Principles of Lightning and Surge Protection
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Lightning strokes, equivalent to 200 kA or 300 kV, cause hazards to the equipment or location, thus lightning protection is crucial for operation. Let us start with what is lightning and why lightning protection is very important. The basic phenomenon behind lightning is that charges accumulated from the cloud and the earth are equal and opposite. This forms a non-uniform potential gradient surface in the air. When this gradient is greater than the potential of the surface, the breakdown occurs and a “streamer” flows from the cloud towards the earth.
A direct stroke occurs when the lightning hits the power systems directly that its immense potential will cause destruction to the equipment or to the facility. In contrast, an indirect stroke occurs from the lightning discharges within proximity of the power line or from the electrostatic discharge on the conductor due to the charged clouds.
The main power system elements requiring lightning protection are power feeds, security systems, telephone lines, data and control systems and RF cables.
Methods of Lightening Protection
The rolling sphere method is used to identify the exact placement of the lightning and surge protection devices near the equipment under operation. Protection of the power line against direct strokes is through ground wire or protector tube. The former produces electrostatic screening, which is affected by the capacitance of the cloud to line and the line to ground. The latter forms an arc between the electrodes, causing gas deionisation.
It is interesting to note that the building and rooftop frame or cladding is preferably metal than insulation type. Installation of a finial at the top of the power tower should have a minimum distance of 1.5 m above the highest antenna or lights. Such rooftop or building frame is made of reinforced steel for protection purpose. Wooden towers without downconductors may cause a fire hazard, as they provide route for incoming charges to ground. In principal, for non-metallic roofs, proper downconductors should be installed at the appropriate location and height.
Antenna lightning protection is provided through spark gap, a gas discharge tube and quad-wavelength shorted stub. The first method uses ball points so that if a strike occurs, high potential forms between them and the ground. The second method causes gas deionisation through arc formation between the electrodes. The last method uses a coax transmission line across the transmission line so that system bandwidth is narrow. A lightning arrester is a device offering lightning protection by regulating spark gaps. The device classification may range from rod gap, horn gap and valve type to metal oxide lightning arresters.
Earthing & Bonding Solutions
Now let us discuss how earthing and bonding solutions for lightning protection should be afforded. The design of earth rods, terminals or clamps should be to route the incoming transients to earth with minimise step and touch potentials. The geometric measurements should comply with the IEEE and NFPA standards. Any earthing system should have proper bonding, as ground potential rise must not be compensated. Again, the number of interconnections and spacing should be designed according to lightning standards.
The device ideal for protection against travelling waves is a surge diverter, connected between line and earth at the substation. Its purpose is to divert the excessive incoming voltage to ground by developing low impedance between the line and earth. Surge protection is essential as overvoltage may damage lightning protection devices and others across the line. Surge measurements can be performed based on the Faraday principle or remote monitoring with sensors. If there are overvoltage devices, they should be placed between surge arresters or diverters and the control equipment. Surge protection for telephone cables is through a setup of a gas arrester, metal oxide varistors and suppressor diodes. The primary protection which surge diverters need to be installed is on the main line. The secondary protection is for the lines, field apparatus and outdoor devices (CCTVs, security devices or towers). In addition, secondary stage protection can be provided with rod gap arresters and the like.