Earthing Practices At Substations
Earthing practices adopted at Generating Stations, Substations, Distribution structures and lines are of great importance. It is however observed that this item is most often neglected. The codes of practice, Technical Reference books, Handbooks contain a chapter on this subject but they are often skipped considering them as too elementary or even as unimportant. Many reference books on this subject are referred to and such of those points which are most important are compiled in the following paragraphs. These are of importance of every practicing Engineer in charge of Substations.
OBJECTIVE OF EARTHING
Prime Objective of Earthing is to provide a Zero potential surface in and around and under the area where the electrical equipment is installed or erected.
To achieve this objective the non-current carrying parts of the electrical equipment is connected to the general mass of the earth which prevents the appearance of dangerous voltage on the enclosures and helps to provide safety to working staff and public.
Importance of Earthing & Practices
· The earthing is provided for
a) Safety of Personnel
b) Prevent or atleast minimise damage to equipment as a result of flow of heavy fault currents.
c) Improve reliability of Power supply
· The earthing is broadly divided as
a) System earthing (Connection between part of plant in an operating system like LV neutral of a Power Transformer winding and earth).
b) Equipment earthing (Safety grouding)
Connecting frames of equipment (like motor body, Transformer tank, Switch gear box, Operating rods of Air break switches, etc) to earth.
· The system earthing and safety earthing are interconnected and therefore fault current flowing through system ground raises the potential of the safety ground and also causes steep potential gradient in and around the Substation. But separating the two earthing systems have disadvantages like higher short circuit current, low current flows through relays and long distance to be covered to separate the two earths. After weighing the merits and demerits in each case, the common practice of common and solid (direct) grounding system designed for effective earthing and safe potential gradients is being adopted.
· Factors that change the requirement of earth electrode
a) If an electrical facility can expand in system, it creates different routes in the electrode. What was formerly a suitable low earth resistance can become obsolete standard.
b) More number of metallic pipes, which were buried underground become less and less dependable as effective low resistance ground connection.
c) Most of the location, the water table gradually falling. In a year or two, area end up with dry earth of high resistance.
d) These factors emphasize the importance of a continuous, periodic program of earth resistance testing.
· The earth resistance shall be as low as possible and shall not exceed the following limits:
Power Stations - 0.5 Ohms
EHT Substations - 1.0 Ohms
33KV Stations - 2.0 Ohms
D/t Structures - 5.0 Ohms
Tower foot resistance - 10.0 Ohms
Step Potential is the difference in the voltage between two points which are one metre apart along the earth when ground currents flowing.
Touch Potential is the difference in voltage between the object touched and the ground point just below the person touching the object when ground currents are flowing.
Specification of Earthing
Depending on soil resistivity, the earth conductor (flats) shall be buried at the following depths.
Soil Resistivity in ohms/metre Economical depth of Burial in metres
1) 50 – 100 0.5
2) 100 – 400 1.0
3) 400 – 1000 1.5
To keep the earth resistance as low as possible in order to achieve safe step and touch voltages, an earth mat shall be buried at the above depths below ground and the mat shall be provided with grounding rods at suitable points. All non-current carrying parts at the Substation shall be connected to this grid so as to ensure that under fault conditions, none of these part are at a higher potential than the grounding grid.
· Taking all parameters into consideration, the size of plate earths are decided as
Power Stations & EHT Station - Main - 100 x 16mm
Auxiliary - 50 x 8mm
Small Stations - 75 x 8mm
· The complete specifications for providing earth mats at EHT & 33KV Substations, Distribution transformers & Consumers premises are reproduced below.
Specification for Earthing System
I) EHT Substation
Earthing of equipment’s in the sub-stations is taken of as discussed below:
1. Power transformers:
i. The transformer body or tank is directly connected to earth grid. In addition, there should be direct connection from the tank to the earth side of the lightning arresters.
ii) The transformer track rail should be earthed separately.
iii) The neutral bushing is earthed by a separate connection to the earth grid.
2. Potential and current transformers :
The bases of the CTs and Pts. are to be earthed. All bolted cover plates of the bushing are also to be connected the earth grid.
3. Lightning arresters :
The bases of the L.As. are to be earthed with conductors as short and straight as Possible (for reducing impedance). The earth side of the L.As. are to be connected directly frolJ1 the equipment to be protected. Each L.A. should have individual earth rods, which are in turn connected to earth grid.
