Overhead Line Insulators: Types, Advantages, Disadvantages

The insulators are the supporting structures in order to prevent the current leakage to the earth. Mainly the overhead line conductors are bare and not covered with any insulating coating, so the line conductors are therefore secured with the insulators.

Insulators are mounted on the cross arms, and the line conductors are attached to the insulators to provide the insulation and necessary clearance between the conductor and the metal part. The purpose of the insulators is

• Insulate the charged conductors from earthed objects.
• Provide mechanical support to the conductors.
• Prevent short-circuiting between the different phase conductors.
• provides the necessary insulation and clearance against the high voltage.

The important properties the insulator must have:

• High mechanical strength: Must bear the load due to the weight of the line conductor, wind force and ice loading.
• High relative permitivity: to provide high dielectric strength
• High insulation resistance:to prevent the leakage of currents to earth.
• Ability to withstand the puncture and flash-over.
• Ability to with stand large temperature variations.

The material used to make Insulators

Toughened glass

Its outer surface is in compression and the interior is in tension, this state is achieved by rapidly cooling the glass insulators after shaping and allowing the interior to cool slowly.

Advantages

• High mechanical strength than annealed glass
• Cheaper than Porcelain in simpler shapes.
• Transparent, avoid partial discharge and other flows can be detected easily by visual examination.

Disadvantage

• Moisture readily condenses on the surface leading to high surface leakage(High leakage current).
• Less reliable in high voltage due to internal strain.
• Very rarely used and are used for lines below 25KV under ordinary conditions.

Porcelain

It is made from a fine homogeneous mixture of wet plastic clay(and silicon with other additives) which is covered with glaze. It is the most commonly used material for the manufacture of insulators. Glazing is necessary to make the surface free from dirt and moisture. Porcelain must be completely air free.

Advantages

• Mechanically strong than glass.
• Less trouble from leakage.
• Less affected by the change in temperature.

Disadvantages

• Glazing is necessary for keeping the insulator free from air impurities and fire gases and dirty liquids.

Steatite

It has much high tensile and bending stress than porcelain. It is usually found combined with oxides in varying proportions.

Advantages

• Can be used when the transmission line takes a sharp turn.
• used at tension tower.

Types of Insulator

1. shackle/spool
2. pin
3. Disc
4. post
5. strain

Shackle

This type of insulator is used in low-voltage lines and is very efficient and economical. Insulator is coated with an extremely hard,smooth glaze. The wet flash-over and dry flash-over for a shackle-type insulator are 10 KV and 25 KV, respectively. Puncture voltage is about 35 kV.Its operating voltage is 1000 volts. Its weight, transverse mechanical load, and total creepage distance are 0.5kg, 1150 kg, and 63 mm, respectively.

Advantages

• A tapered hole in the shackle insulator distributes the load and reduces the possibility of breakage when loaded.
• can be directly fixed to a pole with a bolt or the cross arm.
• can be used in all positions, terminal or angle.

Pin

This type of insulator was used in the earliest days for supporting line conductors. It is mounted on a pin, which in turn is secured to the cross arm of the pole. The pin is screwed on, and the line conductor is placed in the groove at the top of the insulator and tied down with soft copper or soft aluminum binding wire. A soft metal thimble (lead) is employed between the porcelain and the metal pin. The insulator and pin should be mechanically stronger to withstand the force due to the weight of the conductor, wind pressure, and ice loading.

 Compressive force on insulator due to wire

 Used in the poles type structure

 The cost of an insulator depends on the weight of material

 Varies non-linearly 𝐶 ∝ 𝑉𝑛

 Used below 33kV

Advantages

• Most economical, simple, and efficient up to 33KV.
• one pin insulator can do the work of two suspension insulators.
• Requires a shorter pole to give the same conductor clearance above the ground since the pin insulator raises the conductor above the cross arm.

Disadvantages

• when used beyond the operating voltage of 50000 volts become uneconomical.
• Pin-type insulators are very bulky and cumbersome when designed for high voltages.

Disc

The disc-type insulator was developed to overcome the problem of the pin-type insulator. When the voltage is increased, the amount of insulation required increases. At this high voltage, pin-type insulators become bulky and costly. A disc insulator consists of a number of porcelain discs connected in series by metal links in the form of a string. The disc insulator hangs from the crossarm of the supporting structure and line conductor. The distance between the crossarm and the conductor is adjusted by adding more insulators to the string.

