1 Overview
The basic control strategy of the constant pressure water supply control system is: using a motor speed controller and a programmable controller (PLC) to form a control system, optimize the control of the pump group's speed control operation, and automatically adjust the number of pump units to complete the water supply pressure The closed-loop control achieves the purpose of stabilizing the water supply pressure and saving energy when the flow of the pipe network changes. The control target of the system is the outlet pressure of the mains of the pump station. The water pressure value set by the system is compared with the actual value of the total pressure of the feedback. After the difference is input to the CPU, the control instruction is issued to control the number of pump motors and The speed of the variable pump motor is run so that the feedwater main pressure stabilizes at the set pressure value.
With the development of power electronics technology, the theoretical research of power electronic devices and the continuous improvement of manufacturing technology, power electronic devices have achieved great development in terms of capacity, withstand voltage, characteristics, and types. Entering the 90s, power electronic devices have developed toward large capacity, high frequency, fast response, and low loss. As an application of modern power electronic devices and microcomputer technology, organic combination of AC frequency conversion speed control device, with the development of innovative products and promote the application, making AC asynchronous motor speed control field has undergone a huge technological revolution. At present, the motor speed regulators used in automatic constant pressure water supply systems use AC frequency conversion technology, and the system control device uses PLC controllers. PLC can not only realize the logic control of pump sets and valves, but also complete the system's digital PID adjustment function. It can monitor the various operating parameters and control points in the system in real time, and complete the functions of the CRT screen display, fault alarm, and print statements of the system operating conditions. The automatic constant pressure water supply system has a standard communication interface, which can be networked with the upper computer of the urban water supply system, realize the optimal control of the urban water supply system, and provide modern dispatching, management, monitoring and economic operation methods for the urban water supply system.
2. Control scheme
In the pipe network system of a residential community water plant, since the pipe network is closed, the flow rate of the pump station water supply is determined by the user's water consumption, and the pressure of the pump station water supply meets the pressure loss ΔP at the most unfavorable point in the pipe network. There are the following relations between the traffic Q:
ΔP=KQ2;
Where K is a coefficient
Let PL be the lowest pressure required for the most unfavorable point of pressure. Then, the pressure at the outlet of the pump station P should be supplied according to the following relation, which can meet the user's required pressure value for water and have the best energy-saving effect.
P=PL+ΔP=PL+ KQ2;
Therefore, the set pressure of the water supply system should be constantly corrected according to the change of the flow rate. This constant pressure water supply technology is called variable constant pressure water supply, that is, the water supply pressure at the most unfavorable point of the water supply system is constant and the pressure at the outlet of the pump station is Continuously adjustable.
The structural block diagram of a typical automatic constant pressure water supply system is shown in Fig. 1. The system has the function of controlling the constant pressure of the outlet of the water pump outlet and changing the flow of water. The system passes the pressure sensor and flow sensor installed on the water outlet main pipe, and the pressure and flow rate are real-time. The power signal is converted into an electrical signal and input to the input module of the programmable controller (PLC). The signal is compared with the set signal after CPU operation and processing to obtain the best operating condition parameters. The output module of the system Output logic control commands and the frequency set value of the inverter, control the number of pumps running at the pump station and operating conditions of the variable pump, and implement soft start, soft switching and variable frequency operation for each pump according to the CPU instruction. The system can automatically determine the pump's circulation operation according to the user's water consumption changes, in order to improve the stability of the system and the quality of water supply.
3. System function
The system uses FR-500 Mitsubishi inverter. The system has functions:
3.1 automatic switching frequency / power frequency operation function
The inverter provides three different working methods for the user to choose:
Mode 0: Basic mode of operation. The frequency converter always drives a pump in real time and according to its output frequency in real time: control the start and stop of other auxiliary pumps. That is, when the output frequency of the inverter reaches the maximum frequency, an auxiliary pump power frequency operation is started, and when the output frequency of the frequency converter reaches the minimum frequency, the last started auxiliary pump is stopped. This control increases or decreases the number of running pumps. 
Mode 1: Alternating mode, the inverter usually drives a certain pump, and in real time according to its output frequency, the auxiliary pump is running at the power frequency. This mode differs from mode 0 in that if the sequence of the previous pump startup is pump 1 → pump 2. When the output of the inverter is stopped, the next startup sequence becomes pump 2 → pump 1.
