1. Introduction To compensate for the decrease in formation pressure during oilfield production, water injection technology is needed to maintain formation pressure for oil production operations. The level of water injection pressure is a crucial parameter determining the rational development of the oilfield and the status of surface pipelines and equipment. Due to the complex and diverse process conditions that need to be adapted in the design of water injection technology and electromechanical equipment configuration, engineering margins are essential in water injection technology design and electromechanical equipment configuration. These process conditions include: the increase in water injection wells in later development stages; continuous changes in reservoir pressure and oil, gas, and water distribution; changes in the number of wells opened and closed; and the impact of well washing and insufficient water supply, etc. The uncertainty of process conditions causes fluctuations in water injection pressure, uneven and unstable water injection volume, and difficulty in controlling water injection pressure, posing challenges to stable and high-yield oilfield production and management. Because the water injection requirements of oilfields are difficult to accurately predict and control, considering the needs of oilfield development, equipment margins are usually designed based on the maximum possible demand of the oilfield, which is particularly prominent in the design of water injection systems. Oilfield water injection equipment uses centrifugal pumps driven by motors, and high-power systems often operate in a light-load, inefficient state. Water injection pressure is manually controlled by the pump outlet gate, which means adjusting the pump's displacement by changing the pipeline characteristic curve. This makes it difficult to match the pump and motor to operate at the pump's optimal operating point, resulting in low pipeline efficiency and energy losses exceeding 50%. Variable frequency constant pressure technology can effectively stabilize the water injection process and significantly reduce the power consumption of large-capacity fan pumps. The technical approach of the constant pressure variable frequency automatic control system for the water injection station is to fully utilize existing production facilities and adjust the motor speed of the water injection equipment to achieve stable pressure and flow. Simultaneously, soft start and soft stop functions replace reduced-pressure start, ensuring smooth motor start and stop, reducing the impact on the power grid and mechanical equipment, and preventing drastic changes in pipeline pressure, flow rate, and velocity. It eliminates the need for valve shut-off, thus greatly benefiting the prevention of cavitation, water hammer, and surge, and extending the maintenance cycle and service life of the pipeline, pumps, and valves. A complete automatic control system, a demand information platform, improved wastewater treatment efficiency, and strengthened supervision and management mechanisms minimize the labor intensity of on-site employees and improve the enterprise's information management level, resulting in significant economic and social benefits. 2 System Composition and Functional Design The control system structure is shown in Figure 1. The system is divided into two levels: the field level and the control level, which realizes process flow display and control through a human-machine interface (HMI). [IMG=System Structure]/uploadpic/THESIS/2007/11/2007111709002328340F.jpg[/IMG] Figure 1 System Structure The core automation equipment of the system uses Delta electromechanical products. The control level is the key to realizing the system functions, and its main function is to act as the hub between the HMI (Human-Machine Interface) and the field level. It receives parameters or commands set by the HMI, controls the water injection production process, and simultaneously transmits the field status to the HMI. The control part is an integration of Delta PLC and touch screen. It consists of DVP-32EH00R (CPU unit), DVP-04AD-H (analog input module), analog DVP-04DA-H (analog output module), and DOP-A80THTD (touch screen). The control level equipment is installed in the electrical control cabinet in the control room. The Delta PLC's CPU module, DVP-32EH00R, has one RS232 and one RS485 communication interface, while the DOP-A80THTD touchscreen has one RS232 interface and one USB interface. The touchscreen communicates with the controller via the RS232 serial port. The field level is the foundation for realizing the system's functions. The field level consists of an intelligent integrated pressure transmitter and a frequency converter. A Honeywell pressure transmitter is selected as the intelligent integrated pressure transmitter, and a Delta VFD series frequency converter is selected. There is one 160kW water injection motor and two 75kW motors (with one spare). Therefore, one 160kW VFD-1600F43A frequency converter and one 75kW VFD-750 F43A frequency converter are selected to form a one-to-one and one-to-two configuration, respectively, as shown in Figure 2. One PLC controls two frequency converters to drive two water injection motors for variable frequency horizontal pressure water injection. The field-level control uses shielded cable connections. An intelligent integrated pressure transmitter converts the pressure signal into a 4-20mA signal, which is input to the analog input module DVP-04AD-H. The control output from the PLC is converted back to a 4-20mA signal by the analog output module DVP-04AD-H and input to two frequency converters, achieving constant pressure water injection frequency control for two water injection motors. [IMG=Automatic Control System Schematic Diagram]/uploadpic/THESIS/2007/11/20071117090603272344.jpg[/IMG] Figure 2 Automatic Control System Schematic Diagram 3 Process Flow and Control System Design The principle of the horizontal pressure water injection process automation system is shown in Figure 2. The system adopts three control modes: automatic operation, manual frequency conversion operation, and power frequency operation. 3.1 Automatic Operation The system automatically starts the first frequency converter, using PID to control the water injection pressure of the water injection pump. If the output pressure of one pump cannot meet the pressure requirements, the system will automatically start the second frequency converter to meet the pressure requirements. 3.2 Manual Variable Frequency Operation: The inverter can be started independently in manual mode via buttons on the control panel. This method allows for flexible control of the variable frequency output, meeting control requirements under complex conditions. 3.3 Power Frequency Operation: After the inverter starts the motor and it reaches the power frequency, a contactor is used to switch the motor off the inverter and into power frequency operation (see Figure 2). The inverter acts as a soft starter for high-power motors. Through these three control methods, the normal operation of the water injection motor can be ensured, meeting the requirements of uninterrupted production. 3.4 Human-Machine Interface Design: The process flow diagram is shown in Figure 3, the control parameters are shown in Figure 4, and the controller parameter settings are shown in Figure 5. [IMG=Process Flow Diagram]/uploadpic/THESIS/2007/11/20071117091402485996.jpg[/IMG] Figure 3 Process Flow Diagram [IMG=Control Parameters]/uploadpic/THESIS/2007/11/2007111709160184636X.jpg[/IMG] Figure 4 Control Parameters [IMG=Controller Parameter Settings]/uploadpic/THESIS/2007/11/20071117091937638299.jpg[/IMG] Figure 5 Controller Parameter Settings 4 Conclusion The water injection system is controlled using PLCs, touch screens, and frequency converters from Delta Electronics. The single automated platform control structure is simple; energy-saving benefits are significant; and it is convenient for personnel to operate. The project has received attention and praise from oilfield enterprises. The project embodies Delta's energy-saving and automation philosophy, particularly highlighting the advantages of Delta's electromechanical products in complete system control engineering. (Proceedings of the 2nd Servo and Motion Control Forum; Proceedings of the 3rd Servo and Motion Control Forum)