Abstract: With the acceleration of urbanization, in the renovation of old urban areas and the construction of new urban areas, in order to reduce land occupation and adopt GIS (Gas Insulated Switchgear), and to meet the fire protection requirements of cable entry into the city, cable installation on land, oil-free operation, and fire resistance, the adoption of new 110kV transformers in some substations is an unavoidable choice. Besides the existing 110kV oil-immersed transformers , new types of transformers that can be selected include: 1. Dry-type transformers. The main products are resin-cast transformers, which are currently widely used in 35kV distribution systems. Single-phase transformer units are already in operation at 110kV levels. The main problems lie in the excessively high design field strength along the coil surface of the three-phase transformers, leading to issues with long-term insulation reliability and environmental adaptability. The IEC standard is currently being revised, and the environmental, climatic, and combustion tests proposed in the upcoming standard are important methods for assessing the insulation performance of dry-type transformers. 2. Gas-insulated transformers. This mainly refers to SF6 gas-insulated transformers. Future development will focus on newer gas-insulating media with better environmental performance. These transformers are filled with a pressurized gas-insulating medium in a sealed casing, and the gas is circulated and cooled by a pump. The transformer body and radiator must have good pressure-bearing and sealing performance, maintaining a low annual leakage rate. Currently, imported products are in operation in China. Some manufacturers are actively introducing technology to achieve domestic production as soon as possible. The IEC already has corresponding technical standards. 3. High-temperature resistant liquid-immersed transformers. The solid insulation materials used include conventional paper insulation or high-temperature resistant insulation materials, filled with high-ignition-point liquid insulation media such as silicone oil and β-oil. Different combinations are available, including hybrid, semi-hybrid, and composite types. For general-purpose distribution transformers, this reduces size, increases overload capacity, and improves fire resistance, making it an option between dry-type transformers and conventional oil-immersed transformers. Due to its moderate price and good environmental performance, it has considerable development potential. It is also a viable option for 110kV transformers. Semi-hybrid and hybrid high-temperature liquid-immersed transformers are already in use domestically. Corresponding IEC standards are under development. However, issues such as oil chromatography and gas protection exist, and load losses are relatively high. 4. Cable transformers: Some large foreign companies have launched practical products, and some domestic transformer manufacturers are collaborating with universities to develop similar products, hoping to better solve the problems of oil-free and fire-retardant performance of transformers at the 110kV voltage level. However, the concept of longitudinal insulation and conventional winding wave process theory have fundamentally changed for cable transformers. The design and testing techniques for its insulation structure need further research and discussion. II. Transformer Short-Circuit Withstand Capacity Urban power grid transformers, due to their relatively high voltage and low impedance, have a crucial short-circuit withstand capacity. IEC standards and national standards have recently been revised, raising the requirements for transformer short-circuit withstand capacity testing. The number of tests has increased from three to nine, and the voltage value for repeated insulation withstand tests after the short-circuit test has been changed to 100%. Lightning impulse tests and other type tests must also be conducted after the short circuit. The testing and assessment are more stringent. This places higher demands on transformers used in urban power grids, which is undoubtedly beneficial for improving their operational reliability. Whether transformers that passed the short-circuit withstand capacity test under the old standard can pass the test requirements of the new standard is a question worth considering. Of course, the implementation of the standard will require a process. However, from the date the standard comes into effect, testing should be conducted according to the new standard; otherwise, it violates the provisions of the mandatory standard. III. Energy Saving and Consumption Reduction The issue of energy saving and consumption reduction for transformers used in urban power grids has always received widespread attention. Since the 1970s, the country has implemented a policy to phase out high-energy-consuming products. Distribution transformers have undergone two to three major upgrades, and currently, various types of dry-type and oil-immersed transformers are approaching internationally advanced levels. However, there is still a gap compared to the requirements for high efficiency and energy saving. Currently, due to the urgent need for resource and environmental protection, the country is formulating a series of mandatory energy-saving standards for electrical products and guiding the certification of energy-saving labels for high-efficiency energy-saving electrical products, supplemented by certain policy incentives. Distribution transformers are no exception. In energy conservation and consumption reduction work, the following should be noted: 1. Energy conservation and consumption reduction in transformers must not sacrifice product operational reliability. Under any circumstances, the principle of reliability first must be upheld. While taking into account various technical performance aspects of the product, further reduction of losses and improvement of product performance indicators should be pursued to avoid the lessons learned from past upgrades that resulted in decreased dynamic stability, over-excitation capacity, and overload capacity. 