Abstract : This article introduces and analyzes the current status and development trend of ultra-high voltage (UHV) power transmission and transformation technology abroad, especially UHV transformer technology, hoping to provide reference and guidance for the research, design and construction of China's 1000 kV UHV power transmission and transformation system. Keywords : Power system; Ultra-high voltage; Transformer With the construction of large-scale hydropower, thermal power and nuclear power bases, China's demand for long-distance, large-capacity ultra-high voltage and extra-high voltage power transmission is increasing. In the existing power grid, China's highest DC voltage level is ±500 kV, and the highest AC voltage level is also 500 kV. A 750 kV AC voltage level power grid was scheduled to be put into operation at the end of 2005, and the construction of DC ±800 kV and AC 1000 kV and above voltage level UHV power grids is currently being planned, researched and discussed. This article provides an in-depth introduction and analysis of the current status and development trends of ultra-high voltage (UHV) power transmission and transformation technologies abroad, particularly transformer technology, hoping to offer reference and guidance for the research, design, and construction of China's 1000 kV UHV power transmission and transformation system. 1. Current Status and Development Trends of Transformer Technology in Ukraine Ukraine is one of the few countries in the world with experience in developing ultra-high voltage and extra-high voltage power transmission and transformation technologies. The Zabrozhyn Transformer Research Institute in Ukraine is the world's largest transformer research institute. Its main work includes: conducting scientific research, design work, software development, new product development, designing tooling and equipment, researching production processes, manufacturing samples and small-scale products, testing electrical equipment, repairing complex electrical equipment, researching and proposing national standards, product certification, and consulting services. Its major product development and testing projects include: DC ±750 kV, 320 MVA transformer, smoothing reactor, disconnecting switch; 750 kV shunt reactor; 667 MVA, 1150/500 kV autotransformer model; 1800/500 kV autotransformer model; 750～1800 kV bushings; DC 600 kV pulse device; 220～500 kV neutral point bushings; dry-type transformer environmental testing capacity up to 1600 kVA; 750 kV and below electromagnetic current transformers; 500 kV and below electromagnetic voltage transformers; 750 kV capacitive voltage transformers. 2. Current Status and Development Trends of Transformer Technology in Russia In the 1970s, Russia produced single-phase 417 MVA/1150 kV and three-phase 1000 MVA/500 kV transformers. In the 1980s, it produced single-phase 667 MVA/1150 kV and three-phase 1250 MVA/330 kV transformers. 2.1 Improvements in Transformer Design and Manufacturing Based on Operating Experience From 1955 to 1990, the parameters of power transformers produced in factories were significantly improved. The voltage level increased from 220 kV to 1150 kV, the capacity of three-phase transformers increased from 240 MVA to 1250 MVA, and the capacity of single-phase transformers increased from 250 MVA to 667 MVA. Through feedback from operational accidents, the Zabrozhsky Transformer Research Institute in Ukraine conducted in-depth research on a series of issues, achieving excellent improvement results. None of the 240 750 kV power transformers operating in the system experienced accidents in nearly 15 years, achieving a high level of reliability. Based on this, the institute also systematically developed a computer-aided design program for transformers. 2.2 Zabrozhyn Transformer Plant, Ukraine The Zabrozhyn Transformer Plant (ZTR) is the world's largest transformer manufacturer, producing power and distribution transformers, reactors, split reactors, voltage transformers, current transformers, DC converter transformers, enclosed busbars (single-phase or three-phase), etc. 70% of ZTR's products are exported, and 75 countries have imported its transformers, including 73 500 kV power transformers imported by China. ZTR's main products include: ① 6 three-phase 1250 MVA/347 kV transformers (produced in 1980); 21 three-phase 1000 MVA/500 kV generator step-up transformers; and three-phase three-winding 300 MVA, 500/154/38 kV transformers. ② 67 single-phase 417 MVA, 750/500 kV transformers (manufactured in 1978); 1 single-phase 533 MVA, 500/330 kV transformer and 1 single-phase 417 MVA, 1150/500 kV transformer (manufactured in 1981); 26 single-phase 667/180 MVA, 1150/500 kV autotransformers (20 manufactured in 1979, 6 manufactured in 1972). ③ 320 MVA, ±750 kV converter transformer. ④ 120 Mvar, 800 kV and 300 Mvar, 1150 kV parallel reactors and their neutral point reactors. ⑤ OLTC 330 kV, 2000 A. ⑥ 1 single-phase 60 Mvar, 500 kV controllable reactor (manufactured in 1989). ⑦ 143 units of 417/50 MVA transformers (105 units produced in 1972, 38 units produced in 1973). ⑧ 1 unit of 3-phase 25 Mvar, 110 kV controllable reactor (produced in 1997). ⑨ Exported to Brazil a single-phase 500 MVA, 765/345 kV ±13% (1.3% per stage) autotransformer, P0=200 kW, Pk=700 kW. ⑩ Distribution transformers and various special transformers. 