PLC Control Scheme for Parallel Operation of Two Inverter‑Controlled Gas Boosters Supplying Gas to the Main Pipeline

System Overview

In industrial gas transmission systems, operating two frequency‑controlled gas compressors in parallel to supply gas to the main pipeline effectively enhances gas supply stability and system redundancy. Centralized control via a programmable logic controller (PLC) not only enables precise pressure regulation but also optimizes equipment efficiency and reduces energy consumption.

Hardware Configuration and System Architecture

Establishing a stable and reliable control architecture is the foundation for achieving parallel operation. The system primarily comprises the following core components:

• PLC controller:As the control core of the entire system, it is responsible for data acquisition, logical processing, and command issuance.
• Inverter:The motors driving the two gas compressors are controlled independently, receiving frequency setpoint signals from the PLC to achieve stepless speed regulation.
• Sensor Network:Includes a main pipeline pressure transmitter, inlet and outlet pressure and temperature sensors for each booster compressor, flow meters, and motor operating‑status feedback devices.
• Executing Agency:Such as electrically operated inlet and outlet valves, vent valves, and others, which are used in conjunction with the compressor to manage start-up, shutdown, and operational mode switching.

Control Strategy for Parallel Operation

When two compressors operate in parallel, the key challenge lies in coordinating their output to prevent uneven load distribution. Typically, this is achieved by…Master–slave controlorLoad Balancing ControlStrategy.

• Master–slave control mode:Set one unit as the master and the other as the slave. The master performs PID control based on the setpoint for the main pipeline pressure and outputs a frequency signal, while the slave synchronously tracks the master’s operating frequency or current to ensure consistent output between the two units.
• Load balancing mode:The PLC continuously acquires the operating current or power of the two compressors and, using an internal algorithm, dynamically fine-tunes the setpoint frequencies of the two variable-frequency drives to ensure that the load rates of both units remain substantially equal, thereby extending equipment service life.

Variable-frequency control and pressure closed-loop regulation

The primary objective of supplying gas to the main pipeline is to maintain stable pressure within the distribution network. The PLC achieves this through closed-loop pressure control:

• Signal Acquisition and Processing:The main pipeline pressure transmitter converts the real-time pressure signal into a standard analog or digital signal and transmits it to the PLC.
• PID computation:The PID function block within the PLC compares the actual pressure value with the process‑setpoint pressure, calculates the deviation, and performs proportional, integral, and derivative operations to determine the frequency control signal.
• Frequency Issuance and Execution:The PLC sends the calculated frequency command to the variable-frequency drive. When the pipeline network pressure is too low, the drive’s output frequency is increased, thereby raising the compressor speed and flow rate; when the pressure is too high, the frequency is reduced to decrease the air delivery.

Safety Protection and Interlock Mechanisms

Coal gas is a flammable and explosive hazardous medium, making safety interlocks for parallel operation of compressors critically important. The PLC must incorporate comprehensive protection logic:

• Anti-surge control:When the compressor’s operating conditions approach the surge region, the PLC rapidly opens the anti-surge vent valve or the recirculation valve to increase the inlet flow, thereby ensuring safe equipment operation.
• Overpressure and Low-Pressure Protection:When the main pipeline pressure exceeds the safety threshold, the PLC automatically reduces the inverter frequency or even shuts down the system; when the pressure falls below the lower limit, an alarm is triggered and the standby equipment is started.
• Automatic failover:If a major fault occurs in the operating main compressor, the PLC immediately de‑energizes the faulty unit and automatically and smoothly starts the standby compressor, ensuring uninterrupted gas supply to the main pipeline.
• Start–stop sequence interlock:Control shall be carried out strictly in accordance with the process safety sequence to ensure the logical correctness of valve and motor operations.

Debugging and Optimization Recommendations

In practical engineering applications, rigorous commissioning is required prior to system startup. First, individual equipment must undergo no‑load and loaded tests to verify that the inverter parameters and motor rotation direction are correct. Second, during parallel operation, PID parameters should be carefully tuned to prevent pressure oscillations. Finally, by simulating various fault scenarios, the accuracy and response speed of the PLC interlock logic are validated, ensuring the safe, efficient, and stable operation of the entire gas‑transportation system.