Design and application of frequency conversion speed regulation in Fujian Hengyuan Waterworks
PowerSmart series high-voltage high-power frequency converters are voltage source type high-high frequency converters. It adopts multi-level topology with multi-pulse rectification, multiple PWM, unit series superposition, modular design, building block combination, N + 1 backup, module fault automatic removal, uninterrupted operation, speeding start, reverse start, engineering Frequency and frequency conversion without power failure switching, unit automatic bypass, automatic and manual bypass of the whole machine, high voltage power failure recovery, low voltage power failure can continue to run and other functions, at the same time has the characteristics of high power factor, low harmonic pollution, input and The output current waveforms are close to a sine wave. As the "fourth generation medium and high voltage inverter", dual-chip DSP (digital system processor) is used as the core device of the control system, which solves all the shortcomings of the industrial control computer as the control core of the medium and high voltage inverter. In this paper, through the application of high-voltage frequency converters on the scene of high-power motor loads, the working principle is analyzed, the speed regulation performance is introduced, and the voltage source type high-high mode high-voltage converters are introduced more comprehensively.
[Keywords] speed, frequency, energy saving, motor
One. Basic profile
Fujian Hengyuan Lai Water Co., Ltd. is located in Sanming City, Fujian Province. It is responsible for the supply of water for the entire Sanming city and the industrial water supply for Fujian Sangang and Sanhua. There are 6 water pumps in the water plant, including three 6KW high power pump As well as three 3KW low-power water pumps, before the introduction of the high-voltage frequency converter, the water plant used the method of adjusting the valve opening to control the flow of water. Through this method of adjustment, there are mainly the following problems:
1. The feed water pump is used to run at a fixed speed, the valve adjustment has large throttling loss, high outlet pressure, serious pipe loss, and low system efficiency, resulting in waste of energy.
2. When the flow rate decreases and the opening of the valve position decreases, the pressure difference between the front and rear of the adjustment valve increases, the work safety characteristics deteriorate, and the pressure loss is serious, resulting in increased energy consumption.
3. The long-term 40 ～ 70% valve opening speeds up the wear of the valve body itself, resulting in poor control characteristics of the valve.
4. Excessive pressure on the pipe network threatens the sealing performance of the system equipment. In severe cases, the valve leaks and cannot be tightly closed.
5. The service life of the equipment is short, the daily maintenance is large, and the maintenance cost is high, which causes a great waste of various resources.
One of the important means to solve the above problems is to adopt frequency conversion speed control technology. The high-voltage inverter is used to control the frequency of the feedwater pump motor to achieve variable load regulation of the feedwater flow. In this way, it not only solves the shortcomings of difficult control such as poor linearity of the control valve adjustment, large pure hysteresis, but also improves the reliability of the system operation; more importantly, it reduces the pressure flow loss caused by the change in the orifice of the control valve. It reduces the wear of the control valve, reduces the damage of the system to the sealing performance of the pipeline, prolongs the service life of the equipment, reduces the amount of maintenance, improves the economy of the system, and saves energy.
two. Basic principle of frequency conversion speed regulation of pump load
By changing the speed of the pump to adjust the flow rate, this is an advanced electronic control method. The essence of speed control is to change the flow rate by changing the energy of the delivered liquid. Because only the speed changes, the valve opening does not change, as shown in Figure 2, the tube resistance characteristic curve R1-Q will remain unchanged. The head characteristic curve Ha-Q at the rated speed intersects the tube resistance characteristic curve at point A, the flow rate is Qa, and the outlet head is Ha.
