What is Lightening?
Lightening is a phenomenon of electric discharge of a charged cloud to ground. During thunderstorms, the potential of the cloud is raised significantly higher than the ground potential and the surrounding air loses its insulating property. Essentially, the cloud and ground form a capacitor with air as its dielectric. For a lightening to occur, the potential gradient of the air must be broken, which is around 30kV/cm. if the potential difference between the cloud and ground is high enough to break the potential gradient of air, then a huge discharge occurs in the form of lightening strike. Lightening Strike causes a significant overvoltage and if it strikes a building, home or equipment, the huge potential can damage the equipment or start a fire.
How to Protect Against Lightening?
The damage caused by a lightening is destructive and you might have seen trees burning due to lightening. Hence, all the electrical equipment in power stations, substations and also overhead transmission lines must be protected from lightening strikes (both direct and indirect). Some of the commonly used techniques and equipment to protect against lightening are:
Lightening Rods Ground Wires Earth Screens Lightening Arresters
You might have seen tall buildings having a sharp rod mounted on the top. This is known as a Lightening Rod or Earth Rod. The other end of the rod is connected to ground so that when a lightening strikes that building, the potential will be grounded instead of passing through the building. Overhead Ground Wires or Earth Wires are usually placed above the towers that support overhead transmission lines. Earth Screen are usually used to protect power generating stations and substations. It consists of several copper conductors on all the electrical equipment that provide a low resistance path to ground in the event of lightening.
What is a Lightening Arrester?
All the protection devices we saw before provide protection against direct lightening strikes. But they are not that effective against travelling waves. This is where Lightening Arresters come into picture. A Lightening Arrester is a protective equipment that protects electrical equipment from high voltage surges caused by lightening strikes. They are usually installed at the top of transmission poles and buildings and provide an easy path for the surge voltage from lightening to flow to ground. In its basic form, the structure of a lightening arrester is shown in the following image. It consists of a spark gap and a series resistor. One end of the arrester (near the spark gap) will get connected to the equipment (or the overhead transmission line) while the other end is connected to ground through the resistor. Speaking of resistor, it is not a regular resistor but a non-linear resistor as in its resistance is inversely proportional to the voltage i.e., as the voltage increases, its resistance decreases and vice-versa. So, during a lightening strike, the high surge voltage will reduce the voltage of this resistor and makes it a low resistance path to ground. An important thing about the spark gap is that its length is determined in such a way that the regular operating voltage of the overhead transmission line will not initiate an arc but only very high voltages that break the air insulation will initiate the arc. Normally, the lightening arrester doesn’t conduct any current as the spark gap is open and also the resistance of the non-linear resistor is very high. But in the event of a lightening strike, the high voltage will breakdown the air insulation and initiates an arc and also the resistance of the non-linear resistor fall down. As a result, a low resistance path is laid for the surge to flow to ground. Lightening Arresters are also sometime known as Surge Arresters or Surge Diverters.
Location of Lightening Arresters
The best location for a lightening arrester to be is as close as possible to the equipment that needs to be protected. This will minimize the chances of a surge entering the circuit between the arrester and the equipment that need protection (for example, a transformer).
Classes of Lightening Arresters
Lightening Arresters are classified into three types based on their surge voltage and current handling capacities. These are:
Station Type Lightening Arresters Line Type Lightening Arresters Distribution Type Lightening Arresters
Station Type Lightening Arresters
As the name suggests, this class of arresters are employed at power generating stations and sub-stations. Station type Surge Diverters usually have a surge voltage rating in the range of 3kV to 250kV and a discharge current rating of 10,000 Amps.
Line Type Lightening Arresters
The Line type surge diverters are usually used to protect intermediate sub-stations, large transformers and medium power generating stations. This class of arresters have a surge voltage rating of 3kV to 125kV while being able to discharge a current of 5,000 Amps.
Distribution Type Lightening Arresters
These are the smallest rated surge arresters that are usually used to protect distribution transformers and pole-mounted transformers. The surge voltage rating of Distributing type surge diverters is up to 3kV while the peak discharge current is 2,500 Amps.
Types of Lightening Arresters
There are several types of Lightening Arresters that used for different applications. Some of the popular types are:
Rod Gap Arrester (Spark Gap Arrester) Sphere Gap Arrester Horn Gap Arrester Multigap Arrester Expulsion Type Arrester Electrolytic Type Arrester Lead-Oxide Type Arrester Valve Type Arrester Thyrite Lightening Arrester Impulsive Protective Gap with Electrolyte Arrester Pellet Type Arrester Metal Oxide Arrester
Let us now take a closer look at some of the above-mentioned lightening arresters.
Rod Gap Arrester
It is one of the simplest and easy to implement arresters and at the same type it is rugged for protecting insulators in overhead transmission lines and equipment. The following image shows a basic outline of a rod gap arrester. Two conducting rods, of roughly 1.5cm in diameter, are bent at right angles. One rod is connected to the circuit that needs to be protected (line) while the other rod is connected to ground. If the equipment is a transformer, the rods can be placed on either ends of a bushing insulator.
Sphere Gap Arrester
It is another simple implementation where the spark gap is provided by two spheres. One sphere is connected to the line (one secondary winding of the transformer) with a series choking coil while the other sphere is connected to ground.
Horn Gap Arrester
In this type of arrester, the air gap consists of two metal rods that are bent to form a horn like shape. This shape is important as the distance between the rods gradually decreases as we go down. There is a series resistor that will limit the arc current.
Multigap Arrester
As the name suggests, a multigap arrester has multiple air gaps with the help of several cylinders, which are made up of Zinc alloy. The top cylinder is connected to the line while the bottom cylinder is connected to ground with a series resistor. Normally, the cylinder in the center is at earth potential as it is connected to the bottom cylinder through a shunt resistor. In the event of a surge, the air insulation will breakdown and the surge will pass through the cylinders and the air gaps to the ground rather than through the shunt resistor.
Valve Type Arrester
It is also known as a non-linear surge diverter as it consists of a non-linear resistor near the ground terminal. It is relatively expensive to manufacture this type of arrester but its action is very fast.
Expulsion Type Arrester
Also known as Protector Tube type Arrester, the Expulsion type arrester can provide protection for voltages up to 33kV. Unlike other types of arresters, the spark gap in this type is inside a fiber tube. The gap consists of two electrodes, where the top electrode is connected to the line and the bottom electrode is connected to ground. There is also an external series gap, which handles the normal operating voltage. This will eliminate any leakage currents or corona discharge across the main fiber tube.
Conclusion
Lightening Arresters are simple yet very effective devices that protect equipment against lightening strikes. We learned the importance of a lightening arrester, how a typical arrester works, classes of arresters and also different types of lightening arresters. Comment * Name * Email * Website
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