4. Circuit breakers:
The supporting structures, C.T. chambers, P.T. tanks, Cable glands etc., are to be connected to earth.
5. Other equipment’s:
All equipment’s, structures, and metallic frames of switches and isolators are to be earthed separately.
Providing separate earth or connecting to the station earth depends upon the distance of the fence the station earth. If the distance is within feet, an inter-connection made to the station earth. If not, the metallic fences are earthed by means of earth rods spaced at not more than 200 feet. The gates and support pans may be earthed through an earth rod. The cable wires passing under “metallic fence are to be buried below at a depth qf 2’6 or are to bc enclosed in a insulating pipe (P. V.C or asbestos cement) for a distance of not less than 5 feet on each side of the fence.
7. Ground wires :
The ground wires over the station arc connected to the station earth. In order that the station earth potentials during fault condition5 arc not applied to transmission line ground wires and towers, all ground wires coming to the stations shall be broken !It an insulated on the fir5t tower external to station by means of strain disc. insulators.
The followings are the important features in earthing:
1. The earth mat shall be as per the approved layout. The earth mat shall be formed with the steel flats buried in the ground at a depth of 750mm on edge.
2. The earth mat shall extend over the entire switchyard as per the layout.
3. All the junctions of the steel flats while forming the earth mat and taking risers from the earth mat for giving earth connections to equipment, steel structures, conduits cable sheaths shall be properly welded. All joints shall be provided with suitable angle pieces for proper contact between flats.
4. Provisions shall be made for thermal expansion of the steel flats by giving smooth circular bends. Bending shall not cause any fatigue in the material at bends.
5. The earth mat shall be formed by welding 50x8 mm steel flat to the 100 x 16mm peripheral earth conductor. The grounding grid shall be spaced about 5 meters i.e in longitude and about 5 meters in the transverse directions. After the completion of earth mat, the earth resistance shall be measured. In case the earth resistance is more than one ohm the earth mat shall be extended by installing extra electrodes, so that the earth resistance is less than one ohm.
6. All fence corner posts and gate posts shall be connected to the ground by providing 32mm dia M.S rods of 3 metre length near the posts and connected to the main grounding mat.
7. All paint enamel and scale shall be removed from surface of contact on metal surface before making ground connection.
8. The risers taken along the main switchyard structures and equipment structures (upto their top) shall be clamped to the structures at an interval of not more than one metre.
9. 50 x 8mm ground conductor shall be run in cable routes and shall be connected to the ground mat at an interval of 10 metres.
10. Grounding electrodes of 32mm dia 3mtr. long MS rods shall be provided at the peripheral corners of the earth mat. The grounding rods shall be driven into the ground and their tops shall be welded to clamp and the clamp together with the grounding rods shall be welded to the ground mat.
11. Lightening arrestors shall be provided with earth pits near them for earthing.
12. Cast iron pipes 125mm dia and 2.75 metres long and 9.5mm thick shall be buried vertically in the pits and a mixture of Bentonite compound with Black cotton soil a ratio of 1:6 is to be filled 300 mm dia and the pipe for the entire depth. Where it is not possible to go to a depth of 2.75 metres, 1.3 x 1.3 MMS plates, 25mm thick shall be buried vertically in pits of 2 metres depth and surrounded by Bentonite mixture atleast 2 metre away from any building or structure foundation. The plates shall be atleast 15 metres apart. These earth pits in turn shall be connected to the earth mat.
II) Earthing at 33KV Substations
1. Providing of earth pit and earth matting include the following connected works:
a) Excavation of earth pits of size 21/2ft x 21/2ft x 9ft in all type of soils.
b) Providing of CI pipe of 3 inch diameter 9ft length with flange. All connections to CI pipe shall be with GI bolts and nuts.
c) Filling of earth pit excavated with Bentonite with Black cotton soil (1:6) in alternate layers.
d) Providing of cement collar of size 2ft diameter 2ft height 1 inch below the ground level.
e) The top of the CI earth pipe should be at the surface level of the ground.
2. Providing of earth matting with MS flat 75 x 8mm including the following connected works:
a) Excavation of trench in all types of soils of size 2½ ft depth and 1 ft. width.
b) Laying of M.S flat 75 x 8mm in the excavated trench.
c) Inter connecting all earth pits and welding properly at jointing location and junctions.
d) Back filling of earth completely.