Thunder and lightning are common natural phenomena. Due to social and economic development, on one hand there are more and taller skyscrapers which shorten the distance between the ground and thunderclouds; on the other hand, factories and cars emit more exhaust fumes that pollute the air and increase particles in it. This not only facilitates the formation of thunderclouds but also promotes the conduction of lightning currents. Therefore, in the Pearl River Delta where thunderstorms are frequent, the number of thunderbolts is increasing, becoming stronger and lower, posing a great threat to people's production and life. Each year, damage caused by lightning strikes on buildings or equipment becomes increasingly severe. Many units and families suffer from the threats and attacks of lightning, gradually making people realize the importance of lightning protection. Lightning disasters can be divided into direct lightning strikes and induced lightning. Installing qualified lightning rods (belts) on buildings can relatively effectively prevent the harm of direct lightning strikes. However, induced lightning cannot be prevented by lightning rods (belts). Induced lightning has a wide range of impact, regardless of the height of buildings. As long as there are power lines or signal lines introduced, lightning several kilometers away may cause induction, leading to equipment damage.
In electric power distribution lines, commonly used lightning arresters include valve-type lightning arresters, tube-type lightning arresters, zinc oxide lightning arresters, etc. Low-voltage distribution systems recommend using low-voltage zinc oxide lightning arresters. Under normal operating voltage, the resistance of zinc oxide discs is very high, allowing only microampere-level leakage current. However, when powerful lightning currents pass through, they exhibit very low resistance, quickly discharging them into the ground to achieve the purpose of voltage limitation and current diversion. The residual voltage on the disc hardly changes with the size of the passing current, usually remaining below the impulse test voltage of the protected electrical appliance, thus protecting the insulation of the device. After the lightning current passes, it returns to its original insulated state.
Zinc oxide arresters have excellent nonlinear volt-ampere characteristics, stable residual voltage under the variation of impulse current wavehead time, good steep wave response characteristics, no gap breakdown characteristics and arc extinguishing problems. Their resistor chips can absorb a large amount of energy per unit volume and can be used in parallel, so they are particularly beneficial in 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 arresters, relevant technical parameters should be measured to ensure the quality of arrester installation. 1 Insulation resistance measurement: For zinc oxide arresters up to 35kV, use a 2500V megohmmeter to measure, and each section's insulation resistance should be no less than 1000MΩ. Imported zinc oxide arresters generally follow the manufacturer's standards. 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 be no less than 2000MΩ.
2 Measurement of DC and leakage current: Measure the DC voltage U1mA and the leakage current at 75% of U1mA, aiming to check their non-linear characteristics and insulation performance. U1mA is the voltage across the tested arrester when 1mA DC passes through. The regulation stipulates: the 1mA voltage value U1mA compared to the initial value should change by no more than ±5%. The leakage current at 0.75U1mA voltage should be no more than 50A. In other words, when the voltage drops by 25%, the leakage current of a qualified zinc oxide arrester significantly decreases from 1000A to below 50A.
If the U1mA voltage drops or the leakage current at 0.75U1mA significantly increases, it could mean that the arrester's valve disc is damp or aged, or there are cracks in the porcelain. When measuring, to avoid the influence of surface leakage current, the porcelain sleeve surface should be wiped clean or shielding measures added, and attention should be paid to weather effects. Generally, the temperature coefficient of U1mA for zinc oxide discs is about (0.05~0.17)%/°C, meaning that for every 10°C rise in temperature, U1mA decreases by approximately 1%. Necessary recalculations can be made if needed.
3 Measurement of AC leakage current under operating voltage: Using an LCD-4 type detector, the leakage current (total current) of the arrester under operating voltage, along with its active component (resistive current) and reactive component (capacitive current), and power loss Px can be measured. Experimental research shows: when the zinc oxide arrester valve disc gets damp or ages, the amplitude of the resistive current increases rapidly. Therefore, monitoring the resistive current can effectively monitor the arrester's insulation condition.
The regulation stipulates: when the active component of the leakage current doubles the initial value, the power supply should be cut off for inspection. Some domestic units have established certain determination criteria themselves, such as when the peak resistive current of a 330kV zinc oxide arrester exceeds 0.3mA, or for 110~220kV zinc oxide arresters, when the peak resistive current exceeds 0.2mA or the measurement value significantly increases compared to the initial value, a power-off test should be conducted to judge the quality of insulation.
Low-voltage overhead lines are widely distributed, especially in areas prone to lightning where low-voltage lines are separately erected, making them easily struck by lightning. Additionally, when low-voltage overhead lines are directly introduced into users' premises, the insulation level of low-voltage equipment is very low, and people come into contact with them frequently, so it is necessary to consider lightning protection measures against lightning traveling along low-voltage lines into houses. 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 lightning arresters or protective gaps on the low-voltage side. If the transformer's low-voltage side is ungrounded, a breakdown fuse should be installed at the neutral point;
(2) For important users, it is advisable to install a set of low-voltage lightning arresters 50m before the low-voltage line enters the indoor area, and then another set inside;
(3) For regular users, a set of low-voltage lightning arresters or breakdown fuses can be installed at the first support of the low-voltage inlet, or the insulator iron foot of the service drop can be grounded, with the power frequency grounding resistance not exceeding 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 lightning arresters or gap protection, with a gap distance of 1.5~2mm, or discharge gap protection of 500V used in communication equipment can also be adopted.
Power surge protectors are generally connected in parallel with the load, aiming to limit the peak lightning voltage within the range that electrical appliances can withstand. After selecting appropriate surge protectors, considerations must also be given to wiring installation and grounding treatment during installation. According to the protection object and 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 applied to electronic equipment. Shielding can range from entire floors to equipment rooms, cables, etc. Measurement results show: grounding one end of the cable shield can reduce high-frequency interference voltage by one order of magnitude, and grounding both ends can reduce it by two orders of magnitude. Therefore, shielding treatment is an indispensable part of wiring installation and surge protector installation.
After installing the surge protector, a good grounding system must be provided to allow lightning current to quickly flow into the ground. For communication systems' direct grounding, computer network systems' logical grounding, and power sources' working grounding and safety grounding, equal potential treatment should be done.