Abstract: In order to save energy and reduce consumption, the pellet dust removal fan of the pelletizing branch of Baogang Group Guyang Mining Co., Ltd. was retrofitted with frequency conversion technology. The operation data of the frequency conversion retrofit of the pellet dust removal fan was analyzed and calculated. The application of frequency conversion technology in the fan equipment of the pelletizing branch has broad application prospects.
Keywords: High-voltage frequency converter, pellet dust collector fan, speed regulation, energy saving
1. Introduction
With increasingly stringent environmental protection requirements, how enterprises can maximize energy conservation and consumption reduction to lower costs and enhance competitiveness while protecting the environment has become a key challenge. In the metallurgical industry, dust collector fans are energy-intensive devices. Traditional fan airflow control relies on adjusting the size of the inlet damper. Most of the energy is consumed by the damper, and with this control method, the fan's airflow is only a fraction of its rated capacity, resulting in low actual operating efficiency as the fan operates far from its rated point. Adjusting the fan speed to change the airflow is the optimal energy-saving solution. For a long time, people have been actively seeking more scientific speed control schemes to regulate fans and achieve energy conservation and consumption reduction.
2. Basic Information on the Modified Equipment
The pellet dust collector at Baogang Group Guyang Mining Co., Ltd. uses a Y4-73NO29.5F centrifugal fan with a YKK710--8 (10kV) motor. The fan has a power rating of 1250kW, a speed of 745r/min, and a rated current of 88A. The machine operates continuously, and currently, an inlet damper is used to adjust the airflow. Based on operating conditions, the damper opening is typically maintained at around 65%, and the opening is adjusted accordingly for changes in operating conditions. This damper adjustment wastes a significant amount of electrical energy and severely damages the performance of the mechanical equipment, necessitating a modification.
Figure 1 shows a schematic diagram of the performance curves of a dust collector fan. This fan uses inlet guide vanes to regulate airflow. In the figure, 0° represents the fully open position of the regulating valve, and negative values represent the angle at which the regulating valve rotates towards the closing direction. The dashed line in the figure represents the isoefficiency line. The centrifugal fan achieves its highest efficiency at the maximum opening of the inlet regulating valve. The isoefficiency line is nearly perpendicular to the boiler resistance curve, and the efficiency drops rapidly along the resistance line.
Figure 1 Schematic diagram of the performance curve of the dust collector fan
Figure 2 Power consumption effect curves of various adjustment methods for the fan
Figure 2 shows the power consumption effect curves of the fan using various adjustment methods. It can be seen that during variable operating conditions, the greater the adjustment depth, the more significant the energy-saving effect. The closer the motor power consumption is to the ideal curve, the more energy-efficient the fan. Only the variable frequency speed control curve is closest to the ideal curve, exhibiting the lowest operating energy consumption and the best energy-saving effect. Furthermore, the advantages of variable frequency speed control in terms of frequency range, dynamic response, low-frequency speed, slip compensation, power factor, and operating efficiency are unmatched by traditional AC speed control methods. Therefore, using variable frequency speed control for energy saving in fans is currently the best choice.
3. Energy-saving principle of variable frequency speed regulation for fans
The basic principle of variable frequency speed control technology is based on the relationship that the motor speed is proportional to the input frequency of the working power supply: n=60f(1-s)/p, (where n, f, s, and p represent the speed, input frequency, motor slip, and number of motor pole pairs, respectively); the frequency converter changes the motor speed by changing the frequency f of the motor's working power supply.
Variable frequency speed control of asynchronous motors has the advantages of wide speed range, high smoothness and stiff mechanical characteristics. At present, variable frequency speed control has become the most important speed control method for asynchronous motors and has been widely used in many fields.
According to the similarity law of fluid mechanics: Q1/Q2＝n1/n2, the output air volume Q is directly proportional to the rotational speed n;
H1/H2＝(n1/n2)2 The output pressure H is proportional to the rotational speed n2;
The output shaft power P is proportional to the rotational speed n3, where P1/P2 = (n1/n2)3.
