Abstract: This paper analyzes the operation mode of the circulating water system and, combined with the application of high-pressure variable frequency speed control technology in the variable operating condition of the circulating pump, analyzes the methods for achieving economic optimization of the unit from another perspective. It elucidates the feasibility of frequency conversion energy-saving optimization of the circulating water system, providing a reference for the optimized operation of the circulating water frequency conversion system. Keywords: Circulating water system, frequency conversion optimization , operation I. Overview Taking a closed-loop circulating cooling unit as an example, the main function of the circulating water system is to cool the exhaust temperature of the low-pressure cylinder of the turbine, reduce the exhaust pressure of the low-pressure cylinder, and maximize the energy release of the main steam as it passes through the turbine, converting it into the mechanical energy of the turbine rotation to drive the generator. The function of the circulating water pump is to pressurize the cooling water into the condenser to exchange heat with the superheated steam that has done work, reducing the exhaust pressure at the turbine end. The heat-absorbing circulating water is transported to the cooling tower for spraying, and after being cooled by counter-current natural ventilation, it is recycled. The specific system structure principle is shown in Figure 1. Figure 1 For a long time, most units' circulating water systems have operated using a pump start-stop method. Depending on the season and temperature differences, one or two circulating water pumps are activated. Due to factors such as condenser sealing, unit efficiency, and seasonal temperature variations, to ensure safe operation, there are often situations where one circulating water pump is insufficient for the required flow, while two pumps result in excessive flow, and in summer, the flow is insufficient. This makes it impossible to guarantee the long-term economical and stable operation of the unit, and there has been a lack of reasonable control and regulation methods to adjust the power consumption of the circulating pumps according to the unit load, resulting in persistently high energy consumption of the circulating pumps. How to control the condenser vacuum to achieve economical and reliable operation of the circulating water system and reduce the energy consumption level of the circulating water system under low unit load has become an important research topic. With the improvement of unit sealing technology and operating efficiency evaluation systems, and the maturity and widespread application of high-pressure variable frequency speed control technology, it has become possible to achieve economical operation of the unit by controlling the condenser vacuum using automatic operation modes. The application of high-pressure variable frequency speed control technology in the circulating water system, based on factors such as unit load and seasonal environmental temperature changes, can rationally control the circulating water flow to maintain the optimal vacuum level of the condenser exhaust pressure. This can achieve good results in the following aspects: ① Improve unit operating efficiency and reduce coal consumption. ② Reduce circulating water pump unit consumption, saving a significant amount of electrical energy. ③ Reduce cooling tower circulating water evaporation losses. ④ Avoid problems such as excessively low cooling tower return water temperature and freezing in winter. II. System Analysis Through the analysis of the equipment and operating process of the circulating water system, combined with the foreseeable and potential impacts of applying high-pressure variable frequency technology in the circulating water system, a targeted analysis and demonstration are conducted. 1. Circulating Water Pump Speed ​​Regulation The application of variable frequency speed control technology on the circulating water pump is mainly used to adjust the operating parameters of the circulating pump downwards when the unit is operating below the rated load. That is, the operating frequency is adjusted within the range of ≤50Hz. As can be seen from Figure 2, as long as the pipeline working pressure H is higher than the pipeline static pressure HST, safe operation is possible while ensuring the minimum flow rate. When the flow demand decreases, the circulating pump speed decreases accordingly, and according to the similarity law of fluid mechanics, the pressure also decreases. This leads to changes in the operation of the cooling water network and its auxiliary industrial cooling water subsystems. Analysis of the circulating pump, network characteristic curves, and unit flow demand shows that within the unit's 180-300MW load adjustment range, the flow adjustment space of the circulating pump is limited; and while the flow rate decreases with decreasing speed, the pressure decrease is not significant and will not have a major impact. The system can accept the circulating pump operating in a speed-regulating mode within a certain range. Figure 2. 