Abstract: Auxiliary equipment in power plants, such as fans and pumps, consume a large amount of electricity annually. Replacing inefficient hydraulic couplings or fixed-speed systems with variable frequency drives (VFDs) can yield significant economic benefits. This paper analyzes and compares the performance of hydraulic couplings and VFDs to illustrate the importance of promoting the use of VFDs in fan and pump loads. Keywords: hydraulic coupling speed regulation; variable frequency speed regulation; energy saving comparison[b][align=center]An Operational Comparison of AC Drive and Hydraulic Fluid Coupling Controller in Power Plant Blower and Pump Application SUN Guo-qin[/align][/b] (School of M echanicalan dA utanation Engineering,Sh anghaiIn stituteo fT echnology,Sh anghai20 0235,Ch ina) Abstract : Au xiliary equipment in p owerp lant,su cha sb lowersa ndp umps,co nsumesla rgea mounto fel ectricalen ergy,bu tit c anb eim proveda lo tin t ermso fe nergys avinga nde conomics enefitsif low efficiency hydraulic fluid coupling or constant speed system can be replaced by frequency controlled drive system. The article illustrates the importance of the frequency controlled drive system in Blower and pump application by data analysis. Key words : hydraulic fluid coupling speed control; frequency controlled speed; energy saving comparison. In thermal power plants, blowers and pumps are the main energy consumers. Although speed control devices have been added to blowers and pumps, hydraulic coupling equipment is generally used. Moreover, these devices operate continuously for a long time or are subject to speed control, resulting in low operating efficiency and insignificant energy-saving effects. However, their energy-saving potential is even greater under low-load and variable-load operating conditions. Since the 1980s, foreign power plants have been widely adopting blower and pump applications. According to statistics, the total capacity of auxiliary motors in thermal power plants nationwide was 15,000 MW at the end of the 20th century, with an annual total electricity consumption of 52 billion kWh, accounting for 5.8% of the national thermal power generation. The energy-saving effect of auxiliary motors in power plants is significant, but the high price of imported high-voltage frequency converters directly affects the electricity consumption rate, limiting their widespread use in Chinese power plants. In the 21st century, most domestically produced high-voltage frequency converters for fans and pumps use fixed-speed drives, with only a few rapidly emerging, now occupying the market at a rate of hundreds per year. This has undoubtedly played a significant role in promoting energy-saving retrofits for fans and pumps. 1. Working Principle of Variable Frequency Speed ​​Regulation and Hydraulic Coupling Speed ​​Regulation 1.1 Hydraulic Coupling Speed ​​Regulation Principle A hydraulic coupling is a blade-type transmission mechanism that uses the kinetic energy of a liquid (mostly oil) to transfer energy. It is installed between a constant-speed motor (powered by the grid, i.e., running at full speed) and a fan or pump to achieve smooth speed regulation. The core component of a hydraulic coupling is a pair of impellers, which transfer motor energy and change the output speed by controlling the change in the dynamic torque of the working oil in the working chamber. The motor drives its driving impeller (called the pump impeller) through the input shaft of the hydraulic coupling, accelerating the working oil. The accelerated working oil then drives the driven impeller (called the turbine) of the hydraulic coupling, transferring energy to the output shaft and the load. Thus, the torque of the output shaft can be controlled by controlling the amount of working oil participating in energy transfer in the working chamber, thereby controlling the load speed. 1.2 Variable Frequency Speed ​​Regulation Principle The speed expression of an asynchronous motor is: where: f[sub]1[/sub] is the frequency of the power supply (i.e., the output frequency of the frequency converter), Hz; p is the number of pole pairs of the asynchronous motor; s is the slip of the asynchronous motor. When using a frequency converter for speed control, the motor shaft is directly connected to the load, and the motor is powered by the frequency converter. The motor speed is changed by altering the output frequency of the frequency converter (i.e., changing the power supply frequency of the motor). Since the motors of power plant fans and pumps have very high power, typically using 3kV or 6kV power, high-voltage frequency converters must be used for speed control. 2. Energy Saving Comparison of Frequency Converter Speed ​​Control and Hydraulic Coupling Speed ​​Control Besides its own power loss, both frequency converter speed control and hydraulic coupling speed control involve additional power losses. The hydraulic coupling obtains mechanical energy from the motor output shaft, transmits it to the load via hydraulic transmission, and incurs power losses during this process; the frequency converter obtains electrical energy from the grid, transmits it to the load via the motor, and also incurs power losses during this process. The efficiency-speed curves of the two speed control methods across the entire speed range are shown in Figure 1. As can be seen from Figure 1, the efficiency of the hydraulic coupling decreases as the output speed decreases, reaching 0.95 at rated speed. Point A), the efficiency at 75% rated speed is 0.72 (Point B), and the efficiency at 20% rated speed is 0.19 (Point C); while the efficiency of variable frequency speed regulation is still relatively high when the output speed of the motor decreases, with an efficiency of 0.97 at rated speed, greater than 0.95 at 75% rated speed, and greater than 0.9 at 200x6 rated speed. Since the efficiency of the hydraulic coupling decreases quickly when the output speed decreases, the low-speed performance of variable frequency speed regulation is better than that of the hydraulic coupling. 