Earthing in Substations

Introduction:

Earthing is the process of connecting the current carrying and non-current carrying metal parts of an electrical system to dissipate the currents to the ground. Substations are one of the major parts of electrical network. Any abnormality in substation will lead to isolation of all the areas which are under the control of that substation. IS 3043 explains the earthing procedures for substations and some of the recommendations will be discussed below in this article.

Earthing in Sub stations:

The electrical energy from the power plants are stepped up to very high voltage levels and transferred through the transmission lines. These high voltages are stepped down in various levels before reaching the end user. The stepping up and stepping down of voltages at different levels of transmission and distribution are done at the sub stations. Transformers, Circuit Breakers, Isolators, CT, PT, Lightning Arrestors and many protective components are used for this process. Our electrical network is a very complex network and the transmission lines run across the country to interconnect the remote villages with the network. The stability of electrical network is continuously monitored from these stations and necessary actions are taken during any abnormalities.

The transmission lines are more prone to voltage surges due to both switching operations on lines and lightning strikes. These surges should be properly diverted to earth as soon as possible for protecting the components in substation.

The magnitude of fault current in substation during ground fault will be very high and hence proper earthing system with low resistance value should be designed to dissipate the fault current as soon as possible.

The neutral of the transformers will also be earthed at the substations. The ground voltage acts as the reference for system voltage. The ground potential rise in substations during the faults will be of very dangerous levels and it can cause electrocution even without any direct physical contact with any equipment.

In general, earthing installations will be required at power stations and substations for:

The neutral points of each separate electricity system which has to be earthed at the power station or substation.
Apparatus framework or cladding or other non-current carrying metalwork associated with each system, for example, transformer tanks, power cable sheaths.
Extraneous metalwork not associated with the power systems, for example boundary fences, sheaths of control or communication cables.

The earth system should remain effective throughout the life of the plant.

The conductors selected for grounding and bonding in substations should have adequate current carrying capacity and should be able to dissipate the currents without any damage.

On high voltage systems with directly earthed neutrals, potential of earthed metal parts will not be near true earth potential during the passage of earth fault currents. The entire ground potential will get shifted due to the ground potential rise.

The rise of potential of a earthing installation should be as low as practicable since this potential will be applied across the telephone cables or cable sheaths through the common earthing.

If surge protection is provided, the connection of the protective devices to earth should be as direct as possible. The discharge of high currents with high-frequency components requires earth connections of low resistance and reactance.

Recommendations for Neutral Earthing:

Where the neutral points of two electrically separate electricity systems are connected to a common earth electrode system at a site, there is a coupling of the systems in the event of an earth fault occurring on either system by virtue of the rise of earth potential due to the passage of the fault current through the earth electrode system.

If complete separation of electrical systems were required, it would be essential that the neutral points of each system and its associated metalwork be separately earthed. If such a method were adopted, each earthing system would require insulation from other earthing systems to withstand the maximum rise of earth potential occurring in any system by virtue of lightning currents or power system fault currents. Insulation to this level of maximum rise of earth potential is rarely practicable.

The choice of using a common earth or separate earths for the system of different voltages at a transforming point affect:

The probability of breakdown occurring in a transformer between the higher and lower voltage sides due to lighting or other surges.
The safety of consumers or their property supplied by any low voltage system distributed from the station against arise of potential of the earthed neutral by a high voltage system earth fault at the station.

The former risk is reduced by use of a common earth system, and the latter danger only arises if the resistance of the earth electrode system is not sufficiently low to limit the rise of earth potential to a safe value.

There is advantage in using a common earth where the earth electrode resistance, including the parallel resistance of any bonded metalwork, etc, to earth is 1 Ω or less, as is usual at power stations, large outdoor substations or substations supplying a network of cables whose sheaths have a low impedance to earth.

The rise of substation earth potential will not be excessive if the resistance of the earth electrode system is small compared to the total earth fault circuit impedance. Systems of higher voltage (66 kV and above) generally have the neutral directly earthed, since the increase in costs of insulation that would be required for the transformer winding would be considerable.

In rural situations, where overhead lines are used, in certain circumstances, usage of common earth is inadvisable.

Earth Electrodes:

The objective of earthing is to ensure that any voltage appearing on equipment which are accessible should be below a dangerous level. The earth electrodes used in substation shall have the following characteristics.

It should provide low earth resistance under all variations due to climatic conditions for the fault currents.
It should have the current carrying capability for all currents and durations that may arise in normal operating conditions or during fault or surge discharge conditions.
It should be located as short and straight as possible to minimize surge impedance for lighting discharge devices.
Earth electrode installations should be durable and of such material and should be made of materials having good corrosion resistance property.

For high voltage system earthing, special precautions are necessary to restrict the rise of ground potential within safe value.

For lower current rating requirements, driven rods are usually preferred, of the copper-clad steel type. Number of such rods are installed in parallel with an electrode spacing of not less than their length since closer spacing reduces their effectiveness. Earth electrodes in the form of Plates and Pipes are also commonly used for substation earthing.

At large substation compounds, a mesh of earth strips is laid below the ground level. The system neutral terminals and the earth bonding conductors from structures are connected to this earth mesh. It provides an equipotential surface over the substation and the earth strip mesh provides an electrode of suitable resistance and current carrying capacity.

Recommendations:

The earthing system has to be robust and protected from mechanical damage and corrosion.
All the joints should be capable of retaining low resistance after passages of fault current for multiple times.
The required cross-sectional area of the earthing conductor is determined for the maximum duration of the fault current. The generally accepted duration for design purposes are one second for voltages above 33 kV and 3 seconds for lower voltages.
The reinforcement in foundations and piles can be used to provide an effective earthing system. When piles are used for earthing, they should be bonded by welding and connected to earth bonding bars on at least four points.
Copper earth strip in contact with galvanized steel should be tinned to prevent electrolytic action.
Aluminium should only be used above ground and the connections to earth electrodes made above ground with bimetallic joints. Aluminium can be used below ground only if efficiently protected or sheathed against contact with soil and moisture.
All crossings of conductors in the main earth grid should be jointed.
Buried bare copper or steel conductors forming part of the earthing system should be at about 600 mm deep. We should ensure that it will normally be below frost line.
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