High Voltage Insulator


Introduction

The purpose of the insulator or insulation is to insulate the electrically charged part of any equipment or machine from another charged part or uncharged metal part. At lower utilization voltage the insulation also completely covers the live conductor and acts as a barrier and keeps the live conductors unreachable from human being or animals. In case of the high voltage overhead transmission and distribution the transmission towers or poles support the lines, and insulators are used to insulate the live conductor from the transmission towers. The insulators used in transmission and distribution system are also required to carry large tensional or compressive load.

Here our brief discussion will be restricted to high voltage insulators used in transmission lines and substations.

The HV/EHV insulators are broadly divided into two types based on the material used. One is ceramic and the other is polymer (composite) insulator. In Fig-A is shown the sketch of a porcelain disc insulator unit and in Fig-B is shown a glass disc insulator.

Traditionally ceramic insulators of porcelain are used in both transmission and distribution lines.
Now polymer or composite insulators are increasingly used in high voltage transmission systems. The polymer insulators have a fibre rod surrounded by outer sheath of some polymer. Due to the hydrophobic nature of the polymer insulator surface, dry areas are formed between wet areas resulting in discontinuities in wet creepage path. This phenomenon helps improve the performance of the polymer insulator in polluted and coastal areas. The polymer insulators has one great advantage that it is quite lighter in comparison to porcelain insulators. It is reported that the polymeric insulator surface degrade faster in comparison to porcelain insulator. One important disadvantage with porcelain insulator is that the porcelain insulators can bear large compressive force but less tensional force. The porcelain insulators surface is hydrophilic in nature, which means affinity for water. Polymer insulators age faster than ceramic insulators.


Below are few definitions in relation to insulator that one should know which are required here to understand some concepts.

Creepage Length -The creepage length is the shortest distance between two metallic end fittings of insulator along the surface of insulator . In the string of insulators for creepage length calculation the metallic portion between two consecutive insulator discs is not taken into account.
The corrugation below the insulator is for the purpose of obtaining longer creepage path between the pin and cap. The corrugation  increases the creepage length so consequently increasing resistance to the insulator leakage current. The leakage current that flows through the surface of  insulators should be as little as possible.
The creepage distance required  in clean air may be 15 mm per kiloVolt (line voltage). In the polluted air depending on the level of pollution of air the required creepage distance increases.

Flashover distance - It is the shortest distance through air between the electrodes of the insulator. For a pin type insulator shown  in Fig-C the double headed red arrow line is flashover distance.

Flashover voltage - The voltage at which the air around insulator breaks down and flashover takes place shorting the insulator.

Puncture voltage - The voltage at which the insulator breaks down and current flows through the inside of insulator. 


An insulator may fail due to excessive electrical stress, excessive thermal and mechanical stress or degradation due to environmental chemical action of surface of the insulator. The electrical failure can happen between conductor and earth through air or through the volume of insulating material. In one case due to excessive electric stress the insulator may fail when a flashover takes place through the air between the conductor and tower. In other case the insulator may be punctured through the volume. The insulating materials say porcelain has high dielectric strength in comparison to air. The insulators are designed so that it will flashover before it gets punctured. Failure due to flashover is generally temporary and self restoring. But failure due to insulation puncture is permanent and the insulator is damaged and required to be replaced. An insulator which have internal defects like voids and impurities, reduces the electrical strength of the insulator.
The flashover may results in damage of insulator glaze which can be repaired. In polluted regions contaminants deposit on the surface of the insulator that results in reduction of the flashover voltage of the insulator in wet condition. For example if the power frequency flashover voltage of a 33 kV pin insulator is 95 kV in dry then in wet condition the flashover voltage may be reduced to below 80 kV. Insulators are designed to withstand flashover voltage. In this example you can observe that even in the wet condition the flashover voltage (80 kV) is more than twice the insulator working voltage (33 kV).

The other important electrical parameters of insulator are Electromechanical failing load, lightning withstand voltage and switching impulse withstand voltage etc.. HV Line insulator requirement is based upon the creepage length. The switching impulse withstand voltage is particularly more important in case of Extra High Voltage (more than 300 kV) and Ultra High Voltage lines.

Insulators of different design are available for different applications some cases are outlined below.

Suspension Insulator

The suspension insulators are used to support conductors in high voltage transmission lines. The suspension insulators string used in transmission lines are obtained by joining several disc insulator units. according to the type of hardware fittings, usually two varieties of disc insulators are used in HV transmission line. These are cap and pin type and ball and socket type. A porcelain cap and pin disc insulator is shown in Fig-A. Also in Fig-B is shown a glass disc insulator. In the porcelain insulator the somewhat umbrella like upper  part called skirt is glazed and smoothened so that when it rains the dust and salt deposited on it are easily washed away. The contaminants cannot easily penetrate the glazed surface. When it rains the lower corrugated part does not wet and remains dry. This dry portion is the effective creepage length in wet condition.
In the transmission line a string of disc insulators are formed by fitting the pin of one disc to the cap of next disc. Simply by adding more  numbers of discs in the string the insulator string is used for higher voltage. Moreover when one disc is damaged only that particular disc is replaced not the whole string.

Pin Type Insulator

The pin type insulators are suitable for use in low and high voltage distribution systems. Actually in distribution lines you will hardly find any other type of insulators. Pin type insulators are not usually used above 33 kV as the insulator size will become large and costly and unfeasible. See the figure-C for a pin type insulator.

