1. Overview With social progress and development, people have placed higher demands on water supply methods and quality in many aspects. Traditional methods using water tanks, water towers, or gas cylinders for pressurization are costly to construct, prone to secondary water pollution, and unable to handle various emergency situations. A constant pressure water supply solution based on frequency converters can effectively solve the various drawbacks of traditional water supply methods and improve the system's automation management level. The TD2100 series constant pressure water supply dedicated frequency converter, introduced below, possesses many functions not found in general-purpose frequency converters, integrating water supply control and management functions, simplifying the design of constant pressure water supply systems to the greatest extent and improving their performance. 2. Water Supply System Composition and Working Principle The TD2100 series water supply dedicated frequency converter possesses the functions of both general-purpose frequency converters and PLCs in a typical water supply control system, achieving intelligent management of the entire system, including conventional pumps, dormant pumps, fire pumps, and sewage pumps, in a more economical and simpler way. Figure 1 shows a flowchart of the water supply system, which consists of a TD2100 frequency converter, a pressure transmitter, and four water pumps. Three of the four pumps are 15kW conventional pumps, and one is a 3kW dormant pump. Each pump's outlet pipe is equipped with a manual valve for maintenance and water flow adjustment. The pressure setpoint of the water supply system is typically 0.6MPa, and the dormant pump's pressure setpoint is 0.3MPa. During peak water usage periods, two conventional pumps supply water, while during off-peak periods, only one pump operates. When the pressure output by the pressure transmitter equals the pressure setpoint, pump 1 maintains a stable speed. When the water supply pressure is lower than the setpoint, pump 1's speed increases, the water supply volume increases, and the water supply pressure rises until the water supply pressure equals the setpoint, at which point pump 1 operates at the new speed. When the water supply pressure rises, pump 1's speed decreases accordingly until a new equilibrium is reached. When water pump 1 runs to the upper limit of the inverter's set frequency and the water supply pressure is still lower than the set value, water pump 1 switches to mains frequency operation, and water pump 2 runs at the inverter frequency to make the water supply pressure equal to the set value; water pump 3 is the standby pump. When water pumps 1 and 2 are running at mains frequency and the water supply pressure is still lower than the set value, water pump 3 runs at the inverter frequency to make the water supply pressure equal to the set value. Because the water supply system has high requirements for pressure stability, exceeding the upper limit of the pressure will cause production accidents. When it is lower than the lower limit, the water supply is insufficient, and some users have to stop production. Therefore, it is necessary to focus on debugging the highest and lowest frequencies of the inverter's over-limit function. After operation, it was found that under normal circumstances, when the two water pumps are running simultaneously, the operating frequency of water pump 2 is between 30 and 40 Hz. Because the heat dissipation capacity of the motor fan decreases when the motor is running at low speed, the motor temperature will rise. Therefore, the lowest frequency of the inverter is set to 25 Hz. 3 System Functions All functions of the system are controlled and implemented by the TD2100 water supply dedicated frequency converter. Therefore, the introduction of the system's functions is essentially an introduction to the main functions of the TD2100 frequency converter. 3.1 Water Supply Modes The TD21C0 frequency converter offers 8 water supply modes to choose from, meeting various user needs. This system adopts a first-start-first-stop 3-pump frequency conversion cycle mode (as shown in Net 2). In the frequency conversion cycle mode, the frequency converter drives each pump in turn according to a certain sequence. The frequency converter can automatically determine the number of operating pumps within a set range according to the pressure closed-loop control requirements. Only one pump is driven by the frequency converter at any given time. When the frequency-driven pump reaches the set upper limit frequency and an additional pump is needed, the frequency converter switches that pump to mains frequency operation and simultaneously drives another pump to operate at frequency conversion. 3.2 Pump Timed Rotation Function This function is suitable when the capacity of conventional pumps is consistent. It can balance the operating time of each pump, effectively preventing corrosion caused by long-term disuse of standby pumps. This improves the overall utilization rate of the equipment and reduces maintenance costs. When this function is active, all pumps set in cyclic mode participate in timed rotation; in fixed mode, the variable frequency fixed pump acts as the main regulating pump and does not participate in rotation, while the power frequency pump undergoes timed rotation control. Timed rotation is not suitable for pumps of the same capacity. 