The ground grid
The substation grounding system is an essential part of the every electrical system. The proper grounding of a substation is essential and very important for the following two reasons. First, it provides a means of dissipating electric current into the earth without exceeding the operating limits of the equipment.
Second, it provides a safe environment to protect personnel in the vicinity of grounded facilities from the dangers of electric shock under fault conditions.
The grounding system includes all of the interconnected grounding facilities in the substation area, including the ground grid, overhead ground wires, neutral conductors, underground cables, foundations, deep well, etc.
The following information is mainly concerned with personnel safety. The information regarding the grounding system resistance, grid current, and ground potential rise can also be used to determine if the operating limits of the equipment will be exceeded.
Safe grounding requires the interaction of two grounding systems:
- The intentional ground: consisting of grounding systems buried at some depth below the earth’s surface
- The accidental ground: temporarily established by a person exposed to a potential gradient in the vicinity of a grounded facility
It is often assumed that any grounded object can be safely touched. A low substation ground resistance is not, in itself, a guarantee of safety. There is no simple relation between the resistance of the grounding system as a whole and the maximum shock current to which a person might be exposed.
A substation with relatively low ground resistance might be dangerous, while another substation with very high ground resistance might be safe or could be made safe by careful design.
There are many parameters that have an effect on the voltages in and around the substation area. Since voltages are site-dependent, it is impossible to design one grounding system that is acceptable for all locations.
If the geometry, location of ground electrodes, local soil characteristics, and other factors contribute to an excessive potential gradient at the earth surface, the grounding system may be inadequate from a safety aspect despite its capacity to carry the fault current in magnitudes and durations permitted by protective relays.
During typical ground fault conditions, unless proper precautions are taken in design, the maximum potential gradients along the earth surface may be of sufficient magnitude to endanger a person in the area.
Moreover, hazardous voltages may develop between grounded structures or equipment frames and the nearby earth.
|Title:||Best practice in power substation grounding – Richard P. Keil; Commonwealth Associates, Inc.|
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