1. Introduction In recent years, 300MW extraction steam turbine units have become the main generating units for urban heating. Most heating units under construction or planned are 300MW turbine units. A 300MW heating turbine unit is generally equipped with two steam-driven feedwater pumps at 50% of its rated capacity and one electric feedwater pump at 30% of its rated capacity, or three electric feedwater pumps at 50% of their rated capacity. In operating and newly built/expanded thermal power plants in China, the feedwater system regulation methods and the types and configurations of feedwater pumps vary. Although different types and configurations of main feedwater systems can meet the operational needs of thermal power plants, as important auxiliary systems, their investment and operation/maintenance economics differ. In accordance with the national requirement to "vigorously promote equipment upgrades and technological transformations focusing on energy conservation and emission reduction, and accelerate the elimination of high-energy-consuming, high-water-consuming, and high-material-consuming processes, equipment, and products," a circulating soft-start variable frequency speed control scheme is applied to achieve feedwater pump regulation for a 300MW steam turbine unit, replacing small steam turbine regulation and hydraulic coupling regulation. This results in lower initial investment, a simpler system structure, and reduced operating and maintenance costs. Since the 1980s, developed countries have used frequency converters for power plant feedwater pump regulation, gradually replacing small steam turbines and hydraulic couplings. In recent years, high-voltage frequency converters have been successfully applied in my country to the technological transformation of forced draft and induced draft fans, feedwater pumps, condensate pumps, circulating water pumps, and ash pumps in thermal power plants. Their regulation characteristics, energy-saving characteristics, and economic benefits are unmatched by other regulation methods. 2. Main Feedwater System Regulation Methods The main function of the main feedwater system is to deliver water of qualified temperature, pressure, and quality to the boiler's main steam drum, and to maintain the steam drum water level within the allowable range (given range) using the feedwater system regulation function. It is a crucial thermal system for ensuring the safe operation of the boiler unit and the quality of steam and water. The main feedwater regulation system can be divided into two categories: constant-speed feedwater pump regulation systems and variable-speed feedwater pump regulation systems. 2.1 Constant-Speed ​​Feedwater Pump Regulation System The constant-speed feedwater pump regulation system changes the operating point of the feedwater pump by altering the resistance characteristic curve of the main feedwater pipeline while keeping the feedwater pump characteristic curve constant. This regulation method suffers from significant throttling losses and high feedwater pump consumption, which is an indisputable fact and falls into the category of "high-energy-consuming processes that should be phased out quickly." 2.2 Variable Speed ​​Feedwater Pump Regulating System The variable speed feedwater pump regulating system, under the condition that the feedwater pipeline resistance characteristic curve remains unchanged (with the main feedwater regulating valve fully open), changes the feedwater pump speed to alter the feedwater pump characteristic curve, thereby regulating the feedwater flow and controlling the boiler drum water level. Variable speed feedwater pump regulating systems can be divided into two types based on their prime mover: small steam turbine and electric motor. Steam-driven feedwater pumps are driven by a small steam turbine, which receives flow, pressure, and water level signals from the feedwater regulating system and adjusts the feedwater pump speed according to the steam intake of the small steam turbine. Electric feedwater pumps are further divided into two types: hydraulic coupling regulation and frequency conversion regulation. Hydraulic coupling regulation uses oil as the working fluid. A prime mover drives the pump wheel (driving wheel), transferring mechanical power to the working fluid oil, which in turn rotates the turbine (driven wheel). The hydraulic coupling receives flow rate, steam pressure, and water level signals from the feedwater regulation system and adjusts the feedwater pump speed by changing the oil level in the hydraulic coupling through a scoop tube control mechanism. A frequency converter, on the other hand, changes the synchronous speed of the asynchronous motor by altering the power supply frequency (f) and voltage (v) of the asynchronous motor stator, thereby changing the asynchronous motor speed. The frequency converter receives flow rate, steam pressure, and water level signals from the feedwater regulation system and adjusts the motor speed by changing the power supply frequency and voltage. 