This article presents a comprehensive techno-economic comparison of variable frequency speed control (VFD) for electric feedwater pumps in high-pressure boilers of 200MW steam turbine units. Replacing the hydraulically coupled feedwater pumps with VFD-controlled feedwater pumps yields significant energy savings, exceeding 20%. 1. Overview There are nearly 150 200MW steam turbine generator units installed in the domestic power grid. Assuming each unit has 2 × 100% of its rated capacity feedwater pumps, there are nearly 300 hydraulically coupled electric feedwater pumps. Statistics show that feedwater pumps consume 2.5% of the electricity generated and nearly 30% of the plant's total electricity consumption. Assuming an average annual power generation of 126 million kWh per unit and an annual power consumption of 31.5 million kWh per feedwater pump, the annual power consumption of feedwater pumps for 150 units would be 472.5 million kWh. A 0.5% reduction in feedwater pump power consumption would result in an annual power saving of 94.5 million kWh, equivalent to a coal saving of 350,000 tons. Feedwater pumps, as major auxiliary equipment in the production process, consume a significant amount of electricity, directly impacting coal consumption for power generation, power generation costs, and overall energy consumption. Therefore, optimizing the regulation method of electric feedwater pumps and conducting a feasibility study on frequency conversion retrofitting is essential. 2. Feasibility Analysis of Frequency Conversion Speed ​​Regulation Retrofitting for Electric Feedwater Pumps For 200MW steam turbine generator units, the speed regulation method for electric feedwater pumps is generally designed as 2×100% rated capacity with a hydraulic coupling for speed regulation. This traditional regulation method has become the standard design scheme. However, with the maturity and application of variable frequency speed control technology, it is necessary to conduct a comprehensive technical and economic comparison and selection of variable frequency speed control and hydraulic coupling speed control. 2.1 Technical comparison 2.1.1 Variable frequency speed control device Variable frequency speed control uses a variable frequency device as a variable frequency power supply. By changing the frequency f of the power supply to the stator of the asynchronous motor, the synchronous speed n1 changes, thereby changing the speed n of the asynchronous motor and achieving the purpose of speed control. Its principle structure is shown in (Figure 1) Figure 1 Its technical features are: (1) Wide speed control range, which can be adjusted between 1% and 100%. (2) Adjustment accuracy can reach ±0.5% (at 100% speed). (3) Overall efficiency of 97%, power factor above 0.95. (4) It has industrial network and communication interface, which is convenient for realizing closed-loop automatic control. And the protection function is perfect. (5) Long service life, low failure rate, and low maintenance. (6) High power saving rate, which can reach more than 20% compared with hydraulic coupling. (7) There is no high-speed slippage phenomenon of hydraulic coupling. (8) Soft start and soft stop can extend the service life of motor. 2.1.2 Hydraulic Coupling Device The hydraulic coupling uses a squirrel-cage motor as the prime mover and oil as the working medium. The prime mover drives the speed-increasing gear, which drives the pump wheel (driving wheel) to transfer mechanical power to the working medium oil to drive the turbine (driven wheel) to rotate. The driven wheel is connected to the water pump and the speed of the water pump is controlled by the scoop tube. Its structural system is shown in (Figure 2). Figure 2 Its technical conditions are: (1) The slip power loss is large and is dissipated as heat through the oil-water cooling system. (2) It is installed between the motor and the water pump and requires a solid foundation. (3) The pressure oil system and the scoop tube adjustment system require a lot of maintenance. (4) The motor runs at a constant speed and the inrush current is large when starting, which affects the service life of the motor. (5) At high speed, the slip rate affects the slippage by about 3%. (6) The efficiency of the coupler is generally low, 94% at rated speed, and decreases significantly with decreasing speed under variable speed conditions. 2.2 Economic Comparison The economic comparison mainly compares the annual operating costs of the variable frequency speed-regulating feedwater pump and the hydraulic coupler speed-regulating feedwater pump. That is, the comparison of annual power consumption. The annual power consumption of the coupler regulation is shown in Table 1 Table 1 The annual power consumption of the variable frequency regulation is shown in Table 2 Table 2 The power saving of the variable frequency speed regulation is shown in Table 3 Table 3 Through the above comparison, the annual power saving of the variable frequency speed regulation is 6,012,320 kWh, accounting for nearly 0.5% of the annual power generation, with a power saving rate of 20%. Assuming an average electricity price of 0.28 yuan, the annual operating cost savings are 6,012,320 kWh × 0.28 yuan ≈ 1.68 million yuan. The investment can generally be recovered within 2.5 years. 3. Requirements of the high-pressure boiler feedwater pump for the frequency converter The high-pressure boiler feedwater pump, as an important auxiliary machine in the power production process, has very high requirements for the high-voltage frequency converter. The primary requirement is that the high-voltage frequency converter has high reliability, mainly including: (1) The mean time between failures (MTBF) of the frequency converter is ≥80,000 hours. (2) It has strong adaptability to grid voltage fluctuations, that is, it can work normally within a large range of grid voltage fluctuations. This range is generally -30% to +10%. (3) Direct high-voltage frequency conversion output, frequency converter efficiency ≥96%, power factor ≥0.95. (4) It should comply with national standards GB/T14549, GB/T 15543-1995 and IEEE519-1992. Input current harmonics 4%, output voltage and current harmonics 3%. (5) Good output waveform, du/dt <1000 V/microsecond, common mode voltage <500V, no problem of motor overheating or affecting motor insulation. Ordinary asynchronous motors can be used, and there is no limit to the cable length. (6) Speed ​​range 0-100%, speed accuracy 0.5%. (7) Start-up to full load time 