In the design of building power supply and distribution, the design of the grounding system occupies an important position because it relates to the reliability and safety of the power supply system. Regardless of the type of building, a grounding system design is always included in the power supply design. Moreover, as the requirements of the building are different, the functions of the various types of equipment are different, and the grounding system is also different. Especially after the 1990s, the emergence of a large number of intelligent buildings has proposed many new contents for the design of grounding systems. Which of the commonly used grounding methods can be adapted to intelligent buildings? We may wish to analyze the following grounding systems.
1. TN-C system The TN-C system is called a three-phase four-wire system. The neutral line N and the protective ground PE are combined into one, which is called PEN line. Although this grounding system has high sensitivity to ground faults and simple economics, it is only suitable for use in places where the three-phase load is relatively balanced. In the intelligent building, the single-phase load accounts for a large proportion, and it is difficult to achieve three-phase load balance. The unbalanced current of the PEN line plus the high-order harmonics caused by fluorescent lamps, thyristors (thyristors) and the like existing in the line. The wave current, in the case of non-fault, will be superimposed on the neutral line N, so that the neutral line N voltage fluctuates, and the current is large and small, and the neutral point ground potential is unstablely drifted. Not only will the equipment casing (connected to the PEN line) be charged, it will be unsafe for the human body, and it will not be able to obtain a suitable potential reference point. The precision electronic equipment cannot operate accurately and reliably. Therefore, the TN-C grounding system cannot be used as a grounding system for intelligent buildings.
2. TN-C-S system The TN-C-S system consists of two grounding systems. The first part is the TN-C system, the second part is the TN-S system, and the interface is at the connection point between the N line and the PE line. The system is generally used in places where the power supply of the building is led by the regional substation. Before entering the household, the TN-C system is adopted, and the household is repeatedly grounded, and the TN-S system becomes after entering the household. The TN-C system has been analyzed before. The characteristic of the TN-S system is that the neutral line N and the protective grounding line PE can be grounded together when entering the household, and no electrical connection can be made. In this system, the neutral line N is often charged, and the protective ground line PE has no source of electricity. The equipment casing and metal components connected by the PE wire are always not charged when the system is in normal operation. Therefore, the TN-S grounding system significantly improves the safety of people and objects. At the same time, as long as we take the grounding lead, each one is taken out from the grounding body. And the choice of the correct grounding resistance value allows the electronic equipment to jointly obtain an equipotential reference point, etc., then the TN-C-S system can be used as a grounding system for intelligent buildings.
3. TN-S system TN-S is a three-phase four-wire plus PE line grounding system. This system is usually used when an independent power distribution station is installed in a building. The characteristic of the TN-S system is that the neutral line N and the protective ground line PE are not grounded at the neutral point of the transformer, and the two lines no longer have any electrical connection. The neutral line N is charged, while the PE line is not charged. The grounding system is fully equipped with a safe and reliable reference potential. As long as the same technical measures are taken like the TN-C-S grounding system, the TN-S system can be used as a grounding system for intelligent buildings. This type of grounding system is generally used when there are no special requirements for electronic equipment such as computers.
4. TT system is usually called TT system is a three-phase four-wire grounding system. This system is often used where buildings are powered from the public grid. The characteristic of the TT system is that the neutral line N has no electrical connection with the protective grounding line PE, that is, the neutral point grounding is separated from the PE line grounding. When the system is in normal operation, regardless of the imbalance of the three-phase load balance, the PE line will not be charged under the neutral line N. In the case of a single-phase ground fault, the equipment casing may be energized because the protection grounding sensitivity is low and the fault cannot be cut off in time. The TT system in normal operation is similar to the TN-S system, and can also obtain human and material safety and obtain a qualified reference ground potential. With the advent of large-capacity leakage protectors, the system will increasingly become the grounding system for intelligent buildings. From the current situation, because the power quality of the public power grid is not high, it is difficult to meet the requirements of intelligent equipment, so the TT system is rarely used by intelligent buildings.
5. IT system IT system is a three-phase three-wire grounding system. The neutral point of the transformer is not grounded or grounded by impedance, no neutral line N, only line voltage (380V), no phase voltage (220V), protective grounding The wires PE are each independently grounded. The advantage of this system is that when one phase is grounded, the housing will not have a large fault current and the system can operate as usual. The disadvantage is that the neutral line N cannot be dispensed. Therefore it is not suitable for intelligent buildings with a large number of single-phase devices.