2. Energy conservation and consumption reduction in transformers must adhere to a targeted approach. First, address energy conservation and consumption reduction in widely used products to drive energy conservation efforts in similar products. For example, should energy conservation in distribution transformers be mandated across the entire product line, or should a survey be conducted to determine a general series of products, such as small-capacity products, and then establish standards for limiting losses? We should learn from developed countries and attach great importance to the standardization, versatility, and interchangeability requirements of small-capacity distribution transformers, thereby driving energy-saving and consumption-reducing designs for other products. 3. Energy conservation and consumption reduction in transformers must take into account the ripple effects of reducing line losses, minimizing voltage fluctuations, and improving power supply quality in low-voltage distribution systems. Distribution transformer energy conservation should not be viewed in isolation. Standards should provide correct guidance for miniaturizing distribution transformers, increasing their density, limiting power supply radius, reducing low-voltage distribution network line losses, and improving power supply quality. Besides the needs of high-power centralized power supply loads, building design departments should be encouraged to rationally configure transformers according to local conditions, comprehensively comparing the long-term economic efficiency, rationality, and the increased one-time investment in distribution equipment, changing the current abnormal situation of selecting increasingly larger single-unit capacity distribution transformers. 4. The copper-iron loss ratio, i.e., the ratio of transformer load loss to no-load loss, is determined by the transformer's load rate. Generally speaking, a lower load rate requires a higher copper-to-iron loss ratio, while a higher load rate requires a lower ratio. However, this is based on continuous improvements in the performance of silicon steel sheet materials. In the past, the common practice was to have two series: one for urban power grids with high load rates and the other for rural power grids with low load rates. Could we, within the same series, consider the different typical capacities of urban and rural transformers and select transformers with different copper-to-iron loss ratios? That is, transformers used in rural power grids, due to dispersed residences and generally smaller capacities with lower load rates, could use transformers with higher copper-to-iron loss ratios. Transformers used in urban power grids, on the other hand, are generally used in more concentrated residences with higher load rates, so transformers with slightly larger capacities and relatively lower copper-to-iron losses should be selected. In other words, it is not necessary to insist on a uniform copper-to-iron loss ratio for transformers at the low and high capacity ends of the series. When assessing transformer losses, in addition to active power losses (the sum of no-load and load losses), reactive power losses should also be considered. Currently, the multi-stage joint technology for the core can reduce the no-load current to a very low level. Winded core transformers also have very low no-load currents, giving them a certain advantage. Impedance voltage generally doesn't change much and has little impact on reactive power loss, so it can be disregarded. For transformers, active power loss accounts for a larger proportion and is the main factor affecting transformer losses. 5. When launching a new series of energy-saving transformers, the material consumption in the design of various capacity specifications of the series must be considered. While achieving energy saving, material consumption should not be excessively increased. The material consumption per unit capacity should be used as an indicator to evaluate the technological advancement and economic efficiency of the series of products, and should be comprehensively considered. Furthermore, the pursuit of material performance indicators should not be solely based on quantity, and excessive reliance on foreign material suppliers should be reduced. IV. Fully Sealed Transformers To reduce the workload of operation and maintenance of distribution transformers, a considerable number of transformer manufacturers have changed their oil tanks from flat tube type or plate radiator oil tanks to corrugated type or expansion plate radiator oil tanks, eliminating the oil conservator. The corrugated oil tank is available in two types: with or without air cushion. The Transformer Research Institute has formulated corresponding technical standards. In actual production, attention should be paid to the coordination between the throughput of the corrugated oil tank and the expansion of the oil. Excessive positive pressure or vacuum should not be allowed in the tank, otherwise, repeated positive pressure or vacuum can easily damage the seal of the transformer during operation. The throughput of the oil tank is closely related to the elasticity of the steel plates of the corrugated or expansion-plate radiators used. If the expected throughput is not achieved, appropriate oil cups or small oil conservators should be used to partially compensate for excessive drop in the liquid level. For fully sealed transformers with air cushions, attention should also be paid to the sealing condition of the top structure of the oil tank to withstand the test of long-term