3.3 Closed-Loop Control Function The PI regulator inside the frequency converter is specifically designed for process control systems such as temperature, pressure, and flow. It can monitor the output signal of the feedback device and adjust the frequency converter's output according to the feedback requirements. Using proportional-integral-gain regulation, along with feedback calibration and deviation tolerance (if the deviation is within the set range, the PI regulator stops adjusting, and the frequency converter maintains its output), the frequency converter controls its output based on the PI setpoint and feedback value. If the feedback device detects that the feedback value deviates from the ideal point, the PI regulator will adjust the frequency converter's output until the feedback value equals the setpoint. Figure 3 is a block diagram of the TD2100 frequency converter PLD control principle. In Figure 3, one path is the pressure setpoint signal, which determines the pressure setpoint according to the production process requirements. The pressure feedback value detected by the pressure transmitter is compared with the pressure setpoint to obtain the deviation value ΔP. After processing by the proportional-integral controller, a frequency output signal is obtained to control the operation of the motor. 3.4 Water Supply Timed Control Function To adapt to the pressure/flow fluctuation characteristics of domestic water supply, such as flow fluctuations during the three peak water consumption periods during the day, and other special applications, the system provides up to six segments of timed pressure setpoint control to meet usage needs, which is beneficial for water and energy conservation. There is a distinction between regular days and designated days. 3.5 Dormant Pump Control Function The dormant function is mainly used when the water supply decreases sharply at night. The start and end times of the daily dormant operation and the pressure setpoint during dormancy are specified by setting the inverter's internal parameters. During dormancy, the dormant pump operates, and the inverter only monitors the pipeline pressure. When the pipeline pressure is lower than the set dormant pressure, the system automatically wakes up, and the inverter pump starts working. When the pipeline pressure is higher than the set pressure, the system enters dormancy again, meaning only the dormant pump operates. This dormant pump control achieves water and energy saving effects. 3.6 Inlet Tank Water Shortage Detection Function: The inverter's built-in integrated level sensor (also called an external level sensor) easily enables level detection and control. The inverter can automatically control the operation and shutdown of the system based on the detected water level in the tank, effectively preventing damage to the water pump system due to water shortage. 3.7 Automatic Dialing Alarm Function: When the water supply system or inverter malfunctions, it connects to an external modem via the built-in RS232 serial communication interface, automatically activating a pre-set phone number to promptly notify maintenance personnel for appropriate handling. The TD2100 series inverter is designed with various control requirements in mind in mind, exhibiting a highly refined design. Many other functions are also included, but not all are described here. 4. Water Supply Parameter Setting and Commissioning The correctness of the function code settings is crucial to the success of the TD2100. Figure 4 shows the overall steps for setting water supply application parameters. After parameter settings are completed, they should be verified according to functional requirements, especially parameters closely related to the wiring of peripheral electrical equipment of the frequency converter, such as control mode, water supply mode, and closed-loop feedback signal calibration. These should be checked carefully before system commissioning and operation. Commissioning includes the following aspects: Testing the correct rotation direction of each pump motor. For frequency conversion cycle mode, manual soft start should be used for testing. Adjusting the closed-loop control parameters. Before starting operation, the pressure setting should be slightly lower than the actual requirement to avoid abnormal overpressure in the pipeline network. During trial operation, adjust the corresponding parameters in real time according to the instability of the pipeline network closed-loop operation, such as PI parameters, sampling period, deviation band, and given or feedback filter time, until the system is adjusted to a stable and reasonable range and a certain response speed is guaranteed. Water shortage test, pipeline over- and under-voltage test, and timed rotation test should be performed. The corresponding parameters should be modified as needed. Using frequency converters to achieve AC speed regulation often introduces high-order harmonics into the power supply, which not only reduces the power factor of the power grid but also interferes with the normal operation of other electrical appliances. To solve this problem, an AC reactor (optional) can be added to the input of the frequency converter to eliminate harmonic interference. 5. Economic Benefit Analysis The relationship between the conventional pump power INI, water supply Q1, and pump speed n is as follows: [ALIGN=CENTER] [/ALIGN] 6. Conclusion This constant pressure water supply system renovation involved low investment, stable and reliable operation, significant energy saving and consumption reduction, and achieved intelligent control and management of constant pressure water supply. The TD2100 series frequency converter has comprehensive functions and flexible configuration; using the TD2100 series frequency converter to achieve constant pressure water supply is a very advanced and cost-effective solution.