Abstract: This article introduces the development process and improvement results of zinc oxide (ZnO) nonlinear resistor technology at home and abroad; the evolution of surge arrester electric field design technology; the application of three-dimensional electric field analysis software in surge arrester design; the technological progress of porcelain-insulated surge arresters, GIS-type surge arresters, and composite-insulated surge arresters; and the miniaturization, light weight, high reliability, and long life of surge arresters brought about by the improvement of ZnO resistor performance. Finally, the article also introduces the discussion focus, achievements, and future issues to be addressed at each stage of IEC standard development. Keywords: Surge arrester; Zinc oxide resistor; Potential distribution; Electric field strength In the 1960s, Japan pioneered the development of zinc oxide (ZnO) nonlinear resistors. Due to their advantages such as low residual voltage, no follow current, small operating delay, and large current capacity, metal oxide surge arresters (MOAs) assembled from ZnO nonlinear resistors have been widely used and rapidly developed in power systems, replacing SiC surge arresters. Currently, MOAs have become the best-performing and fastest-growing overvoltage protection device in power systems, with an extremely wide range of applications. MOA (Medium-Oxide Arresters) are categorized into four types based on product structure: porcelain-insulated surge arresters, GIS (Gas Insulated Switchgear) surge arresters, composite-insulated surge arresters, and oil-immersed surge arresters (used in transformers). They are also classified by the type of equipment protected: overvoltage protection for distribution systems, overvoltage protection for substations, protection for parallel and series compensation capacitors, overvoltage protection for generators, limiting operational overvoltages generated by motor switching, wave trap protection in line carrier communication, limiting neutral point overvoltages in transformers with ungrounded neutral points, lightning protection for transmission lines, overvoltage protection for converter stations in DC transmission systems, overvoltage protection and energy absorption during the demagnetization process of large generator rotor circuits, and energy absorption in the system during the breaking of ultra-high voltage DC circuit breakers, among others. Because improving the reliability of protected equipment and reducing overvoltage levels can bring significant economic benefits, and because the nonlinear resistance characteristics of ZnO play a crucial role in limiting overvoltages, countries worldwide are striving to research and improve the raw materials and production processes of ZnO nonlinear resistance sheets, aiming to apply the latest technologies to the design of surge arresters, while simultaneously developing new materials to achieve more ideal protection effects. Please visit: Power Transmission and Distribution Equipment Network for more information . 1. Technological Evolution of ZnO Nonlinear Resistor Chips Since its industrialization in the late 1960s, ZnO nonlinear resistor chip technology has undergone continuous improvement and has now reached its fourth generation internationally. The first generation of ZnO nonlinear resistor chips emerged in the late 1960s and continued to be used until the mid-1980s. Its application was a revolution in power system protection against lightning overvoltage and switching overvoltage. However, it still had shortcomings such as an uneven V-A curve, low charge rate, large leakage current, and deterioration in aging performance. The second generation of ZnO nonlinear resistor chips has been in use since its industrialization in the early 1980s. Compared with the first generation, the second generation resistor chips made many improvements in the optimization of additive formulation, resulting in significant improvements in aging life and nonlinear performance. In the mid-1980s, Fushun Electric Porcelain Factory, Xi'an High Voltage Electric Porcelain Factory, and Xi'an Electric Porcelain Research Institute in China introduced the second-generation resistor chip technology from a Japanese company. Through more than 20 years of technological digestion, absorption, and improvement, its technical performance has now been further improved based on the second generation. The third-generation ZnO nonlinear resistor technology emerged in the mid-1980s, represented by Toshiba's technology. Its main features include a flatter V-A characteristic curve, improved protection characteristics, higher charge rate, better aging performance, nearly doubled 2ms square wave withstand capability, and a nearly 50% reduction in resistor volume while absorbing the same amount of energy, saving raw materials and achieving a lighter arrester. The side surface uses low-lead glass glaze, enabling it to withstand 4/10 μs currents, while also enhancing moisture resistance and adapting to various insulating media. Therefore, it can be used in various gases, insulating oils, and directly injection molded silicone rubber. The third-generation resistor is renowned as a high-performance resistor due to these superior performance characteristics. The fourth-generation ZnO nonlinear resistor was industrialized in the 1990s. Based on the third-generation technology, it increased the reference voltage per unit height by 2-3 times by adding new components, reaching 400 V/mm and 600 V/mm, meaning that the resistor height was reduced to less than half of the original height at the same reference voltage. Currently, it is mainly used in tank-type surge arresters for combined electrical appliances. Its application can greatly reduce the size of tank-type surge arresters, enabling them to be miniaturized. At present, research is underway on its application in composite line surge arresters with series gaps. Its development and application will enable line surge arresters to be miniaturized and lightweight, making them