Why do these insulators on a high voltage bus look like mushrooms?

Insulators, as the name suggests, play an important role in power systems by isolating conductors. For example, they can be seen on telephone poles, transformer shells, high voltage iron towers, and even bullet trains. These insulators are more or less shaped like mushrooms with circular plates. So, do they look like this to look good? Obviously not.

It can be said that if there is no insulator, the power system will not work properly. For example, if there is no insulator on the pole, there will be a ground fault between the wire and the cement bar on the pole.

In the case of telephone poles, some people might ask, why don't you just use insulated wires? If 380V/220V low-voltage wire is good, their wire diameter is smaller, the transmission distance is shorter, but if the medium-high voltage overhead line, the original wire diameter is larger, if also add insulation layer, the weight of wire will be greatly increased, and the higher the voltage insulation layer is more difficult to achieve. In addition, the presence of an insulating layer will also reduce the heat transfer rate from the conductor to the atmosphere, and the heat generated by the conductor will be trapped between the insulation layer and the conductor, which may cause the wire to overheat and damage.

Because of this, overhead wires are almost always bare wires. Since the wires are bare, insulation between the wires and the poles or towers has to be addressed. The simple solution is to place an insulator where it is needed. It is placed between the wire and the cross arm, so that the wire is perfectly isolated from the cross arm.

But there is a small problem. There is an electric field between the wire and the cross arm, and its electric field lines are unevenly distributed through the insulator and the surrounding air. All electrical flashovers occur through the air, so we mainly look at the electric field around the air.

d57d1f12e3775ee288d54be3ad0a26b0The closer the electric field is to the insulator, the denser the electric field line is, which means that the closer it is to the insulator, the easier the air molecules are to be ionized and the electric flashover phenomenon occurs. So what to do?

It's really easy, it's just a matter of adding a simple disk. Adding disks significantly reduces the number of electric field lines in the air, even though the field strength is the same.

Adding disks solves the problem of field line distribution, but creates another problem: when it rains, it makes it easier for the water droplets to follow their path.

To solve the problem of water droplets accumulating, think of an umbrella and modify the shape of the disk to make it an "umbrella". In this way, water droplets do not accumulate on the insulator, preventing the water from conducting electricity.

In addition, the chance of water droplets conducting electricity can be minimized by adding more umbrellas, thus creating a complete insulator for a telephone pole.

Another common type of insulator is the suspended insulator, on which the power line is suspended and isolated from the tower. The higher the voltage level, the more insulators are required.

Generally speaking, the number of insulators of various voltage levels is within a certain range. For example, for 10kV overhead lines, the number of suspended insulators is 2, while that of 35kV is not less than 3, 110kV is not less than 7, 220kV is not less than 13, etc. If you are interested, you can check the number of these insulators outdoors. Then look at the voltage level of the output line it is hanging from.

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