When the rotation speed decreases, the head characteristic curve becomes Hc-Q, and its intersection with the tube resistance characteristic curve R1-Q will move down to C, and the flow becomes Qc. At this time, assuming that the flow rate Qc is controlled to the flow rate Qb in the valve control mode, the outlet head of the pump will be reduced to Hc. Therefore, the pressure head is reduced compared with the valve control method: ΔHc = Ha-Hc. According to this, the energy saving can be: ΔPc = ΔHc × Qb. Compared with the valve control method, the energy saved is: P = ΔPb + ΔPc = (ΔHb-ΔHc) × Qb 
Comparing these two methods, it can be seen that under the same flow rate, the speed control avoids the energy loss caused by the rise of the pressure head and the increase of the pipe resistance under the control of the valve. When the flow rate is reduced, the speed control causes the pressure head to be greatly reduced, so it only requires a much smaller power loss than the valve control, which can be fully utilized. Pump efficiency characteristic curve η-Q, as shown in Figure 3. As the speed decreases, the high efficiency section of the pump will move to the left. This shows that the speed control mode can still make the pump run efficiently at low speed and small flow rate.
three. Economical system scheduling scheme
The Jiuzhou high-voltage frequency converter introduced by the waterworks is a "one-to-three" type frequency converter. The frequency converter is equipped with three bypass cabinets, which are connected to three motors, respectively. Conversion, when one pump is in the state of frequency conversion, the other two pumps are in the state of power frequency operation. Since the water plant has already carried out energy-saving operations, two of the pumps (one domestically produced and the other one produced in Germany) have been cut, and the area of ​​the water turbine blades has been greatly reduced, so the rated power of the pump has changed The energy-saving space is not as good as before cutting. After consultation between our frequency conversion commissioners and users, we decided to use one of the uncut # 10 water pumps as the main pump, and the other two # 9, # 13 water pumps as Backup pump.
Sanhua and Sangang are the two largest water supply households in the waterworks. The situation of the two factories is as follows:
1. There are two main water supply units of the waterworks, and the two share a common water inlet pipe.
2. Sanhua, Sangang's mother tube head difference is about 4 meters, Sangang is the lower head.
3. Due to the relatively large water demand in the past, and due to the special working conditions, the water replenishment is not allowed to be interrupted, so a relatively large water storage tank has been established in the plant in recent years. Larger, the water required for Sanhua is about 1-2 times that of Sangang.
4. Since the amount of water was controlled by adjusting the valve in the past, the valves of the two factories are not fully opened.
5. Because the water volume adjustment of the two plants needs to pass through user observers, user operators, user scheduling, water plant scheduling, and water plant operators, information flow is not very fast, and valve adjustment is relatively frequent.
6. Sanhua has a total of nine inlet valves, six of which are used for clean water, and three are used for industrial water in steel mills. A large amount of water overflowed the reservoir, which was a waste of resources.
The high-voltage inverter is a new type of high-tech energy-saving product. Its energy-saving principle is to replace the original control valve adjustment to the motor speed adjustment. Based on the appeal, we have proposed the following economical operation plan:
1. Install the Sanhua and Sangang water flow meters in the operation workshop of the water plant, so that the water supply volume can be observed from time to time, and the water volume can be easily adjusted.
2. Always understand the opening of Sangang and Sanhua inlet valves to ensure that Sanhua valves are fully open.
3. Through a 10-day commissioning process, we found that the amount of water in Sanhua was 2,700 tons, and the amount of water in Sangang was 2,000 tons, which was the best value for long-term water supply.
4. If Sanhua needs to add water, the dispatcher should understand the amount of water needed for Sanhua, for example, 300 tons. Inform the operation workshop of the water plant based on the flow meter display, adjust the frequency of the inverter, and increase the amount of Sanhua to 300 tons.
5. After adding or subtracting water volume, it is necessary to understand the water conditions of the three waters, and when it is satisfied, the normal water supply conditions will be restored.
6. In the long-term water supply, the valve of Sangang may be opened or opened. If the valve is opened, the water volume of Sangang is increased. At this time, adjust the frequency of the inverter to meet the requirements of Sanhua water supply. At this time, the water volume of Sanhua is obviously increased, and it is necessary to adjust the frequency of the inverter immediately to make it reach the normal water supply state, and avoid the Sanhua regulating valve. After Sangang's water supply is stable, it is necessary to prompt him to close the valve to normal conditions, and adjust the frequency conversion unit to restore the normal water supply environment.