easier to install. The fourth generation of resistor elements is called high-gradient resistor elements. The development of resistor element technology is shown in Table 1. In summary, with the rapid development of resistor element technology and the continuous improvement of resistor element performance, the structure of surge arresters has also changed significantly, and the protection performance has become better and better. Source: http://www.tede.cn Table 1 Development of Resistor Element Technology 2 Technological Evolution of Surge Arresters 2.1 Development of Surge Arrester Design Technology Surge arrester design mainly includes electrical performance design and mechanical performance design. Electrical performance design mainly involves potential control of the surge arrester to make its potential distribution as uniform as possible and the electric field strength meet the design value requirements. Before the 1980s, the uniformity of distribution was mainly achieved through circuit calculations and verified using the current method. Later, binary electric field analysis software was used for electric field analysis and calculation, and neon lamps were used to measure the potential of the resistor elements in the surge arrester. This calculation method continued until the 1990s. These methods could only provide rough guidance for the design; further design work required extensive experimental verification and adjustments, resulting in a huge workload. Now, world-renowned manufacturers use precise three-dimensional electric field analysis software for electric field analysis and calculation. This software can analyze the influence of asymmetric equalizing rings and other media on the electric field. By using a small resistor element-fiber method to continuously measure the potential distribution along the resistor element, the local maximum voltage non-uniformity coefficient can be measured. Practice has proven that three-dimensional electric field analysis software can very accurately calculate the potential distribution and electric field strength in surge arresters, as shown in Figure 1, which greatly facilitates the design work. Using high-precision analysis technology to design and select equalizing rings can completely replace ceramic capacitors, thereby achieving optimal potential distribution and electric field strength. Figure 1. Analysis and calculation of potential distribution and electric field intensity in the asymmetric grading ring of a surge arrester using three-dimensional electric field analysis software. 2.2 Technological advancements in porcelain-insulated surge arresters. Surge arresters used at voltage levels of 330 kV and above have extremely uneven potential distribution in their core due to stray capacitance to ground. This causes the resistors on the upper part of the arrester to bear excessively high voltage, which accelerates the degradation of the resistors and ultimately leads to arrester damage. To solve this problem, two aspects need to be addressed: ① improving the charge rate of the resistors; ② reducing the influence of stray capacitance to ground to make the potential distribution as uniform as possible. Before the advent of third-generation resistors, due to limitations in design methods and resistor performance, simply using a grading ring to compensate for the stray capacitance was far from sufficient. Therefore, in the early stages, voltage equalization was achieved by connecting a coaxial capacitor and a grading ring in parallel on the resistor, which could suppress the potential unevenness rate to about 10%. However, its disadvantages were complex structure and extremely high cost of coaxial capacitors. To address the high cost issue, manufacturers later replaced coaxial capacitors with parallel ceramic capacitors next to the resistor elements in the design. However, this also has the following drawbacks: ① Because the capacitors compensate in sections, the potential distribution is discontinuous at the capacitor connection points, resulting in a significant voltage load on the resistor elements at these connection points; ② The charge life of the capacitors and the aging life of the epoxy tubes fixing the capacitors are crucial for the reliability of the surge arrester. With the emergence of third-generation high-performance resistor element technology and the application of three-dimensional electric field analysis software, world-renowned manufacturers have eliminated the parallel capacitor design and adopted equalizing rings to compensate for the effects of stray capacitance. This structure can control the voltage distribution non-uniformity of a 500 kV surge arrester to within 15%. Due to the increased charge rate of the resistor elements and the control of the voltage distribution non-uniformity within 15%, the surge arrester's lifespan requirements are fully met. The advantages of this structure are: ① It allows for a reduction in the diameter of the porcelain bushing, thereby reducing the weight of the surge arrester; ② It reduces nearly 60% of the components, simplifying the structure and improving reliability; ③ The potential distribution on the resistor elements does not change abruptly, ensuring the lifespan of the resistor elements. 