This paper discusses the electromagnetic compatibility between distribution network grounding system and building electrical equipment. The resistance value of grounding grid and the function of grounding grid structure in lightning protection; Discharge process of external lightning protection and internal lightning protection subsystem; The misleading propaganda in grounding technology is pointed out.
Key words: building lightning protection, lightning electromagnetic pulse protection, LEMP lightning protection and grounding, electronic (logical) grounding.
1. Relationship between grounding mode of power supply system and indoor electromagnetic compatibility
The grounding system of distribution network is related to the safety of power grid operation and building power supply [1, 2]. Western countries pay special attention to power supply quality and have strict requirements on power supply frequency and voltage quality; They also attach great importance to electromagnetic compatibility in intelligent buildings, and shielded cables have been widely used in important buildings. Now China's intelligent building design has noticed this. At present, distribution transformers in big cities all adopt small resistance grounding method, and TN-S and TN-C-S systems are widely used in power lines, commonly known as three-phase five-wire system; Pay attention to the laying of indoor electrical lines to meet the specifications of integrated wiring.
If the live wire of a three-phase power supply system is compared to a water supply pipe, the N-wire is like a sewer pipe, and the current it conveys includes the clutter generated by electric equipment such as flashlights and thyratrons, and the overcurrent generated when the power supply system fails. In this way, the power supply is a closed system, and the PE line only plays the role of grounding and fixed potential, and no current flows at ordinary times. If N line and PE line are combined, these clutter currents and overcurrent will generate floating voltage in the ground wire, which will damage the electronic equipment and make it unable to work normally. Therefore, the N line must be insulated and the PE line must be laid independently, which is the significance of establishing the power supply system in tn-s. The way to check whether the N line and the PE line are well insulated is to measure the current in the PE line. Under normal circumstances, there should be no current in the PE line. When we measured the lightning protection grounding of a palace in the lightning protection project of the Forbidden City, we found that the grounding resistance meter swung, so we found that there was a fault of overlapping N line and PE line. PE wire is used as electronic grounding (logic grounding) and safety grounding of equipment at ordinary times, and also as lightning protection grounding when lightning strikes.
Three-phase four-wire pen wire also has the function of safe grounding, but it cannot be used for electronic grounding, because there will be stray current at ordinary times, which can not guarantee the stable operation of electronic equipment. For example 1993, a fire broke out in a bungalow (shop) on the northeast side of Xibianmen overpass in Beijing. The reason is that an overhead truck hung up and a single-phase arc grounding accident occurred, but the electrical switch of the shop could not be turned off immediately. The wires in the ceiling of this bungalow melted due to excessive current, which caused a fire. Rural areas in China should pay special attention to the safety of such overhead power distribution buildings. When U 1ma of SPD is selected for IEC lightning protection standards and current building lightning protection standards in China, only the neutral grounding mode of 220/380 V power grid is considered, but the neutral grounding mode of its superior 10kV power grid is not considered. Even 440V and 470V U 1ma are often selected, which is not suitable. Most distribution networks in China adopt neutral point insulation or arc suppression coil grounding. When a single-phase grounding fault occurs in these power grids, they may run for several hours, and the phase voltage becomes line voltage, causing SPD to burn out. According to the national standard GBll02-2000 formulated by the Ministry of Water Resources and Electric Power and the Ministry of Machinery Industry, considering the charging rate, it is stipulated that 220V MOV is U 1ma=600V and MOV is U1MA =1200 V. This is reasonable, which can not only reduce SPD burning, but also effectively protect against lightning.
Statistics of lightning accidents in buildings show that 80 ~ 90% of accidents are caused by electrical lines, and there are not many accidents caused by direct lightning. It is not a good idea to introduce overhead lines directly into the power supply of buildings in mountainous and rural areas; It is suggested to refer to Reference [3] and use iron pipe for threading and shielding laying instead (see Figure 1): connect an overhead extension ground wire with the length of 15m before the incoming pole to be used as a lightning rod (see Figure 2). The function of this section of the line is to increase the inductance of the main line in front of the shielded iron pipe, so that 1#SPD (zinc oxide arrester) can be easily started and then used before the incoming pole.