alternating hot and cold operation, truly achieving a fully sealed function. V. Components and Materials In recent years, with the development of reform and opening up, many advanced foreign technologies have been introduced, including in the area of ​​transformer components and materials. For example, components such as bushings, tap changers, and radiators, as well as transposed conductors, paper insulation materials, and insulating oil, are now produced and sold in China by many foreign manufacturers. This has greatly promoted the improvement of the quality of transformer components and materials. Besides traditional structural forms, some entirely new component manufacturing technologies have been introduced, such as pluggable cable terminal bushings, which are now available in series from 10kV to 110kV, undoubtedly providing us with more choices. Even smaller components, such as temperature controllers, gas relays, oil flow meters, oil level gauges, oil pumps, fans, and pressure relief valves, have seen the introduction of better products. Overall, this has improved the internal and external quality of transformers. However, it cannot be denied that some small domestic factories are producing inferior components and materials, such as ash powder and high-displacement insulating paper, self-adhesive dripping in self-adhesive transposed conductors, and the use of recycled oil instead of new insulating oil. In the current fiercely competitive market, manufacturers' profit margins are already very small. Some transformer factories with low management levels, in order to achieve profit, are willing to sacrifice the long-term operational reliability and service life of transformers, creating a situation where good and bad products are indistinguishable and counterfeit products are passed off as genuine. Transformers made with some inferior components and materials can still pass routine factory tests, and problems will not immediately surface during short-term operation. Once exposed, similar accidents will occur frequently, posing a potential threat to the safe operation of the power system. We must remain vigilant. We must emphasize component and material standards and strengthen on-site inspections. VI. Single-phase power supply and 20kV high-voltage distribution network power supply Currently, pilot projects for single-phase power supply and 20kV distribution network power supply have been implemented in China. Single-phase power supply is more advantageous for powering villa communities and resorts with relatively dispersed residences. 20kV high-voltage distribution network power supply is beneficial for improving urban power grid planning, power supply nodes, and expanding the power supply range. It should be said that these two pilot projects pose little difficulty for the transformer manufacturing industry. In particular, single-phase transformers with single-phase three-wire power supply, using a wound core structure, have significant advantages in energy saving and noise reduction. It is recommended to continue expanding the pilot scope in new area construction. VII. Partial discharge test of 110kV transformers. New IEC standards and national standards stipulate that 110kV oil-immersed transformers should include partial discharge measurement requirements in addition to the induced withstand voltage test, and this should be a routine factory test item for each transformer. This is beneficial for controlling the manufacturing quality of transformer products and improving product design and process levels. Currently, 110kV transformer manufacturers are actively conducting this previously unexplored test. It is believed that transformer products tested through partial discharge measurement will have higher operational reliability. Power supply departments should also actively create conditions to add the requirement of partial discharge measurement for 110kV transformers to the handover and acceptance tests, thereby controlling the installation quality of transformers. VIII. New Distribution Equipment in Urban Power Grids In recent years, many new types of distribution transformers have been introduced into urban power grid electrical products, such as prefabricated substations and combined transformers, commonly known as European-style and American-style prefabricated substations. In reality, the American-style prefabricated substation is just a simple combination of transformers. It does not yet possess the high and low voltage remote operation capabilities of a prefabricated substation. However, when combined with ring network switches and cable branch boxes, it can form a "modular" power supply network to supply power to residential areas, and it has mature operating experience abroad. Some domestic power supply departments and manufacturers have also developed many different types of simple combined prefabricated substation products to meet various needs; they call this type of product "Chinese-style prefabricated substations." It possesses its own unique characteristics and can adapt to the needs of different regions in China. In addition, underground or semi-underground transformers and prefabricated substations have been introduced to address the power supply needs of streetlights and other applications. Dry-type transformers also come in different types, including cast iron, wound iron, and open-ventilated types. The introduction and development of new power supply equipment has met the needs of urban and rural power grid construction for diversified selection of electrical products. However, the current situation is that the speed of standard setting often lags behind equipment manufacturing and technology import. Standards for new products should be formulated as soon as possible to clarify their performance characteristics and the specific requirements of the power grid.