Tensile force on the conductor due to wire

 Used in lattice or tower-type structure

Advantages

• Above 50000 volts disc type insulator becomes economical.
• In the event of failure of an insulator, one unit, instead of the whole string, can be replaced.
• It is more flexible to the line and reduces mechanical stresses.
• The disc-type insulator when used with steel supporting structures is less liable to be affected by lightning disturbances.
• In case of a rapid increase in load on the transmission line, additional insulation can be achieved by adding one or more discs to the string.
• It can be used where the heavy conductor load is to be sustained. Such as in river or valley crossings.

Disadvantages

• Large spacing between the conductor is required then the pin type insulator due to the large amplitude of the swing of the conductors.

Post

Post insulators are commonly used for supporting bus-bars and isolating switches.It is similar to pin type insulator but has a metal base with a metal cap so that more than one unit can be mounted in series.

Strain

In case of dead end of the line or the sharp corner or the line crosses river,the line is subjected to the greater tension.In such situation pin type insulator cant be used a sit cant bear the tension in the line.For the low voltage up to 11000v shackle type insulator can be used,but for the high voltage strain type insulator are used.The disc of strain insulator are employed in vertical plane whereas the suspension insulators are used in horizontal plane.

Failure of Insulators

Failure of the insulators, results in the interruption of the power supply.It may occurs due to

• Cracking of porcelain,porosity,mechanical stresses,flash-over.

1. Flash-over: Flash-over cause the unequal expansion of the porcelain. This can be avoided by arcing horns or rings.
2. Mechanical stresses:Porcelain has the high compressive strength but although its tensile strength is not adequate and insulator is weak and fails.
3. cracking of Insulators:This is the very common type of failure in the pin type insulator.This occurs due to the unequal expansion of steel,porcelain and cement during varying of the temperature.It results high stresses in the porcelain and results in failure.It can be avoided by using elastic cushions between the shells.
4. Porosity of Material:Porosity in the porcelain may be due to under firing.The pores usually absorb moisture from the atmosphere or the cement,decreasing the insulation resistivity  of the material.It gives rise to leakage current flowing through the porcelain,resulting in the rise in the temperature until the porcelain is punctured.It can be decreased by glazing the insulator.
5. short circuits: Birdage is most common in the pin type insulator. It can be avoided by providing bird guards near the insulator on the cross arm,by increasing the clearance of the conductor from earthed parts or using suspension insulators.
6. Deposition of dust: Improper glazing of the insulator causes the deposition of dust which is partially conducting and reduces the flash-over distance.It can be avoided by cleaning the insulator properly.

Partial Discharge in Insulator

Electric field(E)=V/d

If air is locked in the insulator ,the insulator,the insulation will fail before reaching the given insulation level.

E solid α 1/ε solid

E air α 1/ε air

(E solid/E air) =(ε solid/ε air)>1

Normally its value is 10.

so,the air will experiences the electric field 10 times more than the solid experiences.so partial discharge should be avoided.

 A puncture arc is a breakdown and conduction of the material of the insulator, causing an electric arc through the interior of the insulator. The heat resulting from the arc usually damages the insulator irreparably. Puncture voltage is the voltage across the insulator (when installed in its normal manner) that causes a puncture arc.

 A flash over arc is a breakdown and conduction of the air around or along the surface of the insulator, causing an arc along the outside of the insulator. Insulators are usually designed to withstand flash over without damage. Flash over voltage is the voltage that causes a flash-over arc.

Flash over

 Condition when 𝐸𝑐𝑟𝑒𝑒𝑝 > 𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑎𝑖𝑟
Most high voltage insulators are designed with a lower flash over voltage than puncture voltage, so they flash over before they puncture, to avoid damage.
Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flash overs. The flash over voltage can be reduced by more than 50% when the insulator is wet. High voltage insulators for outdoor use are shaped to maximize the length of the leakage path along the surface from one end to the other, called the creep age length, to minimize these leakage currents.

To accomplish this the surface is molded into a series of corrugations or concentric disc shapes. These usually include one or more sheds; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the ‘cup’ stays dry in wet weather. Minimum creep age distances are 20–25 mm/kV, but must be increased in high pollution or airborne sea-salt areas.

Solid insulator: 𝐸 ∝1 /𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟

Void: Pores at lower dielectric strength is exposed to higher electric field which leads to insulation failure.

ε𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟 = 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑃𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟

ε𝑣𝑜𝑖𝑑 = 𝑝𝑜𝑟𝑒 𝑜𝑟 𝑣𝑜𝑖𝑑

ε𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟 > ε𝑣𝑜𝑖𝑑
𝐸𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟/ 𝐸𝑣𝑜𝑖𝑑 = ε𝑣𝑜𝑖𝑑 /ε𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟

Hence
𝐸𝑣𝑜𝑖𝑑 > 𝐸𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