Abstract : With the continuous expansion of the application fields of high-voltage variable frequency drives (VFDs), axial flow adjustable stator blade induced draft fans for 300MW units have also begun to be used. After adopting VFDs, the air volume regulation of 300MW unit induced draft fans has undergone significant changes compared to the original operation mode, and the reliability of the high-voltage VFD itself will also affect the normal operation of the unit. This paper, using the VFD retrofit project of the induced draft fans at Yangguang Power Plant as an example, introduces how to use a high-voltage VFD to retrofit the induced draft fans according to the characteristics of the relevant power plant systems. VFDs not only significantly achieve energy saving, but more importantly, they have good regulation performance, while also improving the starting of the fan and motor, and extending the service life of the equipment. Keywords : induced draft fan; variable frequency speed control device; 300MW unitAbstract : A long w ith the w ide app licat ion of h igh vo ltage variable frequency device, the use of 300MW unit axial flow induced draft fan is started. ltage variable frequency device it self could affect the no rmal operat ion of the unit. Tak ing Yangguang Power P lant as an example, this is paper describes how to upgrade induced fan by h igh vo ltage variable frequency device based on the characteristic ics of related system in the power plant s. fan and This will improve the efficiency and extend the life of the equipment. Key words : induced draft fan; variable frequency speed control device; 300MW unit. The No. 1 coal-fired unit of Shanxi Yangguang Power Generation Co., Ltd. (hereinafter referred to as Yangguang Power Plant) has a designed output of 300MW. The boiler is equipped with two AN-28 type adjustable axial flow induced draft fans with a rated air volume of 928,800 m³/h and a total pressure of 3196Pa; equipped with YKK800-8-W type motors with a rated power of 2000kW, a rated voltage of 6kV, and a rated current of 254A. The motors have no speed control device, and the air volume is adjusted by changing the angle of the fan's stator blades. Power plant loads typically fluctuate between 50% and 100%. Changes in generator output power necessitate corresponding adjustments to boiler processing, including changes in the boiler's forced draft and induced draft volumes. The output of the induced draft fan is adjusted by changing the angle of the fan blades. While this method offers some energy savings compared to controlling the inlet damper opening, throttling losses remain significant, especially at low loads. Furthermore, the slow adjustment of the stationary blades causes a corresponding delay in unit load. The starting current of an asynchronous motor typically reaches 8 to 10 times its rated current, impacting the plant's power supply. This strong impact torque also negatively affects the lifespan of the motor and fan. When the fan speed changes, its operating efficiency remains relatively stable. The flow rate is directly proportional to the first power of the speed, the pressure to the square of the speed, and the shaft power to the cube of the speed. When the fan speed decreases, the shaft power decreases with the cube of the speed, and the power required by the motor driving the fan can also be reduced accordingly. Therefore, speed regulation is an important way to save energy in fans. Using variable frequency speed control (VFD) allows for linear adjustment of the induced draft fan motor speed. By changing the motor speed, indicators such as furnace negative pressure and boiler oxygen content can be maintained in a certain relationship with the induced draft fan airflow. Since current induced draft fan airflow regulation methods cannot adequately meet the needs of boiler combustion capacity and stable operation, it is necessary to modify the induced draft fan to improve its energy-saving and regulatory performance to meet the overall unit regulation requirements. Variable frequency speed control devices can optimize the motor's operating state, greatly improving its operating efficiency and achieving energy savings. In the past, due to limitations in price, reliability, and capacity, it has not been widely used in my country's thermal power generation market. In recent years, with the rapid development of power electronic devices, control theory, and computer technology, the price of frequency converters has been continuously decreasing, and their reliability has been continuously improving. High-voltage, high-capacity frequency converters have been widely used in power plant auxiliary equipment. Two sets of high-voltage frequency converters were used on the induced draft fans of Unit 1 at the Yangguang Power Plant. The frequency converters are used to change the motor speed, thereby regulating the airflow and air pressure of the induced draft fans. Based on the current operating conditions of the two induced draft fans of Unit 1, the operating current of the induced draft fans is only about 140 A when the unit is operating at full capacity. From a cost-saving perspective, the frequency converter was not selected based on the rated power of the motor, but rather on the actual operating current of the motor. The final selected model is HARSV EST2A 06ö220. This high-voltage frequency converter was manufactured by Beijing Leadway Electric Technology Co., Ltd., and belongs to the HARSV EST2A series voltage source type fully digital control frequency converter, using a high-high configuration and an H-bridge series scheme. The rated capacity is 2250 kVA, the rated voltage is 6 kV, and the rated current is 220A. The renovation period was from the end of May to the beginning of June 2005, a total of 40 days, and it was put into operation simultaneously with the overhaul of Unit 1. The high-voltage frequency converter of the induced draft fan of Unit 1 was put into normal operation on June 10, 2005. 