Grounding Design Calculations – Part Twelve


In Article Grounding Design Calculations – Part One ", I indicated the following: 






Grounding System Design Calculations according to type of the building

The procedures for performing the Grounding System Design Calculations can differ slightly according to the type of the building as follows:

  1. Domestic, commercial and industrial premises,
  2. High and medium voltage electricity substations.








First: Domestic, commercial and industrial premises

We mean by domestic, commercial and industrial premises, all installations up to 1,000 V ac and 1,500 V dc - between phases, with some minor exceptions.





And I explained Methods of Grounding Design Calculations of Domestic, commercial and industrial premises in the following Articles:




You can preview the following Articles for more info:




Second: High And Medium Voltage Electricity AC Substations





I began explaining Grounding Design Calculations for second type of buildings: AC Substations in Article 
Grounding Design Calculations – Part Ten "  where I explained the following:
  • Design Procedures for grounding system design as per IEEE 80: Guide for safety in AC substation grounding,
  • Step#1: Field Data Collection,
  • Step#2: Earthing Grid Conductor Sizing.


Also in " Grounding Design Calculations – Part Eleven ", I explained Step#3: Calculation Of Tolerable Touch And Step Voltages.


Today, I will continue explaining other steps from the design procedures of grounding system for AC Substation.






Design Procedures of Grounding System for AC Substations - Continued





Design Procedures

The design process of a substation grounding system requires many steps. The following steps were established by the IEEE Standard 80-2000 for the design of the ground grid:

  • Step#1: Field Data Collection,
  • Step#2: Earthing Grid Conductor Sizing,
  • Step#3: Calculation of tolerable touch and step voltages,
  • Step#4: Preliminary design of grounding system,
  • Step#5: Calculation of of the preliminary Grid Resistance, RG, of the grounding system in uniform soil.
  • Step#6: Determination of Grid current, IG.
  • Step#7: Calculation of maximum grid potential rise and comparing withthe tolerable touch voltage from step#3. If the GPR of the preliminary design is below the tolerable touch voltage, move to step#12 (no further analysis is necessary). If not, continue to step#8.
  • Step#8: Calculation of mesh and step voltages.
  • Step#9: Comparing the computed mesh voltage from step#8 with the tolerable touch voltage from step#3. If the computed mesh voltage is below the tolerable touch voltage, continue to step#10. If not, move to step#11 for revising the preliminary design.
  • Step#10: Comparing the computed step voltage from step#8 with the tolerable step voltage from step#3.If the computed step voltages are below the tolerable step voltage, move to step#12. If not, move to step#11 for revising the preliminary design.
  • Step#11: Preliminary Design modification, If either the step or touch tolerable limits from step#3 are exceeded, revision of the grid design is required.
  • Step#12: Detailed final design. After satisfying the step and touch voltage requirements, additional grid and ground rods /conductors may be required. The final design should also be reviewed to eliminate hazards due to transferred potential and hazards associated with special areas of concern.


The block diagram in Fig (1) illustrates the Design procedures.


Fig (1)


In this Article and following Articles, I will explain these steps in detail.






Step#4: Preliminary Design Of Grounding System






1- Terms Definitions for Step#4

  • Auxiliary Ground Electrode: A ground electrode with certain design or operating constraints. Its primary function may be other than conducting the ground fault current into the earth. Auxiliary ground electrodes are electrodes that comprise various underground metal structures installed for purposes other than grounding. Typical auxiliary electrodes include underground metal structures and reinforcing bars encased in concrete, if connected to the grounding grid. Auxiliary ground electrodes may have a limited current carrying capability. 
  • Ground Electrode: A conductor imbedded in the earth and used for collecting ground current from or dissipating ground current into the earth.
  • Ground Mat: A solid metallic plate or a system of closely spaced bare conductors that are connected to and often placed in shallow depths above a ground grid or elsewhere at the earth surface, in order to obtain an extra protective measure minimizing the danger of the exposure to high step or touch voltages in a critical operating area or places that are frequently used by people. Grounded metal gratings, placed on or above the soil surface, or wire mesh placed directly under the surface material, are common forms of a ground mat.
  • Grounding System: it is the system that Comprises all interconnected grounding facilities in a specific area.
  • Primary Ground Electrode: A ground electrode specifically designed or adapted for discharging the ground fault current into the ground, often in a specific discharge pattern, as required (or implicitly called for) by the grounding system design. Primary ground electrodes are specifically designed for grounding purposes. Typical primary electrodes include such things as grounding grids, counterpoise conductors, ground rods, and ground wells. 
  • Grounding Grid: A system of horizontal ground electrodes that consists of a number of interconnected, bare conductors buried in the earth, providing a common ground for electrical devices or metallic structures, usually in one specific location.


