Abstract: This paper introduces the composition and implementation scheme of an automated system for the main ventilation fan driven by a variable frequency drive in a coal mine, pointing out that the use of variable frequency drive for the main ventilation fan in a coal mine can not only ensure long-term safe and reliable operation, but also bring good social and economic benefits to the enterprise.
1 Overview
The coal industry is arguably my country's primary energy industry, being both a major producer and consumer of energy, but also possessing significant potential for energy conservation. Coal mine main ventilation fans are among the biggest electricity consumers in coal mine production. Various energy-saving technologies exist for coal mine ventilation fans, but currently, the main fan regulation methods used in China fall into three categories: ventilation network resistance adjustment, fan structure adjustment, and speed regulation. However, using frequency conversion technology (VFD) for energy-saving retrofitting of existing electrical equipment is a fundamental measure to address the high consumption and low efficiency of my country's coal industry.
Coal mining follows the principle of production determined by ventilation capacity; the amount of coal mined is limited by the available ventilation, and ventilation increases with coal production. During coal mining operations, methane gas continuously emerges as coal is extracted, and the amount of methane gas emitted is directly proportional to the amount of coal extracted. Similarly, the amount of fresh air needed to ensure the normal work of each coal miner is also directly proportional to the amount of coal extracted.
In actual energy extraction, the mining area and depth will expand and deepen as the production cycle extends, thus the demand for ventilation will also change. On the other hand, as mine production continues year after year, its production capacity will decrease, and the demand for ventilation will become smaller and smaller during this stage.
This shows that changes in ventilation requirements during coal mine production provide very favorable conditions for energy conservation.
2. Introduction to Coal Mine Ventilation Systems
The main function of a mine ventilation system is to expel or dilute harmful gases and dust generated during energy extraction underground, ensuring a healthy working environment for workers. In addition, the ventilation system can regulate the temperature underground, effectively improving the working environment and thus protecting the physical and mental health of coal miners and ensuring the normal operation of mining equipment, as shown in Figure 1.
There are three types of ventilation systems. Generally, depending on the depth of the mine, the surrounding geographical conditions, and the severity of spontaneous combustion of the extracted energy, one of the following types of ventilation systems is adopted: exhaust, forced, or a combination of exhaust and forced ventilation.
The ventilation system consists of a ventilation network, ventilation power equipment, and related control facilities. Airflow enters the mine through the intake shaft, and harmful gases are discharged through the return shaft with the airflow, thus ensuring the safety of underground operations.
The coal mine ventilation network is a complex system consisting of intake shafts, return shafts, and a network of interconnected underground roadways. The network structure of the underground ventilation system changes continuously as production progresses. With the ongoing underground mining operations, there are changes in the mining area related to preparation, commissioning, termination, and succession. The mining faces also undergo continuous advancement, contraction, and succession processes. Furthermore, various engineering projects, such as the development and extension of underground roadways and changes in ventilation structures, will cause different changes in the network structure of the ventilation system. This inevitably affects underground ventilation parameters in the coal mine, such as airflow distribution and gas concentration, as shown in Figure 1.
Generally, mines are equipped with two main ventilation fans, one for operation and one for standby. There are two types of ventilation fans: single-motor fans and dual-motor fans. A single-motor fan has only one set of blades and is driven by a single motor. Common types of main ventilation fans include centrifugal fans, counter-rotating fans, and axial fans. Among them, counter-rotating fans are dual-motor fans, with each fan containing two motors, for a total of four motors (two fans). A schematic diagram of their structure is shown in Figure 2.
In coal mine production, the required air volume and pressure vary at different stages. To meet these production requirements, coal mine fans are typically adjusted using the following methods:
(1) Gate regulation;
(2) Change the fan speed;
(3) Change the angle of the fan blades;
Of the above solutions, gate regulation is the least efficient. It artificially alters the resistance curve, increasing wind resistance, and the more it is adjusted, the worse the performance becomes. Changing the fan blade angle can change the fan's characteristic curve, allowing the fan to operate at a relatively high efficiency over a wider range, thus achieving energy saving and consumption reduction. Changing the fan speed to operate at the optimal operating point allows the fan to operate at the highest efficiency over the largest range, resulting in the best energy saving effect.