1 Application of High-Voltage Variable Frequency Speed ​​Control System 1.1 Composition of High-Voltage Frequency Converter The high-voltage frequency converter of Beijing Lide Huafu Electric Technology Co., Ltd. consists of three parts: transformer cabinet, power cabinet, and control cabinet. It is a unit series multi-level structure, and its frequency converter principle is shown in Figure 1. 1.2 High-Voltage Frequency Converter and Field Interface Scheme The control part of the high-voltage frequency converter of Beijing Lide Huafu Electric Technology Co., Ltd. consists of a high-speed single-chip microcomputer, human-machine interface and PLC. The microcontroller implements PWM control and power unit protection; the human-machine interface provides a user-friendly, fully Chinese monitoring interface, and also enables remote monitoring and networked control; the built-in PLC is used for the logic processing of switch signals within the cabinet, and can flexibly interface with the user's site to meet specific user needs. This frequency converter uses a Siemens S72200 series PLC, which has good interface capabilities with the DCS system. Based on the characteristics of the fan, operating requirements, and specific requirements of the frequency converter control, corresponding control schemes were adopted. 1.2.1 Interface Scheme between DCS System and Frequency Converter There are a total of 11 signals between the DCS system and the frequency converter, including 9 switching signals and 2 analog signals, as shown in Table 1. 1.2.2 Cable Laying and Materials Used The switch signal cable from the induced draft fan inverter to the DCS system MC cabinet has 14 cores, see Table 2; the switch signal cable from the DCS system RC cabinet to the induced draft fan inverter has 4 cores; the analog signal cable from the DCS system to the induced draft fan inverter and the analog signal cable from the induced draft fan inverter to the DCS system MC cabinet have 4 cores. The internal cable of the DCS system needs to be 800 m. 1.2.3 The content added to the DCS screen is to realize the start-stop control and speed adjustment of the variable frequency induced draft fan. The following are added to the DCS screen: (1) Inverter start-stop operation function block, used for remote start-stop of the inverter; (2) Inverter speed control function block; (3) Inverter minor fault alarm block, major fault alarm block, and power frequency bypass status. 1.3 Inverter Operation Modes and Control Logic Under normal conditions, two fans are put into variable frequency speed control operation. Considering the possibility of inverter failure, the induced draft fan system also has operation modes of 1 inverter and 1 power frequency, and 2 power frequency. Inverter operation modes are divided into local control and remote control. In remote control mode, the speed command signal output by the DCS tracks the inverter speed feedback. In local control mode, remote operation of the inverter is ineffective. When the inverter is controlled by the DCS, there are two modes: automatic and manual. In manual mode, the operator controls the inverter speed by changing the speed control block on the DCS operation screen to achieve negative pressure regulation. 1.3.1 Permissible Conditions for Induced Draft Fan Inverter Start-up Since the prerequisite for inverter start-up is that the high-voltage switch of the induced draft fan motor must be closed and the start-up feedback is 1, the original fan start-up conditions are retained as permissible conditions for induced draft fan inverter start-up, while the ready signal sent locally by the inverter serves as another start-up condition. During the commissioning of the frequency converter remote start-up, it was found that the command in the frequency converter speed setting block might be at a relatively high speed. Starting the frequency converter at this point would cause a significant disturbance to the furnace negative pressure and could easily lead to operational errors. Therefore, a restriction was added to the start-up process requiring the motor speed command to be less than 30%. 1.3.2 Automatic A and B Frequency Converter Speed ​​Adjustment: The automatic A and B frequency converter speed adjustment switch section: When the induced draft fan stator vane adjustment is in automatic mode and the A and B frequency converter speeds are locked in automatic mode, the automatic mode is automatically deactivated when the deviation value in the deviation circuit exceeds a certain value (tentatively set at 50%). When the furnace negative pressure decreases by a certain value, the lockout speed increases after a 3-second delay; when the furnace negative pressure increases by a certain value, the lockout speed decreases after a 3-second delay. Analog input section for automatic speed control of inverters A and B: Since the adjustment object is the same as the automatic induced draft fan blades, the original deviation formation circuit is directly taken out as the deviation of the existing inverter regulation and applied to the existing induced draft fan inverter control. A balancing circuit combining speed is added based on the characteristics of inverters to maintain the output of both sides. A gain circuit for single and dual fan inverter modes is also added independently. Since the original deviation formation circuit includes the feedforward part of the total air volume, it is not added to the new inverter speed circuit. Considering that if a single induced draft fan inverter trips and cannot be restored to inverter mode operation, the current balancing circuit in the original baffle control circuit is changed to a position feedback balancing circuit. Simultaneously, the inverter of the other induced draft fan is gradually increased to the maximum before being put into automatic induced draft mode. Regarding the trip protection related to the induced draft fan inverters 1, 3, and 3: After the inverter of a single fan trips, the corresponding supply fan needs to be tripped in conjunction with it, and the logic for shutting down the corresponding baffle and blades remains unchanged. After the dual-fan frequency converter trips, the corresponding high-voltage switch trips as well, so the original boiler interlock trip circuit remains unchanged. 