Fault Current Distribution

Overview

The Fault Current Distribution (FCD) module uses an accurate Node Voltage (NV) calculation method flexible enough for calculating custom electrical network and earthing configurations.

A user can model the actual fault current that will dissipate into the local earth grid under study (EG LOCAL) based on the upstream cables and overhead line feeders.

How to use

Model the transmission line or lines feeding the Local site (substation). A line consists of a section of cable and/or overhead transmission line selected from several standard configurations. Enter the physical and electrical data required for the respective standard configurations.

Enter the fault current and the respective fault type for that current.

The prospective earth fault current is obtained from the planning department documentation of your project’s power authority.

The earth resistance of the connecting (upstream) substation is required (EG remote resistance).

In general, the EG remote resistance does not have a significant effect on the calculated EG Local current and so therefore it is sufficient to be estimated if an accurate value is not known.

The resistance of the local earth grid under study (EG Local) should be calculated and the fault current distribution calculations should be based on this value. Using the Auto setting, the software will update the fault current calculations with the calculated grid resistance obtained in Display Results.

Local site

The local site pertains to the local earth grid being modelled or designed. For the fault current distribution calculations, the local earth grid resistance (EG local resistance) is required and this should be obtained from the Grid Impedance (Z) calculation in Display Results. The software can automatically update the EG local resistance whenever a main calculations are performed.

A Future increase to the calculated EG local current can be applied to allow for future increases in the system fault level, where an allowance of a 20 % increase is typical.

Sections

Multiple lines can be connected (in parallel) with the local site. A line can consist of cables with either solid, single-point, or cross-bonded screen, or overhead transmission lines with overhead earth wires. A line can consist of multiple sections of cables or overhead lines in a series.

The prospective line fault current is specified. The Fault type is selected and the prospective line fault current for that fault type is specified.

For single phase faults, the faulted phase (whether it is A, B, or C phase) affects the EG Local current and is the worst case when the faulted phase is located farthest away from the screens/earth wires. The software automatically detects and alerts the user to the faulted phase giving the highest EG Local current.

Currents

The calculated branch currents are displayed in a table. A split factor is displayed which is a fraction of the total fault current in percent.

The critical current to consider is EG LOCAL. If under Design Settings, the Grid energisation setting is set to “Use Fault Current module” the EG Local current is used in Display Results.

Voltages

The calculated voltages of earth points and nodes are displayed in a table.

Screen voltages are calculated with respect to the nearest local earth point. Node voltages are with respect to the reference node.