Currently, domestic hydrogen-cooled generators with dual-ring sealing oil structures suffer from varying degrees of excessive hydrogen replenishment. Besides increasing the cost of hydrogen itself, the increased hydrogen consumption in hydrogen-cooled generators leads to lower generator efficiency and higher coil temperatures due to decreased hydrogen purity, severely impacting the generator's safe and economical operation. Furthermore, due to issues with equipment structure, operation, and maintenance, oil can enter the generator, causing a decrease in coil insulation and posing a safety hazard. Therefore, resolving the issue of excessive hydrogen replenishment and eliminating oil ingress in hydrogen-cooled generators with dual-ring sealing oil structures is crucial. 1. Introduction to the Dual-Ring Sealing Oil System The turbine generator sealing oil system consists of two independent yet interconnected oil circuits: one on the air side and one on the hydrogen side. The sealing oil on both sides simultaneously supplies oil to the dual-ring sealing tiles at both ends of the generator. The return oil from the turbine generator set bearings is pressurized by the air-side sealing oil pump, then passes through the air-side sealing oil cooler and filter screen to the air-side oil ring of the generator's steam and excitation end double-flow ring type sealing tile. The air-side sealing oil pressure is controlled by the differential pressure valve's discharge. When the hydrogen pressure inside the generator changes or the air-side sealing oil pressure fluctuates, the differential pressure valve adjusts the air-side sealing oil discharge rate to maintain the air-side sealing oil pressure at least 0.085 MPa higher than the hydrogen pressure inside the generator. The return oil from the air-side sealing oil is discharged to the return oil system of the generator support bearings. After being pressurized by the hydrogen-side sealing oil pump, the hydrogen-side sealing oil passes through the hydrogen-side sealing oil cooler and filter, and then splits into two paths, which pass through the generator steam and excitation end balance valves to the hydrogen-side oil rings of the generator steam and excitation sealing tiles respectively. The function of the steam and excitation balance valves is to track the pressure in the air-side oil rings of the steam and excitation end sealing tiles and adjust the pressure difference between the hydrogen-side oil rings and the air-side oil rings to be no more than ±50 mm water column. The hydrogen-side sealing oil returns to the sealing oil tank. The oil level in the sealing oil tank is controlled by replenishing oil at the outlet of the air-side sealing oil pump or discharging oil at the inlet of the air-side sealing oil pump. 2 Analysis of the reasons for the large hydrogen replenishment and oil ingress of the turbine generator in the dual-flow sealing oil structure 2.1 It is difficult to control the pressure balance of the air-side sealing oil and the hydrogen-side sealing oil in actual operation. According to the design principle of the sealing tile of the dual-flow sealing oil structure, only by maintaining the pressure of the air-side sealing oil and the hydrogen-side sealing oil in the sealing tile to be basically equal and reducing the exchange of air and hydrogen-side sealing oil can we prevent air and other entrained in the air-side oil system from entering the hydrogen-side sealing oil system. However, in actual operation, it is difficult to control the pressure balance between the air-side sealing oil and the hydrogen-side sealing oil due to equipment structure and other factors. When the air-side sealing oil pressure is greater than the hydrogen-side sealing oil pressure, the air-side sealing oil leaks into the hydrogen side within the sealing tile, and air and other contaminants carried by the air-side sealing oil enter the hydrogen-side sealing oil. When the hydrogen-side sealing oil pressure is greater than the air-side sealing oil pressure, the hydrogen-side sealing oil leaks into the air side within the sealing tile. This causes the oil level in the hydrogen-side sealing oil tank to drop, opening the float valve of the hydrogen-side sealing oil tank, allowing pressurized oil from the air-side sealing oil pump outlet to replenish the hydrogen-side sealing oil tank through the float valve. Therefore, regardless of whether the air-side sealing oil pressure is greater than or vice versa, the oil fumes and water vapor carried by the air-side sealing oil returning from the bearing will enter the hydrogen-side sealing oil system through exchange with the hydrogen-side sealing oil. This contaminant will then be drawn into the generator through the oil baffle within the sealing oil, causing hydrogen contamination within the generator, a decrease in hydrogen purity, and an increase in the amount of hydrogen replenished. There are two main reasons for the