Introduction to electrical insulators
In context of power systems, It would not be wrong if we change the expression to “With greater power comes greater issues” as it is always the case with power systems. Transmission of electrical power with greater stability and less losses has always been a challenging issue for power engineers. But another important and cumbersome aspect associated with high electrical power is its transmission in high voltage lines without compromising its safety. Due to high voltage in transmission lines, ordinary electrical insulators are not used as they may easily get conducted because of their low voltage bearing capability and cause serious threat to the system.
Special type of insulators, dedicatedly designed for high voltage overhead transmission lines to prevent them for faults, are used for better reliability and safety purposes. Needless to say, use of insulators in place having poor quality and low mechanical and dielectric strength will allow the current to flow to the ground through poles after breakdown of insulators and become hazardous.
Basic Terminologies of Electrical Insulators
Any material or substance that can conduct electricity through it because of its low resistivity or high conductivity is called a conductor. These are used to transmit electricity from one place to another place.
A material or substance unable to conduct electricity through it because of its high resistivity or very low conductivity is called an insulator. These are used for safety purposes in power systems for prevention of undesired flow of current and provide mechanical strength.
Sometime insulators, because of impurities and wet surfaces, may conduct a small amount of current which is named as leakage current.
Breakdown Voltage of electrical insulators
Application of voltage across a material makes electrons, bound in shells of that material, free to move. As the applied voltage increases, greater number of electrons become free and current increases. In insulators, as voltage applied increases, number of free electrons increases and conductivity goes up. If applied voltage keeps on increasing, there comes a point when number of free electrons become enough to conduct appreciable current through it and conductivity of insulator, at that instance, resides in the range of conductors and insulator now becomes a conductor. The minimum amount of voltage required to make an insulator or portion of insulator an electrical conductor is termed as “Breakdown Voltage” of an insulator.
Puncture Voltage of Insulator
When voltage across insulator becomes so high that an arc is generated across it is known as “Puncturing” of insulator and that minimum voltage at which an insulator gets punctured is known as “puncture Voltage”.
Design Features of Electrical Insulators
Insulators used in overhead transmission lines must have following features incorporated:
- High breakdown voltage to bear high voltage in transmission lines
- High mechanical strength to bear weight of transmission line
- High dielectric strength
- High puncture voltage
- Less impurities
Materials for Insulators
As obvious, insulators made of only those materials having above design features can be used in HV overhead transmission lines. Porcelain is the most common insulator material. Besides, glass, plastic and some composite materials are also used in some cases.
Type of Insulators used in Overhead Transmission Lines
Different types of insulators are used for HV overhead power lines but their use may vary from system to system and therefore, one must select properly the type of insulator for their system. Following are the most common types of insulators used in overhead transmission lines:
- Pin type insulator
- Disc type insulator
- Suspension type insulator
- Strain type insulator
- Shackle type insulator
- Post type insulators
Pin Type Insulators
Pin type insulators commonly used for transmission lines operating on voltage up to 33kV. They become expensive and heavy for the transmission line having voltage beyond 33KV. They are mostly made of porcelain however, other materials like plastic or glass are also used. Pin type insulators have a groove on upper end through which overhead transmission lines pass while bottom end connected to pole. These insulators are used in open air and can get wet during rain and other environmental conditions and as mentioned earlier, wet insulator is a source of conduction towards ground therefore, to over come this problem, pin type insulators are manufactured with sheds or petticoats.
Disc Type Electrical Insulators
Disc type insulators are used for transmission lines operating on voltage beyond 33KV.
Disc type electrical insulators are further categorized into following types:
Suspension Type Insulators
Type of disc insulators that are consist of several porcelain discs connected to each other with metal link in the form of suspension string vertically. Unlike the pin type insulators, conductor or transmission line suspended to bottom of the string of insulating discs and string connected to pole from upper end. Having multiple discs connected in series can increase the break down and puncture voltage of insulator. Each disc is designed for 11KV voltage usually and operating voltage determines the number of discs connected in series. they can easily be replaced in case of damage and more discs can be added if operating voltage increases. They can also allow the conductor to swing, to minimize mechanical stresses, which is not possible in case of pin type insulator. Hence, this type of insulator provides more flexibility than pin type insulator.
Strain Type Insulator
These type of disc insulators are consisted of porcelain discs arranged in the form of string horizontally. These insulators are connected at the end of transmission lines to provide tensile strength to conductor as conductors are subject to greater tensile loads at the end. Operating voltage determines the number of discs in a string and a greater number of strings can be connected in parallel to provide more tensile strength to the conductor.
Shackle Type Insulator
These type of disc insulators are used as strain insulators in low voltage distribution lines. It can be used in both positions i.e., vertically or horizontally and connected or fixed to the pole with bolt.
Post Type Insulator
These types of insulators are mostly used in power system up to 1100 KV lines because of their excellent mechanical properties. They are made of porcelain and composite material (silicon rubber). It can be connected in both positions i.e., vertically or horizontally with the pole.
Different types of insulators are used in overhead transmission lines based on operating voltage of transmission lines. Insulators can be used for insulation as well as for providing tensile strength to the conductors. They are made of porcelain, glass, plastic or composite materials. Disc type insulators have advantages of flexibility over pin type insulators.