(What kind of transmission line is this...... ）
Let's talk about the concept of transmission towers first, the transmission line is supported by the transmission tower section by section, the high voltage level uses the "iron tower", and the low voltage level such as the residential area generally uses the "wood pole" or "cement pole", collectively referred to as the "pole tower". The line of high voltage level needs to have a larger safety distance, so it must be erected very high, only the iron tower can afford dozens of tons of lines, a pole can not be erected so high, nor so much support, so the poles are of lower voltage level. As for how high the voltage is high and how low is low, this will be explained in detail.
(Anhui power transmission to the east 1000 kV UHV AC line)
By the way, the voltage level is the line voltage, the voltage between any two phases in the ABC three phases. The 220 volts used at home is the phase voltage, which is the voltage of any of the three phases relative to the earth. The actual electricity at home is 380 volts (3 times the root number of 220 volts), but when it arrives at the door of the building, the three phases are separated, such as the ABC three phases into the three units of a building. The 380 volt voltage level is also called the 0.4 thousand volt voltage level in the power system, compared with the current 1000 thousand volt UHV transmission line, the difference is 2500 times, tremble~
What we see along the way is generally the transmission tower, as for the tower type or something, there is nothing to say, the cat's head tower, the wine glass tower, the gate tower, and the V-shaped tower are all "pictographic", and you will know it by looking at it. Transmission lines are also divided into DC and AC (DC and AC), DC is easy to recognize but not very common, there are only a few domestic lines, it is not easy to meet.
The figure below is the ± 800 kV Hami South - Zhengzhou UHVDC transmission line.
The tower is T-shaped, and there are two transmission lines hanging below, one side of the positive pole, and the other side of the negative pole, as for why the positive and negative poles should be divided into so many strands, let's wait until we talk about the split wire of the AC line. Look carefully at the tower and there are two small "corners" stretched out on the top, and there is also a "thin wire" on each side, which is not for power transmission, but for lightning protection, also called ground wire.
Let's focus on the AC line, which is almost "Dabao sees you every day".
The first AC line has three phases: A, B, and C, and the top of the transmission tower is a lightning cable. Thunderstorm areas or high voltage level line is two lightning wires, thunderstorm is not serious or low voltage level line can be reduced to a lightning line, this is from the perspective of engineering practice and money saving, anyway, everyone sees the top thin one or two lines to know that it is a lightning line.
The lightning cables are connected to the tower, so that the current in the event of a lightning strike can be directed down the tower to the ground. However, when connected, there is a section of insulator or insulator between the lightning cable and the tower, and if you look carefully, you can see the jumper. The purpose of this is to facilitate the breakdown and discharge in the event of a lightning strike, while reducing transmission losses in normal times. If the lightning cable is directly connected to the tower, the induced current of the wire in the line will flow directly into the ground, resulting in transmission losses.
Lightning cables are generally used in high-voltage level transmission towers in open areas, and there are generally few lightning cables on the electric poles we see: first, the electric poles are generally in the city, and there are other higher buildings that can be struck by lightning;
Below the lightning line is the transmission line, the number of roots are multiples of 3, 3 lines are called a return line, 6 lines are called two circuits, 12 are called four circuits, and each time there are three lines of ABC three phases. The picture at the beginning is called the "four times with the tower" line. The reason why there are multiple circuits on a tower is mainly to consider the transmission capacity and floor area, so the concepts of "line length" and "loop length" are also derived, and the loop length is twice the length of the line for the double circuit of the same tower, and so on. The following figure is also two lines with the same tower four times, if it is of different voltage levels, the voltage of the upper wire should be higher than the voltage of the lower wire, the higher the voltage to the ground safety distance requirements.
Two different layouts of the same tower four-circuit line
In fact, lightning protection wires and transmission wires are also very distinguishable, one is directly on the tower, and the other needs to be hung on the tower with an insulator string. The tower is ground potential, and without the insulation of the insulator string, the wire will be directly short-circuited to the tower. Looking at the transmission line, it is the first place to be able to distinguish the voltage level, which determines which line it is, how much power is approximated, and how far it is transmitted.
How to see the voltage of the transmission line at a glance? The secret is to "look at three things": look at the number of wire splits, look at the length of the insulator string, and look at the height of the tower.
1
Look at the wire is several split
Let's talk about the number of splits first, as shown in the picture below, for each phase of the ABC three-phase wire, it is divided into several strands, for example, the following 1000 kV UHV transmission line is divided into 8 strands, called "eight split" wires. The reason why a phase wire should be split into several strands is to expand the "equivalent diameter" of the wire, which is equivalent to using several thinner wires (relatively thin, in fact, there are also girls' wrist thickness) to enclose an approximate circle, which is equivalent to expanding the "equivalent" diameter of the whole phase wire.
There are several reasons to increase the "wire diameter":
First, alternating current has a "skin effect", because of self-inductance, most of the current flows on the surface of the wire, and there is almost no current in the middle of the conductor.
Second, the current of the high-voltage transmission line is very large, and the resistance of the wire is low, and the resistance is inversely proportional to the area of the wire, so it was originally replaced by a very thick tubular wire, and now the split wire is used (in the substation, the tube bus is still used);
Third, the thicker the wire, the lower the electric field strength on the surface of the wire, and the smaller the corona. Corona is a loss of electrical energy, and of course we want to make the wire thicker to reduce the surface field strength and corona.