To achieve speed regulation of a high-voltage motor, the following three methods are mainly used:
(1) Hydraulic coupling method. As a hydraulic speed regulation solution, it has a large maintenance workload and low efficiency.
(2) Cascade speed regulation. Because it uses a wound-rotor motor, the slip ring maintenance workload is large, which is an outdated technology.
(3) High-voltage frequency converters. With the maturity of high-voltage frequency converter technology and its increasingly high cost performance, high-voltage frequency converters are being used in more and more occasions.
3 System Solution
This article introduces two counter-rotating axial flow fans, model FBCDZ-8-NO28, with a rated speed of 740 rpm, used as the main fan in a large coal mine. Each fan is equipped with a motor with a power of 2 × 630 KW, a voltage of 10 KVAC, and a rated speed of 740 rpm. The total return air volume is 11,500 m³/min, and the total intake air volume is 11,200 m³/min. Due to the extension of the roadway or the layout of the working face, the air volume may change in real time. As one of the most critical and important pieces of equipment for safe production in coal mines, the main fan uses a Siemens perfect harmonic-free series high-voltage frequency converter as its drive system.
3.1 Equipment Composition
The components of the automatic control system for the mine's frequency converter-driven main ventilation fan are as follows:
● Fans: 2 counter-rotating fans
● Motors: 4 explosion-proof units
●Box-type damper: 2 sets corresponding to the blower
●Hydraulic power unit: 2 sets corresponding to the blower
●High-voltage power distribution system
● Switching cabinet
● Frequency Inverter: 4 sets of perfect harmonic-free frequency inverters
●Low-voltage power distribution system
●DC screen
●UPS cabinet
●PLC cabinet
● Console
● Monitoring computer
●Related Sensors
The automation and flow topology diagram of the variable frequency drive main ventilation fan is shown in Figure 3.
3.2 Main Circuit
The project uses two counter-rotating fans, each with a dual-fan, dual-motor structure, for a total of four motors. Four sets of Siemens' Perfect Harmless Series high-voltage frequency converters are used, with an input voltage of 10000VAC, an output voltage of 0-10000VAC, an output current of 70A, and a power of 800KW.
The system's power supply is divided into two parts: power supply for the mains and power supply for the control system. The power supply comes from different power switch cabinets, and the two power sources are drawn from different bus sections to ensure the reliability of the power supply.
In the circuit design of the frequency converter and the motor, a switching cabinet is added. When a frequency converter fails, the frequency converter of other backup circuits can be used for temporary drive. When the frequency converter is repaired, it can be put back into normal use according to the original circuit, as shown in Figure 4.
3.3 Control System
To enhance system reliability, this project uses a Siemens S7-412H redundant controller as the main controller, communicating with the high-voltage frequency converter via PROFIBUS-DP. The control power supply is provided by a step-down transformer in the power switch cabinet as the main power source, and by an uninterruptible power supply (UPS) as a backup.
This PLC system, together with the host computer operator station, field sensors, frequency converters, and other equipment, forms a wind turbine automated monitoring system, performing the following functions:
(1) Remote control function
The operator on the host computer control station can remotely start, stop, switch, and reverse the fan system by clicking the mouse. The system runs automatically according to the interlocking relationship of each device.
(2) The main unit and auxiliary unit are linked and the automatic control function is automatic.
The blower can be automatically interlocked with auxiliary equipment such as air dampers to achieve automatic shutdown and automatic blower switching. Automatic control reduces the blower switching time from 10 minutes with manual control to 3 minutes, significantly improving the reliability of underground air supply.
(3) Automatic reverse ventilation function
According to the operating procedures, automatic reverse ventilation can be achieved.
