INTRODUCTION
The Prime Object of Earthing is to Provide a Zero-Potential Surface in, around, and under the area where the electrical equipment is installed.
Earthing is essential at every stage of electricity generation, transmission, and utilization
IMPORTANCE OF EARTHING IN SUBSTATION
- Personal Safety
- Protection of Equipment :
Prevent or at least minimize damage to equipment as a result of heavy fault current and lightning thus improve the reliability of equipment - Protection of System :
Improve the reliability of power supply.
TYPES OF EARTHING
- Plate Earthing
- Pipe/Rod earthing
- Strip earthing
- Mat earthing
SUBSTATION EARTHING DESIGN
Earthing system design must consider several factors, including soil resistivity, safety considerations, and current carrying capacity, to ensure an effective and safe earthing system.
STEPS INVOLVED IN SUBSTATION EARTHING CONSTRUCTION
- Laying of Earth Mat-Excavation of earth, welding of different size flats, application of two coats of bituminous paint, wrapping of HT taps over it and back filling of earths (including supply of welding 75 x 10mm without supply of GI Flatmachine,welding generator, welding rods and all materials,T&P,Labour) using GI Flats of size 50 x 6mm without supply of GI Flat
- Earthing riser-connection of different size GI Flats from earth mat to equipment, welding of different size flats, application of two coats of bituminous paint, wrapping of HT Tapes over it (with supply of labour and T&P) using GI Flat of size 50 x 6mm without supply of GI Flat
- Connection of GI Flat from Equipment to Earth pit
- Pipe Earthing-including excavation of earth, treatment of bentonate compound, backfilling with borrowed earth, termination to earth mat riser by nut bolting,apply of paint where necessary with supply of all labour and T&P as per ISS-3043 with cost of earthing pipe and GI Flat
SUBSTATION EARTHING RESISTANCE VALUE
The earth resistance shall be as low as possible and shall not exceed the following limits
Power stations (generating station) 0.5 ohms
EHT Sub-station 1.0 ohms
33 KV Stations 2.0 ohms
DT(Distribution Transformer) Structure 5.0 ohms
Tower Foot resistance 10.0 ohms
SUBSTATION EARTHING GUIDE
- Resistivity:
Is the fundamental property of the material - Soil Resistivity:
Resistivity across 1 cubic unit of soil, expressed in Ω-mtr - Purpose:
- Estimating the ground resistance of a proposed substation of transmission n tower.
- Estimating potential gradients including step and touch voltages.
- Computing the Inductive coupling between neighboring power & communication circuits
- Designing cathodic protection systems.
- Geological Surveys
- Equipment Used:
- Four terminal earth tester
SUBSTATION GROUNDING STANDARDS
- IS: 3043 : 1966, 1987 reaffirmed 2006 Code of Practice for Earthing.
- Indian Electricity rules 1956 (as amended up to 2000)
- IS: 2309 1989 (reaffirmed 2005) Protection of Buildings and allied Structures against lightning – Code of Practice.
- IS : 2689: 1989 (reaffirmed March 2010): Guide for Control of Undesirable static Electricity.
- Manual on Earthing of AC Power Systems : CBIP Publication No.302 : 2007 and 311
- BS: 7430 : 1998, Code of Practice for Earthing.( formerly CP 1013: 1965) British Standard Institution London 1992
- BS: 6651: 1992, Protection of Structures. Against Lightning.
- IEEE :80 : 2000( Revision of IEEE Std 80: 1986) Guide for Safety in AC Substation Grounding
- IEEE :142 :2007(Revision of IEEE Std 142 :1991) Grounding of Industrial and Commercial Power System.
- IEEE 1100 : 2005 (Revision of IEEE Std 1100 : 1999) Powering and Grounding Electronic Equipment
SUMMARY
Regular inspection and testing of the earth resistance is essential for maintaining its effectiveness and safety. Earthing systems must be tested to ensure that they have the required resistance and that there are no breaks or other faults in the conductors or electrodes. Regular inspections should be conducted to identify any corrosion or damage to the earthing system components. If any faults are identified during inspection or testing, it is essential to repair or replace the earthing system components for maintaining the safety and effectiveness of the earthing system.