In intelligent buildings, there are many devices that require protective grounding. There are strong electrical equipment, weak electrical equipment, and some conductive equipment and components that are normally not energized. Effective protection grounding must be adopted. If the TN-C system is used, the N line in the TN-C system is used as the ground line at the same time; or the N line and the PE line are connected together in the TN-S system, and then connected to the base board; or no electronic is set The DC ground lead of the device is connected to the DC line directly to the PE line; some simply mix the N line, PE line, and DC ground line together. All of the above practices are not in compliance with the grounding requirements and are wrong. As has been analyzed in the past, in the intelligent building, there are many single-phase electrical equipment, the single-phase load is large, and the three-phase load is usually unbalanced, so there is a random current in the neutral line N. In addition, due to the large number of fluorescent lighting, the third harmonic generated by it is superimposed on the N line, which increases the amount of current on the N line. If the N line is connected to the device casing, it may cause electric shock or fire accident; In the TN-S system, the N line and the PE line are connected together and connected to the equipment casing, so the danger is even greater. Whenever the equipment is connected to the PE line, the casing is charged; the range of the electric shock accident will be expanded; if the N line is The PE line and the DC ground line are all connected together. In addition to the above-mentioned dangers, the electronic equipment will be disturbed and will not work. Therefore, intelligent buildings should be equipped with DC grounding, AC working grounding, safety protection grounding, and lightning protection grounding for ordinary buildings. In addition, since there are many program-controlled switch rooms with anti-static requirements in the intelligent building, the calculations are carried out by the îŒ†æµšîƒªî¤ é¹ª 鹪 鹪 îƒ îƒ î‚€è‰¿ç¼± 艿缱 艿缱 right瞧éƒç’žç¦îƒ¦î‹ƒ  è°¥ è°¥ 芑ビ畹 芑ビ畹 æ“ æ“ â”•ã¶ â”•ã¶ â”•ã¶ â”•ã¶ â”•ã¶ â”•ã¶ â”•ã¶ â”•ã¶ â”•ã¶ è°° 缃 缃 缃 缃 缃 缃 缃 缃 缃 缃 缃 缃
Below, we will then analyze the various grounding measures that should be taken in intelligent buildings.
1. Lightning protection grounding: The grounding for the purpose of quickly introducing lightning current into the earth to prevent lightning damage is called lightning protection grounding.
There are a large number of electronic equipment and wiring systems in intelligent buildings, such as communication automation systems, fire alarm and fire linkage control systems, building automation systems, security monitoring systems, office automation systems, closed circuit television systems, etc., and their corresponding wiring systems. From the completed building, the roof, floor, side wall and ceiling of each floor of the building are almost covered with various wirings. These electronic devices and wiring systems generally belong to the parts with low withstand voltage rating, high anti-interference requirements, and the most fear of lightning strikes. Whether it is direct, string, or counterattack, electronic equipment can be damaged or seriously interfered to varying degrees. Therefore, the lightning protection grounding design of intelligent buildings must be strict and reliable. All functional grounding of intelligent buildings must be based on lightning protection grounding systems and establish a rigorous and complete lightning protection structure.
Intelligent buildings are mostly primary loads, and should be designed according to the protection measures of the first-level lightning protection buildings. The lightning receptors are equipped with a combination of needle and belt, and the lightning protection belt is made of 25×4 (mm) galvanized flat steel. a grid of ×10(m), which is electrically connected to the metal components of the roof and electrically connected to the studs of the building. The down-conducting line uses the steel bars in the studs, the rebar bars, the floor bars and the lightning protection system, and the outer wall All metal components on the surface should also be connected to the lightning protection system. The stud reinforcement is connected to the grounding body to form a cage lightning protection system with multiple layers of shielding. This not only can effectively prevent lightning damage to the equipment in the building, but also prevent external electromagnetic interference.
The power frequency grounding resistance of various lightning protection grounding devices should generally be determined according to the counterattack conditions during lightning strikes. When the lightning protection device uses a total grounding grid with the working ground of the electrical equipment, the grounding resistance should meet the minimum requirements.
2. AC work grounding: Connect a point in the power system directly or through special equipment (such as impedance, resistance, etc.) to the earth as a working ground.
The working ground mainly refers to the neutral point of the transformer or the neutral line (N line) grounding. The N wire must be insulated with a copper core. There are auxiliary equipotential terminals in the power distribution, and the equipotential terminals are generally in the cabinet. It must be noted that the terminal should not be exposed; it should not be mixed with other grounding systems, such as DC grounding, shielded grounding, anti-static grounding, etc.;
In the high-voltage system, the neutral grounding method can make the grounding relay protection operate accurately and eliminate the single-phase arc grounding overvoltage. Neutral grounding prevents zero-sequence voltage offset and maintains a three-phase voltage balance, which makes sense for low-voltage systems and makes it easy to use single-phase power supplies.
3. Safety protection grounding: Safety protection grounding is to make a good metal connection between the metal parts of the electrical equipment that are not charged and the grounding body. The electrical equipment in the building and some metal components in the vicinity of the equipment are connected by PE wires, but it is strictly forbidden to connect the PE wires to the N wires.
