Introduction to Substation Earthing
In this trending era of technology and development, electricity plays a vital role. An electrical power system comprises of a network of electrical elements that can be used to generate, distribute and transmit electrical energy through transmission lines. A substation is a part of an electrical power system that can transform voltage from high to low or from low to high. This helps in the transmission, distribution, and switching of the power in the system. The main components of an electric substation include an insulator, circuit breaker, bus bar, lightning arrester, and a power transformer. There are different categories of substations divided based on the power transfer across the station which include step-up type substation, step down type substation, distribution type transformer, underground distribution type substation, switchyard, customer substation, and system station.
What Is Substation Grounding Or Earthing?
Earthing of a circuit can be defined as physically connecting the circuit with the ground which has zero-volt potential to the ground i.e., the earth. Grounding of a circuit is holding a circuit to a zero-volt potential but not physically connecting it to the ground. The substation grounding system connects all the equipment, lightning mats, overhead ground wires, surge arresters, and all the metallic structures present in the substation like a network and holds them at the zero-volt potential.
Need For Substation Grounding
The major requirements of earthing of a substation are to ensure the safety of the people working in the environment, protection of the equipment in the substation and operational security of the substation. Different requirements that are to be considered for a safe earthing system is:
- Avoiding fatal electric shocks to employees working in the area of earthed facilities during a fault in the power system
- The currents occurring during normal and fault conditions must be taking a low impedance path.
- Ground faults must be cleared by improving the operation of the protective relay scheme
- Reliability and Availability of the electric power system must be enhanced
For calculating the earthing design parameters and the shock potential safety limits, a large variety of national and international limits across the globe are followed which include:
- BS7354 -1990 Code of practice for Design of High Voltage Open Terminal Stations
- EATS 41-24- Guidelines for the design, installation, testing, and maintenance of main earthing stations in substations
- IEEE Standard 80-2000- Guidelines for AC substation grounding
In a substation, all the exposed metal parts, metallic structures, generators, transformers, switchboards, circuit breakers, switches, instrument transformers, lightning arresters, surge arresters, conductors, and reactors are to be grounded using any of the above earthing guidelines so that there would be a proper grounding and there would not be any shock even when there is a fault. For example, a typical substation earthing grid for a 66KV substation is shown in Fig. 1
(image source: NTPC)
Parameters to be considered for the Designing and Construction of a Grounding Network:
- Magnitude and duration of the ground fault current must be computed to select the size of the conductors, straps and connectors used for the protective relay.
- Random loops and circuits in the network should be avoided.
- Ground circuit reactance must be reduced by minimizing the separation between the grounding conductors and their respective phase conductors.
- The return paths of the ground fault current are to be analyzed.
- Grounding network should be extended to all the island network present within the substation.
Methods For Grounding Substation
There are different methods for grounding a substation. The connection to the earth can be made in three major ways which are ring, radial, grid systems.
RADIAL SYSTEM: The radial system has connections with each of the devices present in the substation paired with one or more grounding electrodes. This method of substation earthing is most economical but least satisfactory due to the presence of huge surface potential gradients produced during a ground fault.
RING SYSTEM: The ring system is made up of a conductor that is surrounded by the substation equipment and structures and is connected via short links to each one. This method of substation earthing is economical and efficient as the ground fault currents are given a prearranged path to travel reducing the surface potential gradient.
GRID SYSTEM: A grid system involves grounding a substation where all the equipment in the substation should be grounded individually creating an earth mat. An earth mat is an earthing system where all the conductors are buried horizontally forming a grid like structure to dissipate the fault current into the earth and also form an equipotential bonding conductor system to maintain the earth resistance for all the equipment below a specified value. This system is the most effective as well as most expensive compared to the other systems. The grid equalizes the surface potential gradients and hence protects the people and the equipment from faults.
The earthing system consists of a low impedance path made up of conductors between the metallic structures and the earth. A predetermined circuit is made available for the ground fault current to flow through it rather than the random paths which lack mechanical strength and thermal capacity of carrying the fault current and lead to risking the life of individuals, damage the equipment and in worst case scenario they would cause a fire. This network should be made rigid and with no mistake because, in any case of disturbance in the ground connection, the safety equipment becomes dangerous.
This article is part of our series of articles on Lightning Arresters, Surge Protection & Earthing, you can read more with the following links:
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