Earthing system are provided in a substation for the following main reasons:
- To provide a discharge path for lightning arrests, gaps, etc.
- To provide proper grounding-for substation equipment’s.
- To keep the-non-current carrying metal part. such 'as transformer tank, structures, etc., safe at earth potential even though the insulation fails.
The fault current through the neutral of the transformer to earth is to be distributed over a large area; otherwise the potential gradient of the earth around the earth connection may be dangerous. Earth mat consisting of heavy gauge-bonded conductors or cost iron grid buried at a depth of not less than 30 cm and connected to the earth electrode, serves best to distribute the heavy fault current over a large earth area. For small stations, a greater number of earthing stations are provided at different places and are interconnected which helps to reduce the potential gradient.
BASIC TERMS OF EARTHING:
- Earth: An object is said to be earthed when it is electrically connected to an earth electrode. A conductor is said to be solidly earthed when it is electrically connected to an earth electrode without intentional addition of resistance or impedance in the earth connection.
- Earth Electrode: A metal plate, pipe, any other conductor or an array of conductors electrically connected to the general mass of the earth.
- Earthing Lead: The conductor by which the earth electrode is connected to neutral is called earthing lead.
The word "earthing" and "grounding" have the same meaning. But equipment earthing is different from neutral-point earthing. Equipment earthing refers to connecting the non-current carrying metallic parts to earth available about electrical circuits. For example: motor body, switch gear, Transformer tank, etc. Thus, the purpose of neutral earthing and that of equipment earthing are distinctly different. Equipment earthing ensures safety while neutral earthing is done mostly to ensure that the stator short-circuit current is limited and for stability reasons.
The very purpose of earthing is to safeguard the equipment against possible damage due to electric shock, fire, etc. It is very important to have good and effective earthing or grounding. Today, the neutral grounding is an important aspect of power-system designs because the Performance of the system, in terms of short circuits, stability, protection, etc., is greatly affected "by the state of the neutral. In most of the modem high-voltage systems and the neutral of the system is solidly grounded, i.e., the neutral is connected directly to the ground.
METHODS OF NEUTRAL GROUNDING:
The various methods of grounding the neutral of the system are:
- Solid grounding
- Resistance grounding
- Reactance grounding
- Peterson-coil grounding (or resonant grounding)
- Voltage transformer grounding
- Earthed transformer
Comparison of Neutral Earthing System:
Earthing or grounding means connecting all parts of the apparatus (other than live part) to the general mass of earth by wire of negligible resistance. This ensures that all parts of the equipment other than live part shall be at earth potential (i.e., zero potential) so that the operator shall be at earth potential at all the time, thus will avoid shock to the operator. The neutral of the supply system is also solidly earthed to ensure its potential equal to zero.
- All medium voltage equipment shall be earthed two separate and distinct connections with earth through an earth electrode. In the case of high and extra high voltage the neutral point shall be earthed by not less than two separate and distinct connections.
- Each earth system shall be so devised that the testing of individual earth electrode is possible. It is recommended that the value of any earth system resistance shall not be more than 5 Ω, unless otherwise specified.
- Under ordinary conditions of soil, use of copper, iron or mild steel electrodes is recommended. In direct current system, however due to corrosive action, it is recommended to use only copper electrode. Use similar materials for earth electrode and earth conductors to avoid corrosion.
SPECIFICATIONS OF EARTHING:
The earth rod shall be situated at a distance not less than 1.5 m from the building whose installation being earthed
The size of the continuity conductor shall be 2.9 mm2 (14 SWG) or half of the installation conductor size.
The permissible value of earth resistance is
- Large Power Station - 0.5 Ω
- Major Power Station - 1 Ω
- Small Substation - 2 Ω
- In all other cases - 5 Ω
- Earth continuity resistance - 1 Ω (between earth plate and any earth conductor).
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