Based on the construction practice of the 1000MW unit at Huaneng Yuhuan Power Plant, this paper introduces the structural types of the three main generators, focuses on the selection of materials and equipment under ultra-supercritical high-parameter and large-capacity conditions, and also considers the related system layout and design concepts. The content can serve as a reference for the future promotion of ultra-supercritical technology units. [b]0 Overview[/b] Ultra-supercritical power generation technology has a development history of more than ten years abroad. In 2001, the former State Power Corporation listed the ultra-supercritical coal-fired unit plan as a key national science and technology project during the "15th Five-Year Plan" period, namely the 863 Project. Huaneng Corporation and the former State Power Corporation jointly undertook the technical selection of a sub-project. Through the construction of my country's first 1000MW ultra-supercritical unit in Yuhuan, Zhejiang Province, as the basis for the project, discussions and practices have been carried out, and phased results have been achieved. Regarding the feasibility of the Yuhuan ultra-supercritical project, through extensive practical investigation, parameter comparison, and technical demonstration, we finally determined the Yuhuan unit model to be 1000MW, 26.25MPa, and 600℃/600℃. The selected parameters are similar to those recommended by the 863 Project Group. The unit selected by Huaneng in Yuhuan has higher pressure than Japan and higher temperature than Europe, especially since it is a single-shaft unit, and there is no identical model in the world to refer to. In order to improve the overall technical level of my country's thermal power units, all three main units were ordered domestically. Through the step-by-step manufacturing of the four units, the manufacturers gradually mastered the key technologies and improved the localization rate. The boiler was won by Harbin Boiler Co., Ltd., with Mitsubishi Corporation of Japan as the technical support provider. The steam turbine and generator were supplied by Shanghai Steam Turbine Co., Ltd. and Shanghai Steam Turbine Generator Co., Ltd., respectively, with Siemens of Germany as the technical support provider. [b]1 Boiler Structural Characteristics[/b] The boiler of Huaneng Yuhuan Power Plant is an ultra-supercritical parameter variable pressure vertical coil once-through boiler, single reheat, balanced ventilation, open-air layout, solid slag discharge, all-steel frame, and fully suspended Π-type boiler. The furnace cross-sectional dimensions (width × depth × height) are 32.08m × 15.67m × 66.40m, with a furnace volume of 28,000m³. The maximum continuous evaporation capacity (B-MCR) of the boiler is 2950t/h. The boiler outlet steam parameters are 27.56MPa/605℃/603℃. Both the upper and lower water-cooled walls of the furnace use internally threaded vertical pipes. A mixing header is installed between the upper and lower water-cooled walls. Throttling orifices of different diameters are installed on the water-cooled wall inlet pipe sections leading from the lower headers of each water-cooled wall, according to different circuits. To prevent boiler slagging, the furnace volumetric heat load is carefully selected as 82.7kW/m³, and the furnace cross-sectional heat load is 4.59MW/m². The boiler adopts a built-in start-up system with a starting circulation pump. The combustion method is an octagonal double-flame tangential combustion method without partition walls. PM-MACT type octagonal reverse double tangential arrangement swing burners are used. The NOx emissions from this burner are below 360 mg/Nm3. The superheater system employs a four-stage arrangement to minimize enthalpy increase at each stage, sequentially along the steam flow: horizontal and vertical low-temperature superheaters, partitioned superheaters, screen-type superheaters, and the final superheater. The superheater system has three stages of water spray desuperheating. The reheater is divided into low-temperature and high-temperature reheaters, with an emergency water spray desuperheater between the two reheater stages. Reheat steam temperature is primarily regulated using flue gas dampers. The economizer tube bundle uses seamless, smooth tubes arranged in parallel. The economizer is a continuous coil, drainable type. The selection of the main materials for the heating surfaces is crucial to success. The main materials for the heating surfaces of the high-temperature superheater and the final stage reheater are Super304H and HR3C. Super304H has a higher allowable stress at high temperatures, but it is slightly inferior to HR3C in terms of resistance to steam oxidation and high-temperature corrosion of flue gas. The Yuhuan