw at the bottom of the cabinet to the earthing branch line, and the shielded ground of the field control station shall 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.
About Silicone pet bowl:
Pet is one of the indipensable companionsfor our life,so,many pet products are be invented,such as silicone pet bowl.Silicone pet bowl is suitable for dog,cat and so on.It is made of food grade silicone,non-toxic and odorless,very safe for people.More inportant,if you want to take your pet to have a picnic,silicone pet bowl is your must-have pet supplies,It can be fold up and easy to carry.Besides you can save more space to carry other items.
Silicone Silicone pet bowl introduction:
1.Product name:Silicone pet bowl,silicone dog bowl,silicone cat bowl,soft cat bowl, portable pet bowl,folding dog bowl
2.Place of origin:Guangdong China
3.Color:any pantone color
4.Logo:Printing,debossed,embossed
5.MOQ:500pcs.
6.Package:1 pcs/opp,customized design is available.
7.Design:Customized/stock
8.Certification:FDA,LFGB,SGS,ROHS,etc.
9.Usage:Use for pet.
10Silicone pet bowl for reference.
Silicone Pet Bowl,Silicone Dog Bowl,Silicone Cat Bowl,Soft Cat Bowl,Portable Pet Bowl,Folding Dog Bowl
Dongguan OK Silicone Gift Co., Ltd. , https://www.dgsiliconewearablegifts.com