2.1 Theoretical calculation Energy saving comparison One of the major characteristics of fans and pumps is that the load torque is usually proportional to the square of the speed, and the shaft power is proportional to the cube of the speed. The relationship between their flow rate, pressure, and shaft power is shown in equations (2), (3), and (4). For example: the air volume of a 1000kW fan is reduced from 100% to 70 Since flow rate is proportional to the first power of rotational speed, rotational speed can be reduced by 70%, and load power is proportional to the cube of rotational speed, so load power can theoretically be reduced to 34.3%. The total power input to the power grid is given by the formula: P is the load power (kW); η is the efficiency of the speed control device; ηM is the efficiency of the motor; ηP is the efficiency of the pipeline system. If variable frequency (VFD) speed control is used, its efficiency is calculated as 0.95. Considering the motor efficiency of 0.85 and the pipeline system efficiency of 0.95, the total power input to the power grid is approximately 447.1 kW, resulting in energy savings of 55.29 kWh. Assuming 300 days per year, this translates to an annual energy saving of 3.98 million kWh. If a hydraulic coupling is used, its efficiency is calculated as 0.665. Considering the motor efficiency of 0.85 and the pipeline system efficiency of 0.95, the total power input to the power grid is approximately 638.7 kWh. kW, energy saving of 36.13%, annual electricity saving of 2.6 million kWh. It can be seen that variable frequency speed control saves an additional 1.38 million kWh annually. The comparison of electricity saving between the two speed control methods is shown in Table 1. 2.2 Actual Energy Saving Comparison Taking the actual measurement of a 200 MW unit wind turbine retrofit by a power design institute as an example, the rated parameters of this asynchronous motor are 1250 kW, 6kV, 142 A, rated efficiency 95%, rated speed 742 r/min, and rated power factor 0.85. The input current of the two regulation methods, hydraulic coupling speed control and variable frequency speed control, under different generator loads are shown in Table 2. The comprehensive input power of the motor under the two regulation methods is shown in Table 3. The estimated daily power consumption under the two regulation methods is shown in Table 4. Calculated based on 300 days and 7200 hours of unit operation per year, the application of variable frequency speed control saves 1.4 million kWh annually compared to hydraulic coupling speed control (=3.37 million kWh - 1.97 million kWh). Although the motor power is different, the measured electricity saving ratio is basically consistent with the theoretical calculation. 3 Comparison of Other Performance Aspects of Variable Frequency Speed ​​Regulation and Hydraulic Coupling Speed ​​Regulation 3.1 Starting Performance Direct starting of a motor under load generates a large inrush current, which is detrimental to both the motor and the power grid. On the one hand, this current causes losses and heat generation in the lines and motor, leading to insulation aging. On the other hand, the torque impact during starting increases the possibility of potential faults such as broken rotor squirrel cage bars and unsoldered stator leads. Statistics show that approximately 15% of motor faults are caused by direct starting. Furthermore, the large starting current causes the bus voltage to be too low, which may prevent the motor from starting normally or affect the normal operation of other equipment on that bus. Using a hydraulic coupling for speed regulation cannot directly improve starting performance. The starting current is 5-7 times the rated current. For wound-rotor induction motors, although starting performance can be improved by adding resistance in the rotor circuit, the starting current will still be more than twice the rated current, and an additional starting device is required. Variable frequency speed regulation, on the other hand, can achieve soft starting, meaning the starting current increases slowly, resulting in a smoother starting process with no impact on the power grid or machinery. This reduces the required electrical capacity, especially for new projects, saving on capacity expansion costs. Furthermore, the entire starting process can be controlled, with the starting point and ramp-up time adjustable. 3.2 Adjustment and Control Performance Hydraulic couplings rely on adjusting the amount of oil in the working chamber to change the output speed, resulting in a relatively slow response time that may not keep up with control requirements, and its speed regulation accuracy is also low. Variable frequency speed regulation, however, can be performed very quickly, and with digital control, its frequency stabilization accuracy can reach over 0.1%, thus enabling precise control. 3.3 Power factor frequency converters can maintain a high power factor over a wide speed range (e.g., power factor greater than 0.95 at speeds above 20%), while the power factor of a hydraulic coupler is lower than the rated power factor of the motor at low speeds, and will be lower than 0.7 at speeds below 70%. Therefore, when using a hydraulic coupler for speed regulation and needing to improve the power factor, a power factor compensation device should be added. 3.4 Operational Reliability and Maintenance Hydraulic couplers have complex mechanical structures and piping systems. Long-term reliable operation increases the workload of system maintenance. If a fault occurs, direct constant-speed operation is not possible, and shutdown for repair is necessary. Although high-voltage frequency converters have complex electronic circuits, the technology is becoming increasingly mature. In particular, unit-series multi-level high-voltage frequency converters have automatic unit switching and redundant operation characteristics. They can operate continuously without shutdown in the event of a unit failure, ensuring reliability. Furthermore, maintenance is quite easy, requiring only periodic replacement of the air filter. 4 Conclusion As can be seen from the analysis, the main auxiliary equipment of the power plant adopts high-voltage variable frequency speed regulation, which has a significant energy-saving effect. At present, although the initial investment of hydraulic coupling is lower than that of variable frequency speed regulation, the energy-saving effect and other performance of variable frequency speed regulation are significantly better than those of hydraulic coupling. As mentioned above, a 1000 kW motor using variable frequency speed regulation saves 1.38 million kWh of electricity per year compared to using hydraulic coupling. If the investment in variable frequency speed regulation is 600,000 yuan more, the investment can be recovered in just over a year. From the second year onwards, hundreds of thousands of yuan of operating expenses can be saved each year. Therefore, the overall return on investment is very high, and the resulting economic benefits are very considerable. References: [1] Xu Furong. Application Practice of High-Voltage Variable Frequency Speed ​​Regulation Technology [M]. Beijing: China Electric Power Press, 2007. [2] Feng Duosheng, Zhang Sen. Application and Maintenance of Variable Frequency Drives [M]. Guangzhou: South China University of Technology Press, 2001. [3] Sun Guoqin, Zhou Dexian. Vigorously promote the domestic high-voltage variable frequency speed regulation energy-saving device [J7]. Motor and Control Applications, 2006, 33(6): 3-6.