When the fan airflow needs to be changed, such as by adjusting the damper opening, a large amount of electrical energy is wasted on the damper. However, if variable frequency speed control is used to adjust the airflow, the shaft power can be significantly reduced as the airflow decreases. Therefore, controlling the fan via frequency conversion not only saves energy but also greatly improves equipment performance. The above formulas provide a solid theoretical basis for frequency conversion energy saving.
High-voltage AC variable frequency speed control technology is a speed control technology for AC motors that developed rapidly in the 1990s. Its technology and performance surpass any other speed control method. Variable frequency speed control has gained widespread user and market recognition due to its significant energy-saving benefits, high-precision speed control accuracy, wide speed range, comprehensive power electronic protection functions, and easily implemented automatic communication functions. It also provides users with great convenience and efficient service in terms of safe and reliable operation, installation, use, maintenance, and repair, making it the preferred solution for enterprises adopting motor energy-saving methods.
With the increasing maturity of domestic high-voltage high-power variable frequency speed control technology, this technology has been widely applied in various industries. Considering that the technology of such products in China is relatively mature, and the key components are selected from well-known foreign products, and that they have advantages in terms of price, after-sales service and technical support, after extensive research and comparison, the leaders of Baogang Group Guyang Mining Co., Ltd. finally selected the JD-BP38-1250F (1250kW/10kV) high-voltage frequency converter produced by Xinfengguang Electronic Technology Co., Ltd. to modify the sintering dust removal fan.
4. Variable Frequency Drive Control Scheme for Pellet Dust Collector Fans
The current control method for pellet dust collectors primarily involves adjusting the fan motor frequency in real time using a high-voltage frequency converter based on the required dust collection air volume. With the frequency converter, the inlet baffle remains fully open, and the fan speed is manually adjusted according to production needs. The system can easily change the air supply volume to adapt to variations and maintain efficient fan operation.
The airflow of the dust collector fan is adjusted by regulating the frequency converter of the existing DCS system. The DCS controls the start-up, shutdown, and speed regulation of the frequency converter, and displays the converter's operating data and current status, allowing for real-time monitoring of system operation. Operationally, both remote and local control methods are available, enhancing system safety. The frequency converter includes a built-in PLC for logic processing of switch signals within the cabinet and coordination with various field operation and status signals (such as RS485). It can also expand control switching quantities according to user needs, enhancing system flexibility. Using a high-voltage frequency converter to operate the dust collector fan under varying loads according to actual needs ensures and improves the process while achieving energy saving and consumption reduction.
In addition, to ensure the reliability of the system, the existing control equipment and operating mode of the dust collector fan will be retained. The control loop is designed with a power frequency/variable frequency operation switching option, which is manually switched to enable the dust collector fan to operate at either power frequency or variable frequency.
5. Technical Features of the Fengguang JD-BP38 Series High Voltage Frequency Converter
The Fengguang JD-BP38 series high-voltage frequency converter uses a high-speed DSP as its control core and employs speed-vector control technology and power unit series multi-level technology. As a high-high voltage source frequency converter, its harmonic performance is far below the IEEE 519-1992 harmonic standard. It boasts a high input power factor and excellent output waveform quality, eliminating the need for input harmonic filters, power factor compensation devices, and output filters. It also avoids problems caused by harmonics, such as additional motor heating, torque pulsation, noise, output dv/dt, and common-mode voltage, allowing the use of ordinary asynchronous motors. Fengguang high-voltage frequency converters have been recognized as a Chinese Famous Brand Product. Specifically, in addition to the performance characteristics of ordinary frequency converters, Fengguang high-voltage frequency converters also possess the following outstanding features:
(1) Using a high-speed DSP as the central processing unit, the operation speed is faster and the control is more precise.
(2) Vector control technology measures and controls the stator current vector of the AC motor, and controls the excitation current and torque current of the AC motor according to the principle of field orientation, thereby achieving the purpose of controlling the torque of the AC motor. It has a large starting torque, fast torque dynamic response, high speed regulation accuracy, and strong load-carrying capacity.