2. Condenser Vacuum and the Achievement of the Most Favorable Vacuum. Vacuum degree refers to the percentage ratio of the condenser vacuum value to the local atmospheric pressure, which is a direct variable affecting our control of the circulating water system and is an important parameter influencing the economic benefits of the generator unit. When secondary steam condenses into water through heat exchange with cooling water in the condenser, it changes from a gaseous state to a liquid state, resulting in a rapid volume change. This creates a high vacuum within the condenser. Simultaneously, a high vacuum is established and maintained at the turbine's exhaust port, allowing the steam entering the turbine to expand to the lowest possible pressure, thus increasing the steam's work capacity. In other words, it increases the unit's ideal specific enthalpy drop and improves the turbine's operating efficiency. Currently, condenser vacuum is mainly controlled by adjusting the cooling water flow rate. The heat exchange process is shown in Figure 3. Figure 3 shows that, based on the turbine's operating principle, the condenser pressure Pz during operation mainly depends on the turbine load Dz, the cooling water inlet temperature ts2, and the cooling water flow rate Ds. The lowest temperature at which the cooling water temperature can be reduced mainly depends on the ambient temperature t0. Under a fixed turbine load, the condenser vacuum can only be increased by increasing the cooling water flow rate. However, a higher vacuum is not always better. While increasing the vacuum improves turbine efficiency (ΔN1), it also significantly increases the power consumption of the circulating water pump (ΔN2). Therefore, during normal turbine operation, there exists an optimal operating pressure corresponding to the unit load, maximizing the difference between ΔN1 and ΔN2, i.e., the optimal vacuum, as shown in Figure 4. In practical engineering applications, achieving optimal economic operation by using turbine efficiency (ΔN1) and circulating water pump power consumption (ΔN2) is difficult to achieve through function calculations and process control. Typically, the optimal back pressure range determined at the turbine's factory is used as the control target to improve the unit's operating process parameters, ensuring the condenser pressure changes with the unit load for economical operation. Therefore, using variable frequency speed control technology in the circulating pump to achieve optimal control requires a newer control strategy, and the application of thermodynamic dynamic balance theory can help achieve optimized system control. 3. Condensate Subcooling Issues: Condensate subcooling causes irreversible steam source losses, a minor factor affecting economic efficiency. During normal operation, condenser subcooling is typically 0.5–2°C. For every 1°C increase in condenser subcooling, heat consumption increases by 0.014%, and excessive subcooling leads to increased coal consumption. Many factors contribute to condensate subcooling, with circulating water flow rate and inlet temperature having a significant impact. This issue needs to be considered during system regulation to optimize the unit's overall economic performance. 4. Cooling Tower Evaporation Water Loss and Winter Icing Issues: Thermal power plants are major water consumers, and cooling water loss in cooling towers is one of the main sources of water resource loss in power plants. This includes evaporation losses, wind losses, and blowdown losses; evaporation losses, accounting for over 75%, are closely related to circulating water flow rate and the temperature difference between the cooling tower inlet and outlet. In northern regions, due to the significant temperature difference between summer and winter, the proportion of evaporative heat in cooling towers varies considerably between the two seasons; it is approximately 100% in summer and about 50% in winter. The formulas for calculating evaporative water loss and wind-induced water loss are as follows: Where: Ds—circulating water volume; Cs—specific heat capacity of water; at room temperature, 1 kg of water requires 4.1868 kJ/(kg•℃) to increase its temperature by 1℃; ΔTL—temperature difference between the cooling tower's cooling water outlet and inlet; —latent heat of vaporization of water at temperature tp; tp—average temperature of circulating water; A—ratio of evaporative heat dissipation to the total heat dissipated by the cooling tower; From formula ①, it can be seen that when the ambient temperature T0 is fixed, parameters such as the specific heat capacity of water Cs, latent heat of vaporization α, average circulating water temperature tp, and the ratio of evaporative heat dissipation to the total heat dissipated by the cooling tower A are also determined accordingly. At a normal temperature of 20℃, the constant value of the coefficient can be obtained by referring to a table. Here, it is assumed that this value is a constant related to the ambient temperature, N0. The main water loss of the cooling tower can be obtained from formulas ① and ②, and based on the above assumptions, it can be deduced that: Formula ③ shows that under a certain ambient temperature, the main water loss of the cooling tower depends primarily on the circulating water flow rate Ds and the temperature difference ΔTL between the cooling water inlet and outlet. In other words, reasonably controlling the circulating water flow rate Ds and the temperature difference ΔTL between the cooling water inlet and outlet can effectively