Post Insulator

The post type insulators are mostly used in high and extra high voltage substations. In the substation Post type insulators are used for supporting equipments and Bus conductors. See Figure-D for a post type insulator.

The post insulator is manufactured as single unit from porcelain or composite material. The post insulators are also required to have sufficient bending strength and torsional strength. Both porcelain and polymeric post type insulators are used in practice.



Transmission Tower Types


In the last article we discussed about the transmission line main accessories. Now we will discuss about transmission tower or pylon types.  The transmission tower is an important accessory and the performance of the transmission line depends very much on the design of the transmission tower. The electric transmission towers or pylons can be classified several ways. Here we will try to classify it broadly. The most obvious and visible tower types are
  • Lattice structure
  • Tubular pole structure
Varieties of tower types are used in practice. Traditionally self supporting lattice structures are used for electricity transmission line towers (see Fig-A). You will mostly find the use of self supporting type lattice structures for transmission lines in most of the power companies . The lattice structures can be erected easily in very inaccessible locations as the tower members can be easily transported. Lattice structures are light and cost effective. The main disadvantage of lattice structure is that it requires more ROW (Right Of Way). Right Of Way is the stretch of land acquired along the route length of line keeping the towers in the middle of ROW width. See Fig-D where the width of ROW is shown by double headed arrow. The ROW width is as per the standard set by Local authority or government agency. Clearly ROW is more for higher voltage line.
In the sketch of a single circuit lattice tower (Fig-A), two numbers of ground conductors are used. Theta is the shield angle. For reliably protecting the conductors from lightening this angle θ should be less than 30 degrees. In the Fig-A the phase conductors used are bundled type (twin conductor).


 In many cases due to public resentment  the use of lattice structures has been restricted. So alternative transmission structures are adopted by some power companies.


Steel tubular pole structures  have been used quite successfully by some power companies for high and extra high tension transmission lines.  The installation of these structures are costly but requires less time. See the sketch of a tubular steel pole structure (Fig-B). The tubular structure can be a single tubular form or H-form. Like Lattice tower it can also be designed for carrying two or more circuits. A lattice tower with double circuit is shown in Figure-D. More transmission companies are considering the use of this type of tower especially in populated areas.




The lattice guyed-V transmission towers has also been used by the transmission companies in cases where more space is available.. These are simple, easy and cheaper to install. The guyed towers also require less time for installation. The main disadvantage is that these towers require more space due to presence of guy wires. See the sketch of the tower (Fig-C). This tower uses two string insulators per phase arranged in V form.


Another classification is from the point of view of materials used. The transmission towers are usually made from steel and galvanized steels. Aluminium is also used as construction material for transmission lines. In many countries wooden transmission towers are also used for HV/EHV transmission, if plenty of wood of considerable length(or height) is available at reasonable cost. The wooden towers are mainly single pole or H-frame type.


Even concrete poles/towers are used by transmission companies of some countries for HV/EHV power transmission.

Angle Towers

Another main classification is from the point of view of functioning of tower. That means whether the tower is suspension type, angle type or dead end type. Depending on the deviation angle of the line the respective tower is chosen. The suspension type of towers only carry the load of the conductor in normal situation. However suspension towers are usually designed to work satisfactorily for very small angular deviationn of line. The standard code of practice of different countries has specified the maximum deviation angle for use of suspension towers. The angle towers are used when the line route deviates more than this specified maximum angle. The angle towers can again be sub grouped for different ranges of angular deviation. So the towers can be categorized as small angle, medium angle or large angle towers. The towers used at the termination point of line are dead end towers and are designed to carry large unbalanced load. The dead end towers are the strongest and heavy. In practice large angle towers are designed so that they can be used as dead end towers. Doing so will eliminate the need for designing one more tower type that is dead end. The angle towers use tension insulator strings. See the picture below.



The numbers of transmission towers required to be erected per kilometer depends on the topography of the line route. So the span length of the line depends on the topography. For a particular conductor, the span should be such so that under highest temperature the line maintains minimum clearance (as per local standard) to ground or other nearby objects.


Obviously more towers are required to be erected per kilometer in hilly or other difficult terrain. When the path of line deviates more often from the straight route then the line requires more towers per kilometer. Angle towers are used whenever the line route such deviates so that the suspension tower cannot be used.


From the above discussion it is clear that the choice of transmission tower types depends upon several factors. Also you must have observed that when the tower carries only one circuit then the phase conductors  are usually arranged horizontally(or triangular form). In this arrangement more Right Of Way is required but the tower height will be less, resulting in saving in tower materials so tower cost is reduced. When the transmission tower carries two or more circuits, then the phase conductors are usually arranged vertical (See above photograph and Fig-D sketch). In this configuration the requirement of Right Of Way is less but tower height is more. Usually this is the choice in double or multi-circuit case.  Sometimes double circuits are also arranged horizontally and single circuit vertically according to the availability of Right Of Way and optimized total cost.


Tower Foundation

The type of tower foundation depends on the soil type where the tower is to be erected. Some common  foundation types are for dry soil, wet soil, rocky soil, sandy soil and submerged type. In most cases existing standard design can be adopted to reduce the overall cost of tower installation. In the earth quake prone areas, data pertaining to seismic activity  of the area is very important for consideration in foundation. The foundation cost on river bank or river bed is much more than on plain land.

The design of transmission tower and line is complex which need to consider loading under different conditions. Several softwares are available in the market for the analysis and design purpose.