Frequency conversion upgrade of main fan at Yipinglang Coal Mine, Dongyuan Company

Guan Xianzong, Yipinglang Coal Mine, Yunnan Dongyuan Coal and Power Co., Ltd.

Tian Lirun, Shandong Xinfengguang Electronic Technology Development Co., Ltd.

Abstract   Abstract: This paper analyzes the energy-saving principle of variable frequency speed regulation for main ventilation fans in coal mines, considering their workload characteristics, and elaborates on a high-voltage variable frequency speed regulation control scheme applied to main ventilation fans. The results of variable frequency retrofitting of main ventilation fans demonstrate significant energy-saving effects and high economic and social benefits.

Keywords: Main fan, high-voltage frequency converter, speed regulation, energy saving

1 Introduction

The selection of main ventilation fans in mines should meet the maximum air volume requirements for the entire service life of the mine. From mine construction to production and even mine decommissioning, the required air volume varies significantly at each stage. To meet the maximum air volume requirements within the mine's service life, a larger air volume margin is generally chosen, resulting in wasted electricity when the required air volume is lower. For fans operating for extended periods, energy consumption is staggering. Therefore, finding reliable methods to adjust air volume that both meets the mine's air volume requirements and achieves energy conservation is of great significance.

Currently, there are basically two methods for adjusting the air volume of mine ventilation fans in my country: mechanical adjustment and electrical adjustment. Mechanical adjustment involves adjusting the blade angle and the air network resistance. Adjusting the blade angle can adjust the air volume, but the fan efficiency is low. Adjusting the network increases system resistance, which can adjust the air volume but does not save energy. Electrical adjustment, also known as electrical speed control, involves adjusting the number of pole pairs of the motor or the power supply frequency, i.e., variable frequency speed control. Another method is cascade speed control. Of these two methods, cascade speed control and variable frequency speed control are the best for adjusting air volume.

The initial ventilation system at the Yipinglang Coal Mine of Yunnan Dongyuan Coal and Power Co., Ltd. was a central parallel system. Later, the south and north wing ventilation shafts were developed, adopting a diagonal ventilation system. The south wing ventilation shaft is equipped with Shenyang Blower Works 2K60-55-18 axial flow fans, driven by JRQ1410-6 high-voltage wound-rotor motors. Two fans are used, one in operation and one on standby. Currently, with the extension of the mine's production system, the main fan of the south wing ventilation shaft has begun operation. Because the south wing ventilation shaft fan operates at its rated speed, airflow adjustment relies on stepped adjustments of the fan blade angle. The fan blades can only be adjusted to a low angle, resulting in very low operating efficiency and a significant waste of electrical energy. This is especially problematic given frequent changes in the ventilation network, as adjusting the fan blade angle is difficult to adapt to changes in airflow demand. Therefore, implementing effective fan speed regulation and energy-saving measures is crucial. The parameters of fans #1 and #2 are basically the same; Table 1 shows the basic parameters of the #1 main fan.

2. Research on Energy Saving through Variable Frequency Speed ​​Regulation of Main Fan

2.1 Feasibility Analysis of Energy Saving through Variable Frequency Speed ​​Regulation of Main Fan

In mine ventilation design, it's sometimes difficult to calculate the resistance of the pipe network. Typically, the system's maximum air volume and pressure margin are used as the basis for selection. However, the models and series of fans are limited. Often, when a suitable fan model cannot be selected, an older model is chosen, typically with a margin greater than 20%-30%. Therefore, when these fans are running, the only way to meet the air volume requirements of the production process is to adjust the opening of dampers or duct baffles. Since the mechanical characteristics of a fan are square torque characteristics, adjusting the fan air volume by adjusting the opening of dampers or duct baffles is called throttling regulation. During throttling regulation, the inherent characteristics of the fan remain unchanged; simply closing the damper or baffle opening artificially increases the resistance of the pipe network, thereby increasing system losses and hindering energy-efficient fan operation. Using a variable frequency speed control device, the fan speed is changed, thus altering the fan air volume to meet the needs of the production process. Operating the fan in speed control mode consumes the least energy and yields the highest overall efficiency. There are various speed regulation methods for AC motors, with frequency conversion speed regulation being the most efficient and optimal solution. It can achieve stepless speed regulation of the fan and can be easily integrated into a closed-loop control system to achieve constant pressure or constant flow control.

2.2 Energy-saving principle of variable frequency speed regulation for main fan

Figure 1 shows the characteristic curves of the main fan's air pressure-air volume H-air volume Q.

N1 represents the characteristics of the fan when it is running at its rated speed; N2 represents the characteristics of the fan when it is running at N2 speed with reduced speed; R1 represents the resistance characteristics of the fan when the pipeline resistance is at its minimum; and R2 represents the resistance characteristics of the fan when the pipeline resistance increases to a certain value.