Power Plant boiler uses a combination of HR3C, a mature material with better oxidation resistance, and Super304H. Super304H is shot-peened and used in the lower temperature areas of the metal, while HR3C is used in the easily oxidized high-temperature areas. After the heating surfaces are formed, Super304H and HR3C undergo solution heat treatment. [b]2. Turbine Structural Characteristics[/b] The turbine at Huaneng Yuhuan Power Plant utilizes technology from Siemens, Germany. Its main technical specifications are: ultra-supercritical, single-stage intermediate reheat, single-shaft, four-cylinder, four-exhaust, dual back pressure, condensing, eight-stage regenerative extraction, 1000MW power output, main steam valve inlet pressure 26.25MPa, temperature 600℃, reheat valve inlet temperature 600℃, feedwater temperature 292.5℃. Average back pressure is 5.39/4.4kPa, summer back pressure is 9.61/7.61kPa. This type of turbine is currently the world's largest capacity turbine using a single-shaft series arrangement of a high-pressure cylinder, an intermediate-pressure cylinder, and two low-pressure cylinders. Except for the high-pressure rotor, which is supported by two radial bearings, the intermediate-pressure rotor and the two low-pressure rotors are all supported by single bearings. This support method not only has a more compact structure, shortens the shaft length, improves shaft rigidity, and reduces the impact of foundation deformation on bearing load and shaft alignment, but also facilitates the smooth operation of the turbine rotor and saves on plant investment. The high-pressure and intermediate-pressure cylinders are supported in a traditional manner, with their claw supports resting on two bearing seats at the front and rear of the cylinder. The low-pressure outer cylinder sits directly on the condenser neck, while the low-pressure inner cylinder sits directly on dedicated platforms at the front and rear of the outer cylinder via claw supports and brackets. The inner and outer cylinders are sealed together by an expansion joint, completely eliminating the impact of thermal deformation or displacement of the low-pressure outer cylinder on the low-pressure dynamic and static clearances. The inner cylinder bracket is connected to the intermediate-pressure outer cylinder using a push-pull device to maintain the axial dynamic and static clearances of the low-pressure section. The high-pressure cylinder adopts a single-flow, double-layer cylinder design. The outer cylinder is barrel-shaped, with the front and rear sections bolted together, while the inner cylinder has a vertical longitudinal bisecting structure. The cylinder block is rotationally symmetrical, without connecting large flanges, and the cross-sectional area of ​​the cylinder remains constant, resulting in uniform heating. When the main steam parameters change, the temperature gradient of the cylinder block is very small, and it almost simultaneously reaches the same temperature level as the rotor. Therefore, the unit adapts to rapid start-up and shutdown, and during load changes, without needing to monitor the absolute expansion of the cylinder and the expansion difference with the rotor. The high-pressure cylinder has no regulating stage, with 100° full-circumference steam inlet. The first stage uses a low-reaction blade stage (approximately 20° reaction) to reduce the steam temperature entering the rotor blades. The first-stage stationary blades are angled, with tangential steam inlet, resulting in high efficiency and low steam leakage loss. The intermediate-pressure cylinder adopts a double-flow, double-layer cylinder design. In addition to using a low-reaction blade stage like the high-pressure cylinder and a tangentially inlet angled stationary blade structure, the first stage of the intermediate-pressure cylinder also employs a tangential vortex cooling technology to reduce the temperature of the intermediate-pressure rotor. In addition, the flow passage of the high- and medium-pressure rotor adopts a small diameter and multiple stages; the moving and stationary blades of the high, medium, and low pressure systems (except for the last three stages) are designed as a full three-dimensional saber shape, and the combination of blade stages with different degrees of reaction improves the flow efficiency; two main regulating valves and two reheat regulating valves are horizontally arranged on both sides of the cylinder, with tangential steam intake; the valve body is bolted to the cylinder for easy maintenance; all bearings are directly supported on the foundation through bearing seats, the cylinder does not bear the weight of the rotor, deformation is small, and it is easy to maintain the stability of the dynamic and static clearance. The expansion