(3) Rapid start-up function. It can identify the motor speed and start the motor directly without stopping it. After the frequency converter is subjected to load impact protection, it can be automatically reset and then automatically restarted, thus avoiding losses caused by frequency converter protection shutdown in important situations (such as high-temperature fans in cement plants). The rapid start-up technology can enable the frequency converter to be reset from the protection state and resume load operation within 0.1 seconds.
(4) Seamless switching technology between power frequency and variable frequency. Current high-voltage variable frequency speed control systems generally include a power frequency bypass switching cabinet. When the frequency converter fails, it allows the high-voltage motor to switch to power frequency operation. There are two types of bypass switching: manual and automatic. Manual bypass requires manual operation and is suitable for operating conditions without backup devices or for less critical situations. Automatic bypass can automatically switch to power frequency operation directly after a frequency converter failure. The automatic bypass switching cabinet provided by Xinfengguang Company can not only automatically switch from variable frequency to power frequency operation in the event of a frequency converter failure, but also instantly switch from power frequency to variable frequency operation after frequency converter maintenance is completed. The entire switching process will not affect the operation of the user's equipment.
(5) Power grid instantaneous power failure restart technology: the power grid can be automatically restarted when power fails instantly, and can provide a waiting time of up to 60 seconds.
(6) Automatic line voltage balancing technology (star point drift technology). When a unit in a phase of the frequency converter fails, in order to balance the line voltage, the traditional method is to reduce the voltage of the other two phases to the same voltage as the faulty phase. However, the automatic line voltage balancing technology adjusts the angle between the phases to ensure the maximum line voltage balance output under the premise that the phase voltage output is maximum and unequal.
(7) Oscillation suppression technology: When the motor is lightly loaded or unloaded, local instability may occur, and the current amplitude will fluctuate greatly. The current oscillation may cause the system to trigger protection due to overcurrent or overvoltage. Xinfengguang Company adopts a superior current algorithm to effectively suppress current oscillation and ensure stable and reliable operation of the system.
(8) Multi-machine master-slave control technology: The frequency converter has master-slave control function, and multiple frequency converters can form a master-slave control network through a data bus. One of them is set as the master and the others are set as slaves. The master collects the status information of each slave in real time and sends frequency and torque commands to each slave, realizing power balance and comprehensive control of each frequency converter. This technology is suitable for applications requiring power balance control, such as belt conveyors and friction elevators.
(9) Automatic output voltage regulation technology: The inverter detects the bus voltage of each unit in real time and adjusts the output voltage according to the bus voltage, thereby realizing the automatic voltage regulation function. This avoids the impact of power grid fluctuations on the output voltage.
(10) Fault unit hot reset technology: If a unit fails during operation and the inverter continues to operate by bypassing it, the fault unit can be reset during operation without waiting for the inverter to stop.
(11) Unit DC voltage detection: Real-time display of the DC voltage of the detection system, thereby realizing optimized control of the output voltage, reducing harmonic content, ensuring the accuracy of the output voltage, improving the system control performance, and enabling operation and maintenance personnel to have a comprehensive grasp of the operating status of the power unit.
(12) It has the function of sudden phase-to-phase short circuit protection. If the output short circuit is caused by equipment failure or other reasons, it will lead to a major accident if the frequency converter does not have the function of phase-to-phase short circuit protection. When such a problem occurs, the frequency converter can immediately block the output of the frequency converter to protect the equipment from damage and avoid the occurrence of an accident.
(13) Current limiting function: When the inverter output current exceeds the set value, the inverter will automatically limit the current output to avoid overcurrent protection caused by the inverter during acceleration and deceleration or due to sudden load changes, and minimize the number of shutdowns.
(14) Multiple control modes, including local control, remote control box control, and DCS control. Supports communication protocols such as MODBUS and PROFIBUS. Frequency setting can be set on-site or via communication. Supports frequency preset and acceleration/deceleration functions.
