[align=left] 1. Introduction The process flow of the circulating water pump system is briefly shown in Figure 1. [/align] Figure 1 Process Flow of Circulating Water Pump System The role of circulating water: to provide cooling water for power plant condensers, oil coolers, generators, and other equipment, to meet the requirements of the turbine cooling cycle ratio, and to provide maximum cooling effect to maintain condenser vacuum. If the cooling water supplied by the circulating water pump is insufficient or the water supply is interrupted due to a malfunction, the turbine and generator cannot operate normally. The characteristics of power plant circulating water pumps are a very large water supply, but a low required head (generally only 20mm water column), i.e., a high-flow-rate, low-head pump. Generally, small units use a motherboard system, while large units use a unit system. 2. Current Status of Circulating Water Pump System According to the operating procedures of power plant circulating water pumps: circulating water pumps operate continuously with the unit for a long period of time. Generally, 2 to 3 circulating water pumps are configured with corresponding outlet hydraulic control butterfly valves and inlet electric butterfly valves, and the combination is switched according to the operating procedures. During peak summer periods, three pumps operate, with one on standby and under maintenance; during peak winter periods, two pumps operate, one on standby, and one under maintenance. All valves in the system are fully open. The circulating water system is designed with sufficient margin in mind, considering the harshest summer temperatures and the maximum cooling water flow required by the unit at maximum load. Therefore, when the environment improves, such as with seasonal temperature drops, or when the turbine is operating at less than full load, or during start-up and shutdown, the circulating water flow can be reduced. However, due to constant speed operation, speed adjustment is not possible; once the pumps are started, they operate at full load. Because the circulating water pumps operate at full load for extended periods, the cooling water temperature is very low in winter, which can easily lead to problems such as overcooling of the turbine condensate and excessive dissolved oxygen in the condensate. Using variable frequency speed control to continuously adjust the output of the circulating water pumps can save energy and reduce consumption, while also providing a means of regulating the circulating water flow, keeping the unit in its most economical operating state. The implementation has proven to be quite effective. 3. Using a frequency converter as the pre-drive unit for the ordinary asynchronous motor of the circulating water pump Given the role of the circulating water pump in the system, a frequency converter is used as the pre-drive unit for the ordinary asynchronous motor of the circulating water pump. It receives standard setpoint signals from the regulator (single-loop regulator or other regulating output) or manual controller (manual/automatic seamless switching output) to regulate the circulating water flow, thereby effectively regulating and controlling system parameters. Many power plants have already made attempts in this area, such as Ningbo Shunlong Thermal Power Plant, Weihai Power Plant in Shandong, and Dongyuan Thermal Power Plant. Variable frequency speed control retrofitting of the circulating water system can improve problems affecting the safe and economical operation of the unit, such as high dissolved oxygen in the condensate when the condenser water side is not full, large temperature variations in the circulating water volume in winter and summer, inability to meet the unit's vacuum requirements, and lack of means to regulate the circulating water volume. Given the importance of the circulating water pumps, a frequency converter (VDC) upgrade was implemented to power two pumps via a "one-to-two, with power frequency bypass" primary circuit. During operation, the VDC drives one pump; if one pump fails, the system switches to the other. If the flow rate of one pump cannot meet the process system's requirements, it can be soft-started and run at power frequency before the other pump is soft-started and put into VDC operation. To save costs, only two circulating water pumps were upgraded, while the other two retained their original power frequency operation mode, as shown in Figure 2. Figure 2 shows the primary wiring diagram for the "one-to-two" VDC configuration. As can be seen from Figure 2, the primary electrical wiring for the 380V circulating water pump motor is mainly suitable for circulating water pumps in small units with a single unit capacity of less than 50,000 kW. If the circulating water pump motor is 6kV (all units with a single unit capacity of 100,000 kW or more are 6kV motors), the power supply for the pump motor heater needs to be provided separately. The Beijing Coal Gangue Power Plant, Ningbo Shunlong Thermal Power Plant, and Dongyuan Thermal Power Plant all adopt the electrical primary wiring scheme shown in Figure 2; the Shandong Weihai Power Plant, Daqing Xinhua Power Plant, Yantai Wanhua Power Plant, and Shanxi Yangguang Power Plant adopt the "one-to-one power frequency bypass" electrical primary wiring scheme. The Beijing Coal Gangue Power Plant uses AB frequency converters, the Ningbo Shunlong Thermal Power Plant uses Hitachi frequency converters, the Shandong Weihai Power Plant and Daqing Xinhua Power Plant use Robicon 6kV harmonic-free frequency converters, the Shanxi Yangguang Power Plant uses Leadway 6kV frequency converters, and the Yantai Wanhua Power Plant uses AB's 6kV Powerflex 7000 frequency converter. These power plants use frequency converters to drive ordinary asynchronous motors for circulating water pumps, and have achieved good application results. 4. Automatic Adjustment and Control Strategy of Circulating Water Variable Frequency Speed ​​Regulation System 4.1 Defects of Traditional Control Methods for Circulating Water Systems Traditional control systems for circulating water systems generally do not use adjustment control loops and cannot automatically adjust the water volume. With only a fully open/closed hydraulic butterfly valve at the pump outlet, the pump motor operates at full load as soon as the pump starts. The disadvantages of this control method are: high energy consumption, inability to automatically adjust flow rate, water hammer phenomenon, and impact on the lifespan of the pipeline network and its monitoring instruments. 