system design has a unique technical style: the absolute dead point and relative dead point of the unit are both located at the thrust bearing between the high and medium pressure systems, and the entire shaft system expands towards both ends with this dead point. The low-pressure inner cylinder also expands backward through the push-pull device between the cylinders, so the relative clearance change of the moving and stationary blades is minimized. All sliding support surfaces between the bearing seats and the base plate, and between the low-pressure inner cylinder cat claw bracket and the base plate, are made of low-friction alloy, which can ensure the free expansion of the unit without lubrication. Due to pressure variations, the exhaust steam humidity of ultra-supercritical units is higher than that of subcritical units with the same inlet steam temperature. Therefore, it is crucial that the low-pressure final stage and second-final stage blades possess adequate resistance to stress corrosion and water erosion. Regarding the application of anti-water erosion and anti-corrosion technologies, in addition to structurally designing sufficient flow channels, a considerable axial clearance, and using hollow final stage stator blades, the final stage blades utilize 17-4PH material, which has excellent corrosion resistance. This material exhibits significantly higher fatigue strength in sodium salts and water than 12Cr steel. Furthermore, a novel laser surface hardening technology is employed on the final stage and second-final stage moving blades. Although the world has a wealth of blade technology reserves, the number of blades with actual operational performance is limited. The Yuhuan project utilizes N30 modular blades for the low-pressure cylinder, with a final stage blade length of 1146 mm. Huaneng Yuhuan Power Plant has introduced supplementary steam technology to its steam turbines. This technology involves drawing a portion of fresh steam (5-10% of the intake steam) from between the main steam valve and the main control valve at a certain operating condition (TMCR). After throttling to reduce parameters (steam temperature drops by approximately 30°C), the steam enters the space after the fifth-stage high-pressure rotor blades. The main steam mixes with this steam and continues to expand and perform work in subsequent stages. Supplementary steam technology improves the turbine's overload and frequency regulation capabilities. Furthermore, because the valves can be fully open under various major operating conditions, steam throttling is avoided, improving the heat rate in operating conditions below the supplementary steam initiation point. Although this technology lacks practical application experience with large-capacity, high-parameter steam turbines, and the use of supplementary steam valves in the Yuhuan turbine carries some risk, there are no major difficulties in its principle and structure, so the risk can be judged to be minimal. The supplementary steam valves are only opened in operating areas above TMCR and when frequency regulation is required; most of the time, they are kept in reserve. The turbine of the Yuhuan project has a power output of 950MW under TRL condition (back pressure 11.8kPa). Considering the actual conditions of the cooling water (15m deep seawater) and the cooling area of ​​the condenser in the Yuhuan project, the back pressure in summer is about 8-9kPa. If the turbine operates in sliding pressure mode, the annual output of the unit can reach 1000MW, but there is no frequency regulation capability; if it operates in 5-throttling mode, the summer output is slightly lower than 1000MW; after adopting the steam injection valve technology, the annual output of the unit can reach 1000MW and has frequency regulation capability. [b]3 Generator Structural Characteristics[/b] The generator is not directly related to ultra-supercritical technology and parameters. The cooling method of the Yuhuan generator is water-hydrogen-hydrogen, the stator winding rated voltage is 27kV, the short-circuit ratio is 0.5, the wiring method is 42 slots, YY connection, and 6 terminals. The stator bars use stainless steel water pipes and solid oxygen-free copper wire; the main insulation thickness is 6.5mm, F-class insulation, B-class assessment; manufactured using VPI insulation process; stainless steel hollow conductors are braided with solid copper wire. A vertical spring plate vibration isolation structure is used between the frame and the core, with the natural frequency avoiding 179Hz and the amplitude being 38μm. The stator core structure adopts a stacked method and is axially fixed. The stator core silicon steel sheets use M270-50A type 0.5mm thick laminations, with an air gap magnetic flux density