 Mode 2: Direct mode. When the start signal is input, the inverter starts the first pump. When the pump reaches the maximum frequency, the inverter switches the pump to the power frequency operation, and the inverter starts the next pump frequency conversion operation. On the contrary, when the pump stop condition is established, the Stop the pump that was started first. 
3.2 PID adjustment function
The water pressure signal (4-20MA or -5V) fed back by the pressure sensor is directly sent to the A/D port of the PLC (through the handheld programmer), set the given pressure value, PID parameter value, and calculated by the PLC. The operation of the switching pump completes the system control, and the system parameters are adjusted during the actual operation, so that the system control response tends to be complete. 
3.3 "Sleep" function
When the system is running, it often encounters a situation where the user has a small amount of water or does not use water (such as at night). To save energy, the system sets a “sleep†function that can make the pump pause, when the frequency output of the inverter is lower than the lower limit , The frequency converter stops working, 2#, 3# pumps do not work, the water pump stops (sleeping). When the water pressure continues to increase, it will stop 1 pump. When the water pressure drops to a certain value, it will start the inverter to run 2# pump or 3# pump. When the frequency reaches a certain value, it will start 1# pump to adjust 2# or 3 # Pump speed. 
The lower limit frequency PR507 output of the "sleep value" inverter is set.  
The “sleep confirmation time†is set with parameter PR506. When the inverter output frequency is lower than the sleep value, for example, it is less than the sleep time td, ie, td<tn, the inverter continues to work. When td>tn, the inverter will enter the sleep state. . 
The “wake-up value†is activated by the lower limit of the water supply pressure. When the water supply pressure is lower than the lower limit, the PLC sends an instruction to wake up the inverter.
The test "sleep value" is 10HZ.  “sleep confirmation time†td: 20s  “wake-up value†70%
3.4 Communication Function
The system has a computer communication function, PLC converters are provided with RS232 or 485 interface PLC can be used Siemens S7-200 computer can communicate with one or more sets of systems, using the computer can monitor: current, voltage, frequency, Speed, pressure, etc. can also control various types of inverter parameters.
In addition, the system also has the functions of manual/automatic operation, fault alarm, running status, current, voltage and frequency status display water protection.
4. Operating characteristics
Take the constant pressure water supply system of three pumps as an example. Under the automatic operation mode, the programmable controller controls the soft start 1# pump of the inverter. At this time, the 1# pump enters the variable frequency running state, and the speed of the pump gradually increases when the water is supplied. When the quantity Q<1/3Qmax (Qmax is the maximum flow rate when all three pumps operate at full power frequency), the programmable controller CPU automatically adjusts the operating speed of the #1 pump according to the change in the amount of water supply to ensure the required water supply pressure. . When the water consumption Q is in 1/3Qmax
When the water supply is reduced to 1/3Qmax
5. System Economic Benefit Analysis and System Benefits
5.1 Economic Analysis
Variable pump power N1, water supply quantity Q1 and pump speed n1, the relationship between the following three equations: N1/Q1 = (n1/n)3 Q1/Q = n1/n Where Q - rated flow, Q1
5.2 System Benefits
5.2.1 Constant pressure water supply technology The use of frequency converter to change the frequency of the motor power supply, and to adjust the pump speed to change the pump outlet pressure, than to adjust the valve to control the pump outlet pressure, has the effect of reducing pipeline resistance and greatly reducing the cut-off losses.
5.2.2 Because the variable pump works in variable frequency conditions, when the outlet flow is less than the rated flow, the pump speed is reduced, which reduces the wear and heat of the bearing and prolongs the mechanical life of the pump and motor.
5.2.3 Because of constant pressure automatic control, no operator is required to operate frequently, which reduces the labor intensity and saves manpower.
5.2.4 Water pump The motor adopts soft start method, accelerating according to the set acceleration time, avoiding the current impact when the motor starts, causing fluctuations in the voltage of the power grid, and also avoiding sudden surge of the motor causing surge of the pump system.
5.2.5 Since the variable pump works in the variable frequency operation state, its speed is determined by the external water supply during the operation of the variable pump. Therefore, the system can save considerable electric energy during the operation process, and its economic benefit is very obvious. As its energy-saving effect is obvious, the system has the advantages of quick investment recovery and long-term benefit, and the social benefits it generates are also very large.