Entering the Grid data

Ground Grid data can be entered by either specifying directly their geometrical coordinates or can be imported from an AutoCAD file formatted for use with CYMGRD. This section describes data entry for the case where AutoCAD data files are not available. In CYMGRD, the ‘grid components’ data is classified into five categories: Symmetrically arranged grid conductors, asymmetrically arranged grid conductors, arc conductors, symmetrically-arranged ground rods and asymmetrically symmetrically-arranged ground rods. All are explained in the following sub-sections. Section Symmetrically-arranged grid Conductors explains the import/export of AutoCAD data.

3.6.1 Symmetrically-arranged grid Conductors

This type of array is rectangular, with a number of conductors laid out along the long and short axes, creating a grid. CYMGRD assumes that symmetrically-arranged grid conductors are buried horizontally and are oriented along two perpendicular axes (the X and Y axes in the graphic window). The spacing between the conductors is assumed to be equal along each axis, but the spacing along the Y-axis can be different from the spacing along the X-axis. The data for symmetrically-arranged components is entered using the ‘Symmetrical Conductors’ tab of the

‘Data Entry’ view.

CYMGRD 6.5 – Reference Manual and Users Guide

Symmetrical Conductor data is shown above. Note that the check box ‘Enabled’ is selected, which means that it will be considered in the Grid Analysis. Furthermore, the ‘Primary’

electrode type is selected (default). The drop-down box allows modifying that default to ‘Return’

or ‘Distinct’.

For this example, we have used the symmetrical conductor arrangement to represent the lower rectangular part of an L-shaped grid.

The following set of data is used to define a symmetrically-spaced grid:

Type Primary, Return or Distinct.

[X1, Y1] and [X2, Y2]

Coordinates of two opposite corners of the rectangular array.

Grid conductors parallel to X

The number of grid conductors parallel to the X-axis.

Elements per conductor parallel to X

CYMGRD considers this number of elements in finite-elements analysis, for conductors parallel to the X-axis Grid conductors parallel

to Y

The number of grid conductors parallel to the Y-axis.

Depth The distance between the soil surface and the center of the conductor.

Diameter Ground conductor diameter.

CYMGRD 6.5 – Reference Manual and Users Guide

Note: If the electrodes (Conductors or rods) placed in the grid cannot satisfy a placement pattern with some symmetry, then they should be defined using asymmetrical electrodes.

3.6.2 Asymmetrically-arranged grid Conductors

An asymmetrically-arranged conductor is a single straight conductor stretched between two points defined by two coordinates (X1, Y1, Z1) and (X2, Y2, Z2). Asymmetrical conductors that are slanted may be represented in the model (Z coordinate), which is not the case for the symmetrical arrangements, which are entered using a common burial depth (X,Y). Furthermore, each conductor may have a different diameter, which is not the case for the symmetrical arrangements with a common diameter for all conductors.

Asymmetrical conductor data is shown above. Note that the check box ‘Enabled’ is selected, which means that it will be considered in the Grid Analysis. Also, the ‘Primary’ electrode Type is selected (default). The drop-down box allows modifying that default to ‘Return’ or

‘Distinct’.

For this example, we have used the asymmetrical conductor arrangement to represent the upper left protruding part of an L-shaped grid.

The following set of data is used to define an asymmetrical grid:

Type Primary, Return or Distinct.

[X1, Y1, Z1] and [X2, Y2, Z2]

Coordinates of two ends of each conductor. Conductors may be inclined with respect to the soil surface, which CYMGRD

CYMGRD 6.5 – Reference Manual and Users Guide

Number of

conductor elements

CYMGRD considers this number of elements for conductors parallel to the X (or Y-axis) in finite-elements analysis.

Diameter Ground conductor diameter.

3.6.3 Symmetrically-arranged ground Rods

A symmetric array of ground rods covers a rectangular area in which rods are located in rows parallel to the X-axis with all rods in a row equally spaced. All rods defined in the same array have the same burial depth, length and diameter.

Symmetrical rod data is shown above. Note that the check box ‘Enabled’ is selected, which means that it will be considered in the Grid analysis. In this example, the ‘Primary’

electrode Type is selected (default). The drop-down box allows modifying the default to ‘Return’

or ‘Distinct’.

