Abstract : With the development of power electronic devices, microelectronics technology, electric motor and control theory, computer technology, and automatic control technology, AC motor speed control systems have also made great strides in recent years. Microcomputers and vector conversion control technology have achieved fundamental breakthroughs in high-performance AC drive systems. Due to their significant energy-saving effects, AC speed control systems are now widely used. The Yinchuan Thermal Power Plant uses a high-power frequency converter to control the circulating water pumps of the heating network, achieving the goals of simple speed control operation and energy saving. Keywords: Frequency converter; Speed ​​control; Motor control; Energy saving Based on the application of big power transducers in Yinchuan Cogeneration Power Plant, this paper introduces the principle of three-phase DC-AC converter, vector conversion control theory of asynchronous motor, in order to popularize this technology in power plants. Keywords: three-phase DC-AC converter, vector conversion control theory of asynchronous motor 1 Introduction In electrical drive systems, although DC motors have advantages such as good speed regulation performance and large starting torque, their fatal weakness of mechanical commutation makes maintenance inconvenient, application environment limited, and manufacturing cost high. Compared with DC motors, AC motors (especially asynchronous motors) have the characteristics of simple structure, robustness, and reliable operation. They are also superior to DC motors in terms of single-unit capacity, supply voltage, and speed limit. Therefore, AC motors have been widely used in various sectors of the national economy. With the development of power electronic devices, microelectronics technology, motor and control theory, AC motor speed regulation systems have also developed significantly in recent years. Electromagnetic speed-regulating asynchronous motors, thyristor low-synchronous cascade speed control devices, and variable frequency and variable voltage speed control systems have been widely applied. Variable frequency speed control systems for asynchronous motors with capacities ranging from tens to hundreds of kilowatts, composed of thyristors and high-power transistor inverters, have been put into industrial operation. The capability to manufacture commutatorless motors with capacities of several thousand kilowatts has been achieved. Microcomputers and vector transformation control technology have made fundamental breakthroughs in high-performance AC drive systems. Loads such as fans and pumps, which have traditionally relied on constant speed transmission, have largely adopted AC speed control systems due to energy-saving needs. High-performance AC drives using inverters will inevitably become the mainstream of speed control drives. 2. Problem Statement Power plants are among the places that use asynchronous motors the most, making them ideally suited for variable frequency speed control. Currently, energy conservation and environmental protection are the mainstream of society, and centralized heating through combined heat and power (CHP) units in cities is an inevitable trend. However, how to improve the quality of heating and achieve economic, safe, and efficient operation is a problem faced by every CHP plant. Upon completion of the second phase of the Yinchuan Thermal Power Plant project, the total heating area will reach 405 x 10⁴ m², with 50 water-to-water heat exchange stations and 2 steam-to-water heat exchange stations. The heat exchange stations are unmanned, and all signal parameters are transmitted wirelessly to the power plant's heating network dispatch center. The first station of the heating network is equipped with three steam-water heaters with a heat exchange area of ​​250 m² and three with a heat exchange area of ​​640 m². It uses seven heating network circulation pumps. The rated output flow of the M1, M2 and M3 heating network circulation pumps is 650 m³/h, equipped with three 280 kW motors with a rated voltage of 6.3 kV and a speed of 295 r/min. The rated output flow of the M4, MS, M6 and M7 pumps is 92 m³/h, equipped with four 560 kW motors with a rated voltage of 6.3 kV and a speed of 1470 r/min. As the heating load increases, the heating is normally supplied to the outside in winter through four heating network circulation pumps, with three pumps kept on standby. However, after the heating network is put into operation, the following problems need to be solved. (1) The heating load in winter and summer is very different. The summer heating load is less than 1/20 of the winter heating load. (2) In order to achieve efficient and economical heating, the secondary network circulating pumps all adopt variable frequency technology to adjust the secondary network heating flow according to the temperature. This places high demands on the flow control of the primary network. (3) Since the total heating flow of the primary network circulating pumps is greater than the existing heating flow, the primary network circulating pumps cannot adapt to the flexible adjustment of the heating flow under constant speed operation. Therefore, the variable frequency control method is proposed. 