Corona is a kind of discharge phenomenon, you may hear the sound of "ziz" around the power line on a rainy day, that is the sound of corona, and you can also see the weak light of the wire at night, of course, corona is not only bad, you can talk about it in detail later.
The "zez" sound of corona will also bring radio interference, which is why the DC wire should also be split, all to reduce corona.
There are also 750 kV ultra-high voltage transmission lines below 1000 kV, which is only used in the northwest power grid of the mainland, and there are also 765 kV lines in Europe. The 750 kV voltage level is generally used with six-split wires.
500 kV is a four-split wire according to the regulations, but it is observed that now there are more six-split wires, especially around large cities, and the transmission power may be large.
220 kV is generally two splits, 110 kV and further down the voltage level does not need to split the wire, a single one, corona is not serious.
However, due to the high power of power transmission, 220 kV quadruple division and 110 kV quadruple division can also be seen in some places. Three-split wires are theoretical, but I haven't really seen them much.
2
Look at the number of insulators
What is an insulator? Look at the picture below, the green glass in the red circle is an insulator
An insulator is a special type of insulating control, usually made of glass or ceramic, to increase creepage distance. Its shape is a flying saucer, a flying saucer is regarded as an insulator, and the insulator string is used to string together a number of flying saucers to isolate the wire and the tower. The picture below is the shape of a piece of FRP insulator.
Let's talk about the number of insulators. The number of wire splits is better to count, and you can know how high the voltage level is by counting how many wires. In addition to looking at the number of splits, you can also count the number of insulators, but it is more eye-consuming, and you can know it at a glance if it is familiar.
Commonly used insulators also include ceramic insulators, and the picture below is a ceramic insulator
There are also synthetic insulators
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The thickness of an insulator is about 15 centimeters, and the string of 7 pieces is almost one meter.
In general:
750 kV, 32 pcs
500 kV, 23-25 pcs
330 kV, 17 pcs
220 kV, 13 pcs
110 kV, 7 pcs
66 kV, 5 pcs
35 kV, 3 pcs
If the altitude is high, the area is heavily polluted, or the important tower, the number of pieces should be increased by a few pieces. It can be seen that one insulator can withstand a voltage of 1-15,000 volts, but as the voltage level increases, the nonlinearity of the bearing voltage becomes stronger. In fact, the number of insulator pieces can vary greatly from region to region and environment. Of course, different types of insulators, the number of pieces required is also different, the picture below of the 1000 thousand volt UHV, the power grid Jun counted with the naked eye, about fifty or sixty pieces, really long......
1000 kV AC UHV line insulator
± 800 kV UHVDC line insulators.
3
Look at the height of the tower
In addition to the number of splits and the number of insulator pieces, you can also look at the height of the tower, although the design regulations do not specify how tall the tower should be, but the minimum distance of the transmission line from other objects is specified in great detail.
(500 kV line tower)
The transmission line is about 500 meters in the first gear, and the wire is going to sag, especially when the weather is hot and the electricity load is large, the wire will expand and shrink in heat and cold, and it will sag even more. Therefore, the regulations clearly state the minimum distance between the wires of the transmission lines of each voltage level and various objects.
For example, the minimum distance between a traverse and the ground in a residential area is:
35~110 kV is 7 meters,
220 kV is 7.5 meters,
330 kV is 8.5 meters,
500 kV is 14 meters,
750 kV is 19.5 meters.
(750 kV line tower)
If wire sag and insulator lengths are considered, then:
110 thousand volts are about ten meters above the ground,
220 kV and 330 kV are more than twenty meters,
500 kV is thirty or forty meters,
750 kV is more than fifty meters,
1000 kV is seventy or eighty meters.
(1000 kV UHV line tower)
Converted to the height of the building, it is 5 floors, 8 floors, 12 floors, 18 floors, and 25 floors, which is a bit impressive. However, this is very rough, and the height needs to be adjusted greatly depending on the climate and terrain.
Generally speaking, the easiest thing to see to distinguish the voltage level is the number of wire splits, the most accurate to judge is the number of insulators, and the most shocking is the height of the tower.
We can see the transmission lines, are the terrain is better in the area, you must know that there are transmission lines in the mountains, rivers and rivers, especially for remote areas to send electricity, the tower and wire materials are carried by people on their shoulders, quite hard! It can be said that there are no highways in places, there may be transmission lines, really to pay tribute to the power transmission and transformation workers!
Finally, let's talk about the gadgets on the transmission towers. Look!Guess what this thorn and orange little windmill does?
Anti-bird stinging
Barbs are used on iron towers to prevent birds from nesting, and small windmills are used to drive birds, usually on telephone poles. You can keep an eye out for it next time!
(One day in a certain year, when I went out and looked up, I found a teddy bear on the electric pole, which is also said to be a bird repellent, and immediately vomited three liters of blood...... ）
Why can't the transmission tower get along with the birds? Because bird droppings are an important cause of short-circuit faults in transmission lines. Guano is conductive, and when discharged to the insulator string, it will form a short-circuit channel to the ground of the wire, so the transmission line is afraid of "Angry Birds......
In addition to birds, snakes, rats, weasels, or large birds with too long wingspans can cause transmission lines to short circuits to the ground or from one phase to another. Big trees too, so prune them regularly, too. Hey, it's not easy to be a small partner in operation and maintenance, and you have to deal with all kinds of animals and plants......
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