When the fan operates on the pipeline characteristic curve R1, the operating point is A, with flow rate and pressure of Q1 and H1 respectively. The power required by the fan at this point is proportional to the product of H1 and Q1, i.e., proportional to the area of ​​AH1OQ1. Due to process requirements, the airflow needs to be reduced to Q2. This is actually achieved by increasing the pipeline resistance, shifting the fan's operating point to point B on R2, increasing the air pressure to H2. At this point, the power required by the fan is proportional to the product of H2 and Q2, i.e., proportional to the area of ​​BH2OQ2. Clearly, the power required by the fan has increased. While this adjustment method is simple, it consumes a lot of power and is not energy-efficient, trading high operating costs for a simple control method.

If variable frequency speed regulation is used, the fan speed decreases from N1 to N2. At this time, the operating point moves from point A to point C. The flow rate is still Q2, and the pressure decreases from H1 to H3. At this time, the power required by the fan after variable frequency speed regulation is proportional to the product of H3 and Q2, that is, proportional to the area of ​​CH3OQ2. As can be seen from the figure, the reduction in power is obvious.

3. High-voltage frequency converter control scheme design

3.1 Selection of High Voltage Frequency Converter

The main fan motor in the mine only operates during the motoring process and does not require electrical braking or energy feedback, thus operating in a two-quadrant mode. Therefore, two sets of Fengguang brand JD-BP37-400F high-voltage frequency converters manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd. were selected to modify the south wing fan, and the modification achieved the expected results.

(1) System composition

Its system structure is shown in Figure 2. The Fengguang brand high-voltage frequency converter is a high-to-high voltage source type, consisting of a phase-shifting transformer, power units, and a controller. The front end is powered by a multi-winding isolation phase-shifting transformer, with 15 secondary windings using 30-pulse rectification. Each phase of the power unit is composed of low-voltage power units connected in series, with 5 power units per phase, totaling 15 units across the three phases. The controller uses a high-speed microprocessor for control and communication with sub-microprocessors. The Fengguang high-voltage frequency converter adopts a modular design, offering good interchangeability, simple maintenance, low noise, low harmonic content, and does not cause torque pulsation in the motor, thus requiring no special motor specifications.

2) Power unit circuit

Its circuit structure is shown in Figure 3, which is a basic AC-DC-AC single-phase inverter circuit. The rectifier side uses six diodes to achieve three-phase full-wave rectification. The inverter side achieves inversion through sinusoidal PWM control of the IGBT inverter bridge. If a unit fails, its output can be automatically bypassed, without affecting the continuous operation of the entire unit. Each power unit is identical and interchangeable, which not only facilitates debugging and maintenance but also makes backup very economical.

(3) Input side structure

The input side is powered by a phase-shifting transformer to each power unit, each of which bears the rated motor current, 1/5 of the phase voltage, and 1/15 of the output power. Each of the 15 units has its own independent three-phase input winding on the transformer. The power units and the transformer secondary windings are mutually insulated. The secondary windings use an extended delta connection to achieve multiplexing and reduce the harmonic content of the input current. This multi-stage phase-shifting rectification method significantly improves the grid-side current waveform, bringing the grid-side power factor under load close to 1 and resulting in low input current harmonic content. The measured total harmonic content of the input current is less than 3%.

(4) Output side structure

The output side consists of two output terminals of each unit connected in series, providing power to the motor in a three-phase star connection. By recombining the SPWM waveform of each unit, a stepped PWM waveform as shown in Figure 4 can be obtained. This waveform has good sinusoidal properties and a small dv/dt, which reduces damage to the insulation of cables and motors. It eliminates the need for output filters, allowing for long output cables and eliminating the need for motor derating, making it suitable for retrofitting older equipment. Simultaneously, it significantly reduces motor harmonic losses, eliminating the resulting mechanical vibrations and reducing mechanical stress on bearings and blades.

Stepped PWM waveform of phase voltage output from frequency converter

(5) Controller

The controller core is implemented using high-speed DSP computing, and the carefully designed algorithm ensures optimal motor performance. The human-machine interface provides a user-friendly, fully Chinese monitoring and operation interface, while also enabling remote monitoring and networked control. The controller handles the logic processing of switch signals within the cabinet and coordinates with various on-site operating and status signals, enhancing system flexibility. The controller and its control unit boards utilize large-scale integrated circuits such as DSPs and FPGAs, along with surface-mount technology, resulting in extremely high system reliability.

The controller and power units utilize multi-channel fiber optic communication technology, achieving true electrical isolation between the low-voltage and high-voltage sections. This results in extremely high system safety and reliability, along with excellent electromagnetic interference resistance. Furthermore, each power unit's control power supply employs a unified controller independent of the high-voltage system, facilitating debugging, maintenance, and on-site training, thereby enhancing system reliability.

