Power stations and sub-stations are protected against direct strokes of lightning using:
Right Answer is:
Power stations and sub-stations are protected against direct strokes of lightning using Earthing screen.
Types of Lightning Strokes
There are two main types of lightning strokes that appear on various equipment in the power system. They are viz. Direct Stroke and Indirect Stroke. Direct stroke may appear on line conductor, on tower top or on the ground wire indirect stroke may appear on overhead line conductors.
Direct Stroke on Overhead Conductors
These strokes are most dangerous as their effects are most severe and harmful. In this type of stroke, the discharge or the current path is directly from the cloud to the overhead line. From the line, the current path may be over the insulators down the pole to the ground. The voltage set up is in millions which can cause flashover and puncture of insulators. The insulators may get shattered till the surge is sufficiently dissipated and it travels to both sides. The wave may reach the substation and damage the equipment because of excessive stress produced.
The earthing screen and the ground wire provide protection against direct lightning strokes but do not provide any protection against travailing waves which may reach the electrical apparatus. The lightning arresters or the surge arresters are the ones who provide protection against the travailing waves.
The effect of indirect strokes is similar to that of direct strokes. Their effect is more severe in the case of distribution lines than in the case of high voltage lines. These strokes are due to electrostatically induced charges on the conductors due to the presence of charged clouds. Sometimes currents may be induced electromagnetically due to lightning discharge in the immediate vicinity of the line which results in an indirect stroke.
Commonly used equipment for protection against lightning strokes is as given below.
- Earthing screen.
- Overhead ground wires
- Lightning arresters or surge diverters.
To protect the generating stations and the substations from direct strokes, an earthing screen is used. Overhead ground wires protect the overhead transmission line from direct lightning strokes. The protection against both kinds of strokes direct and indirect in the form of travailing waves is provided with the help of lightning arresters.
It consists of a network of copper conductors, earthed at least at two points, overall the electrical equipment in the sub-station. When a direct stroke occurs on the station, the screen provides a low resistance path by which Lightning surges are conducted to the ground. In this way, station equipment is protected against damage.
An Earthing Mat is the interconnection of the Horizontal and Vertical electrodes. The vertical Electrodes are for the dissipation of fault current into the ground while horizontal electrodes are laid for suppressing the dangerous Touch and Step voltages that are generated due to heavy fault current.
A substation is an assembly of large no of electrical equipment. All the equipment, as well as working personnel, must be safe even in adverse conditions.
The substation grounding system is an essential part of the overall electrical system. The proper grounding of a substation is important for the following two reasons:
1. It provides a means of dissipating electric current into the earth without exceeding the operating limits of the equipment.
2. It provides a safe environment to protect personnel in the vicinity of grounded facilities from the dangers of electric shock under fault conditions.
The grounding system includes all of the interconnected grounding facilities in the substation area including the ground grid, overhead ground wires, neutral conductors, underground cables, foundations, deep well, etc. The ground grid consists of horizontal interconnected bare conductors (mat) and ground rods. The design of the ground grid to control voltage levels to safe values should consider the total grounding system to provide a safe system at an economical cost. After the construction of Earth Mat, Crushed Gravels (of resistivity around 5000 ohms) are laid above the whole mat area. This makes the high resistive path for the dangerous voltages generated(Touch, Step Voltage), which in turn prevents a person from electric shock standing above the Earth mat area.
There are some standards that guide the grounding of the substation.
As per IEEE standards, earth mats are designed with vertical ground rods, and earth mat which brings down the overall resistance of the substation below 1 Ohm. The Earth mat spreads across the substation. All the equipment is connected to the earth mat with two separate earth flats.
A good grounding scheme will ensure faster fault clearing and a low enclosure potential rise.
The limitation of this method A that it does not provide protection against the traveling wave which may reach the equipment in the station.