1 Introduction
Variable frequency drives (VFDs) offer significant energy savings in applications involving fans and pumps, and possess excellent soft-start and soft-stop control characteristics. Therefore, VFDs were initially widely used in the control of fans and pumps in metallurgy, power, petrochemical, heating, and civil applications. Due to the unique environment and specific safety requirements of coal mining, the application of VFDs in coal mines started relatively late. With the deepening development of my country's market economy, increasing production, reducing consumption, and improving efficiency in coal mines have become paramount, making energy-saving retrofitting of equipment imperative.
Coal mine ventilation fans are classified into axial flow and centrifugal types according to their structure. These fans are the main ventilation machinery ensuring coal mine safety, operating 365 days a year. Generally, two sets of mine ventilation fans are provided, one for operation and one as a standby. The rated ventilation capacity (air volume and air pressure) of a mine ventilation fan is determined based on the designed ventilation resistance at the end of the mine's operation and the air volume at full production capacity. In the initial stage of mine operation, the fans have a relatively large spare capacity.
In coal mine production, the required air volume and pressure vary at different stages. To meet these production requirements, coal mines typically employ the following methods for adjustment:
(1) Gate regulation;
(2) Change the speed of the ventilation fan;
(3) Change the angle of the guide vane;
(4) Change the installation angle of the moving blades in the axial flow fan;
(5) Adjust the tail fin angle of the centrifugal fan;
(6) Change the number of moving blades in an axial flow fan;
(7) Axial flow fans change the angle of the stationary blades.
Among these methods, gate adjustment is the least efficient, as it artificially alters the resistance curve, increasing wind resistance, and its performance deteriorates with each adjustment. Lead vane adjustment and tail fin tilting angle adjustment are more efficient than gate adjustments. Changing the blade installation angle and the number of blades can alter the fan's characteristic curve, allowing the fan to operate at higher efficiency over a wider range, thus achieving energy conservation and consumption reduction. Changing the fan speed to operate at its optimal operating point allows the fan to operate at maximum efficiency over the largest possible range, resulting in good energy-saving effects.
2. Advantages of using variable frequency speed control to retrofit mine ventilation fans
With the application of power electronics, microelectronics, information technology and modern control theory in speed regulation systems, and due to the power shortage in recent years, variable frequency speed regulation technology has become a development direction of modern electric transmission. The excellent speed regulation performance makes the energy-saving effect of frequency converters in coal mine production more and more significant. Therefore, the mine ventilation fan is changed to frequency converter control, which combines traditional motor speed regulation technology, modern power electronics technology and computer control technology. When the coal mine ventilation volume changes, the speed is automatically adjusted so that the ventilation fan can run efficiently at an economical speed, thereby achieving the effect of saving electricity. The characteristics of variable frequency speed regulation energy-saving control device are: (1) high speed regulation efficiency; (2) large speed regulation range; (3) high speed regulation accuracy; (4) small starting current and easy to realize closed-loop control. Since the original ordinary AC asynchronous motor can be used, it is particularly suitable for the technical transformation of the original old equipment. It not only maintains the advantage that the original fan and motor do not need to be replaced, but also achieves a significant effect of saving electricity. It is an ideal method for speed regulation and energy saving of mine ventilation fans.
3. User Site Equipment Introduction
Prior to 2003, the ventilation equipment used in the No. 3 shaft of Taoyang Coal Mine, Feicheng Mining Group Co., Ltd., was a turbine-type ventilator (70B2). This type of equipment is inefficient (reaching a maximum of only 60%) and consumes a lot of energy. The main reason for the low efficiency is that it cannot be operated at adjustable speed; the air volume and pressure can only be controlled by adjusting the blade angle. However, the initial selection of ventilation equipment for the mine had a large margin, and it could not reach full load operation for a long time.
The 2K56 series fan is a new type of energy-saving mine fan jointly developed by Shenyang Blower Works (Group) Co., Ltd. and Northeastern University. This machine is a two-stage impeller axial flow fan with cast aluminum alloy blades featuring a twisted airfoil profile. The installation angle can be adjusted between 20° and 50°. When recirculation is required, the fan can be directly reversed without adjusting any fan components. The maximum static pressure efficiency reaches 85.3%. The product has a wide high-efficiency range; the static pressure efficiency of this series of fans is nearly 14% higher than the older 70B2 fan.
