Application of High Voltage Frequency Converter on Reciprocating Air Compressor

1 Introduction

With the mature application of high-voltage frequency converters in various industries, chemical companies generally have air compressor stations that use air compressors to generate compressed gas for use in processes, instruments, etc. The pressure of the compressed gas needs to be controlled within a certain range. Both low and high pressures need to start and stop the compressor (referred to as air compressor). The air compressor of a chemical company is manually started and stopped, and due to the merger of positions, the operation The reduction in personnel has caused inconvenience. Because it is difficult to achieve a complete balance between the air supply volume and the compressor displacement, and it is difficult to manually adjust the return flow to ensure the required pressure and flow stability, the disadvantages of the system are prominent: if the air supply is not large, the gas Repeated compression, low operating efficiency, consumes a lot of electric energy, using a bypass valve to adjust the air supply, the workload is large, the air supply pressure is not well controlled, the air supply quality becomes poor, and the system safety is poor. Therefore, it is more meaningful to carry out frequency conversion technology transformation on the reciprocating compressor system.

2 Load characteristics of reciprocating compressor

Qilu Petrochemical Plastic Factory has a KBC-22X type reciprocating compressor, the adapted motor is a 200KW/6KV 8-pole motor, the rated speed is 735 r/min, and it is controlled by a direct drive method. Through comprehensive investigation and consideration, the user selected our company's JD-BP37-250F high-voltage inverter. Through debugging and application, the inverter has good safety performance, high reliability, reasonable design, long service life of wearing parts, and good starting performance. , The consumption reduction effect is obvious, and the installation, maintenance and maintenance are relatively convenient.

But the general reciprocating compressor is working, the compressor uses the crankshaft to convert the motor's rotary motion into reciprocating motion, and the torque changes with the angle of the crankshaft. In this case, the current of the motor changes with the load. Larger fluctuations. Therefore, the current problems with reciprocating compressors limit the promotion and application of variable frequency technology in compressors. The most serious ones are the torque and speed fluctuations in the low speed range, and the resulting low-frequency noise and vibration problems. At present, the application of high-voltage frequency converter technology to reciprocating compressors is even rarer. Most of the compressors currently on the market run in the middle and high speed areas, and the energy efficiency ratio is generally low.

We found in the initial commissioning: when the working frequency decreases and the speed becomes low, the speed fluctuates. The lower the speed, the greater the fluctuation. The fluctuation at 25Hz is shown in Figure 1, and the fluctuation range is 40r/min. , Indicating that the above situation does exist:

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Figure 1 25Hz speed fluctuation

In order to operate reliably, the fluctuation of the speed must be reduced, that is, the fluctuation of the speed caused by the load torque pulsation and the electromagnetic torque pulsation of the motor must be effectively suppressed. Regarding the suppression of speed pulsation, after consulting some data, some scholars have conducted relevant research and achieved certain results. They proposed the method of combining iterative learning control and PI to suppress the periodic pulsation of the speed, or use the speed signal to reverse the rotation speed. The torque feedforward compensation effectively suppresses the periodic pulsation of the speed, but the control methods in it all require an encoder to provide the current accurate speed signal of the motor, and the control is also more complicated, so it cannot be applied to the general compressor system.

3 Application of wind and wind high voltage inverter

The structure of the JD-BP37 series of high-voltage variable frequency speed regulation system is shown in Figure 2. It consists of a phase-shifting transformer, a power unit and a controller. The 6KV/250KW frequency converter has 15 power units, and each 5 power units are connected in series to form a phase.

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Figure 2 System structure

The core of the controller is realized by a high-speed 32-bit DSP and HMI coordinated operation, and a well-designed algorithm can ensure that the motor achieves the optimal operating performance. HMI provides a friendly all-Chinese monitoring and operation interface, and can realize remote monitoring and networked control. The controller is used for the logic processing of the switch signals in the cabinet and the coordination with various operating signals and status signals on the spot, which enhances the flexibility of the system.

Each power unit is completely identical in structure and can be interchanged. The circuit structure is shown in Figure 3. It is a basic AC-DC-AC single-phase inverter circuit. The rectifier side is equipped with six diodes to achieve three-phase full-bridge rectification. The inverter bridge is controlled by sinusoidal PWM. Each power unit is exactly the same and can be interchanged. This is not only convenient for debugging and maintenance, but also very economical for backup. If a unit fails, the output of the unit can be automatically short-circuited and adjusted The machine can temporarily derate work until it stops running slowly.

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Figure 3 Unit structure

In addition, the multi-channel optical fiber communication technology is used between the controller and the power unit, and the low-voltage part and the high-voltage part are completely and reliably isolated. The system has extremely high safety, and has good anti-electromagnetic interference performance, and the control power supply of each power unit A unified controller independent of the high-voltage system is adopted to facilitate debugging, maintenance, and on-site training, which enhances the reliability of the system. The controller has a power supply system independent of the high-voltage power supply. When high-voltage is not applied, the waveform of each point of the equipment is basically similar to that of high-voltage, which brings great convenience to the reliability, debugging and training of the whole machine.