3.2 Special functions of high-voltage frequency converters

In the process of variable frequency speed regulation of the main fan in the mine, the high-voltage frequency converter, in addition to meeting the general technical standards, also has special functions to meet the operation of mechanical equipment such as fans.

(1) Flying car start function

When the main fan in a mine is stationary, the frequency converter can apply a load to the motor at a set starting frequency, accelerating it until it reaches the set upper limit frequency. However, in actual operation, the main fan needs to start within a certain time. It doesn't start from a standstill (e.g., due to air leakage affecting its rotation, or after being manually stopped and needing to restart at a certain speed), but rather at a specific speed. If a stationary starting frequency is applied at this speed, the frequency converter, without a speed-up start function, will trip due to overcurrent. Therefore, the frequency converter must have the function of tracking the real-time motor speed and adjusting the applied frequency.

"Flystart" of a high-voltage frequency converter occurs when the motor stator is disconnected from the frequency converter or the power grid, the motor stator is "passive," and the motor rotor is rotating, but the speed is random and uncertain. The high-voltage frequency converter is then connected to the motor stator, which transforms the motor stator from "passive" to "active," and the rotating magnetic field of the motor stator is created from nothing. Finally, the rotating magnetic field of the motor stator drives the motor rotor to enter normal driving mode.

As we know from the principles of motor operation, when the speed of the rotating magnetic field of the motor stator differs significantly from the speed of the motor rotor (i.e., when the slip is large), a large current will be generated, but the electromagnetic torque will be small. For example, when a motor is directly started at full voltage under power frequency, the stator current will reach 5 to 7 times the rated value. However, the capacity of a high-voltage frequency converter is generally not selected to be 5 to 7 times the rated current of the motor. A similar situation occurs if the high-voltage frequency converter has a high output frequency (50Hz) during a "fly start" while the motor rotor speed is very slow, inevitably leading to overcurrent tripping. Conversely, if the high-voltage frequency converter has a low output frequency during a "fly start," and the speed of the rotating magnetic field of the stator is lower than the speed of the motor rotor, the motor is in a generating state. The motor rotor will send energy back to the stator side to charge the frequency converter capacitor, causing the frequency converter to trip due to overvoltage pumping from the capacitor. Therefore, the key to the success of a high-voltage frequency converter's "fly start" is that the output frequency and the rotor speed (frequency) are the same. Since the motor rotor frequency is random, a rotor frequency search must be performed. This means that during a "fly start," the motor rotor frequency is searched first. Once the rotor frequency is found, the inverter uses that frequency as its output frequency. This prevents both overcurrent and capacitor voltage surge overvoltage.

On-site commissioning results show that the fly-start function is safe and reliable, and starts smoothly. The inverter enters the frequency search time at 54Hz, and the motor can start directly at the frequency between 10 and 13Hz when the frequency drops, with low starting current and stable speed.

(2) Star point drift function

High-voltage frequency converters have numerous power units, each of which is susceptible to failure. When one unit fails, it can be temporarily bypassed and derated. However, the voltage of the damaged phase will inevitably drop, creating an imbalance with the other two phases and resulting in an unbalanced current at the motor terminals, preventing long-term operation. If the corresponding power units in the other two phases are also bypassed, although voltage balance is achieved, the converter's output power is reduced too much, failing to meet the reliable operation requirements of the mine's main fan. Therefore, the frequency converter for the mine's main fan must have a star-point drift function.

In the design of the main fan inverter, a star-point drift scheme was adopted. Under normal conditions, the three-phase outputs a, b, and c of the inverter are balanced, with the neutral point being the star point. When one unit in phase a fails, the system shifts the star point, performs calculations on phases b and c, and the vector sum output matches that of phase a, ensuring minimal reduction in inverter output power and maintaining maximum output voltage. When two or three units fail, the system performs similar calculations to ensure consistency, improving system reliability.

3.3 Design of Variable Frequency and Power Frequency Main Circuit Schemes

The mine's main ventilation fan has one operating unit and one standby unit, requiring operation such as reversing and tilting, with one fan always in continuous operation. Downtime is strictly prohibited from exceeding 10 minutes. During variable frequency speed control, if the frequency converter trips due to a fault, the operating fan also has the function of automatically switching to mains frequency power supply. Therefore, the electrical main circuit design of the mine's main ventilation fan after being connected to the frequency converter controller is shown in Figure 5.