In order to change the outdated operation of the original ventilation fans, the leaders of Zhongsanjing in Taoyang Coal Mine decided to replace the two old-fashioned turbine ventilation fans 70B2-21NO-24 (500kW/6kV) with axial flow energy-saving fans 2K56-2N0-24, and use frequency conversion speed regulation to control the air volume.
After careful consideration and comparison, the leadership of Zhongsanjing Mine in Taoyang Coal Mine selected the Fengguang brand JD-BP37-500F (500kW/6kV, 2 sets) high-voltage frequency converter system manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd. to upgrade the No. 1 and No. 2 ventilation fans of Zhongsanjing Mine. The high-voltage frequency converters were put into operation in April 2008, and the equipment has been running well ever since. Testing by the Feikuang Group Testing Center confirmed that all performance indicators met design requirements, the ventilation fans are operating efficiently, and the upgrade achieved its intended purpose.
Taoyang Coal Mine has two ventilation fans in the three shafts, one in use and one on standby. The equipment parameters of ventilation fans No. 1 and No. 2 are the same. Table 1 below gives the basic parameters of ventilation fan No. 1.
Table 1 shows the on-site motor parameters, and Table 2 shows the fan parameters.
4. Shandong Xinfengguang Electronics JD-BP37-500F High Voltage Frequency Converter
4.1 Main Performance Indicators of JD-BP37-500F High Voltage Frequency Converter
Inverter power: 500kW
Input frequency: 50Hz
Input voltage: 6.0kV±20%
Output voltage: Three-phase sinusoidal voltage 0-6kV
Rated current: 61A
Output frequency: 0-120Hz
Frequency resolution: 0.01Hz
Acceleration time: can be set according to process requirements.
Deceleration time: can be set according to process requirements.
Fault diagnosis and detection: Automatic detection, automatic location
Grid-side power factor: 0.95 (at high speed)
Overload protection: 150% for 1 minute
Protection rating: IP20
Ambient humidity: 90%, non-condensing
4.2 Performance Characteristics
(1) Fully Chinese WINDOWS operating interface, color LCD touch screen;
(2) High input power factor, low current harmonics, no need for power factor compensation/harmonic suppression device;
(3) Outputs a stepped sinusoidal PWM waveform, without the need for an output filter device, causing no damage to the insulation of cables and motors, reducing motor harmonics, reducing mechanical vibration of bearings and blades, and the output line can be up to 1000 meters long.
(4) Modular design of power circuit, simple maintenance;
(5) The high-voltage main circuit and the controller are connected by optical fiber, which is safe and reliable;
(6) Complete fault monitoring circuit and accurate fault alarm protection;
(7) Built-in PLC, easy to change control logic relationship, adapt to changing field needs;
(8) Flexible selection of on-site control/remote control;
(9) Accepts and outputs 0-5V/4-20mA industrial standard signals;
(10) A built-in PID controller can be installed according to user needs;
(11) Complete general-purpose frequency converter parameter setting function.
5. Modification of the main circuit control scheme
The wind-solar high-voltage variable frequency speed control system consists of a primary circuit incoming cabinet (bypass cabinet), a transformer cabinet, a frequency converter unit cabinet, and an operation control cabinet. During frequency converter maintenance or in the event of a frequency converter failure, the bypass cabinet connects the motor to the mains power grid, ensuring uninterrupted production. During frequency conversion operation, the frequency converter provides comprehensive protection for the motor. The primary circuit of the high-voltage frequency converter bypass cabinet is shown in Figure 1.
Figure 1. Primary circuit diagram of bypass cabinet
DL: User high-voltage vacuum circuit breaker
K1, K2, K3: High-voltage frequency converter with built-in high-voltage disconnect switch
BPQ: JD-BP37-500F High Voltage Frequency Converter
D: 500kW/6kV asynchronous motor
The manual bypass cabinet in Figure 1 contains three high-voltage disconnect switches. To ensure that power is not fed back to the inverter output, K2 and K3 are operated by an electromagnetic interlock 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 operates at the mains frequency. At this time, the inverter is isolated from the high voltage, which facilitates 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 frequency converter output disconnect switch to prevent arcing and ensure the safety of operators and equipment. The operation of the high-voltage frequency converter on site is shown in Figure 2.
