1 process introduction
In the Beiliang Phase 3 system, the air compressor station is equipped with 4 conventional screw air compressors, model ML110. The original design is 4 air compressors running independently, and the working mode is three-in-one. The production process is as follows: the air compressed by the air compressor is separated by an oil separator, cooled by an air cooling device, stored in a gas storage tank, and then distributed to the gas equipment through the main pipeline. Regardless of the change of the gas load at the production site, the three air compressors running online are all running at the same time. The adjustment of the gas load of the field equipment is adjusted by the gas storage tank and the pipeline baffle. The backup machine is also limited to one online air compressor. Switching when a fault occurs.
Although the control system is simple in the current system, there are many problems in the actual operation: when any online air compressor fails, the standby air compressor has a slow switching action, and the system has poor responsiveness; 3 online air compressors The machine can not take a break, it is easy to cause fatigue operation, and the standby machine is waiting for a long time. The mechanical or oil circuit is prone to failure, and the overall system operation safety factor is low. After calculation, the rated gas consumption (air pressure) of the production system is 3 sets. The design richness of the machine is about 20%. As the production gas load changes, the margin amount increases. This part of the energy is lost through the regulating valve or pipeline baffle of the gas storage tank, which makes the system uneconomical.
2 Implementation of safety and economic operation control system
2.1 The composition of the control system
The drawbacks of the original system can be circumvented by the centralized control system using PLC and variable frequency control technology, and the safe and economic operation of the air compressor unit can be realized. The control system is mainly composed of one main control PLC and one inverter. The working principle of the system is as follows: The centralized planning of the air compressors consisting of 4 air compressors is controlled by one PLC controller, and three online operations are maintained. One of them is controlled by the speed control of the PLC and the other is standby. The PLC collects the pressure signal of the main air supply pipeline, calculates, compares, controls the speed control air compressor, performs pressure on it, and adds and unloads control to maintain the system output stability. At the same time, calculate the running time of 4 air compressors, automatically switch and rotate, and realize the cycle operation of 4 air compressors.
2.2 hardware design
2.2.1 Inverter
The FDTA plastic/air compressor-specific VFD-G inverter is used. The inverter has a unique overload capability (150% 60S), built-in PID feedback function, energy-saving operation and power consumption record. In particular, the inverter "torque gain compensation", "slip gain compensation" and the higher "acceleration overcurrent stall prevention" function meet the requirements of the project speed control air compressor load change follow-up requirements. 2.2.2 PLC programmable controller
According to the centralized control function of the unit and the requirements of I/O, DVP48EHOOR2 economical PLC controller is selected. The host also has an RS-485 communication port, which can be connected to Delta VFD inverters. The PLC writes the frequency into the inverter through communication; at the same time, the PLC supports the STL instruction, which is used to make the instruction of the sequential function diagram (SFC, SequentialFunctionChart) design syntax, which is convenient for planning the program flow and making the programming clear and easy to read.
2.3 software design
The programming policy is based on the flow control program as the main program, the communication between the PLC and the inverter, the air compressor unit rotation control, and the alarm accident processing as a subroutine. We made communication, air compressor rotation control and alarm accident processing into standard program modules to enhance program readability.
2.3.1 Air compressor unit rotation control subroutine
(1) DD system initialization (start). Allocate 4 Word memory (air compressor number) D11, D12, D13, D14, and initialize 4 Word contents, corresponding to the status bits of KM1, KM2, KM3, KM4. Define D15 as the fault bit. When fault occurs, set 1. Set D21 as the shift action flag and D22 as the timing flag. The DD system is initialized (end). (2) DD is executed once every 120h (start). Shift 4 Words with a loop shift instruction (note: you must move 1 Word at a time). After the first shift, the data should correspond to the state of KM2, KM3, KM4, and KM1. The next time should correspond to KM3, KM4, KM1, KM2, and so on. DD is executed once every 120h (end). (3) DD is executed once per fault (start). The air compressor switching program is executed once.
2.3.2 PLC and inverter communication subroutine
The communication is mainly to realize the modification of the start and stop and the working frequency of the control inverter. The initial value of the working frequency has been preset in the memory of the PLC system.
(1) DD system initialization (start). 1. Set the communication format MODBUSRTU, 19200, 8, N; the format of the RS485 communication port of the inverter is the same as the PLC communication format. 2. Determine the communication flag M1127, the communication error corresponding flag register: M1129, M1140, M1141 on/off state count, establish a reliable connection of communication. The DD system is initialized (end).
(2) DD performs communication (start) by comparing the value of c0. When M0=on, MODRDK1H2102K2 is executed, and the PLC reads the main frequency and output frequency of the VFD-G inverter. The frequency is stored in D1073~D1076 in ASCII format and automatically converted to hexadecimal number in D1050., D1051. When M2=on, MODWRK1H2001D2 is executed, and the main frequency of the inverter changes with the given change of the D2 value. 1 communication (end) DD PLC repeatedly reads and writes the inverter value.
3 Conclusion
The four air compressor unit control systems mainly composed of PLC and frequency converter have the advantages of good control response real-time performance, high safety and economical energy saving, and effectively solve the unreliable operation of the unit independent operation. High energy consumption. Moreover, the system is simple in structure and low in implementation cost, which makes the system easy to maintain and is a technical method worthy of promotion in the industry.
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