1.4 Automatic Parameter Tuning Test for Induced Draft Fans: Start the frequency converters of induced draft fans A and B, open the stationary blades of C0421 and C0422 (the original baffles of the two induced draft fans) to 100%, and set the furnace negative pressure to -50 Pa; after starting the forced draft fans A and B, adjust the opening of their moving blades C0321 and C0322 (the original baffles of the two forced draft fans) to 10%; set the frequency converters of induced draft fans A and B to the lowest speed of 225 rpm, and simultaneously put the frequency converters into automatic mode. First, perform a setpoint disturbance test, changing the setpoint by 20%, and record the automatic changes; considering the characteristics of pressure regulation, first set the integral time to 4 minutes and the proportional coefficient to 0.13, gradually change the proportional coefficient, and use the critical proportional band method for parameter setting. After the equal amplitude oscillation of regulation appears, perform initial setting according to the critical proportional band algorithm. A set of basic parameters is provided for reference: P = 0.1025, Ti = 100 s. The opening of the moving blades C0321 and C0322 (the original baffles of the two blowers) of blowers A and B is increased by 10% in increments during the upper stroke test. The changes in furnace negative pressure are observed, and the magnitude of the deviation and the time to eliminate the deviation are recorded. After completion, the lower stroke test is performed using the moving blades of blowers A and B for disturbance testing. By changing the opening of one of them by 30%, the changes in the induced draft inverter speed and the response time of the negative pressure are observed. Then, the moving blade disturbance test of the blowers is performed, with the opening increasing by 10% in increments during the upper stroke test. The changes in furnace negative pressure are observed, and the magnitude of the deviation, the time to eliminate the deviation, the inverter command output, and the actual speed value are recorded. After completion, the lower stroke test is performed to verify the proportional gain of single and dual blower operation. Simulating the MFT (Mechanical Furnace Trigger) operating conditions, the forced draft fan was started, and the opening of its moving blades C0321 and C0322 (the original dampers of the two forced draft fans) was adjusted to 50%. The changes in furnace negative pressure and the operation of the induced draft fan over-relaxation mechanism after flameout were observed. After completing the automatic test, a practical operation test was conducted on the interlocking of the induced draft fan frequency converter, and then the induced draft fan frequency converter was put into automatic mode. During the test, one side of the forced draft fan was also disconnected to simulate whether the frequency converter could control the negative pressure to a satisfactory range after one side of the forced draft fan was tripped during operation. The safe operation of the boiler is the fundamental guarantee of the plant's power. Although the frequency converter is reliable, in case of a problem, the safe operation of the boiler must be ensured. Therefore, the switching between power frequency and frequency converter operation must be realized. If one induced draft fan frequency converter fails and cannot be restored in a short time, the induced draft automatic control needs to be adjusted using the original stator blades. Under these circumstances and needs, one induced draft fan frequency converter must be shut down, another frequency converter must be started, and the original induced draft automatic control (stator blades) must be activated to create appropriate disturbances. After testing, some parameters were adjusted and modified. 2. Economic Comprehensive Test Evaluation 2.1 Significant Energy Saving Benefits Table 3 compares the production data of Unit 1's induced draft fan frequency converter from June 10th to 16th with the data of Units 2, 3, and 4. Through the data comparison in Table 3, from the perspective of power saving rate analysis, under the same power generation load of the four units, the average daily power consumption of the two induced draft fans of Unit 1 is 16,431 kWh, while the average power consumption of the two induced draft fans of Units 2, 3, and 4 is 32,450 kWh, saving 16,019 kWh, with a power saving rate of 49137%. 2.2 Investment and Payback Period Estimation: Based on a daily load distribution of 7200 hours of operation per year, using two variable frequency speed-regulating induced draft fans, compared to the previous static blade regulation, calculates to save 4805700 kWh annually. At a power generation cost of 0.120 yuan/kWh, 4805700 × 0.120 = 961140 yuan. The total investment cost of the two frequency converters, including installation and civil engineering costs, is approximately 4.4 million yuan, requiring about 4 years to recover. 3. Conclusion In summary, high-voltage variable frequency drives (VFDs) have great potential in power plant applications and represent the future direction of technological development. They not only offer significant energy savings but also superior regulation performance, while simultaneously extending the service life of the fans and motors. With the development of technology, manufacturing costs have continued to decline, and new products have emerged one after another, greatly simplifying the structure of the equipment, reducing the number of components, and improving the reliability of the frequency converter. About the author: Li Fengming (1964-), male, engineer, from Pingding, Shanxi Province, graduated from the Electrical Engineering Department of Taiyuan University of Technology, and is currently an electrical maintenance specialist at Shanxi Sunshine Power Generation Co., Ltd.