[b]1 Development of Cooling Technology for Hydropower Generators[/b] Cooling methods for hydropower generators include all-air cooling, stator water cooling, rotor air cooling, and stator-rotor water cooling. All-air-cooled hydropower generators have advantages such as simple structure and system, and easy operation and maintenance. However, as they develop towards larger sizes, the large electrical load causes temperature rise, affecting insulation life and limiting the increase in manufacturing capacity. Air cooling is characterized by the requirement that the heat generated by the conductors inside the stator winding insulation must be dissipated to the air through the insulation surface, or further dissipated to the air through conduction through the iron core. This inevitably leads to a rise in conductor temperature. As the unit's three-dimensional dimensions increase, the high temperature rise of the windings can also cause thermal deformation of the stator iron core and excessive thermal stress. The hot and cold cycles during generator start-up and shutdown cause winding expansion and contraction, leading to insulation fatigue and delamination, as well as relative sliding with the stator slots. These have become important problems for large generators, affecting their reliability. Furthermore, the large rotor weight, mechanical stress, and bearing load all increase the difficulty of manufacturing. While water-cooled technology overcomes the aforementioned limitations, it also brings the following problems: ① water pump circulation and water deionization purification system; ② scaling and oxide blockage when water purification is insufficient; ③ insulation failure caused by water leakage. Evaporative cooling technology is steadily developing to inherit the advantages of air cooling while overcoming its disadvantages. Based on the successful development and long-term reliable operation of 10 MW and 50 MW hydro-generator industrial units, the stator winding of Lijiaxia Unit 4 was changed from air cooling to evaporative cooling. On the one hand, this can improve the performance of the original unit, and on the other hand, it can provide experience for the development and operation of larger units through experiments. [b]2 Design of Evaporative Cooling for Unit 4[/b] 2.1 Design Principles Unit 4 of Lijiaxia was changed to evaporative cooling only after three air-cooled units had been put into operation and the elbow tube of this unit had been installed. Because the Lijiaxia Hydropower Station uses a double-row generator layout, changing the cooling method would involve significant structural modifications. Therefore, when switching to evaporative cooling, the overall structural layout, control dimensions, and stator and rotor elevations of Unit 4 were required to remain unchanged, and to be as consistent as possible with the three already operational air-cooled units. This involved optimizing the number of stator slots, the wire gauge and ratio of solid/empty stator conductors, and considering both the normal operation of the evaporative cooling system and the maximum possible output and stator bar cross-sectional current density in the event of a system failure and generator shutdown under full air-cooling conditions. In-depth research was conducted on the structure of the gas and electrical connections from multiple aspects, including structural design, manufacturing processes, installation, and maintenance. The design and research of the evaporative cooling system is the structured implementation of evaporative cooling technology. It mainly includes the optimization of the generator electromagnetic scheme and the optimized design of structural components such as stator bars, sealing joints, fluoroplastic lead pipes, gas collecting pipes, liquid collecting pipes, condensers, connecting pipes, equalizing pipes, exhaust valves, and drain valves. The reliability of the evaporative cooling system is crucial for the successful development of the evaporative cooling generator. Therefore, the design should prioritize a simple and reliable structure and optimize the system design; minimize potential accident points; and minimize on-site welding of the evaporative cooling system. 2.2 Optimization of the Generator Electromagnetic Scheme The generator electromagnetic design directly affects the implementation of the evaporative cooling system, influencing its manufacturability and manufacturing difficulty. The original generator design had 576 slots per layer, divided into upper and lower layers. Due to the excessive number of slots and insufficient space between the conductor bars, the design was changed to hollow conductor bars, which could not meet the construction requirements. Therefore, a 396-slot (upper and lower layers) scheme was proposed. This scheme maintains the rotor essentially unchanged; the upper and lower frames are completely identical to the original air-cooled scheme for stator evaporative cooling. This preserves the original generator structure and the consistency of the power station elevation while effectively reducing the number of generator stator slots, making it fully adaptable to the manufacturing requirements of internally cooled generators. 2.3 Design of Stator Winding Sealing Joints For the evaporative cooling system, the sealing joint is one of the key components. Lijiaxia adopted a structure where hollow conductors are extracted and welded to the sealing joint. To ensure sealing reliability, the connection between the sealing joint and the fluoroplastic tubing uses a compression fitting structure. This fitting underwent rigorous simulated leakage testing before manufacturing to guarantee its sealing performance. 2.4 Exhaust Pipe Arrangement: To ensure the safe operation of the evaporative cooling system, the exhaust pipes must be unobstructed and reliable. Two symmetrically arranged exhaust pipes are selected, with the inner side higher than the outer side to prevent condensation and blockage of the exhaust pipes by the cooling medium. 2.5 Ventilation System Optimization: Since the evaporative cooling medium carries away stator winding losses, the losses that need to be carried away by air cooling are significantly reduced. Analysis and calculation show that the required airflow is 137 m³/s. Due to the thin air at high altitudes, the required airflow is 170 m³/s, while the original airflow for air cooling is 270 m³/s. The airflow is reduced to an appropriate amount while maintaining the original structure. 