reduce the water loss of the cooling tower. Under the premise that the circulating water frequency conversion control system meets the condenser vacuum requirement, water-saving operation of the system can be achieved. Furthermore, in northern winters, due to the cold climate and ambient temperatures approaching 0℃, localized icing of the cooling tower often occurs. Ice curtains form at the air inlet of the cooling tower, reducing the air intake area and causing a decrease in airflow, affecting the cooling tower's operating performance. Inside the cooling tower, this can even cause the packing to collapse, and the concrete inside the tower may have its service life affected by repeated freeze-thaw cycles. To avoid icing in low winter temperatures, an anti-freeze control subsystem is added to the circulating water system control; controlling the condenser return water temperature can effectively prevent icing after the cooling tower evaporates and cools down. III. System Control Scheme This control strategy employs multi-parameter calculation and a single-quantity balance algorithm. It controls the condenser vacuum by changing the circulating pump flow rate, while simultaneously incorporating the law of conservation of energy and thermodynamic conduction theory into the circulating water system control strategy. This means that the circulating pump flow control is no longer solely based on condenser vacuum as the control objective. Instead, it uses parameters such as the varying requirements of unit load on condenser vacuum, the operating differential of cooling circulating water, and ambient temperature as comprehensive adjustment indicators for the main control loop. While meeting the unit's condenser vacuum requirements, it reduces condensate subcooling and keeps the operating differential of cooling circulating water within a reasonable range. This achieves comprehensive system economic efficiency by reducing unit coal consumption and cooling tower evaporation water loss. To ensure economical and safe unit operation under different loads, the unit design and operation include a load-discharge pressure relationship curve. Under the original operating mode, the circulating water system relied on start-stop regulation, which could not guarantee economic efficiency. It could only maintain the condenser vacuum at the lowest possible value through circulating water; this curve was mainly used for unit protection. In circulating water control applications, this curve is used as a reference value for adjustment, and conservative value control is implemented. Figure 5 illustrates that, according to thermodynamic conduction theory, a larger temperature difference or higher flow rate is not necessarily better for heat exchange. Instead, controlling the temperature difference within the range of 4.5–6.5℃, with an inlet-outlet difference of approximately 5℃, is the most economical approach for heat exchange systems. Larger temperature differences reduce system heat exchange efficiency, and excessively high exchange medium flow rates increase operating costs without necessarily improving performance. While maintaining condenser vacuum, the circulating water flow rate is adjusted according to seasonal differences, keeping the inlet-outlet difference within the range of 4.5–6.5℃. Considering the high temperatures in summer and the large demand at full load, variable frequency technology can be used to maximize the potential of the circulating pump and significantly improve system economic indicators. In winter, low temperatures can lead to condensate overcooling and cooling tower icing; the wide adjustment range and fast response speed of variable frequency drives can be fully utilized to meet the needs of low-load operation. In the design of the control strategy, an approximation algorithm is adopted, prioritizing vacuum indicators while also considering auxiliary control indicators. This allows the system to possess diversity and flexibility during adjustment, avoiding large fluctuations and unstable vacuum indicators that could affect unit operation safety and efficiency. IV. Conclusion Converting the circulating water pump to variable frequency operation results in a slight decrease in operating head compared to fixed frequency operation. Consequently, some rubber balls may remain in the condenser capillary tube, reducing the rubber ball recovery rate and impacting operating efficiency. To address this, the status of the rubber ball cleaning device is incorporated into the circulating water control system. This automatically increases the circulating water network pressure based on the rubber ball recovery status, avoiding the adverse effects of decreased flow velocity and ensuring the rubber ball recovery rate. The use of this system solves the long-standing problem of lacking adjustment methods and control theory for circulating pumps, hindering the energy-efficient operation of the circulating water system. Adopting a variable frequency energy-saving optimization system in the current circulating water system can optimize the unit's economic operation, demonstrating significant energy-saving and consumption-reducing economic benefits, and fully leveraging the leading role of high-voltage variable frequency speed control technology in energy saving and consumption reduction in power plant main and auxiliary equipment.