Thursday 11 October 2012

Substation Grounding / Earthing



Substation Grounding/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 ofsurge 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

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 a electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’.

Step 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:

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.

Step Potential and Touch Potential
Step Potential and Touch Potential

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 grounding:

Resistance is connected between the neutral and the ground.

Reactance grounding:

Reactance is connected between the neutral and ground.

Resonant Grounding:

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.
For Studying more about Different Types Of Groundings Please see Power System Earthing

Different Grounding Equipment in Electrical Substation

  • Earthing Electrodes
  • Earthing Mat
  • Risers
  • Overhead shielding wire (Earthed)

Different Equipments and Ground Connections

Apparatus
Parts to be Earthed
Method Of Connection
Power Transformer
Transformer tank
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
Surge Arrester
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
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
Isolator frame, operating mechanism, bedplate
Weld the isolator base frame, connects it to the bolt on operating mechanism base plate and station earth.
Current Transformer
Secondary winding and metal case
Connect secondary winding to earthing bolt on transformer case with a flexible copper conductor.


Power System Earthing

It is necessary to earth a power system at a suitable point by a suitable method as it offers many advantages as
  1. It provide Safety to the electrical equipments against over-current
  2. It provides better safety
  3. It reduces the maintenance expenditure
  4. It improves the service reliabilty
  5. It provides improved lightning protection

Solid Earthing

When the neutral of the power transformer and generator is directly connected to the earth, then the system is said to be solidly earthed. The solidly earthing does not make a zero impedance circuit as generator or transformer would have its own reactance in series with the neutral circuit. The direct earthing of a generator without external impedance causes earth fault current from the generator to exceed the maximum 3-phase fault current if the impedance of the generator is too low. This results in Stator winding damage as the short circuit current during fault will exceed the short circuit rating of the winding for which it was designed. For this system of earthing, it is necessary that the earth fault current shall be in the range of 25% to 100% of the 3-phase fault current to prevent the development of high transit over voltages.

Resistance Earthing

In resistance earthing the neutral of the generator or transformer is connected to the earth trough a resistance in series.
Advantage Of Resistance Earthing are:
  1. It reduces the line voltage drop caused when earth fault occurs
  2. It reduces electric shock hazards to the persons, caused by stray earth fault currents in the return path
  3. It reduces the mechanical stresses in the circuit carrying fault current
  4. It reduces the effect of burning of faulted electrical equipment
The magnitude of the resistance to be used should be such that it should limit the earth fault current to a value which will reduce minimum damage at the point of the fault.
Type Of Earth Resistance Methods
Type Of Earth Resistance Methods

Reactance Earthing:

In reactance earthing a reactor is connected in between the neutral of the machine and earth. A low reactance is connected in series with the neutral of the machine to limit the earth fault current through the generator. This current should not be greater than the 3-phase fault current of the generator. The earth fault current of the earthed system should not be less than 25% of the 3-phase fault current in order to minimize the transient voltages

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