1. System Introduction To improve the production environment, Tuopai Company invested in a clean water technology upgrade project and built a water supply system with a daily production capacity of 25,000 tons, comprising a pumping system, a booster pump system, and an elevated water tank. Based on the company's water demand characteristics, part of the water from the pumping system is directly supplied to the production departments, while the rest needs to be pumped to the elevated water tank via a booster pump. However, the water pressure supplied to the production departments differs significantly from that supplied to the elevated water tank. Furthermore, the elevated water tank is more than ten kilometers away from the pumping station, making the control of the elevated water tank's level, the design of the booster pump system, and how to "link" them with the pumping system challenging problems. Considering these characteristics, and taking into account both technical reliability and economic practicality, we designed an automatic constant pressure control water supply system combining PLC control and frequency converter control. Simultaneously, the system achieves the goal of "remote linkage" control between the booster pump system and the pumping system through the pressure transmission of the main water pipeline in a more economical way. 2 System Scheme The system mainly consists of a Mitsubishi PLC controller, an ABB frequency converter, a Schneider Electric soft starter, motor protectors, data acquisition equipment, and auxiliary equipment. 2.1 Water Pump System The entire water pump system has four 150KW deep well pump motors and two 90KW deep well pump motors, operating in a frequency converter cyclic mode. All six motors can be set to operate in frequency converter mode. One 150KW and one 90KW soft starter are used to start the 150KW and 90KW motors, respectively. When the frequency converter is operating at 50Hz and the pipeline pressure is still lower than the system's set lower limit, the soft starter automatically starts one motor to operate at the mains frequency. When the pressure reaches the upper limit, the mains frequency running motor automatically stops. Each motor is equipped with a motor protector because the motors have high power. Protectors are used to ensure optimal motor operating conditions and extend motor lifespan under conditions such as overload, undervoltage, overvoltage, overcurrent, phase sequence imbalance, phase loss, and motor idling. The system is equipped with a water level display instrument, which can provide high and low level alarms. Simultaneously, the PLC ensures water quality monitoring at reasonable water levels and protects the electrical system for normal operation. The system is also equipped with a flow meter, which can display both cumulative and instantaneous flow over a period of time, allowing for statistical analysis of water output and monitoring of the flow rate of each pump. 2.2 Addressing Different Pressure Water Supply Needs Within the Company: To reliably meet the requirements of water supply pressure (0.4–0.45 MPa) in some areas of the company that is lower than the main water network pressure (0.8–0.9 MPa), a pressure-regulating and reducing valve is installed for adjustment, with an adjustable range of 0.1–0.8 MPa. 2.3 Booster Pump System: Due to the distance between the pumping station and the elevated water tank, the head of the pumps directly supplying water to the elevated water tank is insufficient. Therefore, a booster pumping station is designed at a drop of 36 meters from the elevated water tank, equipped with two vertical centrifugal pumps (one in operation and one as a backup), with a motor power of 75 kW and a head of 36 meters. The control system of this booster pump needs to consider the following conditions: (1) If the water level in the high-level water tank is low and there is water in the main pipe, the inlet electric butterfly valve is opened and the booster pump is started to supply water to the high-level water tank; (2) If the water level in the high-level water tank is full and there is water in the main pipe, an alarm signal is given and the booster pump and the inlet electric butterfly valve are closed; (3) If there is no water in the main pipe, it indicates that the water consumption has increased or the pumping station has stopped supplying water. The outlet electric butterfly valve must be opened to replenish the main pipe from the high-level water tank. Like the water pump, we have equipped the booster pump with a soft starter and a motor protector to ensure that the booster pump operates reliably for a long time. At the same time, it is equipped with a water level sensor, a digital display and a water shortage sensor for the high-level water tank. To ensure constant pressure supply throughout the main water network, when the elevated water tank is full and the main water pipe is filled with water, the booster pump stops. At this time, the main pipe pressure will "stagnate," eventually causing the main pipe pressure to rise. This pressure is then transmitted to the pumping station. The pumping station's control system detects this pressure and performs constant pressure frequency conversion control, thereby achieving constant pressure water supply throughout the entire main water network. This is the key to the entire control system design. 