Overhead Line Insulators
A device Designed to separate and prevent the flow of current between conductors and also provide insulation to the conductor from the ground.
Insulator Materials The insulators are mainly made of either glazed porcelain or toughened glass.
- The materials used for porcelain are silica (20%), feldspar (30%) and clay (50%).
- Porcelain is mechanically stronger than glass, gives less trouble from leakage and is loss affected by changes of temperature.
- Porcelain is the most commonly used for manufacture of insulators.
- The dielectric strength of porcelain should be 15 kV to 17 kV for every one-tenth inch thickness.
Drawback : Any sealed air impurity will lower the dielectric strength of porcelain.
- It has high dielectric strength (35 kV for one-tenth inch thickness).
- Glass being transparent, it is very easy to detect and flow like trapping of air.
- It has lower coefficient of thermal expansion and as a result, the strain due to temperature changes are minimized.
Drawback : The moisture condenses very easily on the surface of glass and hence its use is limited to about 33 kV
Types of Insulators There are mainly four types of insulators used for overhead transmission lines.
Pin Type Insulators
- Consists of a s ingle or multiple shells (petticoats or rain sheds).
- Multiple shells are provided in order to obtain sufficient length of leakage path.
- Multiple shells increases the flash over voltage between the power conductor and the pin of insulator
- Pin type of insulators are used only upto 33 kV lines because for higher voltages they tends to be heavy and more costly.
- The cost increases very rapidly with increase in line voltage. cost µ Vx ; where x > 2
1. Pin type Insulators The pin type insulator is designed to be mounted on a pin which in turn is installed on the cross arm of the pole.
For lower voltages generally one piece type of insulators is use. For high-voltage transmission lines larger, string pin type insulators are used. The highvoltage pin type insulators differ in construction from low voltage type in that they consist of two or three pieces of porcelain cemented together. These pieces from what we call petticoats are designed to shed rain and sleet easily. These are available for use up to 50,000 volts.
The main advantage of the pin type insulators is that it is cheaper.
2. Suspension Type Insulators With the increase in operating voltage, the insulation required increases. Transmission lines use extremely high voltages, 220 kV, for example. At these voltages the pin type insulators become bulky, cumbersome and costly. Besides, the pin which must hold it would have to be inordinarly long and large. In order to meet the problem of insulators for these high voltages, the suspension insulator was developed.
3. Strain Insulators Sometimes a line is to withstand great strain, for instance at a dead end or at a corner or on sharp turns. In such a circumstance for LT (low tension) line shackle insulators are used but for HT (high tension) transmission lines strain insulators consisting of an assembly of suspension type insulators are used.
4. Shackle Insulators The shackle or spool type insulator, which is easily identified by its shape, is usually used on it lines, Both the low-voltage conductors and the house service wires are attached to the shackle insulator.
5. Egg or stay Insulators.
Such insulators are of egg shape and used in guy cables, where it is necessary to insulate the lower part of the guy cable from the pole for the safety of the people on the ground. These are provided at a height of about 3m from the ground level.
Suspension Type Insulators
- Also known as disc or string insulators.
- Most commonly used disc is the cemented cape type.
- Used for lines above 33 kV.
- Each insulator is designed for 11 kV and hence for any operating voltage a string of insulators can be used.
- In case of failure of one of the units in the string, only that particular unit needs replacement rather than the whole string.
- Since the line is sus pended flexible, the mechanical stresses are reduced.
- The operating voltage of the existing transmission line can be increased by adding suitable number of discs in the string instead of replacing all the insulators.
- These are used for supporting the bus bars, and disconnecting switches in sub-stations.
- These are similar to a pin type insulator bus has a metal base and frequently a metal cap so that more than one unit can be mounted in series.
- In extra high voltage sub-stations (400 kV and above polycon post insulators are used.
- These insulators is are puncture proof, solid core insulators for outdoor use.
- These are mechanically strong suspension insulators
- Strain type of insulators are used to take the tension of the conductor at the line terminations and at positions where there is a change in the direction of line.
- The discs of a strain insulator are in a vertical plane.
An insulator string is a chain of two or more strain insulators that are coupled together to increase the total insulation level of the assembly.
- In the figure show below each disc is represented as a capacitor C and the complete string acts as voltage divider.
- The capacitance of each joint to the earthed tower can be assumed to be kC where k lies between 0.1 and 0.2.
- Let Vn is the voltage across n units from the top and vn is drop across nth unit.
- String efficiency of an insulator of n units is defined as
- String efficiency decreases with an increase in the number of units.
- The voltage drop across the unit nearest to the cross arm is minimum and it goes on increasing as we go towards the power conductor.
- The current through topmost unit is minimum and the voltage drop across that unit will also be minimum.
- The lower most unit in a string is fully stressed or fully utilized.
Improvement of string efficiency Whgy it is required?
- The voltage distribution across an insulator string is not uniform.
- The units nearest to the line end are stressed to their maximum allowable value while those near the tower end are considerably understressed resulting of insulating material.
Method to Improve String Efficiency
- By reducing capacitance of shunt branch relative to capacitance of each unit.
- Shunt capacitance can be reduced by increasing the length of cross arm.
- Capacitance grading means an increase in the capacitance of each unit from the tower end towards the line end.
- The voltages across the different units can be made exactly equal by correct capacitance grading.
- This is a seldom method because it implies that all the insulator discs are different from one another.
- This met hod us es a la rge me tal ring surrounding the bottom unit and connected the line.
- This ring, known as a grading ring. It introduces capacitances between different joints and line.
- This has the effect of increasing the effective capacitance of bottom units.
Remember: Every insulators string is provided with an arcing horn at the tower end and the line end so that in the event of insulator flashover, the arc may take the path between the horns and stay clear of the insulator string.
- Static shielding is achieved by changing the arcing horn at the line end to a grading ring.
- The design of the ring should be such that this gives rise to the capacitances which will cancel exactly the charging current in that particular section, such that
where V is the operating voltage and Cn is the capacitance between the guard ring and the pin of the nth unit.
Let V = mv, where k is the number of units used.
wnvkC = (m – n)vwCn
Long Cross-arm Method It is clear from the expression of string efficiency that the string efficiency increase with the decrease in value of K. The ratio of capacity to earth/capacity per insulator, K can be reduced by making use of long cross arms. But the limitations of cost and strength of the supports does not allow the cross arms to be too long.