7. Inverter unit # 10 can provide 0-6000 tons of water. If the water demand is greater than 6000 tons, a power frequency needs to be added to run the small unit. The water volume can be controlled again by adjusting the inverter. Try not to run the inverter at 50HZ for a long time. Under power frequency environment.
four. Benefit calculation after frequency conversion
We know from the principle of water pump speed regulation and energy saving. When the pump is driven by the motor frequency, the output is the rated value, the speed and power consumption are the rated value. When frequency conversion is used, the motor speed can be raised and lowered as needed to change the pump ’s speed. The performance curve makes the rated parameters of the pump meet the technological requirements. According to the similar law of the pump, the relationship between flow rate, head, power and speed before and after speed change is:
Q1 / Q2 = n1 / n2
H1 / H2 = (n1 / n2) 2
P1 / P2 = (n1 / n2) 3
Q1, H1, P1-flow, head, power of the pump at n1 speed
Q2, H2, P2-flow, head, power of the pump under similar working conditions at n2 speed
If the speed is reduced by half, that is: n2 / n1 = 1/2, then P2 / P1 = 1/8, it can be seen that reducing the speed can greatly reduce the shaft power to achieve the purpose of energy saving. It can be seen from the figure above: when the speed is reduced from n1 to n2, the rated operating parameters of the pump Q, H, P are reduced. However, from the efficiency curve η-Q, the efficiency value at point Q2 is basically the same as the efficiency value at point Q1. In other words, when the speed is reduced, the rated operating parameters will be reduced accordingly, but the efficiency will not be reduced, and sometimes even increased. Therefore, under the premise of meeting the operation requirements, the pump can still work at the same or even higher efficiency.
In order to make the measured data more real, we conducted a 72-hour energy-saving test after the stable operation of the high-voltage frequency conversion. Our debugger recorded data within 72 hours, including motor current, water output, motor temperature, mother tube pressure , Frequency, water level depth, etc., in order to more accurately calculate energy saving. As shown in the following table:
The pressure of the mother pipes is between 0.212-0.228MPa, the depth of the water level is 8.88-8.93 meters, the frequency is maintained at 44HZ-46H, calculated according to the annual average unit consumption of 132kwh. On the premise that many energy-saving measures have been taken, such energy-saving effects can be achieved, and the customer is very satisfied.
Fives. Additional effects of using inverter
Not only can the inverter save energy and reduce consumption, but its many additional effects are also commendable.
1. Reduce the current impact when the motor starts
The maximum starting current when the motor starts directly is 7 times the rated current; the star angle start is 4.5 times; the motor soft starter should also reach 2.5 times. Observing the load curve of the inverter starting, it can be found that there is basically no impact when it starts. The current starts from zero and only rises with the increase of the speed. It will not exceed the rated current anyway. Therefore, the frequency conversion operation of the condensate pump solves the problem of large current impact when the motor is started, eliminates the impact stress of the large starting current on the motor, the transmission system, and the host, and greatly reduces the daily maintenance costs.
2. Extend equipment life
The use of frequency converter can make the speed of the motor change along the acceleration and deceleration characteristic curve of the condensate pump, and no stress load acts on the bearing, which extends the life of the bearing. At the same time, the relevant data shows that the mechanical life is proportional to the reciprocal of the speed. Reducing the speed of the condensate pump can double the life of the condensate pump, and the use cost of the condensate pump is naturally reduced.
3. Reduce noise
After the water pump of our factory is changed to a frequency converter, the noise is greatly reduced while the pump speed is reduced. When the speed is reduced by 50%, the noise can be reduced by more than ten absolute decibels. At the same time, it eliminates the slipping and squealing noise during stopping and starting, and overcomes the defects of poor linearity and poor quality of the regulating door, causing hammering and resonance of the pipeline, and causing strong vibration of the water pipe of the water supply system. The noise and vibration are greatly reduced, and the changes are considerable.
In short, the promotion of the use of frequency converters in water pumps can greatly reduce the power consumption rate of the plant, reduce the cost of power generation, and improve the competitiveness of bidding for the Internet.
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