Talking about the "pressure drop" of power cable

For power devices, such as generators, transformers and other cables configured target = '_blank'> power cables, when the transmission distance is far, for example, 900m, you should consider the cable voltage "voltage drop" problem, or cable purchase, after installation Only found that because the pressure drop was not taken into consideration, the equipment could not be started normally, resulting in engineering losses. However, how to correctly calculate the voltage drop of the power cable is a real problem. In addition, in actual work, the author also encountered a case where the cable voltage drop exceeds the allowable range due to improper cable laying. This article attempts to talk about personal views on these two issues.

one. Why does the power line produce a "voltage drop"?

In English, “voltage drop” is the voltage drop, and “drop” is the meaning of “pulling down.”

The voltage drop of the power line is due to the resistance of the conductor. Because of this, no matter what kind of material (copper, aluminum) the conductor will cause a certain voltage loss of the line, and this loss (voltage drop) is not greater than 5% of its own voltage is generally not generated on the electric drive of the line Consequences. For example, for a 380V line, if the voltage drop is 19V, that is, the circuit voltage is not lower than 361V, there will be no big problem. Of course, we hope that the smaller the pressure drop, the better. Because the pressure itself is a kind of power loss, although it is inevitable, we always hope that the pressure drop is within an acceptable range.

two. In which situations need to consider the voltage drop?

In general, when the line length is not very long, due to the very limited voltage drop, the “pressure drop” problem can often be ignored. For example, the line has only tens of meters. However, on some relatively long power lines, some users often only care about how to select the cable (type, size) when configuring the power line, but often neglect or ignore the problem of cable voltage drop. Once the cable is laid, it is only discovered when the device is started: or because the voltage is too low, the device cannot be started at all; or the device can be started but is in a low-voltage operating state. When this happens, it becomes very passive. So under what circumstances need to consider the voltage drop in advance?

First, the long power lines need to consider the pressure drop problem. The so-called "long line" generally refers to the cable line is greater than 500 meters. Second, the pressure drop should also be considered in applications where high voltage accuracy is required. For example, some electrical devices require voltage. When the voltage drop exceeds the device's allowable range, the device cannot start. In addition, cables are used to drive important mechanical equipment. When the voltage is lower than a certain value, the equipment can still operate. However, because it is in a “low voltage” state, a long time will damage the equipment. If the equipment is expensive, or if the equipment is damaged, it will cause serious economic losses, so you must pay attention to the pressure drop in advance.

three. How to calculate the pressure drop of the power line?

In general, the pressure drop of the calculation circuit is not complicated. Follow these steps:

1. Calculate the line current I, the formula: I = P / 1.732 × U × cos θ, where: P-power, with "kilowatts" U-voltage, unit kV cos θ - power factor, with 0.8 ~ 0.85

2. Calculate the line resistance R, the formula: R = ρ × L / S, where: ρ-conductor resistivity, copper core cable with 0.01740 into, aluminum conductor with 0.0283 into the L-line length, with "meter" into the S-cable Weigh section 3. Calculate line pressure drop, formula: ΔU = I × R, for example: a certain length of power line 600m, motor power 90kW, working voltage 380v, cable is 70mm2 copper cable, try voltage drop.

Solution: First find the line current I

I = P / 1.732 × U × cos θ = 90 ÷ (1.732 × 0.380 × 0.85) = 161 (A)

Find the line resistance R

R=ρ×L/S=0.01740×600÷70=0.149 (Ω)

Now you can ask for the pressure drop:

ΔU=I×R=161×0.149=23.99(V)

Since ΔU=23.99V has exceeded 5% of the voltage 380V (23.99÷380=6.3%), the voltage requirement cannot be satisfied.

Solution: Increase the cable section or shorten the length of the line. The reader can calculate his own calculations.

four. The various reasons for the large pressure drop of the power line in the design of power lines do not exceed 5% of the pressure drop. However, why would the voltage drop be too large or even fail to start the equipment after the cable is laid? From the above calculation process, it is not difficult to find that too small a cable cross section or a long line can cause a large voltage drop in the line. In addition, there are no other reasons?

1. During or after the installation of the cable, the cable is damaged by external forces and the insulation is damaged, but it does not immediately cause a short circuit.

In this case, because of the “leakage current” phenomenon, the line voltage is naturally lost, which is like a broken water pipe, and the water pressure at the far end is obviously decreased; the larger the hole, the greater the pressure drop. . To verify if this is the case, the verification method is simple: Measure the insulation of the cable core. If the insulation level at this time is found to be lower than that before the installation, the cause is found. Really verifying that the insulation level has dropped, the problem at this time is not "pressure drop", but to find ways to find the damaged location of the cable to deal with, or cable short circuit will be caused sooner or later due to the expansion of cable insulation defects "s consequence. In my practice there is such an example.

2. The other reason is that after the cable is laid, there are many remaining cables and the remaining cables are closed in small circles near the switchgear cabinet. This is a case I personally experienced.

A Taiwan-funded enterprise in Shanghai produces an automotive component whose processing process requires high ambient temperature and humidity. Therefore, there is a special air-conditioning system for exhaust and humidification in the production workshop. The power cable uses the company's 4-core 240 square copper cable (VV22 0.6/1kV 4 x 240mm2). The cable was shipped two years ago. After receiving the user's complaint, we immediately arrived at the scene to understand the situation. At the scene, it was learned that the length of the power cable was about 420 meters, and the load of a single air conditioner was 60 kW. The factory used six such high-power air-conditioning devices, which were equipped with two of the aforementioned cables, namely, "one to three." The measured output voltage of the transformer is close to 400V at the site, but the terminal voltage is only 320 to 340V. It is impossible to start the air conditioner at all: Don't say that 3 units are being dragged, that is, an air conditioner cannot start, production is at a standstill, and the factory is very anxious. Based on this clue, our on-site calculation of the pressure drop will not exceed 3% in any case (the reader can try to calculate according to the above explanation). Then where is the problem?

First of all, there is a long period of time on the cable line that is buried underground in the concrete. Will it be caused by partial damage to the cable during the laying of the cable and a decrease in insulation? After field measurement, the cable insulation is in good condition. This ruled out this possibility. At this point, the problem has not been solved. Do not say that the factory is anxious, we are also very anxious: to solve problems for users is also an incumbent responsibility of our cable companies! On the second day, we returned to the scene again, this time mainly looking at the route along the 420-meter path of the cable. Since most of the cable is buried directly underground, we can see only the transformer's transformer start and switchgear terminals. First, look at the transformer room and found no abnormalities. But then came to the switchgear of the power distribution room, finally found the crux of the problem: due to the length of cable procurement to leave enough room, more than 10 meters left after laying the cable, the factory is not willing to cut off, will be redundant The cables are all labeled as "circles" that are less than 1m in diameter. This "circle" is the "culprit" that caused the voltage drop! Because the cable is bent too much, it causes the current "resistance" of the current (this is also the reason that all power cables have a minimum bending radius). With this kind of judgment, the factory is required to take measures to "loose" and "loosen" the cables. The factory also accepted our proposal. Two days later, when we arrived at the scene, the air conditioners were all able to work normally, and the workshops resumed production.

From the above example, we can find that improper laying methods can also cause “abnormal pressure drop” on the line.