Thunder and lightning are common natural phenomena. Due to the development of the social economy, on the one hand, skyscrapers have been springing up and getting taller, shortening the distance between the ground and thunderclouds; on the other hand, the exhaust gas discharged from factories and cars has increasingly polluted the air, increasing the particles in the air, which not only facilitates the formation of thunderclouds but also promotes the conduction of lightning current. Therefore, in the Pearl River Delta where there are many thunderstorms, the number of thunderstorms is increasing, becoming stronger and lower, posing a great threat to people's production and life. The damage caused by lightning strikes to buildings or equipment becomes more serious every year. Many units and families have suffered from the threats and invasions of lightning, making people gradually realize the importance of lightning protection. Lightning disasters are divided into direct lightning and induced lightning. Installing qualified lightning rods (belts) on buildings can relatively effectively prevent the damage caused by direct lightning. However, induced lightning is something that lightning rods (belts) cannot prevent. Induced lightning damages a wide range, regardless of the height of the building, as long as there are power lines or signal lines introduced, lightning several kilometers away could induce damage to equipment.
In power distribution lines, commonly used lightning arresters include valve-type arresters, pipe-type arresters, zinc oxide arresters, etc. For low-voltage distribution systems, it is recommended to use low-voltage zinc oxide arresters. Zinc oxide discs under normal operating voltage have high resistance, allowing only microamp-level leakage currents. However, during powerful lightning currents, they exhibit very low resistance, quickly discharging into the ground, achieving the purpose of voltage limitation and current diversion. The residual voltage on the disc barely changes with the size of the passing current, often remaining below the impulse test voltage of the protected electrical appliance, protecting the insulation of the equipment, and after the lightning current passes, it returns to its original insulated state.
Zinc oxide arresters have excellent nonlinear volt-ampere characteristics, stable residual voltage with changes in impact current wavehead time, good steep wave response characteristics, no gap breakdown characteristics and arc extinguishing problems. Their resistor discs can absorb large amounts of energy per unit volume and can be used in parallel, so they are particularly beneficial for protecting ultra-high voltage long-distance transmission systems and large-capacity capacitor banks. They are also suitable for protecting low-voltage distribution networks and are an important protective measure for low-voltage distribution networks.
Before using the arrester, all relevant technical parameters should be measured to ensure the quality of the arrester installation.
1. Insulation Resistance Measurement
For zinc oxide arresters of 35kV and below, use a 2500V megohmmeter to measure, and the insulation resistance of each section should not be less than 1000MΩ.
For imported zinc oxide arresters, the insulation resistance of each section usually follows the manufacturer's standard. For example, Japan’s Meiden specifies: for ZSE-C2Z type 294kV zinc oxide arresters, a 1000V megohmmeter should be used, and the insulation resistance should not be less than 2000MΩ.
2. Measurement of DC Voltage and Leakage Current
Measure the DC voltage U1mA and the leakage current at 75% U1mA voltage to check their nonlinear characteristics and insulation performance.
U1mA is the voltage across the tested arrester when 1mA DC flows through it. According to the regulations, the 1mA voltage value U1mA compared to the initial value should not change by more than ±5%. The leakage current at 0.75U1mA voltage should not exceed 50A. In other words, when the voltage drops by 25%, the leakage current of a qualified zinc oxide arrester decreases significantly, from 1000A to less than 50A.
If the U1mA voltage drops or the leakage current at 0.75U1mA increases significantly, it may indicate that the arrester valve disc is damp or aged, or there are cracks in the porcelain. When measuring, precautions should be taken to prevent the influence of surface leakage current, such as wiping the porcelain sleeve clean or adding shielding measures, and attention should be paid to weather effects. Generally, the temperature coefficient of U1mA for zinc oxide discs is about (0.05～0.17)%/℃, meaning that for every 10℃ increase in temperature, U1mA decreases by about 1%, and if necessary, conversion calculations can be performed.
3. AC Leakage Current Measurement Under Operating Voltage
Using the LCD-4 type detector, the leakage current (total current) of the arrester under operating voltage, its active component (resistive current), and reactive component (capacitive current), and power loss Px can be measured.
Research shows: when the zinc oxide arrester valve disc is damp or aged, the amplitude of the resistive current increases rapidly. Therefore, monitoring the resistive current can effectively monitor the insulation condition of the arrester.
According to the regulations, when the active component of the leakage current doubles the initial value, power should be turned off for inspection. Some domestic units have established their own judgment criteria, such as some units specify that when the peak resistive current of 330kV zinc oxide arresters exceeds 0.3mA, or for 110～220kV zinc oxide arresters, the peak resistive current exceeds 0.2mA or the measurement value significantly increases compared to the initial value, power should be turned off for testing to determine the insulation quality.
Low-voltage overhead lines are distributed widely, especially in areas with frequent thunderstorms where independently erected low-voltage lines are easily struck by lightning. At the same time, when low-voltage overhead lines are directly introduced into users' premises, the insulation level of low-voltage equipment is very low, and there are many opportunities for human contact, so it is necessary to consider lightning protection measures against lightning traveling along low-voltage lines into homes. Specific methods are as follows:
(1) For 3～10kV Y/Y or Y/Y connected distribution transformers, it is advisable to install a set of valve-type arresters or protective gaps on the low-voltage side. If the low-voltage side of the transformer is ungrounded neutral point, a breakdown fuse should be installed at the neutral point;
(2) For important users, it is advisable to install a set of low-voltage arresters 50m before the low-voltage line enters the indoor area, and another set inside;
(3) For ordinary users, a set of low-voltage arresters or breakdown fuses can be installed at the first support point of the low-voltage inlet, or the iron foot of the service drop insulator can be grounded, and the power frequency grounding resistance should not exceed 30Ω;
(4) For areas prone to lightning strikes, motors or watt-hour meters directly connected to overhead lines should preferably be equipped with low-voltage arresters or gap protection, with a gap distance of 1.5～2mm, or a 500V discharge gap protection used for communication equipment.
Power surge protectors are generally connected in parallel with loads, aiming to limit the peak voltage of lightning within the range that electrical appliances can withstand. After selecting qualified surge protectors, considerations should also be given to line laying and grounding treatment issues during installation. Depending on the protected object and its sensitivity to lightning voltage, appropriate shielding treatment should be considered. Shielding refers to using various shielding bodies to block and attenuate electromagnetic interference and overvoltage energy added to electronic equipment. Shielding can be as large as an entire building floor, or as small as equipment rooms, cables, etc. Measurement results show that grounding one end of the cable shield can reduce high-frequency interference voltage by one order of magnitude, and grounding both ends of the shield can reduce it by two orders of magnitude. Therefore, shielding treatment is an indispensable part of line laying and surge protector installation.
After installing the surge protector, a good grounding system must be provided to allow the lightning current to quickly flow into the ground. For the direct grounding of communication systems, logical grounding of computer network systems, and working grounding and safety grounding of power sources, equal potential treatment should be done.