When earth fault occurs
Right Answer is:
The voltage potential at the earth mat increases due to grounding
When an earth fault occurs the voltage potential at the earth mat increases due to grounding.
The substation ground mat is a series of interconnected ground wires buried in the earth of the substation floor.
In order to form ground electrodes, several vertical galvanized steel pipes are inserted in the ground and their heads are solidly connected to the ground mat by means of horizontal grounding rods/grounding strips. Ground mat (ground grid or ground mesh) is also called earth mat (earth grid or earth mesh).
The primary purpose of the substation ground is to provide electric shock protection for personnel working in and around the substation, including during lightning strokes, short circuits, equipment failures, and many situations of human error or carelessness.
A second purpose of the substation ground is to limit insulation stress by readily conducting lightning surges to earth via the station surge Lightning arresters.
Discharge through the lightning arresters into the ground mat will raise the substation ground mat potential appreciably above “remote earth.” The potential difference between the main substation and “remote earth”
Numerically, the ground potential rise is equal to the product of the grid resistance times the maximum grid current.
This protection is provided by the multiple interconnections (grounding) of all accessible surfaces within the substation in such a way as to limit to a safe level the voltage differences that can appear from point to point on these surfaces.
Increase in voltage potential at the earth mat caused by electrical faults that occur at electrical substations, power plants, or high voltage transmission lines.
When the fault occurs, short circuit current flows through the equipment and into the grounding electrode. The resistance of the Earth is non-zero, so current injected into the earth at the grounding electrode produces a potential rise with respect to a distant reference point.
There are some standards that guide the grounding of the substation.
As per IEEE standards, earth mats are designed with vertical ground rods, and earth mat which brings down the overall resistance of the substation below 1 Ohm. The Earth mat spreads across the substation. All the equipment is connected to the earth mat with two separate earth flats.
A good grounding scheme will ensure faster fault clearing and a low enclosure potential rise.
The limitation of this method is that it does not provide protection against the traveling wave which may reach the equipment in the station.