Grounding Grid Study and Analysis Service

Since many years, i.e. from the start of high voltage power station development, the grounding grid analysis began with the theoretical or experimental analysis based on which different authors rendered rough expressions. These expressions were carried out on a single horizontal or vertical grounding rod. With the knowledge obtained from the behaviour of single vertical or horizontal grounding rod, different authors suggested additional approximate analytical representations for the grounding grids. Proposed analytical expressions had different restrains regarding dimensions and geometry of the grounding grid. It became clear that all these empirical formula have considerable drawbacks when applied to the complex geometry grounding grid.

Why is Grounding Grid Design Analysis and Study Necessary?

The aim of a ground grid design is to limit electric shock related accidents. The circumstances leading to the shock accidents are:

• High fault current to ground.
• Large soil resistivity.
• Existence of a person connecting 2 points of large potential difference.
• Lack of adequate contact resistance or other series resistance to regulate current through the body to a safe value.
• Time duration of the fault and body current for an adequate time to cause harm at the given current intensity.

The grounding system can be designed such that the step and touch voltages are kept within the calculated safe limits.

The benefits of this study include:

• Design time savings
• Material cost savings by determining the minimum size of conductors and number of ground rods to keep the potential values below the IEEE limits.

What is Done During Grounding Grid Analysis and Study?

The analysis contains of these steps:

• Inspection of soil characteristics.
• Calculation of peak ground current.
• Initial design of the ground system.
• Determination of resistance of the ground system.
• Determination of step voltages at the margin.
• Determination of internal step and touch voltages.
• Fine-tuning of preliminary design.

Due to the fact that grounding grid is buried in the earth, its mathematical treatment occupied prominent position from the very beginning of the grounding grid analysis. Hence, various soil interpretation techniques have been surveyed as well. They are basically concerned with the problems associated with a boundary between soil and air. While speaking with techniques adapt for the transient grounding grid behaviour, one needs to distinguish three very different approaches:

• Circuit approach: It is said to be fast and relatively easy to understand, but at the same time least accurate.
• Electromagnetic approach: Main difficulty in applying electromagnetic approach stems from the complex mathematical apparatus, which transforms Maxwell equations in system of algebraic equations, and its associated numerical implementation. Here one can distinguish between finite element approach, boundary element approach and approach according to the method of moments.
• Hybrid approach: It is a synergy between the two above mentioned approaches and can vary in complexity as well as in accuracy.

From the point of view of problem formulation, methods could be regarded as a

• Differential problem formulation
• Integral method
• Integro-differential method

How is Grounding Grid Analysis Done?

Following steps are performed during the analysis procedure:

• Soil resistivity: The potential rise of a grounding system during ground fault conditions is directly proportional to the resistance of the grounding system. Resistance of the grounding system is also important for the satisfactory operation of over current devices. Hence, it is essential to predict the resistance of the grounding system before its actual installation
• Effect of voltage gradient: The ground resistance is not affected by the voltage gradient unless it is greater than 103 Volts/cm. When this value is exceeded, arcs will start at the electrode surface and progress into ground so as to increase the effective size of the electrode which the soil can withstand.
• Effect of moisture: The resistivity of the soil rises abruptly when the moisture content falls below 22% by weight. It is therefore necessary that the electrode system should be buried deep enough to ensure contact with permanently moist soil. Where this is not possible, greater dependence will usually be placed on a well distributed system of vertical rods bonded to the ground grid and reaching deep layers.
• Effect of temperature: Resistivity of soil rises abruptly when the temperature falls below 32°F. The ground grid electrode system should extend below the frost line wherever feasible to minimize seasonal variation of the grounding system resistance. Further, the overall resistance of the ground grid varies with season due to the summer and winter temperature variations.
• Determination of maximum ground fault current: The following steps are involved in determin.