1. Overview
The basic control strategy of the constant pressure water supply control system is as follows: A control system composed of a motor speed regulator and a programmable logic controller (PLC) is used to optimize the speed control of the pump sets and automatically adjust the number of pumps in operation, completing closed-loop control of the water supply pressure. This achieves stable water supply pressure and energy savings when the pipeline flow changes. The system's control objective is the outlet pressure of the pump station's main pipe. The system compares the set water supply pressure value with the actual feedback pressure value of the main pipe. The difference is input into the CPU for processing, and then control commands are issued to control the number of pump motors in operation and the speed of the variable displacement pump motors, thereby stabilizing the main water supply pressure at the set pressure value.
With the development of power electronics technology and the continuous improvement of theoretical research and manufacturing processes for power electronic devices, significant advancements have been made in capacity, voltage withstand capability, characteristics, and types. Entering the 1990s, power electronic devices have evolved towards larger capacity, higher frequency, faster response, and lower losses. As an application of modern power electronic devices combined with microcomputer technology, AC variable frequency speed control devices have undergone a major technological revolution in the field of AC asynchronous motor speed control due to product development, innovation, and widespread application. Currently, the motor speed control devices used in automatic constant pressure water supply systems all employ AC variable frequency technology, while the system control device uses a PLC controller. The PLC can not only realize the logical control of pump groups and valves but also complete the system's digital PID regulation function. It can monitor various operating parameters and control points in real time and perform functions such as CRT screen display of system operating conditions, fault alarms, and report printing. Automatic constant pressure water supply systems have standard communication interfaces, allowing them to network with the host computer of urban water supply systems, achieving optimized control of urban water supply systems and providing modern means for scheduling, management, monitoring, and economical operation of urban water supply systems.
2. Control Scheme
In the pipeline network system of a residential water plant, since the pipeline network is closed, the flow rate of water supplied by the pumping station is determined by the water consumption of the users. The pressure of water supplied by the pumping station is such that the pressure loss ΔP at the most unfavorable pressure point in the pipeline network and the flow rate Q are related as follows:
ΔP=KQ2;
In the formula, K is a coefficient.
Let PL be the minimum pressure required at the most unfavorable pressure point. Then, the water supply pressure P of the main outlet pipe of the pump station should be supplied according to the following formula. This will meet the user's water demand pressure and achieve the best energy-saving effect.
P = PL + ΔP = PL + KQ2;
Therefore, the set pressure of the water supply system should be continuously adjusted according to the change in flow rate. This constant pressure water supply technology is called variable constant pressure water supply, which means that the water supply pressure at the most unfavorable point of the water supply system is constant while the pressure of the main outlet pipe of the pump station is continuously adjustable.
A typical block diagram of an automatic constant pressure water supply system is shown in Figure 1. The system controls the pressure of the main outlet pipe of the water pump to remain constant while providing variable flow water supply. The system uses pressure and flow sensors installed on the main outlet pipe to convert non-electrical signals of pressure and flow into electrical signals in real time. These signals are then input to the input module of a programmable logic controller (PLC). After processing by the CPU, the signals are compared with the set signals to obtain the optimal operating parameters. The system's output module outputs logic control commands and the frequency setpoint of the frequency converter to control the number of pumps in operation and the operating conditions of the variable pumps. It also enables soft start, soft switching, and variable frequency operation of each pump according to CPU instructions. The system can automatically determine the cyclic operation of the pumps based on changes in user water consumption to improve system stability and water supply quality.
3. System Functions
This system uses the FR-500 Mitsubishi frequency converter from Japan. The system has the following functions:
3.1 Automatic switching between variable frequency and mains frequency operation function
The frequency converter offers three different operating modes for users to choose from:
Mode 0: Basic Operating Mode. The frequency converter always drives one pump and controls the start and stop of other auxiliary pumps in real time based on its output frequency. That is, when the frequency converter's output frequency reaches its maximum, one auxiliary pump starts operating at mains frequency; when the frequency converter's output frequency reaches its minimum, the last auxiliary pump started stops. This controls the number of pumps operating at mains frequency.
Method 1: Alternating mode. The frequency converter typically drives a fixed pump and adjusts the auxiliary pump's operation at the mains frequency based on its output frequency. This mode differs from Method 0 in that if the previous pump start sequence was pump 1 → pump 2, when the frequency converter output stops, the next start sequence becomes pump 2 → pump 1.
Method 2: Direct method. When the start signal is input, the frequency converter starts the first pump. When the pump reaches its highest frequency, the frequency converter switches the pump to mains frequency operation and starts the next pump for frequency conversion operation. Conversely, when the pump stop condition is met, the first pump to start is stopped first.
