To protect power facilities from the impact of lightning overvoltage and system overvoltage, metal oxide lightning arresters are commonly installed and used. Especially in 10kV distribution networks, gapless metal oxide surge arresters are commonly used. With the passage of operation time, line tripping and grounding accidents caused by damage to metal oxide surge arresters often occur in 10kV distribution networks, seriously affecting the safe operation of 10kV distribution networks.

Based on the comprehensive damage situation of gapless metal oxide lightning arresters, those with good quality have less damage, while those with poor quality have more damage; Less damage on sunny days and more damage on thunderstorm days; Less damage occurs when there is no operation, and more damage occurs when there is operation; There is less damage during normal operation, and more damage during abnormal operation.

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1. Dampness

Metal oxide arrester is made of silicone rubber as the enclosure of the arrester. The quality of the silicone rubber sleeve enclosure is poor, mainly because small factories counterfeit and shoddy products. The technology used by the manufacturer is not perfect, or the anti aging performance of the sealing materials used is unstable, resulting in poor sealing when the temperature difference changes greatly or the operation is close to the end of the product life. The rough processing of both ends of the lightning arrester allows moisture or water to penetrate, causing internal insulation damage and accelerating the deterioration of the resistor, leading to damage. From the wreckage of the lightning arrester after the accident, it can be seen that there are no traces of current flow on the valve plate, and no discharge marks were found on the aluminum sprayed surfaces at both ends of the valve plate after high current passed through. However, there are obvious flashover marks on the inner wall of the silicone rubber sleeve or the side of the valve plate, and rust spots or zinc white on the metal accessories, which are the effects of moisture on the metal oxide lightning arrester.

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2. The values of rated voltage and continuous operating voltage are too low

The rated voltage of metal oxide lightning arrester is an important parameter indicating its operating characteristics and also an indicator of its ability to withstand power frequency voltage. The ability of the valve plate of metal oxide lightning arrester to withstand power frequency voltage is closely related to the duration of the operating voltage. Continuous operating voltage is also an important characteristic parameter of metal oxide surge arresters, and the selection of this parameter has a significant impact on the reliability of metal oxide surge arresters. The effective value of the power frequency voltage allowed to be continuously applied to the terminals of the lightning arrester during operation, which covers the highest value of the power frequency voltage that may be continuously applied to the metal oxide lightning arrester during the operation of the power system.

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3. Voltage fluctuations in the power grid

The voltage fluctuation range of the distribution network is very wide, and for metal oxide lightning arresters, if they are required to absorb a large amount of energy in a stable state, it may cause thermal collapse. When using gapless metal oxide surge arresters, it is necessary to meet the fluctuation range of the system's operating voltage. Otherwise, due to the high steady-state voltage, not one lightning arrester will be damaged, but many lightning arresters will be damaged at the same time.

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4. Aging issues

The increase in the ratio of the peak value of the reference voltage or power frequency reference voltage increases the burden, accelerates the aging rate, and forms a vicious cycle, ultimately leading to the heating and collapse of the metal oxide lightning arrester. The aging rate of lightning arrester valve plates is a key factor affecting their lifespan. Oxide lightning arresters operate in the later stages of their product life, and valve plate degradation causes an increase in leakage current, even leading to internal discharge. When the discharge is severe, the gas pressure and temperature inside the lightning arrester increase sharply, causing breakdown of the oxide lightning arrester body. When the internal discharge is not too severe, it can cause single-phase grounding of the system

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5. Poor impact resistance

Oxide surge arresters often experience accidents under operating overvoltage or lightning conditions. The reason for this is that during the manufacturing process of the valve plate, the quality control points of each process are not strictly controlled, resulting in a weak ability of the valve plate to withstand impulse voltage. In the process of frequently absorbing overvoltage energy, the deterioration of the valve plate is accelerated and damaged. When resonance occurs due to disconnection, grounding, or other reasons in the power grid, its amplitude can reach three times the phase voltage, which may cause damage to metal oxide lightning arresters.

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6. Poor self overvoltage protection capability of metal oxide lightning arresters

For overvoltage with limited energy, such as lightning overvoltage and operational overvoltage, lightning arrester leakage can provide voltage limiting protection. Overvoltage with infinite energy (with supplementary energy), such as transient overvoltage (a general term for power frequency overvoltage and resonant overvoltage), whose frequency is either an integer multiple or a fractional multiple of the power frequency, forms transient overvoltage. The power frequency power supply can automatically supplement the overvoltage energy, so that the amplitude of the lightning arrester leakage overvoltage does not decay or weakly decays. If transient overvoltage enters the lightning arrester protection action zone, it will inevitably repeatedly operate for a long time until thermal collapse, so transient overvoltage has a fatal harm to gapless metal oxide lightning arresters. ?

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7. Unqualified grounding resistance causes counterattack

The grounding body of the lightning arrester is the only channel for discharging lightning current. If the grounding resistance is too high or the grounding device is unqualified, when lightning enters, the lightning current will be discharged to the ground through the lightning arrester and the grounding electrode. Due to the high grounding resistance, it cannot be discharged, and some lightning will flow in the opposite direction to the lightning arrester or distribution transformer, causing counterattack and damaging the lightning arrester, and sometimes even destroying the distribution transformer.

