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One of the major contributing factors in the design of safe and reliable grounding in AC substations is the soil resistivity and the effects it has on both the ground potential rise in and around the substation as well as the influence it may have on system protection. In general, the lower the impedance of a substation ground grid with respect to remote earth, the better it is. However in some instances where the soil resistivity is very high, the ability to attain low ground grid resistance may be difficult and costly.
The generally accepted practices to reduce the ground grid resistance include adding soil enhancing materials, using close grid spacing, and installing deep ground wells in an attempt to provide a low impedance path to remote earth. While the benefits of closer grid spacing on the grounding system performance can be calculated with a good degree of accuracy, the effects of ground enhancing material and ground wells on the grounding system are more ambiguous. This may result in a costly design, perhaps with little gain.
This paper focuses on the design of AC substation ground grids with very high soil resistivity. It also explores the fact that it is possible to design a ground grid system that has a relatively high resistance but is also safe for equipment and personnel.
The design of a safe and reliable grounding system seeks to make, “the magnitude and duration of the current conducted through the human body…less than the value that can cause ventricular fibrillation of the heart.” If the effective resistance of a human body is assumed constant, then there are two methods to ensure that the criterion listed above is met:
- Decrease the touch or step voltage the body can be subjected to. This can be achieved by either limiting the grid current or reducing the grid resistance.
- Decrease the amount of time to clear a ground fault, which can be used by fast tripping of a protective device.
|Designing Safe and Reliable Grounding in AC Substations With Poor Soil Resistivity: An Interpretation of IEEE Std. 80|