Lightning arrester is a device which provides easy conducting path for the flow of current when the system voltage is more than designed value and acts as insulator when the system voltage is at normal level. Lightning arrester is also known as surge diverter. Lightning arresters protect system equipment from the high transient voltage which is produced in the system due to striking of lightning on the line. Lightning arrester works as an insulator under normal conditions and as conductor under abnormal conditions. The lightning arrester are placed as near as possible to the equipment to be protected against overvoltage. If the lightning arrester is installed to protect the transformer, it is placed on the terminals of the transformer.
There are various types of lightning arrestor are used in the power system. The most common used types of lightning arrester are:
- Rod Gap Arrester
- Horn Gap Arrester
- Multigap Arrester
- Thyrite Arrester
- Valve Type Arrester
1. Rod Gap Arrester
The rod gap arrester as the name suggests consist of two rods which are bend at right angle. These rods are placed at the bushings of transformer or across the string of suspension type insulators. Rod gap arrester are the simplest type of transformer because of its construction. In rod gap arrester, One rod is connected to the line and the other rod is connected to the ground. The gap or distance between two rods is adjusted according to system voltage.
The rod gap arrester are used as a back up protection because of following limitations:
- The surrounding atmosphere affects the performance of arrester.
- The polarity of the surge also affect the operation of lightning arrester.
- The heat produced by the arc melts down the tip of rod which increase the rod gap and thus disturbs the setting of lightning arrester.
- Once the arc is established between the rods, it may continue even when the system reached at its normal voltage.
2. Horn Gap Arrester
Horn gap arrester consists of two metal rods which are bent in the shape of horn, this is the reason why these arrester are called horn gap arrester. The metal rods are placed in a vertical plane. One rod is connected to the line through resistor and a choke coil and other rod is connected to the ground. The resistor which is connected in series between line and rod is a limiting resistor. This resistor is used to limit the current to a small value. The choke coil between line and rod is used to suppresses the high frequency waves. The gap between the horn rods are set according to system voltage.
Due to the horn type construction, The lower point of rods AB has small gap while the higher point CD has larger gap. The gap between the two rods are set in a such way that the flash over occur between point AB at a voltage nearly 1.5 to 2 times to that of normal voltage. At abnormal condition, An arc is struck between point AB and goes up to point CD, After that arc extinguished.
Horn gap arrester are used as back up protection because of its poor performance. These arrester have following limitations:
- The atmospheric conditions affects the performance of Horn gap arrester.
- Due to open environment, Corrosion occur in metal rod horns which affects the setting of lightning arrester.
- The gap can be bridged by birds which affect the operation of lightning arrester.
- The breakdown voltage of gap changes with change in frequency.
3. Multi-Gap Arrester
Multi-gap arrester consists of a multiple numbers of metal spheres. These metal spheres are separated by air gap, That is why this arrester is called multi gap arrester. The first metal sphere is connected to the line and the last metal sphere is connected to ground through a series resistance Rse. The series resistance is basically a limiting resistor which is used to limit the current in the system but it also reduces the efficiency of the lightning arrester. To compensate the adverse effect of series resistance on efficiency, One resistance is connected in parallel with the circuit. This resistance is known as shunt resistance Rsh and is connected across a certain number of gap. The gap between metal spheres are set according to line voltage. Multi gap arrester can be used up to 33kV lines.
Multigap arrester has following limitations:
- The atmospheric conditions affect the breakdown voltage of multi gap arrestor. In rainy season, due to moisture the breakdown voltage of arrester decreases.
- The gap between the metal spheres can be bridged by birds which affect the operation of lightning arrester.
- Due to corrosion of metal spheres, The setting of lightning arrester get affected.
4. Thyrite Arrester
Thyrite arrestor consists of thyrite discs which are connected in series. Thyrite is basically an inorganic compound of ceramic nature. It act as a insulating material at low voltage and as a conducting material at higher voltage. Due to this property, it is an excellent choice for a lightning arrester. The thyrite discs has a diameter of 15 cm and thickness of 19mm. The number of discs depends up on the system voltage. To give an electrical contact between thyrite discs, Both sides of discs are metal sprayed. The disc assembly is placed inside a porcelain container.
The thyrite type arrester has no time lag in its performance. When the abnormal high voltage occur in the system due to lightning, the resistance if thyrite discs decreases immediately and provide easy conducting path. After that, the thyrite discs regain the properties of insulator.
5. Valve Type Arrester
The valve type arrester consists of a number of flat discs which are stacked one above the other. The flat discs are made of porous material and separated by the thin mica rings. The mica rings are used to provide insulation during normal operating conditions. The flat discs are installed inside porcelain casing with rain sheds which protect the discs from moisture and humidity. The single disc can withstand up to 350 volts. The discs inside porcelain casing are arranged in such a way that, there is no discharge occur at normal system voltage. Under normal voltage, the discs offer high resistance path and no current flow through the valve type arrester but at abnormal high voltage due to lightning, these discs act as conductor.
The valve type arrester are mostly used in substations these days because they have following advantages over other arresters:
- The initial cost of valve type arrestors is low as compare to other arresters.
- These arresters are very robust in construction.
- The maintenance cost of these arresters is very low.
- The valve type arresters does not require daily supervision.