(1) Description of main components of the electrical main circuit

In the main electrical circuit diagram, ZJC and FJC are the main fan forward and reverse rotation contactors under power frequency conditions, enabling switching between the two high-voltage circuits; ZJC and FJC are the main fan forward and reverse rotation contactors under power frequency conditions, and they are electrically and mechanically interlocked; BJC is the high-voltage inverter feeder contactor; XJC is the isolation contactor, mainly to prevent high voltage from entering the inverter power unit and damaging the IGBT if the gap between the vacuum tubes of ZJC and FJC is too small when the fan is running at variable frequency; BJCC1 and BJCC2 are variable frequency operation contactors, with six-pole contacts, and every two contacts are connected in series to form a phase circuit, mainly to prevent high voltage from entering the inverter power unit if the gap between a single contact of BJC is too small when the fan is running at power frequency; JC1 and JC2 are rotor frequency-sensitive reactor disconnection contactors. Two sets of high-voltage inverters are used to control the two fans separately, one in operation and one on standby, designed as a one-to-one configuration.

(2) Electrical scheme for power frequency operation

When the main fan needs to run under power frequency conditions, BJC, BJCC1, and BJCC2 are disconnected, and ZJC or FJC and XJC are connected. Fan #1 or #2 is powered on and starts. JC1 and JC2 disconnect the rotor frequency-sensitive reactor in two stages, first and second stage, according to the set start time, to complete the fan start-up process.

(3) Electrical scheme for variable frequency operation

When the main fan needs to run under frequency conversion, ZJC, FJC and XJC are disconnected, BJC, BJCC1 or BJCC2 are connected, and JC1 is connected at the same time. Fan #1 or #2 starts under frequency conversion at the set frequency and accelerates to the set upper limit frequency within the set time to complete the fan start-up process.

(4) Automatic switching scheme for frequency converter faults

To prevent prolonged downtime caused by inverter failures during main fan operation, the system is designed with an automatic switching scheme between inverter and mains frequency power supply. When the inverter trips due to a fault, BJC, BJCC1, or BJCC2 disconnects, ZJC (FJC in reverse ventilation) and XJC automatically connect, JC1 closes, automatically short-circuiting the rotor frequency-sensitive reactor. The system then applies mains frequency power to the currently operating fan, allowing the fan motor to restart directly at its current speed. This reduces downtime and enables seamless switching between inverter and mains frequency power supply circuits, ensuring continuous mine ventilation.

4. Operational Effects of Variable Frequency Drive Retrofit

After the main fan was upgraded with a frequency converter, the high-voltage frequency converter was successfully put into operation in February 2012 and has been running normally ever since, achieving the expected goals of the upgrade. In summary, the frequency converter upgrade has the following advantages:

(1) Significant energy saving effect. Before the renovation, the operating efficiency of the fan in the south wing ventilation shaft was less than 50%. After adopting the high-voltage variable frequency speed control device, the fan efficiency increased to more than 78%, and it was calculated that the annual energy saving was more than 650,000 kWh.

(2) After the frequency converter is put into operation, the air inlet gate of the fan is fully opened. The frequency converter sets the frequency according to the production needs and adjusts the speed of the motor to realize automatic control of the fan speed.

(3) Reduced impact on the power grid. When the motor is started directly at the power frequency, it will generate 4 to 7 times the rated current of the motor. This current value will greatly increase the electrical stress of the motor windings and generate heat, thereby reducing the life of the motor. However, after using frequency conversion, the motor achieves soft start and can start at zero speed and zero voltage (of course, the torque can be appropriately increased) until the operating current is reached, with almost no impact on the power grid.

(4) Reduced maintenance. With the adoption of variable frequency speed control, the vibration, noise and temperature of the fan are significantly reduced due to smooth start-up and low-speed operation, which correspondingly extends the life of many components, especially seals and bearings. This effectively extends the maintenance cycle, reduces the amount of maintenance, and saves a lot of maintenance costs.

(5) The frequency converter has multiple protection functions, which are very comprehensive. Compared with the original old system, the frequency converter has multiple protection functions such as overcurrent, short circuit, overvoltage, undervoltage, phase loss, and temperature rise protection, which protects the motor more accurately.

5. Conclusion

After implementing variable frequency speed control in the south wing ventilation shaft of the Yipinglang Coal Mine of Yunnan Dongyuan Coal and Power Co., Ltd., not only were ventilation safety requirements met, but considerable economic benefits were also achieved. Currently, many main fan units in my country's coal mines have excess capacity but operate inefficiently for extended periods, resulting in significant energy waste. AC variable frequency drives (VFDs) are widely welcomed by users due to their strong applicability, high reliability, ease of operation, and high energy efficiency.

About the Author

Guan Xianzong, male, is an electromechanical engineer working at Yipinglang Coal Mine of Yunnan Dongyuan Coal and Power Co., Ltd.

Tian Lirun, male, is a technical support engineer employed by Shandong Xinfengguang Electronic Technology Development Co., Ltd.

 Address: Wenshang Economic Development Zone, Shandong Province

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 Contact person for this article: Guo Peibin

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