(4) Online monitoring and display function
The system monitors and analyzes the operating parameters of the ventilation fans online, and generates alarms. In the event of a major accident, the backup fan can be automatically activated. It monitors the equipment's operating status and parameters, as well as ventilation parameters such as air volume, air pressure, gas concentration, and airflow temperature, uploading these parameters to a monitoring computer that connects to the coal mine production scheduling system.
5) Other functions such as reporting, printing, and storage.
Analysis of key points and difficulties in the project
4.1 Regarding system reliability issues
The long-term reliable and stable operation of the main ventilation fans in coal mines to ensure sufficient airflow is a crucial aspect of coal mine ventilation safety control. Therefore, the design of the fan frequency converter system is primarily focused on ensuring the reliable operation of the fan system.
In addition to considering redundancy in the power supply, main circuit design, and control system design, the reliability of the frequency converter itself must also be extremely high. The Siemens GH180 perfect harmonic-free frequency converter system boasts exceptional safety and reliability.
Changhao's design advantages:
● It adopts a series power unit structure and a modular design for the power units, using mature and highly reliable low-voltage components such as IGBTs and diodes, resulting in extremely high reliability.
● It features power unit bypass technology with an automatic bypass time of less than 250ms, which can physically and completely bypass faulty power units. It also features three-phase line voltage balancing (center point drift) technology, allowing for different numbers of power units per phase.
● In terms of power supply adaptability, it allows for grid fluctuations of ±10%, and can still be used with derating even when the power supply voltage is as low as 55% to 90%.
● Automatic restart function: The system has an automatic restart function after a momentary power outage, which can effectively avoid electrical shocks caused by lightning or other load changes.
Source instantaneous fluctuation phenomenon
● The rotating load function allows the inverter to measure the speed of a motor that is already rotating. This enables the inverter to output a voltage at the same frequency as the motor's rotation, minimizing the impact on the motor when the inverter supplies power.
4.2 Fan switching operation
According to relevant regulations, coal mines must conduct underground ventilation reversal drills every year. This is to ensure that, in the event of a disaster, the ventilation fans can quickly reverse the ventilation, providing sufficient airflow to the underground mine. The inverter-driven motor solution can easily achieve reverse rotation. Through the human-machine interface, the main ventilation fan is first decelerated to zero, and then reversed to a negative frequency speed. This reduces the workload of manual operation and the probability of error, while also improving the safety of ventilation reversal.
Furthermore, in actual coal mine production, it is inevitable that the main ventilation fans will require periodic or irregular maintenance and shutdown operations due to malfunctions. Excessive shutdown time is generally unacceptable for high-gas and high-outburst mines. Variable frequency drive ventilation fan automation systems can conveniently achieve "one-button" shutdown operation and automatic shutdown function based on ventilation fan malfunctions. This allows for rapid switching of the main mine ventilation fan, reducing shutdown time from 10 minutes with traditional equipment to approximately 1-3 minutes.
The key to effectively shortening the time during the reversing operation lies in the coordinated control of the fan's start-up and shutdown with the damper. The operation process is briefly described below:
Reverse air operation: Adjust the deceleration of fan #1 - after fan #1 drops to 50 rpm, apply the brake to stop - adjust fan #1 to start reversing - fan reaches rated speed - fan enters normal reverse air operation state.
Reversing operation: Adjust the deceleration of fan #1 - after fan #1 drops to 50 rpm, stop the machine by applying the brake - close damper #1 - open damper #2 - start fan #2 when damper is 1/3 open - fan reaches rated speed - fan enters normal working state.
4.3 Wind speed control scheme
The variable frequency technology used in coal mine ventilation control systems allows for the control of fan rotation speed. Speed and pressure sensors are installed at the fan outlet, and the system uses signals from these sensors to assess the specific ventilation conditions in the coal mine passage. When the airflow speed fails to meet the passage's ventilation standards, the fan speed can be quickly adjusted to ensure safe production requirements in the coal mine. The ventilation control system does not have strict requirements on airflow speed; the acceptable range is quite wide, and frequent speed adjustments are generally unnecessary.