2.6 Cooling Medium: The cooling medium for the evaporative cooling system is Freon R-113. It possesses excellent insulation and fire-resistant arc-extinguishing properties, is non-corrosive when anhydrous, and has a boiling point of 47.3℃ at atmospheric pressure. When the motor cooling system operates below 70℃, the pressure is between 0 and 0.02 MPa gauge pressure, effectively operating as a pressureless sealing system, facilitating manufacturing and installation while maintaining high reliability. The motor has 792 (originally designed as 1152) stator bars, each with a separate air branch, divided into upper and lower interfaces. The lower interface is the liquid inlet, connected to the liquid collector via an insulated lead-in pipe; the upper interface is the air outlet, connected to the gas collector via an insulated lead-in pipe. The gas collector and liquid collector are connected via a condenser, forming a closed-loop circulation system. The stator bars are divided into upper and lower layers, and the gas collector and liquid collector are also divided into inner and outer rings, connected to the upper and lower layers of bars respectively. [b]3 Stator Rollout of Evaporative Cooling Unit[/b] 3.1 Preparatory Work Before Rollout of Bars The stator rollout of Unit 4 at Lijiaxia is a national public project, and the installation personnel must strictly follow the relevant drawings, procedures, and specifications to complete the work. In evaporative cooling generators, the lamination process is the same as that of air-cooled generators. Before rollout, in addition to surface inspection of the bars, measurement of insulation resistance value in the cold state, AC withstand voltage test, and corona voltage test, an airtightness test must also be performed. The air source is compressed nitrogen, the pressure is 1 MPa, the pressure holding time is 3 minutes, and there must be no leakage. The gas is released from the other end to verify the flowability of a single bar. 3.2 Rollout Process and Withstand Voltage Test The rollout process and withstand voltage test are the same as those for ordinary bars. 3.3 Installation of the Gas Piping: Install the upper gas collecting pipe and lower liquid collecting pipe, and connect the PTFE pipe. First, install two 1/12 upper gas collecting pipe sections (outer ring) using temporary supports. Temporarily seal the outlet of one gas collecting pipe section with a flange, and seal the outlet of the other gas collecting pipe section with a flange equipped with a leak detection nozzle. Install the 1/6 lower liquid collecting pipe (outer ring) and return port, fix them with temporary supports, and temporarily weld and seal both ends. When installing the above two pipe sections, stagger them by 9 rods in the circumferential direction to form a relatively closed system through the corresponding lower rods after connecting the PTFE pipe, thereby reducing the need for sealing and facilitating leak detection. Remove the plugs at the rod joints, heat one end of the plastic pipe to about 150°C for 1 minute to soften the pipe, and then quickly slip it onto the joint. After attaching the clamp, the assembly of one pipe is complete. A leak test is performed using compressed air at 0.4 MPa for 120 minutes; no leaks are detected (using detergent and an ultrasonic leak detector). Nitrogen and an appropriate amount of SF6 are then introduced, pressurized to 0.3 MPa, and held for 4 hours without leaks (using a laser leak detector for directional patrol testing and a halogen leak detector for point testing). If a joint leaks, it should be reinstalled. All pipes are installed using this method. Stainless steel welding is performed using a CO2 gas shield. After the entire evaporative cooling system is installed, a system airtightness test is conducted. Dry nitrogen and an appropriate amount of SF6 are introduced, pressurized to 0.25 MPa, and a final patrol test is performed (using a laser leak detector for directional patrol testing and a halogen leak detector for point testing). After confirming no leaks, the pressure is held at the initial pressure of 0.25 MPa for 72 hours; the gauge pressure drop must not exceed 3%, fully meeting the requirements. 3.4 Freon Refilling: Open the condenser vent valve, connect the stator lower ring pipe drain valve to the Freon tank using a clean, pressure-resistant inlet pipe, open the Freon bottle valve, and adjust the pressure reducing valve to bring the nitrogen pressure into the storage tank to 0.02–0.05 MPa. At this point, Freon gas will flow into the cooling system. Monitor the liquid level; when the level reaches approximately 4.4 m from the lower ring pipe, close the drain valve. 4. Operational Test of the Evaporative Cooling Unit The Lijiaxia 400 MW evaporative cooling turbine generator unit officially commenced commercial operation on December 10, 1999. By the end of November 2000, Unit #4 had accumulated 5,643.2 hours of operation, generating 1,322,418,900 kW·h. Its maximum output load was 380 MW (liquid level: 2.32 m, pressure: 0.05 MPa, stator coil maximum temperature: 58℃). The typical load range was 100–330 MW. Measured winding temperatures ranged from 50–58℃, and pressure variations ranged from 0.02–0.06 MPa, all meeting design values. Unit #4 experienced a system leak during the initial power generation phase. On December 23, 1999, operators noticed a significant drop in liquid level. After continuous operation and assessment, a normal shutdown was performed followed by nitrogen purging and inspection. A loose flange bolt on the upper ring pipe was found to be causing the leak. After repair and refilling, operation returned to normal. From January 3, 2000 onwards, the system remained reliable, well-sealed, stable with low temperature rise, and the unit operated smoothly. [b]5 Conclusions[/b] ① Evaporative cooling is not only technically feasible for large-scale hydro-turbine units, but also has significant effects. ② As a new technology, evaporative cooling has many aspects worth studying and exploring in large-scale generator units. [b]References[/b] ［1］ Gu Guobiao. Development of internal cooling technology for hydro-turbine generators［J］. Journal of Electrical Machines and Control, 1997, (3).