4.2 Automatic Adjustment Control Strategy The purpose of modifying the variable frequency speed control system for the circulating water system is to achieve automatic water flow adjustment, maintain optimal vacuum, and maintain the optimal circulation ratio. Figure 3 Relationship between pump flow rate, power, pressure, and speed Based on the relationship between pump flow rate, power, pressure, and speed shown in Figure 3, and the motor power consumption characteristic curves under different load conditions, we can obtain: q∝k×n (flow rate proportional to speed) h∝k×n² (pressure proportional to the square of speed) p∝k×n³ (power proportional to the cube of speed) Where: q—flow rate; h—water pressure; p—motor power; k—proportional coefficient. Therefore, it can be seen that as long as the pump speed is adjusted, the flow rate expected by the user can be obtained and energy can be saved. 4.3 Circulating water pump speed adjustment methods There are two types of circulating water pump speed adjustment methods: manual adjustment method and automatic manual adjustment method with backup. (1) Manual adjustment method The frequency of the inverter is manually adjusted by using a hand controller, which can adjust the speed of the circulating water pump. (2) Automatic manual adjustment method with backup The purpose of the frequency conversion transformation of the circulating water system is to realize the automatic adjustment of water volume, that is, to optimize the operation under the conditions of unit load change and circulating water temperature change (such as temperature change in winter and summer). In order to ensure the efficient, reliable and safe operation of the system, it is necessary to design an automatic water volume optimization control system. (3) It is explained that the circulating water of the thermal power plant is the basic condition for ensuring the safe and economical operation of the unit. Due to the many factors affecting the circulating water flow requirements, pressure and temperature (such as load change, system disturbance, large inertia and pure time delay of the system), it is difficult for the automatic control system based on the conventional PID algorithm to be put into long-term stable automatic operation under the conditions of changing working conditions and seasonal changes. Manual control is too slow and the changes are too large, which can easily cause system instability. (4) The control strategy of automatic optimization control is based on the relationship between the unit's optimal vacuum (multiplier) and the circulating water volume, with the unit's optimal vacuum as the objective function. The objective function of the optimized operation of the circulating water system is the indirect function relationship obtained when the difference between the turbine's power generation and the circulating water pump's power consumption reaches its maximum under the premise that the turbine's heat consumption remains unchanged. Since the optimization of the entire circulating water system is equivalent to the optimization of all subsystems, the mathematical model of the circulating water system optimization is established according to the following four aspects: the characteristics of the circulating water volume and the generator power under a certain turbine exhaust volume (constant heat load); the relationship characteristics between the circulating water pump flow rate and power consumption; the head characteristics of the circulating water pump; and the resistance characteristics of the circulating water system network. (5) Adjust the speed of the circulating water pump, thereby adjusting the circulating water flow rate, maintaining the optimal circulation multiplier, and maintaining the optimal vacuum of the unit's condenser. (6) The strategy diagram during engineering implementation is shown in Figure 4. In Figure 4, q is the circulating water flow rate; p is the pressure; and t is the circulating water inlet and outlet temperature. Figure 4. Strategies for Project Implementation (7) The strategy for project implementation is based on the relationship between the unit's optimal vacuum (multiplier) and circulating water volume, with the unit's optimal vacuum as the objective function. The control system mainly collects the changes in turbine regulating stage pressure, atmospheric pressure, condenser vacuum, unit load, and circulating water pump inlet and outlet water temperature parameters, performs optimization calculations, determines the fitting curve of optimal vacuum (multiplier) and circulating water volume, and controls the frequency converter through proportional-integral control to maintain a constant cooling effect and maintain the optimal vacuum. The optimal vacuum is obtained by calculating and analyzing the condenser's heat transfer coefficient, exhaust pressure, and other parameters, using a successive cyclic approximation optimization calculation method. The circulating water flow rate is calculated by detecting the unit's exhaust enthalpy drop, unit exhaust flow rate, and circulating water inlet and outlet water temperatures. Circulating water flow rate q = k × h × q1 / (t1 - t2) Where: h is the exhaust enthalpy drop; q1 is the exhaust flow rate; t1 and t2 are the circulating water inlet and outlet temperatures; k is a coefficient. (8) The control system is based on PID regulation calculation and performs optimized coordination control. Its output controls the frequency of the frequency converter, adjusts the pump speed, adjusts the circulating water flow, and maintains the optimal vacuum. This system helps prevent the problem of high dissolved oxygen in condensate due to low circulating water temperature in winter, especially under low load conditions. (9) To facilitate the implementation of automatic optimization control and make it convenient for operators to use and manage, a small distributed control system is proposed. 5. Conclusion The energy saving of the power plant circulating water system is mainly reflected in the efficient operation of the circulating water pump and the effective regulation of the flow. With the participation of variable frequency speed regulation technology, the appropriate speed ratio is obtained through optimized control to ensure that the circulating water pressure and flow meet the cooling effect requirements, maintain the optimal vacuum and optimal circulation ratio of the condenser, and save energy. Through the attempts of many power plants, the automatic optimization variable frequency control system can change the original situation of large energy waste and low automation of the circulating pump system. This system has strong practical significance and is also one of the foundations for realizing the unattended control system of circulating water pumps. This is just a starting point to promote the application of new technologies in power plants.