pressure imbalance between the air-side and hydrogen-side sealing oils. Firstly, the balancing valve in the hydrogen-side sealing oil system has poor adjustment accuracy. Currently, the required accuracy for the balancing valve is ±50 mmH2O (±490 Pa). During operation, due to the small gap between the piston and cylinder of the balancing valve, even slight impurities can increase the piston's movement resistance or even cause it to jam, resulting in poor adjustment accuracy of the balancing valve. This prevents it from effectively maintaining the pressure balance between the air and hydrogen-side sealing oils, leading to hydrogen contamination and increased hydrogen replenishment. The second major reason for the pressure imbalance is the measurement error of the air and hydrogen-side sealing oil pressures. Maintaining oil pressure balance between the sealing pads and the shaft is crucial to reducing the mutual movement of the air and hydrogen-side sealing oils during unit operation. However, due to the equipment structure, currently only the pressure at the air and hydrogen-side sealing oil inlets on the sealing pads can be measured as the adjustment signal for the balancing valve. This inevitably leads to measurement errors, preventing the balancing valve from effectively maintaining the pressure balance between the air and hydrogen-side sealing oils, thus increasing the amount of hydrogen replenished by the generator. 2.2 Increased Clearance Between Sealing Sheet and Generator Rotor The oil flow along the shaft between the sealing sheet and the rotor shaft, moving from the air side to the hydrogen side, is called axial flow. When the pressure difference between the air and hydrogen side sealing oils remains constant, the exchange volume of the air and hydrogen side sealing oils is directly proportional to the clearance of the sealing sheet. For a 300MW steam turbine, the diameter clearance of the sealing sheet is 0.15-28mm. When the sealing sheet clearance increases from 0.15mm to 0.28mm during operation, the sealing oil flow rate will increase significantly. Due to the unavoidable pressure difference between the air and hydrogen side sealing oils, the increase in sealing oil flow rate will lead to a multiple increase in the exchange volume of the air and hydrogen side sealing oils. Air and moisture carried in the air side sealing oil will enter the hydrogen side sealing oil through exchange, and then enter the generator hydrogen through contact between the hydrogen side sealing oil and hydrogen, contaminating the hydrogen and reducing its purity. An increase in the amount of sealing oil will cause obstruction in the static pressure return oil pipeline. When the oil level in the generator's hydrogen-side return oil chamber (defoaming box) rises above the lowest position of the journal, oil will enter the generator. 2.3 High Generator Sealing Oil Temperature The viscosity of the sealing oil decreases as the oil temperature increases. To maintain a certain sealing oil pressure within the same flow area, a larger flow rate of sealing oil is required when the sealing oil temperature is high. Similarly, an increase in sealing oil temperature will lead to an increase in the gap between the sealing tiles, which also requires an increase in the sealing oil flow rate to maintain a certain sealing oil pressure. Generator manufacturers generally specify that the normal value of the air and hydrogen-side sealing oil temperature of hydrogen-cooled generators is between 27-50℃. For a 300MW steam turbine generator cartridge sealing oil system, the outlet oil temperature of the cooler of its air and hydrogen-side sealing oil system is controlled by a water return regulating valve, and is generally maintained at around 42℃. The viscosity of the oil at 42℃ is lower than that at 27℃, and a larger sealing oil flow rate is required to maintain a certain sealing oil pressure. Similarly, due to the increase in sealing oil temperature, the inner diameter of the sealing tile will increase. To ensure that hydrogen does not leak from the generator, the sealing oil flow rate needs to be increased to maintain a certain pressure. Therefore, excessively high sealing oil temperature will lead to an increase in sealing oil flow rate. According to the analysis in section 2.2, this will also cause a decrease in the purity of hydrogen in the generator or oil ingress into the generator. 2.4 Low Exhaust Fan Output From the perspective of the 300MW steam turbine generator sealing oil system, the air-side sealing oil pump takes oil from the hydrogen-oil separator. The main function of the exhaust fan of the hydrogen-oil separator is to extract trace amounts of hydrogen from the air-side oil to prevent hydrogen from returning to the main oil tank with the lubricating oil. Increasing the output of the exhaust fan in the hydrogen-oil separator creates a large negative pressure, causing air in the air-side oil to be extracted along with hydrogen. This reduces the air content in the air-side sealing oil. According to the analysis in section 2.1 (generator hydrogen pollution mainly occurs because air carried by the air-side sealing oil exchanges with the hydrogen-side sealing oil, then pollutes the hydrogen through further exchange with hydrogen), this will reduce hydrogen pollution. 