On the other hand, the control system is connected to the gas monitoring system. When gas suddenly surges out of the underground coal mining face, the gas monitoring system automatically adjusts the frequency of the frequency converter, increases the speed of the main ventilation fan, and dilutes the gas concentration in time to ensure safe production.
4.4 Socioeconomic Benefit Analysis
4.4.1 Economic Benefits of Soft Start Function
As is well known, frequency converters have excellent soft-start capabilities. Determining the optimal S-acceleration curve based on the main fan's electrical parameters achieves the best soft-start effect, resulting in low starting current and good energy savings during startup.
In addition, the soft-start function ensures that the impact on the mechanical system is minimized, reducing maintenance and repair costs.
4.4.2 Socio-economic benefits brought by speed regulation function
Most main ventilation fans currently have adjustable blades. However, if only the blade angle is adjusted, the motor speed remains at the rated speed, while the operating point of the mine ventilation system varies with different production periods. With a constant speed, a single main ventilation fan cannot meet the needs of all periods, especially since the fan operates in a low-efficiency range during certain times. In such cases, adjusting the blade angle alone still results in significant energy consumption. Therefore, variable frequency drive (VFD) technology can be used in conjunction with blade angle adjustment. Based on the required air volume and pressure underground, the fan blades are pre-adjusted to the optimal angle. Then, the optimal operating point and main ventilation fan speed are selected according to the main ventilation fan's characteristic curve. Finally, the operating frequency of the VFD is determined, achieving the most economical operating point. This is because we know that as long as the actual operating frequency is below the rated frequency of 50Hz, there is an energy-saving advantage, especially for square torque loads like fans.
The use of variable frequency speed control reduces the number of times the fan blade angle needs to be adjusted during traditional shutdowns, greatly improving the safety factor of mine ventilation.
In addition, the reduced operating speed of the ventilator improves the lubrication conditions of the operating parts of the equipment and reduces the failure of the transmission device.
4.4.3 Socioeconomic benefits of shortening turnaround time
The significantly shortened downtime of the main ventilation fan in the coal mine greatly reduces gas accumulation and gas exceedances caused by ventilation interruptions and airflow turbulence during downtime, eliminating safety hazards, improving the safety and reliability of the entire ventilation system, and effectively ensuring safe production in the coal mine, thus having positive social benefits. At the same time, by eliminating the problem of gas exceedances during fan downtime, it also saves the time, manpower, and resources previously wasted on gas venting due to gas exceedances, achieving economic benefits of savings and revenue generation.
5 Project Operation
After being upgraded and put into use at the end of 2010, the system has been operating well, proving reliable and stable. During its years of operation, the switching cabinet has never even been used. Based on the mine's current production situation, the system operates at 41Hz, with a motor power of 315KW and a current of 41A. Figure 5 shows the real-time monitoring screen of fan #2.
6. Application Summary
With the vigorous advancement of supply-side structural reform in my country, the past production methods characterized by excessive capacity and intensity of mining, coupled with a large number of underground workers and low efficiency, will gradually be optimized, leading to a gradual increase in the requirements for the mechanization and automation of coal mines. Against this backdrop, the frequency converter-driven automation transformation of coal mine ventilation systems will become a practical requirement, and the increased level of automation will provide a strong guarantee for safe production in coal mines.
In the above case, the variable frequency drive main ventilation fan automation control system mainly realizes functions such as automated operation, control, and alarm. We can further develop a ventilation fan fault early warning expert system based on this, safeguarding coal mine safety production. Utilizing real-time monitoring of various parameters by the automation system, including fan air pressure, air volume, speed, shaft temperature, ventilation temperature and humidity, and three vibration parameters, as well as motor electrical parameters, speed, shaft temperature, and three vibration parameters, and further based on the principles of ventilation fan mechanical equipment accident diagnosis, a certain mathematical model can be established to form a ventilation fan fault early warning monitoring system. This system can diagnose and analyze potential faults in advance, detect subtle changes in the degree of ventilation fan damage, and thus propose maintenance suggestions. This not only avoids unplanned shutdowns of the ventilation fan but also improves the utilization rate of the ventilation fan and the production efficiency of the mine.