2.3 Technical and Economic Analysis of Different Speed ​​Regulation Methods Small turbine regulation, hydraulic coupling regulation, and frequency converter regulation can all achieve feedwater pump regulation functions and achieve the purpose of regulation. However, their initial investment, maintenance costs, and operating costs differ. 2.3.1 Investment Economic Comparison (1) The investment structure of steam-driven feedwater pump mainly consists of feedwater pump, small steam turbine, extraction steam pipeline, regulating pipeline system, front pump deceleration mechanism, small steam turbine steam sealing system, condensate release steam system, small steam turbine exhaust pipeline, etc., and the initial investment is also relatively large due to the large increase in the main plant space caused by factors such as the footprint and space of the small steam turbine. (2) The investment structure of hydraulic coupling speed-regulating feedwater pump mainly consists of feedwater pump, motor, hydraulic coupling, switch cabinet, cable, etc. (3) The investment structure of frequency converter speed-regulating feedwater pump mainly consists of feedwater pump, motor, frequency converter, switch cabinet, cable. Due to the soft start of the frequency converter, the starting capacity of the plant transformer can be reduced without considering the feedwater pump starting inrush current problem, and the transformer capacity can be reduced accordingly, which can also reduce the corresponding initial investment. 2.3.2 Maintenance Economic Comparison (1) The annual maintenance cost of steam-driven feedwater pump is 3 times higher than that of hydraulic coupling. The main components include feedwater pumps, small steam turbines, pre-pump reduction mechanisms, lubrication oil systems, small steam turbine condensate and steam release systems, and small steam turbine steam sealing systems. Compared with the main steam turbine, it is small but complete. Compared with electric feedwater pumps, it undoubtedly increases the workload of maintenance, spare parts, and consumable materials. In addition to regular minor maintenance, small steam turbines also require regular (4-year) major overhauls to inspect the flow passages. (2) The annual maintenance cost of hydraulic couplings is one-quarter of that of small steam turbines. In addition to feedwater pumps, the maintenance of hydraulic coupling speed-regulating feedwater pumps mainly involves the oil system and cooling water system of hydraulic couplings. Compared with small steam turbines, the maintenance workload is smaller, and its maintenance cost accounts for only one-quarter of that of small steam turbines. (3) The annual maintenance cost of frequency converters is the lowest. Frequency converters are high-tech power electronic products with a design life of 15 years. If the design and installation location is appropriate (no dust), maintenance is generally not required, and the annual maintenance cost can be ignored. 2.3.3 Comparison of Operational Economy According to relevant literature reports, the configuration of steam-driven feedwater pumps is not economical in operation compared with the configuration of electric feedwater pumps with hydraulic couplings; under the same conditions, the hydraulic coupling speed-regulating electric feedwater pump consumes 15% more electricity than the variable frequency speed-regulating electric feedwater pump. In summary, steam-driven feedwater pumps have high initial investment, complex systems, large space and area, and high maintenance costs; hydraulic coupling speed-regulating feedwater pumps have high initial investment, complex systems, high maintenance costs, and poor energy saving rate; variable frequency speed-regulating feedwater pumps have low initial investment, simple systems, low maintenance costs, and high energy saving rate. Therefore, the configuration order of speed-regulating feedwater pumps for 300MW steam turbine units should be: (1) variable frequency speed-regulating feedwater pumps; (2) hydraulic coupling speed-regulating feedwater pumps; (3) small steam turbine speed-regulating feedwater pumps. 3. Structure of 300MW Steam Turbine Unit Feedwater System The structure of the 300MW steam turbine unit feedwater system is shown in Figure 1. The feedwater system flow is as follows: oxygen supply, booster pump, boiler feedwater pump or electric feedwater pump, high-pressure heater, main feedwater regulating valve and its bypass valve, economizer inlet header, economizer outlet header, and boiler drum. Figure 1 shows the feedwater system structure of a 300MW turbine unit. 4. Variable Frequency Speed ​​Control Scheme for 300MW Turbine Unit Feedwater Pumps Hydraulic coupling speed control requires one hydraulic coupling per pump. Variable frequency speed control, however, is different. Whether equipped with two 100% rated capacity feedwater pumps or three 50% rated capacity feedwater pumps, only one variable frequency drive is needed to easily achieve soft start and variable frequency speed control, which is impossible with hydraulic couplings. 