by the 3 Phase Associates "TEAM"
Importance of Proper Grounding
Just as it’s important to ground equipment and panels for lower voltages (50 – 600 VAC) per NEC 250 for proper grounding and bonding in order to reduce electric shock for human safety and proper equipment protection, likewise, it’s extremely important to ensure proper substation grounding is performed due to the higher voltages and increases in fault current. Since electrical current flows through the path of least resistance or to ground, it’s important to maintain a proper ground. The NEC 250 recommends the grounding of panel and equipment to maintain a resistance of 25-ohms or less. In substations the grounding should maintain a resistance of 1 to 5-ohms, depending on size and voltage levels. The main goal of grounding is to ensure that the path of least resistance for electrical current is not through personnel or equipment.
Unlike grounding for lower voltage panels and equipment, designing a proper substation grounding system requires a copper grid mesh to be installed underground at a minimum of 18 inches below grade. Additional 10-foot ground rods or electrodes (~5/8 inch thick bar rods) driven vertically in the ground are also necessary at different locations in the substation and outside the substation fencing to help achieve the 1 to 5-ohms or less resistivity. More ground rods may or may not be required depending on the type of soil and soil resistivity levels within the substation.
Designing an Adequate Substation Ground Grid
A well-designed substation ground grid can uphold human safety for authorized personnel working in or around substations. A good grounding system can also help prevent equipment damage during fault conditions, direct lightning strikes, etc. During fault conditions, the ground mat (copper mesh) should provide the lowest possible resistance to earth in order to minimize the ground potential rise (GPR) or diminish the large potential differences between various grounding points (touch and step potential) within the substation. This ensures safe and acceptable limits for human touch and step potential conditions. (See illustrations below from IEEE Standard 80-2000 for touch and step potentials).
Human Safety
The dangerous voltage potential gradients or GPR throughout the substation are caused by electrical faults, and the goal of the grounding design is to reduce these gradients to harmless or negligible electric shocks to humans. “Effects of an electric current passing through the vital parts of a human body depend on the duration, magnitude, and frequency of this current. The most dangerous consequence of such an exposure is a heart condition known as ventricular fibrillation, resulting in immediate arrest of blood circulation.” - IEEE Standard 80-2000. This can happen with just 100 milliamps of electrical current passing through the heart and may even cause death.
A good test for adequate substation grounding systems provides a ground resistance of 1 to 5-ohms for human safety. As adequate assumptions and inputs are used for designing a substation ground grid system, the ground resistance of 1 to 5-ohms should be achievable based on the major factors below. There are others, but these cover the main items for an adequate ground grid design:
- Proper copper conductor sizes selected for the
ground grid - Proper spacing of copper conductors for an
adequate mesh design (X and Y spacing) - Designing the ground grid to be as large as
possible by covering the entire substation including going beyond the
substation fencing - Adequate grounding connections of all equipment,
steel, fencing, gates/posts, etc. - Installing adequate grounding rods at various
locations within the substation - Adequate soil resistivity tests performed to
achieve optimal conditions - Maximum fault current data calculated or
provided
Computer Aided Software Analysis
Because there are many factors and calculations that go into
developing a proper ground grid design, software programs are used to aid the
substation design engineer. Many variables coupled with various software
packages may produce slightly different results. There are also available
tutorials and guides to assist the substation design engineer with the ground
grid design.
Most of the design inputs and assumptions for developing a substation
ground grid design will usually be provided by the utility or power company. These
assumptions and inputs are required to help facilitate the findings (aided by
grounding analysis software) to ensure an adequate ground grid design for human
safety during faulted conditions. The grounding analysis software can provide supporting
figures and tables including the acceptable safety tolerances recommended by
the IEEE Standard 80 “Guide for Safety in AC Substation Grounding”. The
software analysis helps illustrate and support the grounding study results for your
substation project.
Before the software can aid the substation design engineer, the
electrical single line and ground grid for the substation will have to be designed and modeled
within the software. The soil resistivity test results will also be required as design inputs. Other design inputs required for the grounding analysis software are:
- External generating sources and/or transmission line source kVA and impedances
- Line to line and line to ground voltage levels
- Maximum fault current data for the primary and secondary sources
- Equipment ratings (Voltage, kVA, delta or wye configurations, impedances, etc.)
- Soil resistivity along with soil layer data
A typical bare copper grid mesh design may consist of sizes 1/0 AWG up to 500 kcmil AWG depending on the magnitude of fault currents and soil resistivity levels within the substation. The grounding connections to the equipment and steel will have similar size copper connections or copper pigtail connections to achieve under 5-ohms of ground resistance. Also, certain equipment, such as, switches will require grounding platforms for additional safety.
Conclusions and Recommendations
In conclusion, the ground grid design analysis and findings should be compared to the safety tolerable levels for proper human care to ensue an adequate ground grid design. The findings should be favorable within tolerable levels for the step voltage, touch voltage, mesh voltages, GPR, etc. (See illustrations below from IEEE Standard 80-2000). The grounding analysis and ground mat design must demonstrate that all correlated results meet or exceed the safety tolerable levels for the substation design.
It is recommended to perform routine substation grounding and soil resistivity tests. This is to ensure future test results remain favorable despite the potential for fluctuations in weather patterns and soil modifications. Future weather patterns over time can cause soil changes, thereby, affecting the overall ground resistivity. Periodic testing can provide future trends indicating possible negative impacts to the overall substation ground grid design.
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