3 Problem Solving In Figure 1, the M1, M4, M5, M6, and M7 heating network circulating pumps operate at constant speed, while the M2 and M3 heating network circulating pumps can operate at either constant or variable speed. Multiple methods are used for combined control. When the primary flow rate of the heating network is not higher than 650 m³/h, the frequency converter is started for speed regulation; when it is higher than 650 m³/h but less than 920 m³/h, one 920 m³/h constant-speed pump is started; when it is higher than 920 m³/h but less than 1840 m³/h, two constant-speed pumps and one frequency converter are used; when it is higher than 1840 m³/h but less than 2760 m³/h, three constant-speed pumps and one frequency converter are used. [Figure 1 Wiring of the heating network circulating pump system] The Yinchuan Thermal Power Plant selected Siemens' Maestr EVR type frequency converter. This is a low-voltage current source frequency converter with a maximum output power of 40kW, using an AC-DC-AC frequency conversion method. It also employs a high-low-high wiring configuration. This wiring method steps down the high voltage to 380V via a transformer, then converts the frequency through the frequency converter, and finally steps it up to 6kV using a step-up transformer to drive the motor. To eliminate harmonic interference during the frequency conversion process, a filter is used. This type of frequency converter is reasonably priced and technologically mature. 4. Frequency Converter Speed ​​Control Methods and Energy-Saving Principles 4.1 Frequency Converter Speed ​​Control Methods Frequency converter speed control achieves the purpose of adjusting the output speed of the AC motor by changing the frequency of the power supply input to the AC motor. The output speed of the AC asynchronous motor is determined by the following formula: n = 60f(1-s)/p Where: n - motor output speed; f - input power supply frequency; s - motor slip; p - number of pole pairs of the motor. As can be seen from formula (1), the output speed of the motor is related to the input power frequency, slip rate, and number of pole pairs of the motor. Therefore, the direct speed regulation methods of AC motors mainly include: pole pair speed regulation (adjusting p), rotor series resistance speed regulation, cascade speed regulation or internal feedback motor (adjusting s), and frequency conversion speed regulation (adjusting f). Frequency converter speed regulation receives AC power with a power frequency of 0.5 Hz from the power grid and, through appropriate forced conversion methods, converts the input power frequency AC power into AC power with adjustable frequency and amplitude, which is then output to the AC motor to realize the variable speed operation of the AC motor. There are two main conversion methods for converting power frequency AC power into variable frequency AC power output: one is called direct conversion method, also known as AC-AC frequency conversion method. It is a method of directly forcing the input power frequency power into AC output that needs to be frequency converted through controlled rectification and controlled inversion. Therefore, it is called AC-AC frequency conversion method. Another type is called indirect conversion, also known as AC-DC-AC frequency conversion. It first converts the input AC power into DC power through a fully controlled/semi-controlled/uncontrolled rectifier, and then converts the DC power into AC power output with adjustable frequency and amplitude through an inverter unit. 4.2 Principle of Speed ​​Regulation and Energy Saving According to the basic laws of fluid mechanics, the water pump is a square torque load. Its speed n has the following relationship with the flow rate Q, pressure (head) H, and shaft power P: Where: Q1, H1, P1 - flow rate, pressure (or head), and shaft power of the water pump at speed n1; Q2, H2, P2 - flow rate, pressure (or head), and shaft power of the water pump under similar operating conditions at speed n2. From formulas (2), (3), and (4), it can be seen that the flow rate of the water pump is directly proportional to the rotational speed, the pressure (or head) is directly proportional to the square of its rotational speed, and the shaft power is directly proportional to the cube of its rotational speed. From formula (4), it can be seen that, under the condition that other conditions remain unchanged, by reducing the running speed of the motor, its energy-saving effect is related to the cube of the speed reduction. The variable frequency drive speed regulation can solve this problem well and achieve the purpose of energy saving, and the energy-saving effect is very obvious. 5 Variable Frequency Drive Control of Heating Network Circulating Water Pump Operation and Energy Saving As shown in Figure 1, the wiring of the heating network circulating pump system is as follows. The heating network circulating pumps M1, M2, and M3 are HPK-Y200-15 type, with a flow rate of 650 m³/h, a head of 116 m³/h, and a speed of 2950 r/min, and are equipped with Y3553-2 type high-voltage motors with a power of 280 kW. The M4, M5, M6, and M7 heating network circulation pumps are all QSGT920-125 type, with a flow rate of 920 m³/h, a head of 125 m³/h, and a speed of 1470 r/min, equipped with a Y4(X)5 type 560 kW high-voltage motor. Under normal circumstances, the frequency converter controls the M3 heating network circulation pump for speed regulation, while the other heating network circulation pumps can operate at a constant speed at the