Medium voltage and High-voltage frequency inverter

- Nov 20, 2017 -

Inverter is the use of power semiconductor devices on-off effect of power frequency power conversion to another frequency control device. With the rapid development of modern power electronic technology and microelectronic technology, the high-voltage high-power frequency conversion speed regulation device is constantly maturing. The high-voltage problem, which has always been difficult to solve, has been solved well in recent years by series connection of devices or series connection of cells.


Fundamental


High-voltage high-power frequency control device is widely used in large-scale mining plants, petrochemicals, municipal water supply, metallurgical steel, power and energy industries such as various fans, pumps, compressors, rolling mills and so on.


Pump loads widely used in metallurgy, chemical industry, electricity, municipal water supply and mining industries account for about 40% of the energy consumption of the whole electric equipment, and the electricity cost even accounts for 50% of the cost of making water in the waterworks. This is because: on the one hand, the design of the equipment usually leaves some margin; on the other hand, due to the change of working conditions, it is necessary for the pump to output different flow rates. With the development of the market economy and the improvement of the degree of automation and intelligence, the speed control of the pump load by using the high-voltage frequency converter not only improves the process and improves the product quality, but also is the requirement of energy saving and equipment economic operation. The inevitable trend of sustainable development. There are many benefits to speed control of pump loads. From the application examples, most have achieved good results (and some energy-saving up to 30% -40%), significantly reducing the cost of waterworks water system, improve the degree of automation, and is conducive to the pump and pipe network Buck operation, reducing leakage, burst tube, extend equipment life.


Adjustment method


Pump load flow control methods and principles


Pump load is usually the liquid flow rate of the control parameters for this, often using the valve control and speed control of two methods.


Valve control


This method is by changing the size of the outlet valve opening to adjust the flow. It is a long-term mechanical method. The essence of valve control is to change the size of fluid resistance in the pipeline to change the flow. Because the pump speed constant, its head characteristic curve H-Q remains unchanged.


When the valve is fully open, the tube resistance characteristic curve R1-Q intersects the head characteristic curve H-Q at point A, the flow rate is Qa and the pump outlet pressure head is Ha. If the valve is closed, the characteristic curve of the tube resistance becomes R2-Q, and the point of intersection with the head characteristic curve H-Q is moved to point B where the flow rate is Qb and the pump outlet pressure head rises to Hb. Then the amount of pressure head increases: ΔHb = Hb-Ha. As a result, the loss of energy is shown as the female part: ΔPb = ΔHb × Qb.


Speed control


By changing the pump speed to adjust the flow, which is an advanced electronic control method. The essence of speed control is to vary the flow by changing the energy of the liquid being delivered. Because only the speed changes, the valve opening unchanged, as shown in Figure 2, the tube resistance characteristic curve R1-Q will remain unchanged. Head speed characteristic curve at rated speed Ha-Q and the pipe resistance characteristic curve intersect at point A, the flow rate is Qa, the outlet lift is Ha.


When the 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 flow to Qc. At this time, assuming that the flow rate Qc is controlled to the flow rate Qb in the valve control mode, the pump outlet pressure head will be reduced to Hc. Therefore, the pressure head is reduced compared to the valve control method: ΔHc = Ha-Hc. According to this can save energy: ΔPc = ΔHc × Qb. Compared with the valve control method, the energy saved is: P = ΔPb + ΔPc = (ΔHb-ΔHc) × Qb.


Comparing the two methods, the speed control avoids the energy loss caused by the increase of the pressure head and the increase of the tube resistance under the valve control under the same flow rate. When the flow rate is reduced, the speed control greatly reduces the indenter, so it only needs a much smaller power loss than the valve control.


Efficiency analysis


Pump Efficiency Analysis at Variable Speed


As the speed decreases, the pump's high-efficiency section will move to the left. This shows that the speed control mode at low speed low flow, still can make the pump high efficiency operation.


Research on Water Supply Mode under Frequency Conversion


In the multi-point, multi-pumping stations constitute the water supply system, the pump station outlet pressure to be controlled in order to adapt with the pipe network system to achieve better system performance indicators, which can be divided into constant pressure water supply, variable Pressurized water supply and sub-period variable pressure water supply.


Constant pressure water supply


The pump station outlet head pressure remains unchanged, the system control objectives. The given outlet pressure head is Hg.


When the flow rate Q changes, the lift characteristic H1-Q due to speed changes moves up and down, the pump operating point will be in the H = Hg line for horizontal movement (A, B, C, D). Although this meets the flow requirements, it is a waste of energy because of the steepness of the tube resistance R.


Constant pressure water supply system is more convenient to implement, easy and multi-pumping station water supply, large-scale pipe network system coordination, has a certain versatility and practicality, so some speed control pump equipped water plant happy to adopt this method, at Under the constant pressure control mode, there is a certain gap between the parallel characteristics of the pump and the actual characteristics of the pump due to the constant pressure head at the exit of the pump station, and the energy-saving effect is not as good as that of the transformer water supply system.


Transformer water supply


In order to save energy, export pressure should be reduced as the flow rate decreases (or at least not increased). At this time, the outlet of the pump station can be used as a "transformer water supply". When the rotational speed drops, the head characteristic moves downward, and the pipe resistance characteristic R1-Q intersects at point C, and the flow rate decreases from Qa to Qc (letting the flow rate Qc be equal to the QB during constant-pressure control). Transformer control to form a larger pressure difference H = Hac, thus saving the energy shown in Figure 5, the Yin Xian part. Transformer water supply pressure drop due to the export, inhibit the pipe resistance changes in favor of the loss and pump additional loss, energy-saving effect is significant.


Collapse summary


Through analysis, the frequency changer in the pump load speed regulation process, is the water supply way optimized, has already achieved the better energy conservation effect.

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