Design and Analysis of High-voltage Variable-frequency Energy-saving Modification of Mine Screw Air Compressor System

Abstract : This paper introduces the reform plan of high-voltage frequency converter used in coal mine underground air compressor of Henan Hebi Zhongtai Mining Co., Ltd.; introduces the basic structure and working principle of multi-level high-voltage frequency converter drive system, application notes and experimental data, and finally Gives energy-saving analysis.
Keywords :multilevel; frequency converter; energy saving; closed loop; sinusoidal pulse width modulation
Design and Analysis of High Voltage Converter in Coil Mine Screw Air Compressor
NAN Yong-Hui, FAN Jin-Zhi, LI Jun-Wei, PENG Li
(Zhuzhou National Research Center of converter Co.Ltd, Zhuzhou, Hunan 412001, China)
The waveforms of the drive system experiment are also described and analyzed. The Finally the energy-saving is analyzed
Keywords: multilevel;converter;energy-saving;closed loop ;SPWM
1. Introduction The motor system energy-saving project is one of the ten key energy-saving projects of the “11th Five-Year Plan” initiated by the National Development and Reform Commission. In many large coal mines, power companies and other enterprises, there are many high-power screw air compressors to provide pneumatic pressure for downhole pneumatic equipment, and its annual power consumption is considerable. Because there are many equipments in the downhole, and the wind time is not fixed, the air volume is unstable, so the air compressor must run continuously. When the air volume is small and the air compressor is operated for a long period of time under low load, the power loss increases and the wear of the equipment is exacerbated, which increases the operating cost. If the equipment is shut down or some of the equipment is shut down when the load is small, the operation will be frequent, the equipment life will be shortened, and the normal operation of the wind equipment and pneumatic tools will be affected.
The scheme of using a cascaded multi-level high-voltage frequency converter to drive a high-voltage and high-power motor has the advantages of no impact on the grid, high reliability, significant energy saving effect, small output harmonics, high input power factor, and therefore has become the mainstream of development.
The high-voltage frequency converter is the transmission system of two sets of screw-type air compressors for the Hebi No. 4 Mine of Henan Coal Chemical Group. It mainly provides pneumatic power for the downhole equipment. It was originally used for direct start of the power frequency, and the long-term power frequency is running at a constant speed. There is a large starting current. Due to the serious waste of energy and unstable gas supply pressure, the frequency conversion was adopted to improve economic and social benefits. This article mainly introduces the frequency conversion retrofit project.
2. The necessity of frequency conversion of air compressor
2.1 The basic working principle of the screw air compressor The main working methods of the screw air compressor are start, stop, load, and unload. The intake valve is equipped with a load valve and the inlet valve is controlled by loading a solenoid valve. When the air compressor is running, if the discharge pressure is lower than the lower limit of the set target pressure, the air compressor controller energizes the loading solenoid valve, opens the inlet valve, and the air compressor is in full load; if the exhaust pressure Above the upper limit of the set target pressure, the air compressor controller commands the solenoid valve to be de-energized, thereby closing the inlet valve, and the air compressor is in a no-load unload state. At this time, the air compressor is actually idling.