4.1 One-to-Three Circulation Soft Start Variable Frequency Speed ​​Control Scheme A 300MW turbine unit is often equipped with three 50% rated capacity feedwater pumps, two in operation and one on standby. The so-called "one-to-three cycle soft-start variable frequency speed control" method utilizes a single frequency converter to achieve soft start and variable frequency speed control for three feedwater pumps, as shown in Figure 2. The key feature of this scheme is that a single frequency converter can achieve both soft start and variable frequency speed control for any one of the three pumps, improving the utilization rate of the frequency converter. It achieves both soft start and variable frequency speed control for the feedwater pumps. The operating mode of this scheme is as follows: when the boiler unit is in slip-start mode and below 50% rated load, one feedwater pump operates at variable frequency speed; from 50% to 100% rated load, two feedwater pumps operate in a single frequency-controlled mode (one at fixed frequency and one at variable frequency). The fixed-frequency pump, with a fixed flow rate, has a head determined by the boiler unit operating parameters (the sum of the steam drum pressure, the steam drum water level, the feedwater pump water column height, and the pipeline resistance). The variable frequency pump adjusts the feedwater flow rate to maintain the boiler water level. Figure 2. Wiring diagram of a three-phase circulating soft-start variable frequency speed control feedwater pump. 4.2 Synchronous switching between power frequency and variable frequency is key to circulating soft start . The key to achieving circulating soft start variable frequency speed control using a single variable frequency drive is the synchronous switching technology between power frequency and variable frequency. This is because the boiler unit cannot be interrupted in water supply during operation. Therefore, when switching the feedwater pump from variable frequency speed control to power frequency operation, the power cannot be cut off instantly; that is, the variable frequency switch cannot be disconnected first and then the power frequency switch closed. Instead, the power frequency switch of feedwater pump #1 in variable frequency speed control operation must be synchronously connected in parallel with the power frequency switch of feedwater pump #1. After closing the power frequency switch of feedwater pump #1, the variable frequency switch of pump #1 must be disconnected. This achieves a synchronous and smooth transition and switching of the feedwater pump from variable frequency operation to power frequency operation. The key to achieving synchronous switching is the synchronous switching software system and the PLC control system. The main function of the synchronous switching software system is to automatically synchronize with the power grid through the running variable frequency speed control system, ensuring consistent phase sequence (determined during wiring), equal voltage (achieved by the synchronous switching software system), and a frequency difference of less than 0.5 Hz, until the phase error is less than the set value (achieved by the synchronous switching software system). The PLC then issues a command, and the power frequency power switch closes synchronously. After verifying normal synchronous closing, the variable frequency switch is disconnected. 4.3 Principle of Parallel Operation of One Constant Speed ​​Pump and One Variable Speed ​​Pump The parallel operation of two constant speed pumps and two variable speed pumps is familiar. Parallel operation of one constant speed pump and one variable speed pump is also feasible as long as the head is the same. The regulating function (manual and automatic) of the main feedwater pump of the 300MW unit, under the condition of constant steam drum pressure and fully open main feedwater regulating valve, adjusts the feedwater flow by changing the feedwater pump speed to maintain the boiler drum water level. During boiler startup and below 50% rated load, one pump (No. 1) operates using frequency conversion. When the boiler load exceeds 50% of the rated load, the flow rate of one pump (No. 1) is insufficient, requiring the startup of another pump (No. 2) for parallel operation. At this time, the operating frequency conversion pump (No. 1) is switched to fixed-speed operation. With a fixed flow rate, its head is determined by the sum of the steam drum pressure, steam drum water level, feedwater pump column height, and pipeline resistance. The other pump (No. 2) is soft-started using a frequency converter for frequency conversion speed regulation, adjusting the flow rate as needed to maintain the boiler water level. The fixed-speed pump (No. 1) operates at its optimal point