In intelligent buildings, there are many devices that require safety protection and grounding. There are strong electrical equipment, weak electrical equipment, and some non-charged conductive equipment and components, all of which must be secured and grounded. When the insulation of electrical equipment that is not grounded by safety is damaged, the casing may be charged. If the human body touches the outer casing of this electrical device, it may be electrically injured or life-threatening. As shown in Figure 6. In a power system where the neutral point is directly grounded, the grounding short-circuit current flows through the human body and flows back to the neutral point; in the power system where the neutral point is not directly grounded, the grounding current flows into the earth through the human body and is formed by the line-to-ground capacitance. Access, both of which can cause physical shock.
If the insulation of the electrical equipment with the grounding device is damaged, the grounding short-circuit current will flow along both the grounding body and the human body at the same time, Id=Id'+IR, we know: in a parallel circuit, through each The current value of the branch is inversely proportional to the magnitude of the resistance, ie
In the formula: Id—the total value of the current in the ground loop Id′—the current flowing along the grounding body—the current flowing through the human body rR—the resistance of the human body rd—the grounding resistance of the grounding device can be seen from the above formula, the grounding resistance The smaller the current flowing through the human body, the smaller the current resistance of the human body is hundreds of times larger than the grounding resistance, and the current through the human body is hundreds of times smaller than the current flowing through the grounding body. When the grounding resistance is extremely small, the current flowing through the human body is almost equal to zero. Id≈Id'. In fact, since the grounding resistance is small, the voltage drop generated when the ground short-circuit current flows is small, so the voltage of the device casing to the earth is not high. When a person stands on the ground and touches the outer casing of the device, the human body is subjected to a low voltage and is not dangerous.
Adding a protective grounding device and reducing its grounding resistance is not only an effective measure to ensure the safety and effective operation of the intelligent building electrical system, but also a necessary means to ensure equipment and personal safety in non-intelligent buildings.
4. DC grounding: In an intelligent building, it contains a large number of computers, communication equipment and building automation equipment with computers. In these series of processes, such as inputting information, transmitting information, converting energy, amplifying signals, logic actions, and outputting information, these electronic devices are rapidly performed through micro-potentials or micro-currents, and devices often work through an interconnection network. . Therefore, in order to make it highly accurate and stable, in addition to a stable power supply, it must have a stable reference potential. A large-section insulated copper core wire can be used as the lead, one end is directly connected to the reference potential, and the other end is used for DC grounding of the electronic device. The lead should not be connected to the PE wire. It is strictly forbidden to connect with the N wire.
5. Shield grounding and anti-static grounding: In intelligent buildings, electromagnetic compatibility design is very important. In order to avoid the dysfunction of the equipment used and avoid even equipment damage, the equipment constituting the wiring system should be able to prevent internal conduction. And external interference. These disturbances are either due to coupling between the wires or due to capacitive or inductive effects. Its main sources are ultra-high voltage, high-power radiated electromagnetic fields, natural lightning strikes and electrostatic discharge. These phenomena can cause significant interference to devices designed to transmit or receive very high transmission frequencies. Therefore, these devices and their wiring must be protected from various interferences. Shielding and proper grounding are the best protection against electromagnetic interference. The equipment casing can be connected to the PE wire; the shielding grounding of the wire requires reliable connection between the two ends of the shielding pipe and the PE wire; the indoor shielding should also be reliably connected to the PE wire at multiple points. Anti-static interference is also important. In a clean, dry room, people walking and moving equipment will generate a lot of static electricity. For example, in an environment with a relative humidity of 10 to 20%, a person's walking can accumulate an electrostatic voltage of 35,000 volts. If there is no good grounding, it will not only cause interference to the electronic device, but even damage the device chip. Grounding an electrostatically charged object or an object that may generate static electricity (non-insulator) through the static conductive body and the earth to form an electrical circuit is called an anti-static ground. Anti-static grounding requirements In a clean and dry environment, all equipment enclosures and indoor (including floor) facilities must be reliably connected to multiple points on the PE line.
The grounding resistance of the grounding device of the intelligent building is as small as possible. The independent lightning protection grounding resistance should be ≤10Ω; the independent safety protection grounding resistance should be ≤4Ω; the independent AC working grounding resistance should be ≤4Ω; the independent DC working grounding resistance Should be ≤ 4Ω; anti-static grounding resistance is generally required ≤ 100Ω.
The electrolyte material inside the electrolytic capacitor, which has charge storage, is divided into positive and negative polarity, similar to the battery, and cannot be connected backwards.A metal substrate having an oxide film attached to a positive electrode and a negative electrode connected to an electrolyte (solid and non-solid) through a metal plate.
Nonpolar (dual polarity) electrolytic capacitor adopts double oxide film structure, similar to the two polar electrolytic capacitor after two connected to the cathode, the two electrodes of two metal plates respectively (both with oxide film), two groups of oxide film as the electrolyte in the middle.Polar electrolytic capacitors usually play the role of power filter, decoupling (like u), signal coupling, time constant setting and dc isolation in power circuit, medium frequency and low frequency circuit.Non-polar electrolytic capacitors are usually used in audio frequency divider circuit, television S correction circuit and starting circuit of single-phase motor.
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