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8. Installation error

Due to its small size and unclear umbrella skirt, as well as some electricians with low technical skills and a weak sense of responsibility, gapless metal oxide lightning arresters are installed in the opposite direction. When there is rainwater, the umbrella skirt is filled with water, causing discharge and arc creeping. Over time, the silicone rubber jacket of the metal oxide lightning arrester is damaged due to arc creeping discharge. During the inspection of the oxide lightning arrester in the distribution transformer, the author found that some of it was scrapped due to the reverse installation of the umbrella skirt, causing arcing discharge on the silicone rubber jacket.

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1. Correct selection of lightning arrester

The main difficulty in selecting lightning arresters is to determine the range of transient overvoltage. It is necessary to ensure safe and reliable operation under high operating overvoltage and atmospheric overvoltage, while also ensuring that the valve plate does not operate under transient overvoltage. At present, the selection and design of lightning arresters must ensure that the metal oxide lightning arrester does not operate when the system experiences the highest overvoltage during a 2-hour single-phase grounding, otherwise the oxide lightning arrester will experience thermal collapse accidents. In the design of model selection, the products with stable operation for many years shall be preferred. When selecting the manufacturer, the manufacturer with advanced process equipment and perfect detection means shall be selected to ensure that the selected metal oxide arrester has high anti-aging and impact resistance performance, so as to ensure stable operation within the life cycle of the product. Choose a series gap metal oxide lightning arrester. The valve plates of gapless metal oxide surge arresters are subjected to grid voltage for a long time, with harsh working conditions, low inflection point voltage, high operating frequency, and may also suffer from transient overvoltage hazards and temperature thermal damage, which rapidly accelerate the aging of the valve plates. Some have a shorter lifespan than silicon carbide surge arresters. The gap of the series gap metal oxide lightning arrester can ensure that the valve plate only withstands high voltage during the overvoltage protection process, with a very short time. In other situations, the valve plate is in a low potential state (composite gap resistance voltage division) for the grid voltage, greatly improving the long-term working conditions of the valve plate. The protection action of the series gap metal oxide lightning arrester only discharges lightning current without continuous current, with a light operating load. The gap does not need to have the ability to extinguish arcs and cut off continuous current. The 10 kV metal oxide lightning arrester has only one gap, and its operating characteristics can maintain long-term stable operation.

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2. Install a detachment device

To prevent accidents from escalating when metal oxide lightning arresters are damaged, it is recommended to install a disconnector at the bottom of each lightning arrester, so that when the arrester is subjected to abnormal voltage, it can be disconnected from the operating power grid in a timely manner. The main characteristic of lightning arrester failure is an increase in leakage current, which is difficult to detect during operation and may operate for a long time with problems, leading to the expansion of accidents. Therefore, it is necessary to monitor its operating conditions. The metal oxide lightning arrester is equipped with a disconnector. When it fails or is damaged, the disconnector will automatically operate (not more than 8 minutes at 30 mA) to exit operation, in order to avoid greater losses and accidents and improve operational safety and reliability. It can reduce the trouble of conducting various electrical performance tests or online monitoring for metal oxide surge arresters on a regular basis.

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3. Strengthen the management of metal oxide lightning arresters

Strengthening monitoring is one of the important measures to ensure the safe and reliable operation of metal oxide lightning arresters. Before operation, metal oxide lightning arresters must be inspected and tested, and the inspection and testing results should be recorded in the technical file. A technical file must be established for each metal oxide lightning arrester operating on the distribution network. The factory report, regular test report, and online monitoring operation records must be stored in the technical file as a reference for regular inspections until the lightning arrester is taken out of operation. Strengthen the professional and technical training for electricians, enhance their sense of responsibility, and establish dedicated (part-time) administrators for the management of metal oxide lightning arresters in each unit, with strict assessment. ?

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4. The grounding resistance should meet the requirements of the regulations

The grounding body of the lightning arrester should consider long-term durability, using hot-dip galvanized round steel with a diameter of 16 mm, which should be driven into the ground 7-8 meters underground (in plain areas) for transformers of 100 kVA and above, ensuring a grounding resistance of ≤ 4 Ω. For transformers below 100 kVA, the grounding resistance should be ensured to be ≤ 10 Ω. The lead wire should be made of 4 mm × 40 mm hot-dip galvanized flat iron points, and all connections should be firm. Special inspections and tests should be conducted before thunderstorms every year, and any problems should be promptly dealt with.

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The reasons for damage to metal oxide lightning arresters include lightning, transient overvoltage of the system, moisture, inherent faults, and counterfeit products produced by small factories. However, the reason for a certain proportion of damage is still unknown. The discreteness of the degradation rate of metal oxide lightning arresters, as well as the randomness of lightning, operating overvoltage, harmonics, and operating environment, all determine the reliability of the safe operation of metal oxide lightning arresters. Therefore, it is necessary to study, experiment, explore, and summarize in future work practice to prevent and improve unsafe factors during operation.

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