Bs of 1.2T, and the core ends are shielded. The rotor's large teeth have damping grooves with damping copper bars. The damping groove wedges extend outwards to the retaining ring's lap surface for good contact, functioning as damping windings (there are no damping bars inside the embedded grooves). The rotor slot wedges are made of nickel-silicon copper. The retaining ring structure and material are Mn18Cr18 non-magnetic austenitic steel retaining rings with a comb-tooth structure and a suspended retaining ring. The rotor shaft material is 26NiCrMoV, an integral forging without a center hole. The hydrogen supply system features a CO2-displacement hydrogen system with automatic hydrogen replenishment and a molecular sieve hydrogen dryer, with a booster fan in the hydrogen dryer. The excitation method is brushless excitation, with an excitation voltage of 418V, a current of 5653A, and a demagnetization time constant of 8.73. The selection of the main auxiliary equipment for Huaneng Yuhuan Power Plant considered safety and reliability to support the safe operation of the main unit, fully taking into account the improvement of domestic auxiliary equipment manufacturing capabilities, and also aiming to reduce project costs so that the unit has sufficient competitiveness after being put into commercial operation. [b]4 Material Selection for Thermal Pipelines[/b] P92 steel was chosen for the main steam pipeline. While there is sufficient domestic experience with conventional P91 steel, its application range is limited to 540–600℃ according to relevant standards, and its allowable stress at 610℃ is relatively low. The calculated wall thickness reaches 108mm, resulting in long welding times and high welding costs for single-pass welds. More importantly, it is difficult to guarantee weld quality. Furthermore, the large wall thickness is detrimental to the flexibility of the piping system, and the large thermal expansion of the piping system is difficult to absorb, which would generate significant thrust on the unit. Experts believe that the wall thickness of the main steam pipeline should not exceed 80mm. At the same time, P91 material ages rapidly at 600℃, and thick-walled P91 main steam pipelines experience high thermal stress, making long-term operational safety difficult to guarantee. Therefore, P91 was not chosen as the main steam pipeline material for Yuhuan. In addition, while P92, P122, and P911 all meet the temperature requirements, P911 has a higher carbon content, and P122 has a higher chromium content, resulting in poor weldability and slightly lower structural stability during long-term high-temperature operation. Furthermore, the calculated wall thickness of P911 reaches 92mm. In contrast, P92 has high high-temperature creep strength, and its welding and other heat treatment processes are similar to P91, with a calculated wall thickness of only 72mm. From a technical perspective, P92 is more suitable as the main steam pipe material. For the reheat section, due to the lower steam pressure, P91 with a moderate wall thickness is suitable. If P92 with a thinner wall thickness is chosen, the pipe fittings will be more difficult to process, and the ellipticity of the pipes will be difficult to guarantee. The reheat cooling section uses A691Cr1-1/4CL22, and the feedwater pipes use 15NiCuMoNb5. These two materials are mature materials with proven applications in China. [b]5 Conclusion[/b] In summary, considering the large capacity and high parameters of the 1000MW ultra-supercritical unit in the Yuhuan project, attention was paid to the selection of high-temperature and high-pressure materials; full consideration was given to the conventional configurations of the technical support providers, such as the mature combination of Siemens generators and rotating exciters; the turbine and generator both utilize Siemens technology, and the electromechanical integration ensures the stability of the shaft system; based on thorough research, domestic or joint-venture products were prioritized for major auxiliary equipment while ensuring safety, supporting domestic manufacturing and reducing the initial investment of the Yuhuan project; the design incorporated new concepts of foreign power plant layout and management, improving the unit's automation level and overall environmental protection level. As the Huaneng Yuhuan Power Plant project is still under construction, many issues require further discussion and exploration. The views expressed in this article are for discussion only, and we welcome the attention of all parties. The aim is to ensure that my country's first domestically produced 1000MW ultra-supercritical unit using imported technology truly achieves the goals of safety, reliability, efficiency, and maturity.