6. Dust collector fan frequency conversion retrofit main circuit
To maximize equipment utilization and ensure normal fan operation even in the event of a frequency converter failure, thus guaranteeing production, the original power frequency hydraulic resistance starting system and the motor integrated protection device (activated during power frequency operation) are retained. A one-to-one manual/frequency converter switching scheme is adopted. In the event of a frequency converter failure, the system can manually switch back to the original power frequency hydraulic resistance starting mode, ensuring continued operation of the dust collector fan. The primary circuit of the system's manual bypass cabinet is shown in Figure 3.
Figure 3 Primary circuit of manual bypass cabinet
The bypass cabinet contains three high-voltage disconnect switches. To ensure no reverse power is fed to the inverter output, K2 and K3 employ an electromagnetic interlocking operating mechanism. When K1 and K3 are closed and K2 is open, the motor operates at the inverter frequency. When K1 and K3 are open and K2 is closed, the motor is started at the power frequency using a liquid resistance cabinet. At this time, the inverter is isolated from the high voltage, facilitating inspection, maintenance, and debugging.
The bypass cabinet must be interlocked with the upstream high-voltage circuit breaker DL. When DL is closed, it is absolutely forbidden to operate the bypass disconnect switch and the inverter output disconnect switch to prevent arcing and ensure the safety of operators and equipment. To achieve inverter fault protection, the inverter is interlocked with the user switch DL. In the event of an inverter fault, the inverter will trip DL. During power frequency bypass, the inverter should allow DL to close and remove the trip signal to DL, allowing the motor to start normally via power frequency after DL is closed.
When selecting a frequency converter system, the liquid resistance cabinet should not be activated; the liquid resistance cabinet is only effective when the power frequency mode is selected.
7. Effects of frequency converter retrofit
During normal production, the dust collector fan at Baogang Group Guyang Mining Co., Ltd. operates continuously with the inlet damper opening at approximately 65%. The measured actual airflow is 3,478,000 m³/h, the input current is approximately 65A, the power factor is 0.78, and the power consumption is 878.1 kW per hour. After frequency conversion modification, the same dust collection requirements are met at an operating frequency of 40.0 Hz, with an input current of 28.2A, a power factor of 0.96, and a power consumption of 469.1 kW per hour. Figure 4 shows a photo of the high-voltage frequency converter operating on-site.
Figure 4. Photo of the high-voltage frequency converter of the dust collector fan in operation.
To examine the energy-saving effect of the frequency converter upgrade, the pellet plant branch compiled comparative data before and after the upgrade, as shown in Table 1.
Table 1 Comparison data of dust collector fan before and after modification | |||||
damper opening (%) | Operating frequency (Hz) | Operating current (A) | Power factor | Power consumption (kW) | |
Before renovation | 50 | 50 | 65 | 0.78 | 878.1 |
After renovation | 100 | 40.0 | 28.2 | 0.96 | 469.1 |
Table 1 shows that the motor operating current is significantly reduced and the power factor is significantly improved, demonstrating the obvious effect of the upgrade. The pellet dust collector fan operates 24 hours a day, 330 days a year. The annual electricity savings after the frequency converter upgrade are calculated as follows:
(878.1kW - 469.1kW) × 24h × 330d = 3,239,280 kW•h
Based on a price of 0.5 yuan/kW•h, the annual electricity cost savings are:
3.23928 million kW·h × 0.5 yuan/kW·h = 1.61964 million yuan.
8. Conclusion
After the dust collector fans at the Pelletizing Branch of Baogang Group Guyang Mining Co., Ltd. underwent frequency conversion upgrades, the impact on the power grid during fan startup was reduced, the mechanical strength and wear of the fans were decreased, and the service life of the fans was extended. Equipment operating noise was also significantly reduced, and energy-saving effects were remarkable. With the increasing emphasis on energy conservation and emission reduction in the national "13th Five-Year Plan" and the intensification of market competition, the application of high-voltage variable frequency speed control will become increasingly widespread.
References
[1] Xinfengguang Electronics User Manual [Z] Xinfengguang Electronics Technology Co., Ltd.
About the Author
Ma Jiankai, male, with a university degree, is a power supply engineer and deputy director. He works in the Mechanical Department of Baogang Group Guyang Mining Co., Ltd.
