A lightning arrester or lightning diverter is used for the protection of insulation and conductors of the electric power systems and telecommunication systems from the damaging effects of high voltage surges.
The purpose of the Lightning arrester is to discharge overvoltage to earth and thus prevent service interruption and damage to station equipment.
Stations connected over the head, transmission lines are exposed to overvoltage generated during the under storm. These voltages may cause damages to station equipment. To protect these valuable equipment lightning arresters are installed between line and earth near the equipment to discharge the overvoltage to earth.
Representative values of a lightning stroke:
Voltage – 2 x 10^8 volts.
Current – 2 x 10^4 Amps
Duration – 10-5 seconds.
Power – 8 x 10^5 kW
Working of Lightning Arrester
When a high voltage surge strikes on conducting line it travels along the conductor in the form of wave and reaches the point at which a lightning arrester is installed. The arrester provides a conducting path to the waves of relatively low impedance between the line and the ground. The surge impedance offered by line restricts the amplitude of the current flowing to the ground.
The lightning arrester provides a path of low impedance only when the traveling surge reaches the surge diverter, neither before it nor after it.
What exactly does a lightning arrester do ?
- It Diverts the Lightning to Ground
- It Does Clamp (limit) the Voltage produced by the Lightning.
- It Only protects equipment electrically in parallel with it.
- It Does not Absorb the Lightning.
- It Does not Stop the Lightning.
TYPES OF LIGHTNING ARRESTER
The following types are protective – against Lightning surges.
- Rod Gap
- Horn Gap
- Explosion Gap or Protective tube
- Surge Absorber
- Lightning arrester or Surge Diverters
The protection of line insulation insulators, equipment, and bushings is given by rod gaps or coordinating gaps. The conducting rods are provided between the line terminal and the earthed terminal of the insulator with an adjustable gap. The medium of the gap is air. The rods are approximately 12mm, diameter or square. The gap is adjusted to breakdown at about 20% below flash overvoltage of an insulator. The distance between the arc path and insulator should be more than 1/3 of the gap length.
The gap between horns is less at the bottom and it is large at the top. During a high voltage surge, an arc is produced at the bottom. This arc travels some distance along with the horn due to electromagnetic field action and heat of the arc. Thereby the length increases. The arc may blow out. Horn gap is in between 135 to 160 mm.
Expulsion gap or Protective tube
An important of the rod gap is the expulsion tube which consists of
- A series gap external to the tube which is good enough to withstand normal system voltage.
- There is no possibility of corona or leakage current across the tube.
- A tube that has a fiber lining on the inner side which is a highly gas evolving material.
- The breakdown voltage of a tube must be lower than that of the insulation for which it is used.
- When a high surge voltage is an incident on the expulsion tube the series gap is spanned and the arc is formed between the electrodes within the tube.
What is surge absorber?
A surge absorber is a device that absorbs energy contained in a traveling wave.
Corona is the result of absorbing energy in the form of corona loss. A short length of cable between the equipment and the overhead line absorbs energy in the traveling wave because of its high capacitance and low inductance.
Another method of absorbing energy is the use of Ferranti surge absorber which consists of an air-core inductor connected in series with the line and surrounded by an earthed metallic sheet called a dissipater. The dissipater is insulated from the inductor by the air as shown in the figure.
Lightning Arresters or Surge Diverter
There are three classes of lightning arresters are available now a days
The voltage ratings of section type arresters in between 3kV to 245kV and they are designed to discharge a current of 10,000 amps, they are commonly used for the protection of substation and power transformer.
The voltage ratings vary from 3kV to 123kV and can discharge current 5,000 amps. They are used for the protection of distribution transformer, small power transformer and some times the small transformer
The voltage ratings vary from 0.175kV to 0.660kV and a discharge current 1500 amps. They are used mainly by a pole-mounted substation for the protection of the distribution transformer.
Advantage of ZnO over the SiC Arrester
- High non-linearity in V-I characteristics
The non-linear index α defined as I = KVα is 30 to 40 for ZnO based elements while the same is < 5 in the case of SiC elements
- Absence of series/ spark gaps
ZnO arresters do not constitute series gaps due to high non-linear while the series gaps are essential in the case of SiC arresters
- Superior performance under polluted environments
The deposition of the polluted layer on the porcelain housing of SiC arresters leads to a significant increase in the internal temperature due to the presence of series gaps. The absence of series gaps in the ZnO arresters ensures better performance in the polluted conditions
- Absence of follow on current
As soon as the surges are absorbed the ZnO arrester regains its impedance (i.e., pre-breakdown condition) and ready for subsequent surge suppression. Contrarily, in the case of SiC arresters, the current continues to flow till the first cycle Zero even after the passage of surge. The absence of follow on current ensures smooth operation of the lightning arresters.
- High energy handling capability
ZnO ceramic elements are capable of withstanding higher energies in the range of 150 jouls/ cm^2
- Longer life
ZnO arrester have inherently better service life in the absence of series gaps and follow on current
- Superior protective characteristics
Combination of high non-linearity and faster response enables the arrester to offer controlled protective characteristics