with a fixed flow rate, while the frequency conversion pump (No. 2) adjusts its flow rate accordingly. The intersection of the characteristic curve of the fixed-speed pump and its operating head is the operating point of the fixed-speed pump, and its corresponding flow rate is the flow rate of the fixed-speed pump. Conversely, the intersection of the characteristic curve of the frequency conversion pump and its operating head is the operating point of the frequency conversion pump, and its corresponding flow rate is the flow rate of the frequency conversion pump. The sum of the flow rates of the fixed-speed pump and the variable-speed pump is the total flow rate. As shown in Figure 3. Figure 3 Schematic diagram of the parallel operation and regulation principle of the fixed-speed pump and the variable-speed pump 4.4 Features of the one-to-three cycle soft-start variable-speed water pump The application of the frequency converter to realize the cycle soft start and speed regulation of the electric water pump is a revolution in the starting and regulation method of the electric water pump in the main water supply system. This method fundamentally solves all the drawbacks of the speed regulation of the hydraulic coupler. Its main features are: (1) It realizes the soft start of the water pump, reduces the impact current of starting under constant pressure, shortens the starting time, and reduces the iron loss, copper loss and damage to the main insulation during the starting process. (2) It realizes the stepless speed regulation of the water pump, with high speed regulation accuracy (0.01 Hz) and solves the problem of the hydraulic coupler losing rotation. (3) High efficiency (frequency converter efficiency can reach 98%), high power factor (frequency converter power factor can reach 0.98), and high energy saving rate (energy saving rate is more than 15% higher than that of hydraulic coupler under the same conditions). (4) It has industrial network and communication interface, which can easily realize DCS start, stop, interlock, lock, open loop, closed loop, manual and automatic control. (5) It has complete protection functions, including overcurrent, abnormal output voltage, overload, motor overheating, abnormal cooling fan, power outage detection, grounding detection and other functions. (6) Low maintenance, low failure rate and long service life, generally up to 80,000 hours without failure. (7) Annual operation and maintenance costs are much lower than those of hydraulic coupler. 5. The two-to-three soft start variable frequency speed regulation water pump scheme is also feasible. Theoretically, the one-to-three soft start variable frequency speed regulation water pump is feasible, but there is no operational practice yet. People are worried about what to do if the frequency converter fails. In fact, it is very simple. Adding a frequency converter will solve the problem. The schematic diagram is shown in Figure 4. Figure 4: Schematic diagram of the two-to-three soft-start variable frequency speed control feedwater pump scheme. In this scheme, when starting the boiler unit, feedwater pump #2 (#3) can be started with a variable frequency drive. When the load reaches 50% of the rated capacity, feedwater pump #1 can be started. After normal operation, feedwater pump #2 (#3) can be switched to mains frequency operation, while feedwater pump #1 operates with variable frequency speed control to maintain the boiler water level. Variable frequency feedwater pump #3 (or #2) is used as a backup. Although this scheme adds one frequency converter, its initial investment is not higher than that of a steam-driven feedwater pump, making it a viable option in terms of overall technical and economic comparison. 6. Conclusion Variable frequency speed control is the most advanced, reliable, and efficient speed control technology of our time. Low-voltage frequency converters have been widely used in thermal power plants, achieving good results in automatic control and energy saving, and are widely recognized by society. High-voltage frequency converters (VFDs) are being widely used in the technical renovation, construction, and expansion of units in thermal power plants, including forced draft fans, induced draft fans, pulverizers, ash pumps, circulating water pumps, turbine condensate pumps, and feedwater pumps in mains lines. The application of VFDs in power plants is receiving increasing attention. Energy conservation and emission reduction are fundamental national policies and long-term strategies; they should be given the same fundamental importance as environmental protection. The main feedwater pump is a crucial auxiliary machine that requires regulation and is also the auxiliary machine with the highest single-unit power consumption in thermal power plants. Using VFDs for one-to-three circulating soft-start VFD speed regulation or two-to-three VFD speed regulation operation is the best choice for main feedwater pump speed regulation.