Electrode Sizing

If desired, prior to designing the grounding grid, the minimum required conductor and/or rod size can be determined. Simply enable one or more electrode types provided in the

‘Electrodes’ tab of the ‘Data Entry’ view. CYMGRD calculates the minimum required ground conductor or rod size in accordance with IEEE 80-2000.

The selection of the suitable conductor material and size should satisfy the following criteria: electrical conductivity, corrosion resistance, current carrying capacity and mechanical strength.

Any conductor should be capable of conducting the entire ground fault current without exceeding a specified temperature.

As per ANSI/IEEE Std. 80-2000: ,

A Is the conductor section (in cmils) ILG is the RMS fault current (in A)

Kf constant dependent of the conductor material ( Kf = 7.01 for Copper, Soft Drawn)

tc fault duration (in sec.)

The size of the ground electrode must be specified prior to the grounding system design.

CYMGRD calculates the minimum required size of the ground conductor or rod in accordance to IEEE standards.

To determine the minimum required electrode size, the constant parameters of the material of the electrode (conductor/rod), the Ambient-temperature, the Maximum fault-current and the Fault-duration are required.

CYMGRD 6.5 – Reference Manual and Users Guide

The default value for the fault current is 1000 [amps], and the Fault-duration is equal to the Shock-duration as default. However the user should change the values to the desired values in the Buses tab in the Data Entry window. (See below)

In order to consider auto-recloser reaction – if any – the Fault-Duration is assumed to be equal to the summation of the Shock-Durations.

Notes: y The Fault-Duration in the Buses tab cannot be less than the Shock-Duration in the Soil Parameter dialog box.

y Ambient temperature can be specified in the Soil Parameters dialog box.

In order to specify the electrode material, the user can choose one of the materials from the CYMGRD library in the Electrodes tab. (See below). In addition, the user can change the material parameters in the CYMGRD library to specify a user-defined material.

The following figure shows the CYMGRD library (Electrodes data entry tab), which includes the list of the most common grounding electrode materials and corresponding parameter values.

After all the required parameters are specified, the result will appear in the Output window under the Electrode Sizing tab. There is no need to run electrode-sizing analysis. The following figure shows an electrode-sizing result.

CYMGRD 6.5 – Reference Manual and Users Guide

After the electrode material and size have been chosen by the user, the diameters of the electrodes are required. CYMGRD has a feature to help entering the diameter of the electrodes.

When one or more ‘Conductor’ and/or ‘Rod’ items are selected in the Electrodes data entry tab and that the Electrode Sizing report has been generated (a valid ‘Soil Model’ analysis must be available for the active study), a list of corresponding ‘Materials’ and ‘Sizes’ will be available for selection in the data entry windows for all matching Electrode types.

By picking a ‘Material’ from the list, the ‘Nominal Size’ (this is the default setting as reported in the Electrode Sizing results) for the Conductor will be set and its ‘Diameter’ will be adjusted accordingly.

Proceeding to change the ‘Size’ will alter the Conductor ‘Diameter’. Modifying the

‘Diameter’ directly will cancel both the ‘Material’ and ‘Size’ selections.

CYMGRD 6.5 – Reference Manual and Users Guide

3.3.1 LG fault parameters

LG fault current and corresponding X/R are the results of fault analysis and are required for Electrode Sizing analysis.

In the “Buses” tab of ‘Data Entry’ view, the user must enter data for all the buses in the substation. CYMGRD will automatically choose the bus that requires the thickest electrode and apply it towards the Electrode Sizing analysis.

As shown under the Buses data entry tab above:

• When the ‘Enabled’ box is checked, it means that the bus data will be considered in the analysis.

• Usually a substation has two or more buses. CYMGRD identifies each bus and the corresponding parameters by a ‘Bus ID’. The results of the analysis appear in the

‘Electrode Sizing’ tab in the ‘Reports’ view with corresponding Bus ID (See following image).

• ‘LG Fault Current’ is the total single line- to-ground fault current in amperes.

• ‘Remote Contribution’ is the summation of the contributions (of the LG Fault Current) from the transmission lines (not the local transformers within the substation) divided by total fault current and multiplied by 100.

• ‘LG X/R’ is ‘(2x1+Xo)/(2R1+Ro)’ for the corresponding single line-to-ground fault current.

Note: CYMGRD does not use the following parameters for Electrode Sizing, however, in order for the bus data as a whole to be saved, they must be supplied. CYMGRD uses this additional data for grid analysis when a ‘Current Split Factor’ needs to be determined.

• ‘Transmission Lines’ is the number of the lines connected to the bus.

• ‘Rtg’ is the ground electrode resistance of the above transmission line (Default = 100 Ohms).

CYMGRD 6.5 – Reference Manual and Users Guide

• ‘Distribution Feeders’ is the number of the feeders connected to the other side of the transformers which, in turn, is connected to the bus.

• ‘Rdg’ is the ground electrode resistance of the above feeders (Default = 200 Ohms).

3.3.2 Electrode Material

To determine the minimum required electrode size, a correction factor (i.e. Decrement factor), the constant parameters for the electrode material and ambient temperature value are required:

• The ambient temperature is defined in the Grid Parameters dialog box (Default = 40 degrees Celsius). The ‘Grid Parameters’ dialog box can be accessed under the

‘Parameters…’ item of ‘Grid’ menu.

• The type of the material along with its parameters is specified in the “Electrodes” tab of the ‘Data Entry’ view (See below).

• CYMGRD uses the information in the ‘Buses’ tab to calculate the Decrement factor in accordance with the standard. This factor is used to take into account the DC components, resulting in the asymmetrical fault current for the corresponding fault duration.

The following image shows the CYMGRD ground conductor library (“Electrodes” tab). In this example, ‘Copper commercial hard-drawn’ is selected for the conductor sizing and ‘Copper-clad steel’ is selected for the rod sizing.

Note: Certain parameters, such as the Melting Temperature (Tm) can be modified in order to better define the materials in use. Any altered values will be saved only as part of the active study.

3.3.3 Electrode Sizing report

After all the required data for the Electrode Sizing has been specified, the result of the analysis automatically appears in the ‘Electrode Sizing’ tab of the ‘Reports’ view.

CYMGRD 6.5 – Reference Manual and Users Guide