Substation Grounding / Earthing
Substation Gronding/ Earthing
The sole purpose of substation grounding/earthing is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation. The substation earthing system is necessary for connecting neutral points of transformers and generators to ground and also for connecting the non current carrying metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth. Earthing of surge arresters is through the earthing system. The function of substation earthing system is to provide a grounding mat below the earth surface in and around the substation which will have uniformly zero potential with respect to ground and lower earth resistance to ensure that
- To provide discharge path for lightning over voltages coming via rod-gaps, surge arresters, and shielding wires etc. .
- To ensure safety of the operating staff by limiting voltage gradient at ground level in the substation
- To provide low resistance path to the earthing switch earthed terminals, so as to discharge the trapped charge (Due to charging currents even the line is dead still charge remains which causes dangerous shocks) to earth prior to maintenance and repairs.
Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth potential with respect to a remote earth fault. Persons touching any of the non current carrying grounded parts shall not receive a dangerous shock during an earth fault. Each structure, transformer tank, body of equipment, etc, should be connected to earthing mat by their own earth connection.
Generally lower earth resistance is preferable but for certain applications following earth resistance are satisfactory
Large Power Station s– 0.5 Ohm
Major Power Stations - 1.0 Ohm
Small Substation – 2.0 Ohm
In all Other Cases – 8.0 Ohm
Step Potential and Touch Potential
Grounding system in an electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’.
Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault current through the grounding system.
Touch potential is a potential difference between the fingers of raised hand touching the faulted structure and the feet of the person standing on the substation floor. The person should not get a shock even if the grounded structure is carrying fault current, i.e, The Touch Potential should be very small.
Types of Grounding:
Un earthed Systems:
It is used no more. The neutral is not connected to the earth, also called as insulated neutral system.
Solid grounding or effective grounding:
The neutral is directly connected to the earth without any impedance between neutral and ground.
Resistance is connected between the neutral and the ground.
Reactance is connected between the neutral and ground.
An adjustable reactor of correctly selected value to compensate the capacitive earth current is connected between the neutral and the earth. The coil is called Arc Suppression Coil or Earth Fault Neutralizer.
Different Grounding Equipment in Electrical Substation
- Earthing Electrodes
- Earthing Mat
- Overhead shielding wire (Earthed)
Electrical Questions With Answers
Different Equipments and Ground Connections
Parts to be Earthed
Method Of Connection
Connect the earthing bolt on transformer tank to station earth. Connect the neutral to earthing system
High Voltage Circuit Breakers
Operating mechanism, frame
Connect the earthing bolt on the frame and the operating mechanism of Circuit Breaker to earthing system
Lower Earth Point
To be directly connected to the earth mat
Support of bushing insulators, lightning arresters, fuse, etc..
Device Flange or Base Plate
Connect the earthing bolt of the device to the station earthing system
Potential transformer tank, LV neutral.
Connect the transformer earthing bolt to earthing system Connect LV neutral of phase lead to case with flexible copper conductor
Isolator frame, operating mechanism, bedplate
Weld the isolator base frame, connects it to the bolt on operating mechanism base plate and station earth.
Secondary winding and metal case
Connect secondary winding to earthing bolt on transformer case with a flexible copper conductor.