Types of Insulators used in Power Systems

We have various types of overhead line and substation insulators. Generally they can be classified as:

• Post insulators
• Cap and pin insulators
• Long rod insulators

Post Insulators

The post insulators comprise of pedestal posts and solid core cylindrical types.

Pedestal post insulators

The pedestal post insulator stacks employed in substations are available as single units with a range of lighting impulses withstand ratings (LIW) from 60 to 250 kV per unit. The figure below shows an example of pedestal post insulator.

The pedestal post insulator has a high bending strength of up to 310 KN. The units have standardized top and bottom fixing arrangements such that insulator stacks may be built up with bending strengths varying from the maximum required at the base to the minimum at the top. As many as 12 such units may be needed to form a post insulator for a 550 kV-rated voltage system.

Solid core cylindrical post insulators

The shed shapes in this type are normally simplified for ease of production since the units are cast in cylindrical form and machined on vertical milling machines before firing. The cylindrical post insulators may also be made from individual sheds cemented together. This permits complex shed profiles to be obtained at the expense of cost and use of special cements to overcome any degradation problems in service.
Also, the creepage distance (distance measured along the underside of the insulator sheds) can be increased by using alternate long short (ALS) insulator shed profiles without increasing the overall insulator stack height.

Standardized top and bottom fixing arrangements permits stacks to be formed from an assembly of single units. Typically one unit may be employed for a 72 kV-rated voltage system and up to four units for a 550 kV system. The number of units employed is usually determined in conjunction with the manufacturers of the insulator.

Hollow insulators

Hollow insulators are used by substation equipment manufacturers to house post type current transformers (CTs), voltage transformers (VTs) & capacitor voltage transformers (CVTs), cable bushings, circuit breakers supports with central operating rods and interrupting chamber assemblies, isolator supports, etc.

Cap and Pin Insulators

Cap and pin insulators of porcelain or glass dominate as overhead line suspension or tension sets above 33 kV. They are also used for substation bus-bar high level strained connections. Practically any creepage distance may be obtained by arranging the needed number of individual units in a string.

The upper surface shed shapes are similar with the top surface having a smooth hard surface to prevent the accumulation of dirt & moisture and a slope greater than 5° to support self-cleaning. The undersides have a considerable variation in shape which depends on the aerodynamics and creepage distance requirements.

Suspension insulator sets are seldom used in substation designs and substation bus-bar tension sets avoid the use of the anti-fog profiles because the deep rigs may not be naturally cleaned by rainfall when mounted nearly horizontally with short spans.

Overhead line can and pin insulators are specified with corresponding higher failing loads of 125 KN and 190 KN. Since substations short span applications do not require high strength cap and pin units, the insulators are usually specified with 80 KN minimum failing electromechanical test load to meet the 3 x safety-factor requirement.

Related: Surge Suppression in Power Systems

Long Rod Insulators

The long rod insulators are similar to porcelain solid core cylindrical post insulators except, the top and the bottom fittings are of the cap and pin type.

The long rods are an alternative to the conventional cap and pin insulators with possibility of providing longer creepage paths per unit length however long rod insulator sets have not demonstrated in overhead lines any noticeable improvements in performance under heavy pollution conditions. Besides the mechanical performance of porcelain under tension is such that brittle facture could easily cause a complete failure of the whole unit leading to outage condition. Conversely, the cap and pin insulators using toughened glass or porcelain are designed such that they do not present a brittle fracture characteristic.

Since cap and pin insulators are able to support the full tensile working load with glass shed shattered or all the porcelain shed broken away, they are more frequently used for overhead line work.

Related: Switchgear

Insulator Materials

There basic materials are used in the manufacture of the insulator types discussed above. These materials are:

• Polymeric composite
• Glass
• Porcelain

Polymeric & Resin materials

Overhead line polymeric insulators are also called silicone insulators. They offer several advantages:

• Reduced weight
• Have a high creepage offset
• Offer resistance to the effects of vandalism since the sheds do not break on impact
• Some composite polymer insulators can perform well in instances of pollutions

Epoxy resin cast insulators are extensively used in indoor substation equipment up to 66 kV and metal enclosed switchgear. Epoxy resin has been used to a limited extend on medium voltage current transformers installed outdoors specifically on neutral connections where the insulation is not subject to the same dielectric stress as the phase conductor supports.

Cap and pin insulators used on low voltage distribution lines and long rods at transmission voltage levels may employ composite insulators based on high tensile strength core of glass fiber and resin. The insulator sheds are bonded to this core and are made from silicone & ethylene propylene flexible elastomers.

Glass and Porcelain

Toughened glass has the advantage for overhead lines that broken insulators tend to break completely upon impact and therefore they are more easily spotted during maintenance inspections. However, glass insulators are rarely used in substations equipment since on breaking they leave only around 15 mm between the top metal cap and the pin. Porcelain insulators, which may be chipped but not shattered, are therefore preferred for substation use.

Bottom Line

The conductors are normally attached to their support by means of an insulator unit. For overhead lines up to 33 kV and for outdoor substation equipment the insulator that is usually used is the post insulator type. For overhead lines above 33 kV and substation aerial bus-bars, suspension or tension cap and pin or long rod insulator units are used. The insulator sets must be able to support the conductor under the most difficult loading conditions; besides voltage flashover must be prevented under the worst weather and pollution circumstances with the leakage currents kept to negligible proportions.

Related articles:

• DC Transmission and Distribution
• Overhead vs. Underground Power Distribution Systems
• AC Transmission and Distribution Systems