industrial frequency. Only when the M3 heating network circulation pump is under maintenance, malfunctions, or fails and stops, can it be disconnected via an electrical switch. The frequency converter then switches to control the MZ heating network circulation pump, still achieving speed regulation through frequency converter control. The heating load differs significantly between winter and summer, with the summer load being less than 1/20th of the winter load. Furthermore, heating in winter begins in early January each year and ends at the end of March of the following year. During these 5 months, the outdoor temperature fluctuates from high to low to high, and within a single day, the outdoor temperature fluctuates from high to low to high during the day and night. To ensure the quality of the heating system, i.e., the indoor temperature does not fall below 1°C, the flow rate and temperature of the heating network are adjusted according to the changes in outdoor temperature. The flow rate is between 120 m³/h and 280 m³/h, and the pressure is around 0.7 MPa. Two or three fixed-speed circulating water pumps operate, and one variable-speed circulating water pump is controlled by a frequency converter. In summer, only a portion of domestic hot water is supplied, and the circulating water pump is controlled by a frequency converter, with one pump operating. A primary network flow rate of 24 m³/h is sufficient. The frequency converter controls the circulating water pump to adjust in real time, improving the accuracy of the adjustment. Under the power frequency operation, the M1 circulating water pump operates at a constant speed with a flow rate of 650 m³/h, which is the minimum flow rate under power frequency operation. The average daily electricity consumption of the lowest-flow-rate fixed-speed pump is 280 x 24 = 6720 kWh. During the heating season, a heating network circulation pump controlled by a frequency converter is used for flow regulation. The actual daily electricity consumption of this pump is 3680 kWh (according to the electricity meter). After the heating season, the circulation pump controlled by the frequency converter operates, and its actual daily electricity consumption is 1860 kWh (according to the electricity meter). The daily electricity savings of the frequency converter-controlled heating network circulation pump during the heating season are: 6720 - 3680 = 3040 kWh; the daily electricity savings after the heating season are: 6702 - 1860 = 4806 kWh; the annual electricity savings per pump controlled by the frequency converter are: 5 x 30 x 3040 + 7 x 30 x 4860 = 1,476,600 kWh. Based on the on-grid electricity price of 0.402 yuan/kWh for Yinchuan Thermal Power Plant, the annual electricity savings for one heating network circulating pump controlled by a frequency converter is: 1,476,600 x 0.402 = 593,593.2 yuan. Based on the over-generation electricity supply of 0.175 yuan/kWh for Yinchuan Thermal Power Plant, the annual electricity savings for each heating network circulating pump controlled by a frequency converter is: 1,476,600 x 0.175 = 258,405 yuan. 6. Impact on Equipment Using frequency converters simplifies operation, allows for full valve opening, reduces valve throttling losses, and provides uniform speed regulation. It reduces wear on pump impellers and other rotating equipment, thus reducing maintenance. It avoids the adverse impacts of high current and torque from direct power frequency starting on motors, cables, switches, and mechanical equipment. Furthermore, when the heating network circulating water pump controlled by the frequency converter operates at low load for extended periods, the motor torque is much lower than the torque at rated load, reducing the hazards associated with power frequency starting. This greatly improves the health level of the equipment, which not only extends the service life of the motor, but also reduces the wear of the bearings and improves the reliability of the heating supply. At the same time, it extends the maintenance cycle of the equipment, reduces the amount of maintenance of the equipment, and lowers the maintenance cost of the equipment. 7 Conclusion Since the installation and commissioning of the frequency converter in January 1999, the frequency converter has performed well, has high operational reliability, and requires little maintenance. (1) It has achieved the goal of saving electricity, increased the grid-connected electricity of Yinchuan Thermal Power Plant, and improved economic benefits. (2) It has improved the precision of regulation, ensured the quality of heating, and reduced the labor intensity of personnel frequently opening and closing valves. (3) It makes it easier to maintain the pressure of the external heating network and reduces the amount of equipment maintenance. (4) If the blowers, induced draft fans, feed water pumps, circulating water pumps, etc. of Yinchuan Thermal Power Plant are all controlled and regulated by frequency converter technology, the cost will be recovered in 2 to 4 years, reducing plant electricity consumption, improving economic benefits, and achieving the goal of energy saving and consumption reduction. References : [1] Huang Jun. Semiconductor converter technology. Machinery Industry Press, 1986. [2] Zhao Liangbing. Fundamentals of modern power electronics technology. 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