Notes:

  1. Grids buried horizontally near the earth’s surface are also effective in controlling the surface potential gradients.
  2. A typical grid usually is supplemented by a number of ground rods and may be further connected to auxiliary ground electrodes to lower its resistance with respect to remote earth.









2- Data Needed For Preliminary Grounding System Design

The following Data are needed before starting the preliminary grounding system design:

  1. A layout of the site,
  2. Substation grid area ,
  3. Soil resistivity measurements at the site,
  4. Resistivity of any surface layers intended to be laid,
  5. Location of the feeding station,
  6. Near-by utility,
  7. Communication network in the area,
  8. Feeding characteristics,
  9. Any special considerations.









3- Typical Configurations For The Grounding System Of AC Substations


First: Conventional Earthing Configuration:

The conventional system of Earthing calls for digging of a large pit into which a GI pipe or a copper plate is positioned amidst layers of charcoal and salt. It is cumbersome to install only one or two pits in a day.
Types of conventional Earthing are shown below:

  1. Pipe Earthing,
  2. GI Pipe Earthing,
  3. Cast Iron Plate Earthing,
  4. Copper Plate Earthing,


The conventional system of GI pipe Earthing or copper plate Earthing requires maintenance and pouring of water at regular interval. This configuration is inadequate in providing a safe grounding system for AC Substations.


Second: Earth Mat Design Configurations:

This design has two configurations as follows:


Configuration#1

Several electrodes, such as ground rods, are connected to each other and to all equipment neutrals, frames, and structures that are to be grounded; the result is essentially a grid arrangement of ground electrodes, regardless of the original objective.
If the connecting links happen to be buried in a soil having good conductivity, this network alone may represent an excellent grounding system and is suitable for medium size substation.

Configuration#2:

A Substation earth system that have a combination of:

  • Buried horizontal conductors in rows and columns and a number of vertical ground rods penetrating lower soils.
  • A sold metallic plate or a system of closely spaced bare conductors that are connected to and often placed in shallow depths above a ground grid or elsewhere at the earth’s surface, in order to obtain an extra protective measure minimizing the danger of the exposure to high step or touch voltages in a critical operating area or places that are frequently used by people.
  • Grounded metal gratings placed on or above the soil surface, or wire mesh placed directly under the surface material, are common form of a ground mat.



This congiuration#2 has the following advantages:

  • While horizontal (grid) conductors are most effective in reducing the danger of high step and touch voltages on the earth’s surface, provided that the grid is installed in a shallow depth [usually 0.3–0.5 m (12–18 in) below grade], sufficiently long ground rods will stabilize the performance of such a combined system. For many installations this is important because freezing or drying of upper soil layers could vary the soil resistivity with seasons, while the resistivity of lower soil layers remains nearly constant.
  • Rods penetrating the lower resistivity soil are far more effective in dissipating fault currents whenever a two-layer or multilayer soil is encountered and the upper soil layer has higher resistivity than the lower layers. For many GIS and other space-limited installations, this condition becomes in fact the most desirable one to occur, or to be achieved by the appropriate design means (extra-long ground rods, grounding wells, etc.).
  • If the rods are installed predominately along the grid perimeter in high-to-low or uniform soil conditions, the rods will considerably moderate the steep increase of the surface gradient near the peripheral meshes.
  • Space saving on the ground level due to substantial reduction of earth pits which leads to easy of coordination.








4- The Grounding System Components For AC Substations

Generally, the grounding system for AC Substations consists of the following components (see fig.2):



fig.2

  • Auxiliary Ground Electrode: see above definition,
  • Primary Ground Electrode: see above definition,
  • Grounding Grid: usually, it consists of a conductor loop surrounding the entire grounded area, plus adequate cross conductors to provide convenient access for equipment grounds, etc. (see above definition),
  • Ground Mat: see above definition,
  • Grid Connections: it includes connections of the buried earthing grid to metallic parts of structures and equipment, connections to earthed system neutrals, and the earth surface insulating covering material.