3.2 PID Control Function
The water pressure signal (4-20mA or -5V) fed back from the pressure sensor is directly sent to the PLC's A/D port (or via a handheld programmer). The given pressure value and PID parameter values are set, and the PLC calculates the necessary pump switching to complete system control. System parameters are adjusted during actual operation to ensure a complete system control response.
3.3 "Hibernation" function
During system operation, situations often arise where user water consumption is low or no water is used (such as at night). To save energy, the system is specifically equipped with a "hibernation" function that allows the water pumps to pause operation. When the inverter's frequency output falls below its lower limit, the inverter stops working, pumps #2 and #3 cease operation, and the water pumps stop (entering hibernation mode). When the water pressure continues to rise, pump #1 will stop. When the water pressure drops to a certain value, the inverter will first start running pump #2 or #3. Once the frequency reaches a certain value, pump #1 will start to adjust the speed of pumps #2 or #3.
The "Sleep Value" setting is the lower limit frequency of the inverter output, set by PR507.
The "Sleep Confirmation Time" is set using parameter PR506. When the inverter's output frequency is lower than the sleep value for a period less than the sleep time td (i.e., td < tn), the inverter continues to work. When td > tn, the inverter enters sleep mode. The "Wake-up Value" is triggered by the lower limit of the water supply pressure. When the water supply pressure falls below the lower limit, the PLC issues a command to wake up the inverter.
The tested "sleep value" is 10Hz.
"Hibernation confirmation time" td: 20s
"Wake-up value" 70%
3.4 Communication Function
The system has computer communication capabilities. Both the PLC and the frequency converter are equipped with RS232 or 485 interfaces. The PLC can be Siemens S7-200. The computer can communicate with one or more systems. The computer can simultaneously monitor current, voltage, frequency, speed, pressure, etc., and can also control various parameters of the frequency converter.
In addition, the system also features manual/automatic operation, fault alarm, operating status, current, voltage, and frequency status display, and water shortage protection.
4. Operational characteristics
Taking a constant pressure water supply system with three pumps as an example, in automatic operation, the programmable logic controller (PLC) controls the frequency converter to soft-start pump #1. At this time, pump #1 enters variable frequency operation, and its speed gradually increases. When the water supply Q < 1/3Qmax (Qmax is the maximum flow rate of all three pumps operating at full frequency), the PLC CPU automatically adjusts the operating speed of pump #1 according to the change in water supply to ensure the required water pressure. When the water supply Q is within 1/3Qmax...
When the external water supply is reduced to 1/3 of Qmax, 5. System economic benefit analysis and system advantages.
5.1 Economic Benefit Analysis
The relationship between the power N1, water supply Q1, and pump speed n1 of the variable pump is as follows:
N1/Q1 = (n1/n)3
Q1/Q = n 1/n
In the formula, Q is the rated flow rate, and Q1N is the shaft power at the rated flow rate Q.
n—Rated speed of the water pump
Since when the rated flow rate Q=100%, n=100%, and N=100%, if n1=90%n, Q1=90%Q and N1=72.9%N, then 27.1% of electricity can be saved. If n1=80%n, Q1=80%Q and N1=51.2%N, then 48.8% of electricity can be saved.
5.2 System Advantages
5.2.1 Constant pressure water supply technology uses a frequency converter to change the power frequency of the motor, thereby adjusting the pump speed and changing the pump outlet pressure. Compared with the method of controlling the pump outlet pressure by adjusting valves, it has the effect of reducing pipeline resistance and greatly reducing interception losses.
5.2.2 Because the variable pump operates under variable frequency conditions, when its outlet flow rate is less than the rated flow rate, the pump speed decreases, which reduces bearing wear and heat generation, and extends the mechanical service life of the pump and motor.
5.2.3 Because constant pressure automatic control is achieved, frequent operation by operators is not required, which reduces the labor intensity of personnel and saves manpower.
5.2.4 The water pump motor adopts a soft start method, accelerating according to the set acceleration time, to avoid the current surge during motor start-up and the impact on the grid voltage fluctuation, and also to avoid the pump system surge caused by sudden motor acceleration.
5.2.5 Because the variable frequency pump operates in a variable frequency mode, its speed is determined by the external water supply during operation. Therefore, the system can save considerable electrical energy during operation, resulting in significant economic benefits. Due to its obvious energy-saving effect, the system offers a quick return on investment and long-term benefits, generating substantial social benefits as well.
