Electrical Insulators and Types of Insulators in overhead line

Electrical insulators

What are Electrical insulators 

Electrical insulators are materials whose internal charges are not allowed to flow freely when subjected to an electrical field. In other words, they don’t literally allow electrical current to flow through them. This type of material does not conduct electricity or heat through them.

However, there are materials that do allow electrons to flow through them, they conduct. This implies that they allow something to be transferred through them. The allow of electrons flowing through the type of materials, is called Electrical conductor. 

A major reason why when you place hands on a current-carrying conductor, you won’t experience any form of electrocution. This reason is due to an insulator enclosing the conductor. They confine the Electrical current, even without allowing the current to flow through them. Insulators are commonly found on cables, this differentiates the cable from a normal conductor. Moreover, conductors with insulators are called cables.

Naturally, almost all metals allow electrical current to flow through, metals like silver, copper, mercury, aluminum, gold and iron are good conductors of electricity. During discontinuity of electrical currents, electricity doesn’t flow and this is because, air doesn’t conduct electricity. In other words, Air is an insulator. Although the human body is a good conductor of electricity, they allow electrical current to flow through them, acting as a pathway to complete an electrical circuit. So the human body are not to be classified as an insulator. 

On the other hand, materials such as rubber, wood, glass, plastic, and cotton do not allow electrical current to flow through them. So we can classify them as an insulator. Although in the real world, perfect insulator doesn’t exist. However, there are also materials that are close to perfection due to their high resistivity to electrical current, materials like paper, glass and Teflon and classified under this. 

Modern Electrical Insulators 

Insulators help to electrically support and separate electrical conductors. Meanwhile, Insulators are not only used for low voltage rating, but they are also used for high voltage and extra-high voltage rating even up to 330KV. This makes the study more important in the Electrical power sector.

With the recent growth in the power sector and the improvement of power substations, power generation, transmission, and distribution have increased in several forms. This growth is still exponentially increasing as the future gets closer. A world-renown company in the power sector that came into existence in 1985, known as the Modern Insulator Limited, with a technical collaboration with Siemens. 

This company helps in the manufacturing of high voltage and extra-high voltage insulators. They are among the best manufacturers in the world as regards insulators.

Difference between insulators and conductors

However, both conductors and insulators consists of atoms and molecules. These atoms and molecules has a positively charged nucleus, with electrons surrounding the nucleus. Most importantly, whether a conductor or an insulator, the positively charged nucleus can’t move around, even during thermal vibration. 

In conductors, despite the static nature of the positively charged nucleus, the negative charged electrons can relatively move freely with almost no resistance. However, for that of insulators, both nucleus and electrons can’t move freely from their initial state. 

Moreover, a conductor has to be subjected to an electrical force field such as batteries for electrical currents to flow through them. But even with this field, insulators will still not conduct.

Insulators are used to avoid electrical interactions between circuits, while conductors are used to create electrical interactions between circuits. However, for insulators, despite that, the electrons can’t move relatively, but when subjected to an electric field, the nucleus exhibits a slight shift from his initial state. This slight shift will cause more electrons to concentrate on the other side.

Moreover, when insulators are brought to a high electrical field, there’s a tendency for them to conduct, this is known to be the breakdown voltage. The reason why I said earlier that perfect insulators do not exist.

Most importantly, insulators prevent humans from getting electrocuted, while conductors can cause electrocution.

Properties of a good insulators

  • A good insulator should have a strong mechanical strength. This mechanical strength is something to lay great consideration, due to the load acted upon.
  • Should have high insulating resistance, thereby preventing leakage current to flow through it. 
  • A good insulator should have high dielectric strength, which is the maximum level of current that insulators can handle under some electrical conditions without breaking down electrically.
  • Insulators should have low relative permittivity, which is the ability for a material to conduct electricity 
  • A good insulator should have high air permeability.
  • Insulators should have the tendency to withstand normal voltage and overvoltage.
  • The safety factor of an insulator should be high, and this factor is the ratio between the puncture strength to the flash overvoltage. 
  • A change in temperature shouldn’t affect insulators. However, it should be a perfect homogeneous material without cracks or holes.

Continuing in this discussion, we shall see more of the line insulators, which provides necessary insulation between line conductors and supports. These are mainly used in power generation, transmission, and distribution systems.

Type of line insulators

  1. Suspension type 
  2. Strain insulators 
  3. Pin type/Post type insulators 
  4. Shackle insulators
  5. Overhead line insulators 

In an electrical line system, insulators prevent current leakage from conductors to earth. Although line conductors in an overhead system are bare, which implies that they don’t have any insulating coating around them. Since these conductors are bare, there’s a tendency for electrical current to leak through the line support structure. 

As such, line insulators as listed above are used as a means of insulating medium between the line conductor and the supporting structure, thereby providing necessary clearance between them. However, the application of these types of line insulators factored on their voltage rating. Also, proper selection of these insulators is important for the perfect operation of the overhead line system.

Pin type insulator 

Post insulator
Pin insulators

The pin type of insulator is used for voltage transmission-33KV and distribution-11KV. They are more economical, simple and effective for this voltage rating. Moreover, the size differs from both 11KV and 33KV. The pin insulators used for 11KV is smaller in size compared with the 33KV. 

They are mounted on the cross arm of the supporting line structure to raise the conductor above the cross arm with the help of spindle. At the very top of this insulator is the Groove. The groove house the conductor and the conductor is tied to the groove with a copper or aluminum wires, both annealed wires depends on the type of line wire used. 