5- Preliminary Design Parameters Of Grounding System Of AC Substations

Many parameters are affecting grounding system design and performance which are as follows:

  1. Area of grounding system,
  2. Spacing between adjacent conductors,
  3. Depth of burial of grid,
  4. Total length of horizontal buried conductors,
  5. Number of parallel conductors in one direction,
  6. Number of vertical ground rods,
  7. Length of vertical ground rods,
  8. Conductor diameter,
  9. Ground Rod Diameter,
  10. The thickness of the surfacing material,



Notes:

  • The area of the grounding system, the conductor spacing, and the depth of the ground grid have the most impact on the mesh voltage, While parameters such as the conductor diameter and the thickness of the surfacing material have less impact.
  • The area of the grounding system is the single most important geometrical factor in determining the resistance of the grid. The larger the area grounded, the lower the grid resistance and, thus, the lower the ground potential rise GPR.
  • The initial estimates of conductor spacing and ground rod locations should be based on the current IG and the area being grounded.








6- Basic Ideas And Concepts For The Preliminary design parameters of Grounding System For AC Substations


To establish the basic ideas and concepts, the following points may serve as guidelines for starting a typical grounding grid design:


1- For Grid Area:

  • The substation should surround the perimeter and take up as much area as possible to avoid high current concentrations. Using more area also reduces the resistance of the grounding grid.
  • The ground grid should encompass all of the area within the substation fence and extend at least 0.91 meter (3.0 feet) outside the substation fence. A perimeter grid conductor should be placed 0.91 meter (3.0 feet) outside and around the entire substation fence including the gates in any position. A perimeter grid conductor should also surround the substation equipment and structure cluster in cases where the fence is located far from the cluster.
  • The grid should extend over the entire substation and beyond the fence line. Multiple ground leads or larger sized conductors would be used where high concentrations of current may occur, such as at a neutral-to-ground connection of generators, capacitor banks, or transformers.
  • The entire area inside the fence and including a minimum of 1.0 meter (3.3 feet) outside the fence needs to be covered with a minimum layer of 10 cm (4 inches) of protective surface material such as crushed rock (or approved equal) possessing a minimum resistivity of 3,000 ohm-meters wet or dry.



2- For Ground Mesh Conductors:

  • The ground grid consists of horizontal (grid) conductors placed in the ground to produce square mesh. This can be visualized as a checkerboard pattern. One row of horizontal conductors is equally spaced 3 to 15 meters (9.8 to 49.2 feet) apart. A second row of equally spaced horizontal conductors running perpendicular to the first row is spaced at a ratio of 1:1 to 1:3 of the first row’s spacing, unless a precise (computer-aided) analysis warrants more extreme values. For example, if the first row spacing was 3 meters (9.8 feet), the second row spacing could be between 3 to 9 meters (9.8 to 29.5 feet).
  • Typically conductors are laid in parallel lines. Where it is practical, the conductors are laid along the structures or rows of equipment to provide short ground connections.
  • A typical grid system for a substation may include 4/0 bare copper conductors buried 0.3–0.5 m (12–18 in) below grade, spaced 3–7 m (10–20 ft) apart, in a grid pattern. At cross-connections, the conductors would be securely bonded together.
  • Grid conductors should be buried a minimum of 0.46 meter (18 inches) to 1.5 meters (59.1 inches) below final earth grade (excluding crushed rock covering) and may be plowed in or placed in trenches. In soils that are normally quite dry near the surface, deeper burial may be required to obtain desired values of grid resistance.



3- For Vertical Ground Rods:


  • Vertical ground rods may be at the grid corners and at junction points along the perimeter. Ground rods may also be installed at major equipment, especially near surge arresters. 
  • In multi-layer or high-resistivity soils, it might be useful to use longer rods or rods installed at additional junction points. 
  • Vertical ground rods should be 1.6 cm (5/8 inch) diameter by at least 2.5-meter (8.0-foot) long copper, steel, or other approved type. Where used, they should be installed with tops 5 cm (1.97 inches) minimum below grade and bonded to the ground grid connectors. 
  • A good design practice is to space rods not closer than their length. 
  • An additional determinant is having enough rods so that their average fault current pickup would not exceed 300 amperes, assuming all ground system current entering the grid through the rods.