3. Measures to prevent generator oil ingress, reduce hydrogen pollution, and decrease hydrogen replenishment : 3.1 Ensuring a suitable clearance between the sealing tile and the shaft: 3.1.1 Ensuring the sealing tile clearance meets requirements during maintenance: For a 300MW turbine, the clearance between the sealing tile and the shaft diameter should be 0.105-205mm. During maintenance, the sealing tile clearance should be strictly adhered to according to the standard, and should be as close to the lower limit as possible. This reduces the sealing oil flow and prevents defects such as high sealing tile temperature, sealing tile wear, and even excessive generator shaft vibration caused by an excessively small sealing tile clearance. 3.1.2 Adopting High-Precision Sealing Oil Filters Currently, the air-hydrogen side sealing oil of the 300MW sealing oil system uses scraper-type filters. However, these scraper-type filters can only be considered coarse filters and cannot effectively filter out the tiny particles in the sealing oil. The abrasion caused by the relative flow of these tiny particles in the sealing oil with the sealing tiles and journals exacerbates the wear on the sealing tiles and journals, leading to an increase in the clearance between the operating sealing tiles. It is understood that scraper-type filters have been phased out abroad, and there are examples in China of power plants replacing them with fiber filters with a filtration accuracy of 0.01mm or less. 3.2 Improving the Adjustment Accuracy and Operational Reliability of the Balancing Valve Improving the adjustment accuracy of the balancing valve can effectively reduce the amount of cross-flow of air and hydrogen side sealing oil and prevent hydrogen contamination. This can be achieved in the following two ways: 3.2.1 Preventing Balancing Valve Jamming and Adjustment Malfunction 3.2.1.1 In the initial operation of the sealing oil system after maintenance, manual adjustment using the balancing valve bypass valve can be adopted to prevent jamming of the balancing valve components due to uncleanliness of the system after maintenance. 3.2.1.2 Adoption of a New Type of Balancing Valve It is reported that a domestic unit has successfully developed a balancing valve with a continuously rotating valve core. This balancing valve uses sealing oil as the power oil to drive the valve core to rotate at a certain speed, which can prevent impurities in the sealing oil from causing the valve core to jam. 3.2.2 After maintenance, a balancing valve adjustment test is conducted to ensure pressure balance of the air and hydrogen side sealing oils. The purpose of the balancing valve is to prevent the exchange of air and hydrogen sealing oils within the air and hydrogen side sealing oil rings in the sealing tiles. Based on this principle, the oil supply and drainage valves of the sealing oil tank can be closed, and the balancing valve can be adjusted according to the changes in the sealing oil tank level. Ultimately, the oil level in the sealing oil tank is basically stable, achieving the goal of reducing the exchange of air and hydrogen side sealing oils within the sealing tiles. Through testing, a pattern can be found: during normal unit operation, fine-tuning the balancing valve according to whether the sealing oil tank is being replenished or drained can also reduce the exchange of air and hydrogen side sealing oils within the sealing tiles. 3.3 Controlling the sealing oil temperature can be achieved by conducting tests to monitor the sealing oil temperature within the upper and lower limits of the standard requirements. Under the condition that the generator shaft vibration does not increase, the sealing oil temperature should be maintained at the lower limit of the standard as much as possible, thereby reducing the sealing oil flow, reducing generator oil ingress, and reducing hydrogen pollution. 3.4 Increasing the exhaust fan pressure: Increasing the exhaust fan pressure of the hydrogen-oil separator can increase the negative pressure of the hydrogen-oil separator, reduce the air and water content in the air-side sealing oil, and thus reduce hydrogen pollution caused by the exchange of air and hydrogen-side sealing oil. 4 Conclusion In summary, regarding the hydrogen pollution and generator oil ingress defects existing in generators, taking corresponding preventive measures based on the characteristics of the dual-flow ring sealing oil system can effectively reduce generator hydrogen pollution and oil ingress defects, and improve the safety and reliability of generator operation.