6 Energy Saving Calculation
6.1 Energy Saving Calculation Formula for Fans
As mentioned earlier, variable frequency speed control is an effective energy-saving measure for fan flow rate changes. According to GB12497-1995 "Economic Operation of Three-Phase Asynchronous Motors," the following calculation formula applies to the management of economic operation of motors.
The relationship between the motor input power P1V and the flow rate Q when using baffles to regulate flow is as follows:
6.2 Energy Saving Calculation
When the air volume is adjusted by damper, the required shaft power of the fan is 401kW. When the air volume is adjusted by frequency converter, the energy saving rate is 0.364 compared to adjusting the air volume by damper.
(1) Calculation of annual electricity savings (based on 300 working days per year)
Annual working days and hours, power saving rate = (2) Calculation of annual electricity saving cost (electricity price is calculated at 0.5 yuan/day)
Electricity price per unit of electricity consumption = yuan
6.3 Actual Energy Saving Situation
After the wind turbine was upgraded and put into operation with a frequency converter, the current dropped from 55A to about 26A, the operating frequency was 35Hz, the input current was 26A, the input voltage was 6.3kV, the output current was 49A, and the output voltage was 4kV. The power of the wind turbine motor dropped from 411.52kW to 260.68kW. The actual annual electricity savings amounted to 543,024 yuan, which is basically consistent with the theoretical calculation value.
7. Other advantages of frequency converter retrofit
(1) When the fan is started by using a frequency converter, a frequency conversion soft start can be achieved, which avoids the impact of the starting current on the power grid and control equipment, and can effectively extend the service life of electrical equipment.
(2) Adjust the air volume as needed, with a wide speed range and high speed adjustment accuracy, avoiding waste. After the frequency conversion modification, the air volume of the fan no longer needs to be adjusted by changing the blade angle, but is achieved by the frequency converter by adjusting the speed of the motor. The adjustment linearity is good, so the air volume can be adjusted at any time according to the needs of underground production, avoiding waste.
(3) The working intensity of the fan is reduced and its service life is extended. After the frequency conversion modification, the fan operates at low speed for most of the time, which greatly reduces the mechanical strength and electrical shock of the fan, greatly extends the service life of the fan, and improves the safe operation coefficient and operating cycle of the fan.
(4) The frequency converter has various protections such as short circuit, overload, overvoltage, undervoltage, and phase loss, which effectively protect the motor.
(5) Improved the on-site working environment. Due to the reduced fan speed, the fan noise was greatly reduced, which was welcomed by the on-site staff.
(6) The number of operational steps in reversing the airflow has been reduced. Previously, reversing the airflow required operating the vertical damper, horizontal damper, and reversing damper of the system, which involved many procedures and often resulted in the wrong damper being reversed. Now, with the use of axial flow fans and frequency converter control, reversing the airflow is extremely simple. Just adjust the operating frequency to a negative frequency on the human-machine interface, and the motor will first decelerate to zero and then reverse to the adjusted frequency. During this process, there is no need to adjust the entire damper system, which reduces the workload and lowers the failure rate during operation.
8. Conclusion
After undergoing frequency conversion modification, the ventilation fans at the Taoyang Coal Mine of Feicheng Mining Group Co., Ltd. not only achieved excellent energy-saving results but also significantly improved the stability of the entire ventilation system. my country is a major coal producer and also a country with relatively high electricity consumption per ton of coal. If coal mines require speed-adjustable equipment, most can be retrofitted with frequency conversion speed control devices, resulting in substantial energy savings.
About the Author
Hu Lingzhi, male, is a technical support engineer working at Shandong Xinfengguang Electronic Technology Development Co., Ltd.
Address: Wenshang Economic Development Zone, Shandong Province 272500
Telephone: 0537-7237196 Article Contact Person: Guo Peibin Telephone: 0537-7237007