2.2 The original power frequency drive equipment program analysis system is two sets of the same Shanghai Fusheng rotary screw air compressor drive system, driven by the asynchronous motor air compressor rotation, air compressor and the control system compressed air downhole pneumatic Equipment gas supply.
Induction motor parameters: rated voltage 6KV, rated current 43A, rated power 375KW.
The air compressor system operates as follows:
(1) One device is running and one device is standby; if a device fails, it will exit the operation and start another device.
(2) In order to improve the service life of the air compressor transmission equipment and facilitate maintenance, the two equipments are gated once every three days to switch the operating equipment to the standby equipment and the standby equipment to the operating equipment.
(3) Pneumatic equipment can operate under the pressure of more than 0.6MPa. Downhole equipment has several peaks for gas use. When the gas is low, the downhole pressure is high. The air compressor control system adjusts the pressure through loading and unloading, and there is a regulation speed. With the disadvantages of slower, low adjustment accuracy and frequent solenoid valve operation, the actual daily output pressure is between 0.60MPa and 0.75MPa.
(4) The motor has been running at a constant power frequency of 50Hz. The average rated current of the motor is 38A per day. The starting instantaneous current is about 200A, and the average power is about 310KW.
According to the principle of motor drive control, the output power is proportional to the two main factors: output speed and output torque (torque and load pressure are proportional). The original motor drive system has a downward adjustment in speed and pressure, there is a huge energy waste. The use of variable frequency speed regulation for continuous pressure and speed can greatly save energy.
3. The principle and application of high-voltage frequency converter system are designed to be redundant. We choose our company's GVF-500-6-F2 cascaded multi-level high-voltage frequency converter with a rated capacity of 500 kVA and 6 power cells per phase. According to the characteristics of the mine pneumatic equipment system, the system adopts pressure closed loop, the given pressure is 0.62MPa, the inverter has a built-in digital PID regulator, and only the pressure given inverter can be set on the interface of the inverter to adjust the output frequency to stabilize pressure.
3.1 Inverter retrofit electrical scheme The main electrical wiring diagram of the inverter is as follows:

Figure 1 main electrical wiring diagram of the inverter
Fig1 main electric circuit of inverter

Note: 1QF is 1# startup cabinet circuit breaker, 2QF is 2# startup cabinet circuit breaker, and QS1~QS6 are manual disconnectors. QS5 and QS2 are mechanically interlocked; QS4 and QS1 are mechanically interlocked; QS6 and QS3 are mechanically interlocked; QS5 and QS4 are interlocked by program locks; QS2 and QS1 are interlocked by program locks.
Select 1 # high-voltage motor inverter operation requires switching operation sequence: off QS4, combined QS5, combined QS6, 1QF.
When the inverter is equipped with 1# motor, if the inverter needs to be operated and repaired, then disconnect 1QF, combine QS1, and combine 2QF to make 2# frequency drag.
With this variable frequency speed regulation scheme, the reliability of the equipment can be further improved.
The 6kV high-voltage frequency converter consists of 6 power units per phase (only 2 modules are shown in Figure 2 per phase). The multi-level high-voltage inverter is mainly composed of a bypass switchgear (as described in Fig. 1), a phase-shifting transformer, a power unit, and a controller. As shown in Figure 2.

Figure 2 cascaded multi-level converter system structure
Fig2 Frame of the cascaded multilevel inverter

The phase-shifting transformer has 6 windings per phase (only 3 are drawn in each phase in Figure 2), and the windings are staggered by 10 degrees in order to achieve 36-pulse rectification. The output of the phase-shifting transformer is connected to each power unit, and the controller drives each power unit through the optical fiber. (Figure A phase each unit connected to the phase-shifting transformer, B, C the same reason). Each power unit consists of a three-phase bridge rectifier and an H-bridge inverter. It can be seen that each unit's rectified DC voltage is independent, which is conducive to the control of IGBTs. Each phase is superimposed by a low voltage into a high voltage. Each power unit also has a bypass control (not shown in the figure). When a problem occurs in a unit, the unit is bypassed, and the remaining units can continue to work, balancing the output line voltage by balancing algorithm control. This enhances the reliability of the system.
3.2 Basic control principle of multi-level inverter The speed formula of asynchronous motor is: (2)
Where: n is the motor speed, f is the stator frequency, s is the motor slip, P is the motor pole pair.
For asynchronous motors, P is generally fixed, s adjustable range is very small, f variable range is very large. Therefore, the adjustment of f is now the most important speed adjustment method. According to the motor theory, in order to maintain a certain torque capacity, it is necessary to maintain the magnetic flux density is basically constant, so it is necessary to maintain the ratio of stator voltage and stator frequency is basically constant, that is, variable voltage control (VVVF).
Multi-level inverter generally adopts carrier phase shift SPWM control technology. The basic principle is that each phase consists of N cascade units, and the phase angle of triangle carrier of each unit is 360/N in sequence, and the same phase uses the same sine. Modulate the signal and use the waveform generation method in the SPWM technology and the waveform superposition structure in the multiplexing technology to generate the SPWM waveform so as to realize the multi-level SPWM output; the reference waves between the A, B, and C phases differ by 120 in sequence.
For inverters with each phase unit cascaded by N module units, the maximum number of phase voltage levels is 2N+1, and the maximum number of line voltage levels is 4N+1. Therefore, the more output modules the output waveforms are, the closer to the sine wave is. This helps reduce dv/dt and harmonics. Figure 3 shows the 6-cell cascade output line voltage. It can be seen that the output waveform is very close to a sine wave.