The distance between the hole of the spindle from the edge of the cross arm is 0.5 ft(6 inches). Knowing that for 11KV cross arm which is 6ft, the spindle at each end is 0.5ft, so we are left with 5ft. This implies that 2ft6inches from each other, although when you place 2ft6inches, you find out that the spindle of the insulator is already at the center of the pole. As such you can’t attain 2ft6inches equally. 

In such case, what you do is to move the middle spindle to 2ft6inches to one end of the cross, while on the next electric pole, you transpose, by moving the spindle to the other side on the next pole. This will, therefore, balance the inductance of the line.

However, a three-phase power system will require a three-pin type insulator mounted on the cross arm. The properties of this type of insulator should be able to withstand mechanical stress and wind.

For voltage greater than 33KV, the pin type becomes uneconomical and can result in failure in the insulating properties, this can result in a short circuit. 

Post insulators 

Although the post insulators are similar to that of the pin insulators. However, instead of a groove to house the conductor like that of the pin type, they have a metal cap and also a metal base.

The post insulators have more applications in substations, they are used here to support bus bars and disconnect switches. They are also used for an operating voltage up to 33KV.

The materials used for pin type are nonconducting materials such as porcelain, plastic, glass, wood or polymer. However for post insulators, the porcelain element used are in the form of cones fitting, with special cement bounding it. 

Suspension type of Electrical Insulators

Suspension Insulators
Suspension type of Electrical Insulator

For operating voltage rated above 33KV, the suspension type is used. They consist of number of porcelain discs. However, these discs are flexibly connected in series by metal linking all disc together in a string form. Mostly used on power towers, and they house the conductor on the lower end and they are placed on the cross arm of the power tower. 

The number of discs used here is dependent on, type of transmission structure, the operation voltage level of transmission and also weather condition. However, each of the disc insulators in the form of a string is designed to handle 11KV. So for the 66KV power system, the tower will consist of six-disc of insulators per phase, arrange in form of a string. So the operating voltage can increase in accordance with adding more dice.

This type of insulator, is more economical for voltage rating above 33KV. Moreover, for cases where a disc is damaged, the entire disc string need not be replaced. However, since they carry the conductor at the lower end, they, therefore, experience less mechanical stress since the conductor is at a height lesser than that of the cross arm.

String Efficiency

When a transmission line above 33KV is operational. What happens is that the disc in form of a string, closer to the conductor, experiences a high voltage effect. This effect exponentially reduces as you move upward. This uneven distribution on the string discs is known as String Efficiency

The string Efficiency, on the other hand, is inversely proportional to the voltage fluctuation between discs. This inverse proportionality implies that when the voltage fluctuation between discs is low, the string Efficiency is high. 

The higher the string Efficiency to 1, the better the insulation. However, poor string Efficiency can be a burden to the disc closer to the conductor and can result in insulation failure of the disc. Again, poor string Efficiency can inefficiently lead to proper usage of the insulator.

Factors affecting string Efficiency

The string Efficiency of a suspension type of insulator depends on the following;

  • The ratio of self-capacitance to shunt capacitance 
  • Number of discs 
  • Weather condition 

How to calculate String Efficiency

The string Efficiency is literally the ratio between the algebraic sum of the voltage across the string to multiple of the number of String present and the voltage across the unit nearest to the conductor.

String \quad Efficiency\quad =\quad \frac { { V }_{1} + { V }_{ 2 }+{ V }_{ 3 } }{n*{V}_{ 1 } }

Where;

{ V }_{1} + { V }_{ 2 }+{ V }_{ 3 } represent the algebraic sum of the voltage across the string.

n represents the total number of discs in the string

Strain insulators 

This type of insulator is used for operating voltage for transmission-33KV and distribution-11KV. However, they are used for both terminal and sectional points. For terminal points, terminal H-pole is used, and this is the point where the line is being strained. Usually at the beginning and when the line end.  Although they are used at the dead-end of the line, or where there is a corner or sharp curve.

Certainly, you start a line with a terminal H pole and end it also with a terminal H pole. In other words, strain insulators are used to help withstand tension. Strain insulators consist of discs, but the number of discs present is factored on the operating voltage level. Three-disc per phase is required for 33KV line, while one disc per phase is used for the 11KV line. This implies also that each disc is designed to handle 11KV and they are placed horizontally.

Shackle insulators 

Shackle insulator
Shackle insulator

The shackle insulators are used for the mini distribution power system, with operating voltage lower than 11KV. However, during the early days, the shackle insulators were used as strain insulators. They are fixed directly to the pole or cross arm with a bolt, either vertically or horizontally. With soft binding wires, the conductors are fixed to the groove.

The shackle insulators consist of

  • Bolt
  • Shackle 
  • D-strap

You can learn more on Electrical/Electronic Engineering, by checking our Electrical/Electronic Engineering category, you can find this under Academic page. Thanks for coming around.

FAQs on Electrical Insulators

Are humans insulators?

The human body consists of electrons, having the tendency to move when subjected to an electrical field. In other words, the human body creates a pathway for the current to flow to earth. So you can simply say that the human body can conduct. However, since insulators are classified as materials that do not conduct, the human body is considered not to be an insulator. Although, this conducting property of the human body, has a lesser influence to low voltages…

What are electrical insulators used for?

As earlier discussed, electrical insulators help to electrically support and separate electrical conductors. However, Insulators are not only used for low voltage rating, but they are also used for high voltage and extra-high voltage rating even up to 330KV. This makes the study more important in the Electrical power sector. However, they help to protect humans from being exposed to an electrical shock…

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