The following set of data is used to define symmetrically-arranged rods:

Type Primary, Return or Distinct electrode.

[X1, Y1] and [X2, Y2]

Coordinates of the two opposite corners of the area where the rods are placed.

Rod rows parallel to the X-axis

Number of the horizontal rod rows on the display.

Number of ground rods per row

Number of rods along each row (Defined above).

CYMGRD 6.5 – Reference Manual and Users Guide

Length Ground rod length.

Depth Burial depth (the distance between the soil surface and the top of the rods).

Diameter Ground rod diameter.

3.6.4 Asymmetrically-arranged ground Rods

An asymmetric array of ground rods is a single row of equally spaced rods. The position of the first rod is given by the coordinates (X1, Y1, Z1) and the position of the last rod in the row is given by the coordinates (X2, Y2, Z2). The upper end of each rod lies on the straight line between these two points. All rods defined in the same array have the same length and diameter. If a single rod is specified (Number of Rods along axis = 1), then only the starting point coordinates (X1, Y1, Z1) need to be entered.

Asymmetrical rod data is shown above. Note that the check box ‘Enabled’ is selected, which means that it will be considered in the Grid analysis. The ‘Primary’ electrode Type is selected (default). The drop-down box allows modifying the default to ‘Return’ or ‘Distinct’.

For this example, we have used the asymmetrical rod arrangement because all the rods placed in the grid were strategically positioned at specific coordinates. It is seen in the data that we have entered the rods one at a time using different coordinates for the beginning and the end points.

The following set of data defines a row of rods:

CYMGRD 6.5 – Reference Manual and Users Guide

[X1, Y1, Z1] and [X2, Y2, Z2]

Coordinates of the two end points of the row of rods.

Number of rods along axis

Number of rods in the row.

Elements per Rod in upper soil layer

Number of elements for rods in upper soil layer for the finite-elements analysis.

Elements per Rod in lower soil layer

Number of elements for rods in lower soil layer for the finite-elements analysis.

Length The rod length.

Diameter The rod diameter.

3.6.5 Rod Encasement

In order to improve the impact of a rod in the grid, the rod may be installed in a cylinder of semiconductor material buried in the soil. See the following picture from IEEE 80.

This is of particular interest in medium and highly resistive soils.

To enter a rod encase in CYMGRD:

1) Activate the check box Material- Encased for the rod.

2) Enter the Material Thickness (the cylinder radius).

CYMGRD 6.5 – Reference Manual and Users Guide

3) In the Grid Parameters dialog box, enter the Resistivity of the material around the rod in the encasement (cylinder). The default value is 100 [Ohm-m].

CYMGRD 6.5 – Reference Manual and Users Guide

3.6.6 Arc Conductors

An arc conductor is a circular or arced conductor laid in the ground.

Arc conductor data is shown above. Note that the check box ‘Enabled’ is selected, which means that it will be considered in the Grid analysis. The ‘Primary’ electrode Type is selected (default). The drop-down box as allows modifying that default to ‘Return’ or ‘Distinct’.

The following set of data defines an arc conductor:

Type Primary, Return or Distinct electrode.

[X1, Y1] Coordinates of the arc center.

Starting angle Beginning of the arc in degrees.

Ending angle End of the arc in degrees, assuming a counter-clockwise rotation.

Radius The radius of the arc.

Number of

conductor elements

Number of conductor elements the arc is to be approximated with as an inscribed polygon.

Depth The arc burial depth (common for both ends).

Diameter The arc conductor diameter.

CYMGRD 6.5 – Reference Manual and Users Guide

Note: A positive value of Z denotes a position below the surface of the soil for all electrode types and arrangements. No negative Z is permitted.

Both ends of an asymmetrical grid conductor must be in the same soil layer.

Only ground rods are permitted to bridge two separate soil layers.

The minimum number of conductor elements that an arc can be approximated to is 3.

Electrodes are color-coded in the graphic window. ‘Primary’ electrodes are red, ‘Return’ electrodes are blue and ‘Distinct’ electrodes are green.