15-30 seconds. (8) The frequency converter should have an automatic start-up function. It should fully meet the automatic start-up requirements for Class 1 motors in the "Technical Regulations for the Design of Auxiliary Power Supply in Thermal Power Plants", specifically including: ① Meeting the minimum bus voltage (65%) requirement. ② Meeting the no-load automatic start-up requirement. That is, automatic start-up formed when the standby power supply automatically switches to the working section that has lost power in the no-load state. ③ Meeting the undervoltage automatic start-up requirement. That is, automatic start-up formed when a sudden low voltage occurs during operation and the fault is cleared and the voltage is restored. ④ Meeting the load-bearing automatic start-up requirement. That is, automatic start-up formed when the standby power supply is already carrying a partial load and then automatically switches to the working section that has lost power. ⑤ Meeting the maximum time limit (9s-10s) for the low voltage protection to trip the circuit breaker. That is, automatic start-up is completed before the low voltage protection trips. This ensures the safe operation of the unit. ⑥ The automatic start-up process is undisturbed and impact-free, which is conducive to the restoration of the auxiliary power supply voltage. 4. Issues to be Noted in the Design and Retrofitting of High-Pressure Feedwater Pumps 4.1 High-Pressure Feedwater Pump Variable Frequency Speed ​​Control Design High-pressure boiler feedwater pump variable frequency speed control can be divided into variable frequency speed control with a speed-increasing gearbox connecting the feedwater pump and motor via the speed-increasing gearbox, and variable frequency speed control without a speed-increasing gearbox directly connecting the feedwater pump and motor. 4.1.1 Variable Frequency Speed ​​Control with Speed-Increasing Gearbox Connecting the Feedwater Pump and Motor Currently, most high-pressure boiler electric feedwater pumps in operation in China are asynchronous motors. Since the speed of high-pressure boiler electric feedwater pumps is mostly between 3000-6000 r/min, variable frequency speed control of asynchronous motors requires a speed-increasing gearbox between the motor and the feedwater pump. The booster pump can be arranged separately or coaxially with the feedwater pump motor. To save power consumption of the booster pump, a coaxial arrangement is preferable. When designing and manufacturing a coaxial arrangement, attention should be paid to calculating that the booster pump must meet the effective net positive suction head (NPSH) and required NPSH of the feedwater pump. This ensures that the booster pump meets the necessary net positive suction head (NPSH) for the feedwater pump under the minimum speed condition corresponding to the recirculation flow rate. 4.1.2 The variable frequency speed control gearbox directly connecting the feedwater pump and motor, as a gear speed-increasing device, undoubtedly incurs mechanical losses (approximately 3%). To further reduce the power consumption of the feedwater pump, an asynchronous motor (6kV 60Hz) with a synchronous speed of 3600rpm is used, equipped with a high-pressure feedwater pump of the same speed via variable frequency speed control, eliminating the booster gearbox and achieving direct connection between the motor and the feedwater pump. Compared to variable frequency speed control with a booster gearbox, this method improves efficiency by 3%. This variable frequency speed control scheme adds one frequency converter, reduces one booster gearbox and coupler set, reduces two pairs of couplings, and reduces the pressure oil system compared to the coupler speed control scheme. The feedwater pump uses inducer technology, which also eliminates the need for a booster pump, as well as corresponding pipelines, valves, thermal instruments, coupler foundations, etc. Overall, the variable frequency speed control directly connecting the feedwater pump and motor only increases costs by 3.5 million compared to hydraulic coupler speed control. Since variable frequency speed control can save 1.68 million yuan in operating costs annually, the additional initial investment of 3.5 million yuan can be recovered in 2.1 years (this does not include the corresponding cost of the 3% energy saving due to the efficiency improvement after eliminating the speed-increasing gear). 4.2 Variable Frequency Retrofit of Electric Feed Pumps for High-Pressure Boilers in Operation To reduce the power consumption of electric feed pumps for high-pressure boilers in operation, it is essential to retrofit them with variable frequency technology. The first step in retrofitting electric feed pumps for high-pressure boilers in operation is to select a frequency converter that meets the requirements of the high-pressure feed pump and to choose an appropriate location to install the high-pressure frequency converter, and to carry out corresponding modifications to the primary and control systems. This is undoubtedly crucial. The important part is the corresponding modification of the hydraulic coupling. The main content of this modification is to eliminate the regulating part of the hydraulic coupling and retain the speed-increasing part. That is, to remove the pressure oil system of the hydraulic coupling, remove the scoop tube speed regulation system, and replace the pump wheel and turbine with a coupling connection. The coaxial gear oil pump is eliminated, and a spare gear oil pump is added and interlocked with the original electric gear lubrication oil pump. 5. Conclusion Based on the above technical and economic analysis, changing the speed regulation of the high-pressure boiler feedwater pump in a 200MW steam turbine unit from hydraulic coupling to frequency converter is technically feasible and economically viable. The optimal approach is to use frequency converter speed regulation with direct connection between the electric motor and the feedwater pump, considering both technical and economic factors. To achieve the goal of reducing energy consumption by 20%, one of the two hydraulically coupled electric feedwater pumps (both with 100% rated capacity) can be converted to frequency converter regulation for operation, while the other hydraulically coupled electric feedwater pump can serve as a standby pump. This will effectively reduce the power consumption of the feedwater pumps and achieve the goal of energy conservation and consumption reduction.