2.3 Technological Advances in Tank-Type Surge Arresters Currently, gas-insulated metal-enclosed switchgear (GIS), using SF6 gas with excellent insulation properties as the insulating medium, is increasingly widely used worldwide due to its advantages such as small footprint, maintenance-free operation, and high reliability. For GIS with voltage levels of 110–1050 kV, the size of the surge arresters used with it is becoming increasingly larger as the voltage level increases. In GIS with voltage levels of 330 kV and above, if the surge arrester core adopts a single-column structure, the surge arrester body will be very tall. To solve the height problem, if ordinary gradient resistors (200 V/mm) are used, only a multi-column parallel or tortuous series structure can be adopted, which will also result in disadvantages such as large cylinder diameter, many parts (requiring the addition of insulating pads in the middle), and the inductance of the intermediate connecting wires affecting the protection level. High-gradient resistor elements are half the height of ordinary gradient resistor elements, making it possible to use a single-column structure design for the core of 800 kV and below tank-type surge arresters, thus simplifying the arrester's structure. The diameter of an 800 kV GIS surge arrester is only 1400 mm and its height is 3600 mm, achieving miniaturization and facilitating transportation. To further miniaturize tank-type surge arresters, Toshiba Corporation of Japan developed ultra-high gradient resistor elements with a gradient of 600 V/mm, reducing the size of the 800 kV GIS surge arrester to 1200 mm in diameter and 2700 mm in height, a 45% reduction in volume. In ultra-high voltage power transmission, the protection level of surge arresters directly affects project costs. Due to the requirements for surge arrester energy absorption capacity and protection level, world-renowned manufacturers use multi-column parallel (generally 4-column parallel) resistor element structures, which are very large and therefore pose significant transportation difficulties. The emergence of ultra-high gradient resistor elements has significantly reduced the size of surge arresters. It is foreseeable that surge arresters designed with ultra-high gradient resistor elements will be widely used in ultra-high voltage transmission systems. 2.4 Technological Advances in Composite-Insulated Surge Arresters Addressing the shortcomings of porcelain-insulated distribution surge arresters, such as the lack of pressure relief devices, high failure rate, and the risk of porcelain insulator explosion during faults, General Electric (GE) developed a composite-insulated surge arrester for distribution systems in 1979. Due to its advantages such as low internal moisture risk, no explosion hazard in low-voltage arrester skirts, small size, light weight, and good pollution resistance, its development is rapid worldwide. In my country, substation surge arresters have been developed up to the 220 kV voltage level. Their small size, light weight, and ease of on-site installation allow them to be installed on transmission line towers, effectively improving the line's lightning withstand level. Especially in areas with intense lightning activity where reducing grounding resistance is difficult, substation-type surge arresters provide a powerful means to address transmission line accidents caused by lightning strikes, reduce line overvoltage, and protect insulator strings. China has developed surge arresters up to the 500 kV voltage level. With the increase in voltage levels and the widespread adoption of compact towers, the disadvantages of conventional gradient resistance elements in surge arresters—such as excessive length and inconvenient installation—have become apparent. Currently, Japan is conducting research on applying high-gradient resistance elements to surge arresters and is also conducting preliminary research and development work for the formulation of IEC standards. 3. Changes in IEC Standards The first standard for gapless metal oxide surge arresters was the Japanese national standard established in 1984, version number JEC-217. It is the world's earliest standard for gapless surge arresters, including porcelain-insulated and GIS-type arresters. The first IEC standard for gapless metal oxide surge arresters was published in 1991, version number IEC 60099-4. It was developed by integrating standards for gapped surge arresters and various national standards for gapless surge arresters. It covered all surge arresters up to 500 kV. This standard explicitly stated sealing requirements, and the pressure relief test followed the regulations for gapped surge arresters, but it did not specify pollution test methods. IEC 60099-4 underwent its first revision in 1998, which clarified the pollution test methods. A second revision was conducted in 2001, which specified test methods for all types of surge arresters (including composite, oil-immersed, and GIS types), and discussed test methods for composite-insulated surge arresters. However, it still did not specify pressure relief and mechanical strength tests. IEC 60099-4, second edition, was published in 2004. This edition enriched the relevant regulations for various surge arresters, revising the pressure relief test and mechanical strength test. Standards for surge arresters for line voltage levels of 54 kV and below were published in 2002. Currently, Japan, France, and the United States are leading the revision of standards for line arresters, aiming to standardize all levels of line arresters. One important aspect is the discussion of applying high-gradient resistor elements to line arresters, reducing the arrester's height for better application in high-voltage and ultra-high-voltage systems. 4. Conclusion The successful development and widespread application of gapless metal oxide surge arresters represent the most outstanding scientific research achievement in power system overvoltage protection, elevating power system overvoltage protection technology to a new level. With continuous improvements in resistor element performance and advancements in design technology, surge arrester structures are becoming simpler, smaller, more reliable, and offer superior protection performance. The application of new materials and technologies has expanded the protection range of surge arresters, further reducing overvoltage levels in power systems and bringing greater economic benefits. About the authors: Song Jijun (1962-), male, senior engineer, engaged in surge arrester technical management; Su Yahong (1954-), male, master's degree, senior engineer, long-term experience in surge arrester design, development, and management; Pan Yangguang (1966-), male, master's degree, senior engineer, engaged in surge arrester testing and quality management.