Figure 3 46Hz output voltage waveform
Fig3 output voltage waveform when the frequency is at the 46Hz

3.3 Frequency conversion design and use instructions (1) After analysis and test, set the high pressure frequency conversion system to pressure closed loop, the user enters a given pressure at the interface (recommended value of 0.62Mpa), the inverter built-in digital PID regulator, the output of the regulator is Given the output frequency of the motor, the pressure is adjusted by frequency modulation. When the actual pressure is small, the negative feedback regulator increases the output frequency to increase the output pressure to maintain the given pressure. When the actual pressure is high, the negative feedback regulator reduces the output frequency to reduce the pressure to reduce the energy consumption.
(2) The setting of the acceleration/deceleration time must satisfy both the process requirements and the system reliability. If the acceleration time is set too small, a large slip in the process of starting or accelerating will cause excessive current. Similarly, if the deceleration time is set too small, during the deceleration process, when the motor is running in the generator state, reverse charging to the inverter may cause the module capacitor voltage to be too high, affecting the system reliability. After tests and analysis, the local system acceleration time range: 60S to 150S; deceleration time: from 120S to 300S.
(3)Because there is a pressure judging device before the air compressor control system is started, the inverter is generally allowed to start until the pressure is less than or equal to 0.45 MPa.
(4) The order of isolation switch and circuit breaker must be correct; after the frequency conversion is adopted, the motor shaft temperature decreases (the original shaft temperature averages 95 degrees, and the average temperature after the transformation is 80 degrees) as the motor energy consumption decreases. Other friction losses decrease. It is very beneficial to improve the life of the equipment, so the switching period of the two air compressors is recommended to be changed to 10 days.
4. System test and energy saving calculation The whole system was debugged on the spot and the test waveforms were collected.
4.1 System test
Fig. 4 is the current waveform of asynchronous motor starting process after adopting frequency conversion transformation, Fig. 5 is the steady state 36Hz electric current output waveform.

Figure 4 Start-up process output dynamic current waveform
Fig4 output current waveform during the startup

Figure 5 Output current waveform at steady state 36Hz
Fig5 output current waveform when the frequency is at the 36Hz

Figure 4 shows that the start-up current is small, and there is a large amplitude in the middle (peak value is about 70A). The vibration is because the motor generally has low-frequency vibration phenomenon. The motor has mechanical vibration between 7Hz and -12Hz. . Because the inverter has internal current feedback suppression measures to make the motor run in a very safe range, it is much better than the original direct power frequency startup process.
Fig. 5 is the electric current waveform that the electric machinery runs steadily at 36Hz, the electric current RMS value is 29.9A, can find out the electric current sine is very good, output harmonic wave is very small, very favorable to the electrical machinery.
4.2 Energy Saving Calculation The original power frequency operation of the system: the air compressor pressure is between 0.62MPa and 0.75MPa, the average rated current of the motor is 38A a day; the average power is 310KW
After the inverter is used, the pressure is basically 0.6MPa--0.65MPa, the motor average current is 32A, the input average current is 23A, and the average input power is 200KW.
The frequency converter calculates one year according to the work 350 days, then one year total energy-saving:
(310-200)*24*350 = 924,000 kilowatt-hours are calculated at 0.5 yuan per kilowatt hour, which can save electricity for 462,000 yuan a year.
5. Conclusion Practice has proved that the use of high-voltage frequency converter for mining screw air compressor has high reliability, significant energy-saving effect, so that the air compressor system achieves a soft start, greatly reducing the equipment maintenance, maintenance costs and other advantages, has obvious Direct and indirect economic benefits, in line with the interests of the country and the enterprise, have broad application prospects.
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