Substation grounding design, as outlined in IEEE Standard 80 (IEEE Guide for Safety in AC Substation Grounding), is a critical aspect of ensuring the safety and reliability of electrical substations. The primary objectives of grounding in a substation are to provide safety for personnel, protect equipment from damage during electrical faults, and ensure reliable operation of the system. IEEE 80 provides detailed guidelines for designing an effective grounding system that can dissipate fault currents safely into the earth without creating dangerous touch and step potentials.
Key Concepts in Substation Grounding According to IEEE 80
Purpose of Substation Grounding
- Personnel Safety: Prevent hazardous voltages from being transferred to personnel who might be in contact with grounded objects.
- Equipment Protection: Ensure that electrical equipment, such as transformers, breakers, and control panels, are protected from fault currents.
- Control of Ground Potential Rise (GPR): Maintain the ground potential at a safe level during fault conditions, thus limiting step and touch voltages.
Key Definitions
- Ground Potential Rise (GPR): The increase in potential of the substation ground with respect to a remote ground during a fault.
- Step Voltage: The potential difference between two points on the ground separated by a distance of one step (approximately one meter).
- Touch Voltage: The potential difference between a grounded structure and a point on the ground that a person could touch.
- Mesh Voltage: The maximum potential difference within a mesh of a grounding grid, which must be kept below dangerous levels.
Grounding System Design Process IEEE 80 outlines a systematic process for designing an effective grounding system in a substation, including the following steps:
a. Data Collection
- Soil Resistivity Measurements: Soil resistivity is a key factor in determining the design of the grounding system. IEEE 80 recommends performing a soil resistivity test, typically using the Wenner method, to assess how well the soil can dissipate fault current.
- Fault Current Levels: The expected maximum fault current in the substation must be determined for the design, including duration and magnitude of fault conditions.
- Grid Layout and Substation Area: The layout of the substation and the expected footprint of the grounding grid should be established early in the design process.
b. Grounding Grid Design
- Grid Conductors: The primary grounding grid is typically composed of horizontal bare copper conductors placed below the surface of the earth. The grid must be designed to carry fault current safely while limiting GPR and step and touch potentials.
- Rod Placement: Vertical ground rods are often added to the grid to improve grounding by providing additional paths for current to dissipate into deeper, lower-resistivity layers of soil.
- Conductor Sizing: According to IEEE 80, the cross-sectional area of conductors must be adequate to handle the maximum fault current without overheating or fusing.
c. Step and Touch Voltage Calculation
- Acceptable Step and Touch Voltages: IEEE 80 provides formulas for calculating safe step and touch voltage limits based on parameters like soil resistivity, fault current magnitude, and the body resistance of a person.
- Grid Configuration Adjustments: If the calculated step or touch voltages exceed safe limits, the design of the grid must be adjusted by adding conductors, rods, or increasing the size of the grid.
Grounding Grid Geometry
- Mesh Size: The size of the mesh in the grounding grid (i.e., the spacing between parallel conductors) is an important factor in controlling touch and step voltages. IEEE 80 recommends keeping mesh sizes small in areas where personnel may frequently be present.
- Depth of Conductors: Grounding conductors are typically buried at a depth that provides good dissipation of fault currents and reduces the potential for damage due to surface activity.
- Peripheral Conductors: A ring conductor placed around the perimeter of the substation is often used to help equalize potential and control step voltages in the surrounding area.
Mitigation Techniques If the calculated step and touch voltages exceed the permissible limits, IEEE 80 suggests several mitigation techniques:
- Reducing Mesh Size: Decreasing the distance between grounding conductors will help distribute fault currents more evenly, reducing potential gradients.
- Increasing Ground Rods: Adding more ground rods or increasing the length of rods can improve grounding effectiveness.
- Surface Layer Treatment: Placing a high-resistivity layer (such as gravel or asphalt) on the surface of the substation yard can help limit touch and step voltages by increasing the resistance between a person’s feet and the ground.
Ground Potential Rise (GPR) The GPR is the product of the fault current and the grounding system resistance. IEEE 80 defines acceptable limits for GPR based on safety considerations:
- Single-Line-to-Ground Faults: The most significant cause of GPR occurs during single-line-to-ground faults. Ensuring that GPR stays below dangerous levels is a priority in substation design.
- Limiting GPR: Reducing the substation’s grounding system resistance is one way to control GPR. This may involve expanding the grounding grid, adding rods, or improving the soil conductivity.
Grounding of Equipment and Structures
- Bonding: All metallic structures and equipment, such as transformers, circuit breakers, and fences, must be bonded to the grounding grid to prevent dangerous potential differences during faults.
- Fences and Gates: Fences that are near or part of the substation must be grounded properly, as they can present a touch hazard if not connected to the grounding grid.
Inspection and Maintenance
- The grounding system must be inspected periodically to ensure it remains effective over time. Corrosion, physical damage, or changes in soil conditions can affect the performance of the grounding grid.
- Maintenance: If the grounding system degrades over time, it must be repaired or upgraded to maintain the safety of personnel and equipment.
Conclusion
Grounding design according to IEEE 80 is essential to ensuring both the safety of personnel and the protection of substation equipment. By properly designing the grounding grid, controlling step and touch voltages, and mitigating ground potential rise, substations can operate safely under fault conditions. The guidelines provided by IEEE 80 help ensure that grounding systems are both reliable and effective, even under extreme operating conditions.
