Electrical grounding is a broad field, but with regard to the power industry a grounding (earthing) system analysis refers to a specific subject. To gain an understanding of a grounding system analysis, we should start by answering: What is a grounding system?
A grounding system is a network of ground electrodes, which are simply conductors imbedded into the earth. Grounding systems are an important part of the power infrastructure and are found at substations, switchyards, generation sites, and industrial facilities. As an example, at a substation, bare copper is directly buried into the earth typically as a grid or mesh, as illustrated in the image below:
Grounding electrodes are typically horizontally placed bare copper conductor buried 18 – 24 inches below grade with vertically installed ground rods. Some areas require one or multiple ground wells to achieve the grounding system design objectives.
Grounding systems are designed and assessed to improve electrical safety and operation. The primary purposes of a grounding system include:
- Helping to ensure personnel and public safety.
- Facilitating proper equipment operation under normal and faulted conditions (some protection schemes require sufficient ground current to detect and operate for a fault).
- Preventing or reducing equipment damage or fault escalation from a power system fault.
- Preventing or reducing equipment damage from lightning effects.
A grounding system analysis or study is the evaluation of the grounding system in meeting its design objectives. In the power industry, the primary focus is addressing the aspect of personnel and public safety. IEEE Std 80 provides guidance for safety related to grounding in AC substations. This standard highlights the dangerous conditions that may occur during a ground fault that can severely or fatally injure individuals in the area or in contact with metallic objects.
During a ground fault, current flows into or out of a grounding system and the electrical potential of the grounding system and surrounding soil are elevated relative to remote earth. This is referred to as the ground (earth) potential rise, and is illustrated in the image below:
Bonding and grounding equipment at a site elevates all metallic objects to the ground potential rise. Knowing that current will travel in all available paths, sufficient voltage gradients may be present on the earth’s surface to produce catastrophic current to flow through personnel or public within the affected area. A lower grounding system impedance results in a lower ground potential rise, but designing to a specific impedance, such as 5 ohms or less, is not a measure of an effective grounding system for personnel safety. Determining the touch and step voltages that may occur at a grounding system, compared to the permissible limits, is the is the correct measure of a grounding system efficacy. Generally, three variables drive the grounding system performance:
- Grounding system physical design and geometry
- Soil electrical characteristics
- Ground fault current magnitude and duration
It is important to note that each component is complex, often varying over time, and significantly affects the conclusions of a grounding analysis. Engineers performing a grounding system study must consider the accuracy of the data and how changes in theses variables can affect a study’s conclusion. In addition, the design must verify all equipment is bonded, size the equipment and below grade ground conductors, and possibly evaluate effects on adjacent facilities.