3 System Functions 3.1 Fully Automatic Smooth Switching and Constant Pressure Control: The pressure signal (4-20mA) from the main water network pressure sensor is sent to the digital PID controller. The controller performs PID calculations based on the pressure setpoint and the actual detected value, and sends a signal to directly control the speed of the frequency converter to stabilize the network pressure. When water consumption is low, one pump operates stably under the control of the frequency converter. When water consumption is high enough that even the frequency converter running at full speed cannot guarantee the pressure and stability of the pipeline network, the PLC simultaneously detects the lower pressure limit signal from the controller and the high-speed signal from the frequency converter. The PLC automatically switches the pump originally operating in frequency converter mode to mains frequency mode to maintain pressure continuity. At the same time, it starts a standby pump using the frequency converter to increase the water supply of the pipeline network and ensure pressure stability. If both pumps are still running, the pump operating in frequency converter mode is switched to mains frequency mode in sequence, while the other standby pump is switched to frequency converter mode. When water consumption decreases, the first sign is that the frequency converter is already operating at the lowest speed signal. If the upper pressure limit signal still appears, the PLC first stops the pump operating at mains frequency to reduce the water supply. If both signals still exist, the PLC then stops another motor operating at mains frequency until the last pump supplies water at constant pressure using the main frequency converter. In addition, the control system is designed with six pumps in two groups. The cumulative running time of each pump's motor can be displayed, and the pumps rotate every 24 hours. This ensures that the water supply system has backup pumps and that all pumps in the system have the same running time, thus ensuring the reliable lifespan of the pumps. 3.2 Semi-automatic operation When the PLC system malfunctions, the automatic control system fails. At this time, the system operates in a semi-automatic state, meaning that one pump has a variable frequency automatic constant pressure control function. When water demand is insufficient, one or more fixed frequency pumps can be manually activated. 3.3 Manual operation When the pressure sensor or frequency converter fails, to ensure water supply, the six pumps can operate in manual fixed frequency mode. 4 Implementation results Actual operation has proven that this control system constitutes the most economical structure for the automatic control of multiple deep well pumps. The software design fully considers the instantaneous pressure and current impact during the switching between variable frequency and fixed frequency. The use of soft start for each pump is the key to solving this problem. The setting of the upper and lower limit frequencies of the frequency converter and the upper and lower limit control points of the digital PID control also plays a significant role in the system's error range. ① The use of variable frequency constant pressure water supply eliminates pressure fluctuations in the main pipeline, ensuring water quality and significantly improving energy efficiency, while also extending the service life of the main pipeline and its valves. ② Pressure-reducing valves economically solve the problem of varying water pressures. ③ The application of variable frequency constant pressure control principles effectively addresses the long-distance communication between the booster pump station and the pumping station, achieving remote interlocking control. ④ Continuous level display is installed in the pumping station, transmitting signals to the PLC to prevent motor burnout due to water shortage. The set water intake position ensures water quality. ⑤ The motor has both a motor protector and a soft starter, overcoming the high current surge during startup and extending the motor's service life. ⑥ PLC-controlled automatic pressure control enables unmanned remote operation. The PLC has a reserved RS485 interface for connection to the company's central dispatch center computer network. ⑦ Due to the closed-loop constant pressure control, the system achieves significant energy savings while meeting the pressure requirements of the main water pipeline, resulting in annual electricity savings of 610,000 kWh, equivalent to RMB 360,000. ⑧ By using frequency converter control, water volume can be fully adjusted in different seasons, holidays, day and night, and commuting times. Based on a daily water saving of 100 tons, an annual water saving of 36,500 tons can be achieved.