35 kV, 31.5 kA enclosed busbar. ZTR's core binding uses adhesive tape. A thin paper tube is filled inside the core column, then adhesive tape is applied. The core clamps are tightened with steel strips, similar in structure to transformers produced by Siemens and ABB. For winding voltages of 500 kV and above, a non-guided oil flow structure is used, so the inner and outer diameters of the windings are supported, and the oil baffles are not visible inside the winding coils. Transformer accessories are relatively outdated; the drain valve is a water valve, the oil tank is roughly machined, and the top of the oil tank has a large slope, making operation difficult for workers. The transformer uses magnetic shielding; the silicon steel sheets are approximately 80 mm wide and 15–30 mm thick, with the ends welded together. The transformer bushing structure is relatively outdated; the upper part of the bushing has a wire equalization cover, but no equalization ball; the bushing oil pressure relies on a separate small oil conservator. 3. Current Status and Development Trends of Transformer Technology in Japan To meet the growing electricity demand of the 21st century, Tokyo Electric Power Company (TEPCO) developed Japan's first 1000 kV transmission system, which is currently operating at the Shin-Haruna substation 1000 kV test field to test the performance and reliability of the 1000 kV equipment. Mitsubishi Electric has developed various 1000 kV electrical equipment. Toshiba's Ako plant manufactures a 1000 kV on-load tap-changing single-phase shell-type transformer for qualification testing. The specifications, construction, installation, and testing of this 1000 kV transformer manufactured by Toshiba are described below. 3.1 Specifications The basic specifications of Toshiba's 1000 kV on-load tap-changing single-phase shell-type transformer are shown in Table 1. The selection of the high-voltage and medium-voltage side capacities is primarily to meet the maximum transmission capacity requirements. The third winding capacity of 1200 MVA (40% of the high-voltage and medium-voltage side capacity) is chosen primarily to meet the maximum apparent capacity required for 1000 kV transmission lines. If the low-voltage side winding rated voltage were chosen to be 63 kV as in a 500 kV transformer, it would result in a large fault current; 147 kV is chosen to avoid increasing the size of equipment connected to the low-voltage side winding. The impedance value (short-circuit voltage percentage) is chosen at 18% to consider the maximum stability of the power grid, determined by factors such as the suppression of ground fault currents and the economic efficiency of transformer design. Because the 1000 kV substation will be built in a mountainous area, all transformer components must be transported by rail or large trailers. To meet transportation constraints, the main body of the 1000 kV transformer was divided into two units, each equipped with an automatic on-load tap changer. The selection of the long-term AC power frequency withstand voltage was based on fault analysis of the future 1000 kV transmission system, and partial discharge must not occur during the test. The lightning pulse withstand voltage was obtained from transient voltage analysis of the 1000 kV transmission system with high-performance surge arresters. 1950 kV was selected for the high-voltage side, and 1300 kV for the medium-voltage side. The 65 dB noise level is primarily to minimize substation noise, which can be achieved by installing steel plate barriers around the transformer. 3.2 Construction The voltage and capacity of the 1000 kV transformer are twice that of the 500 kV transformer, making it the largest transformer currently in use in Japan. However, transportation and installation space constraints require that the transport dimensions cannot be larger than those of the 500 kV transformer. Therefore, the single-phase transformer was divided into two units, each with the same capacity as a 500 kV, 1500/3 MVA transformer. These two units can be operated in parallel via a T-buffer with an oil-gas gasket. The 1000 kV transformer must be able to withstand twice the voltage of the 500 kV transformer while meeting the minimum insulation distance requirements for transportation. Therefore, the winding arrangement and insulation construction should minimize local electric field concentration, and numerous barriers are arranged to properly separate the oil space; a purification process is also used to reduce impurities in the oil, which helps ensure a greater insulation margin. If only insulating paper is used, the insulation of the 1000 kV conductor is unacceptable; multiple layers of barriers are used to reduce the insulation distance between the conductor and the transformer casing. 4. Conclusion Currently, 1000 kV transformers capable of long-term excitation testing have been produced, and testing has shown that their economical operation is reliable. The development, manufacturing, transportation, and assembly technologies of 1000 kV transformers can also be used to improve the quality of transformers at voltage levels of 500 kV and below. About the authors : Feng Qingdong (1964-), male, PhD, senior engineer, IEEE member, mainly engaged in research on power systems and automation; Wang Wei (1979-), male, PhD candidate, main research area is power systems and their automation.