PUBLISHED ON July 12, 2022  ELECTRICAL MACHINES

What are the electric insulators?

You might have seen brown shiny devices around you on an electric pole, on transformers, and even in electric trains. They are nothing but insulators.

In this article, I am going to explain why insulators are used, their different types, and also why they have a peculiar shape. So let's begin.

Why don't transmission wires have an insulation shield?

If we provide insulation to the entire length of wire, it would unnecessarily increase their weight. Moreover, it also decreases the rate of heat transfer from the conductor to the atmosphere. The heat generated by the conductor is trapped between the insulator and the conductor, which could damage the wire. However, there is an issue with naked transmission line when any conductor is placed near it. The high-powered electrons start jumping from wire to conductor, which leads to sparks as shown in fig 2. The simple solution for this issue is to place an insulator wherever required.

Role of insulators

Insulator is placed between the wire and pole as shown in fig 3 below. The wire is perfectly isolated from the pole. However, there is a small issue. Let’s study the nature of the electric field lines with the help of FEA results (refer fig 4). Here, the electric field lines pass through the insulator and air as well. You might have seen that all the electrical flashovers occur through the air.

Hence, let’s examine the air's electric field in detail. Consider two points A and B in the air volume as shown in fig 5a. The intensity of the electric field lines at both points is the same, but more field lines pass through the point A. This means atom A is experiencing more force and becoming more stressed than B. That means A is more likely to leave its electron first and initiate the flashover process(refer fig 5b).

Similar case happens with cylindrical insulator design. Which we will discuss in the design section.

Why does the insulator have an umbrella shape?

Previously we have used simple cylindrical insulators and discussed issues related to it. As you can see in fig 6a, the number of electric field lines are quite high near the surface of the insulator and the chance of flashover is high here. We can solve this issue with the addition of a simple disc. This FEA result from emworks clearly shows what happens to the electric field lines when a disc is present (refer fig 6b). You can see that the addition of disc has reduced the number of electric field lines in the air significantly even though the electric field intensity is the same.

This reduction will greatly reduce the chance of flashover. Here another issue comes with the insulators:the conducting path of water droplets when it rains(refer fig 7). If I slightly modify the disc design, it will solve the issue. We know that an umbrella is our best tool when we need to escape the rain. Let's modify the disc design and make it an umbrella shape as you can see in fig 8.

The water drops do not accumulate on the insulator, so the chance of a conductive water path is reduced. Further, the rain effect can be minimised by adding more of these smaller umbrellas. We have successfully developed an insulator for electric poles.

Bushing insulator

You must have seen insulators on top of the transformers.The transformer insulators are known as bushings. But why do transformers need insulators? What will happen in the transformer if the insulators are not used? There will be arcing between the high voltage power line and the low voltage transformer body. Thus, the conductor should be isolated from the body of the transformer. Due to this, the geometry of the transformer insulators are different. They will allow current flow in an axial direction and provide good insulation radially(refer fig 9). However, the electric pole insulators are providing insulation in axial direction.

Suspension insulator

Another popular type of the insulator is the suspension insulator. In this type, the power line is suspended from the insulators (refer fig 10a). Suspension insulators have multiple discs connected with metallic pins. However, the presence of these pins causes a big electrical issue in this design. Let’s do an FEA simulation once again with the help of emworks as shown in fig 10b. You can see from this result that There is an unequal voltage distribution between the discs, which causes unequal stress. Let's see how this unequal voltage occurs and how we can avoid this.

Here, you can see in fig 11a, two metallic pins of the insulator acting as capacitor plates with a dielectric between them (refer fig 11b). Similarly, there is one more type of capacitor that forms between the tower and metallic pins, in which air acts as a dielectric.

The unequal voltage distribution is caused by this capacitance effect. Luckily a simple metallic ring can neglect this capacitance effect and the voltage issue(refer fig 12). This ring forms another capacitor opposite to the shunt capacitor.

I hope you enjoyed this insulator article. Thanks for reading.

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