However, to get started on the preliminary design, the following steps can be taken:

  1. On a layout drawing of the substation site, draw in the largest square, rectangular, triangular, T-shaped, or L-shaped grids that will fit within the site.
  2. Place grid conductors to produce square mesh of approximately 6.1 to 12.2 meters (20 to 40 feet) on a side.
  3. Set the grid depth, h, equal to 45.72 cm (18 inches).
  4. Set the thickness of surface material equal to 10.16 cm (4 inches).
  5. Place ground rods around the perimeter of the substation. As a general rule, place a ground rod at every other perimeter grid connection and at the corners of the substation.



Notes:

  • These represent the outer grid conductors and will define the area of the grid to be used in the calculations. A square, rectangular, triangular, T-shaped, or L-shaped grid site generally requires no additional conductors once the design is complete,
  • For irregular sites, once the design has been completed, additional conductors will be run along the perimeter of the site that were not included in the original grid design and connected to the grid,
  • The safety level for equipment and people can be enhanced using unequally spaced grounding grids by decreasing earth surface potential gradients and making conductor leakage current distribution more uniform,
  • A non-uniform conductor spacing, having more conductors at the edges of the grids, provides the most efficient design,
  • An unequally spaced grounding grid is an economical and reasonable technique as shown in simulation,
  • The design of rectangular grids buried in uniform soil without ground rods could be adjusted and applied to any practical substation grounding grid design.









7- Examples for Preliminary design parameters of Grounding System
 For AC Substations


By applying Basic Ideas And Concepts For The Preliminary design parameters of Grounding System, we can get preliminary Grid design as in below examples.


Example#1: Square Shape Grid Without Ground Rods:




 Fig (3)

 A square earthing grid with no ground rods with the following parameters is proposed as shown in Fig (3):

  • Design layout of 70 m × 70 m grid,
  • Equally spaced conductors with spacing D = 7 m,
  • The grid wire pattern is 11 x11 (11 parallel rows and 11 parallel columns),
  • Grid burial depth h = 0.5 m,
  • Grid conductors will be 120 mm2,
  • No ground rods,
  • The total length of buried conductor, L T, is 2 x11x70 m = 1540 m.



Example#2: Square Shape Grid With Ground Rods: See Fig (4)



 Fig (4)


The preliminary design in fig (2) will be modified to include the following parameters:
  • 20 ground rods, around the perimeter of the grid,
  • Ground rod length 7.5 m (24.6 ft).



Example#3: Rectangular Shape Grid With Ground Rods:



Fig (5)


A Rectangular shape grid with ground rods with the following parameters is proposed as shown in Fig (5):

  • Design layout of 63 m × 84 m grid,
  • The grid wire pattern is 10 x13 (10 parallel rows and 13 parallel columns),
  • Equally spaced conductors with spacing D = 7 m,
  • Grid burial depth h = 0.5 m,
  • Grid conductors will be 120 mm2,
  • The grid conductor combined length is 13x63 m + 10x84 m = 1659 m,
  • 38 ground rods,
  • Ground rod length 10 m.



Example#4: L-Shaped Grid With Ground Rods:



Fig (6)


L-shaped grid with ground rods with the following parameters is proposed as shown in Fig (6):

  • Design layout of 35 m × 35 m + 105 m x 35 m grid,
  • The grid wire pattern is 16 x11 (16 parallel rows and 11 parallel columns),
  • Equally spaced conductors with spacing D = 7 m,
  • Grid burial depth h = 0.5 m,
  • Grid conductors will be 120 mm2,
  • The grid conductor combined length is 6 x 105 + 5 x 35+ 6 x 70 + 10 x 35 = 1575 m,
  • 24 ground rods, around the perimeter of the grid,
  • Ground rod length 7.5 m (24.6 ft).







In the next Article, I will explain Other Steps from the Design Procedures of Grounding System Design for AC Substation. Please, keep following.


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