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Fault Current Distribution Analysis

FCDIST calculates fault current distribution in multiple terminals, transmission lines and distribution feeders using minimum information and a simple set of data concerning the network. By providing the user with an accurate estimate of fault current distribution, and hence the actual fault current flowing into a grounding system, FCDIST makes it possible to avoid over-design of the grounding system due to otherwise necessary conservative assumptions regarding the fault current entering the grounding system under study. This can result in significant time and financial savings.

Schematic diagram of FCDIST circuit model

Technical Highlights

Fault and unbalance current distribution is determined by the grounding of the various sections of the transmission line and associated substations, and also the inductive coupling between the phase wires and the sky wires. The model created by FCDIST accounts for inductive coupling, assigns unique ground impedances to the central site and terminal stations along the transmission line, accounts for the grounding of all structures along the transmission line, and takes into consideration all the other key elements necessary to calculate fault current distribution accurately. By using a single phase (which can be used to represent all three phases) and a single (possibly bundled) neutral wire, and constant parameters for all sections, the amount of necessary input data is maintained at a relatively low and easily manageable level.

Technical Features

  • The FCDIST circuit model consists of three basic elements: i) a central site (faulted substation or transmission line structure), ii) terminal stations (sources of faulted currents), and iii) transmission lines connecting the central site to the terminal stations. At least one, and possibly several, terminal stations may exist in each FCDIST model.
  • Each arm of the network is energized at terminal station by current sources whose current can be specified with an arbitrary magnitude and phase relative to each other. Each source is connected via a lead with a specified series impedance to the transmission line neutral. This lead also has a mutual impedance with respect to the phase wire.
  • The transmission line is modeled as a single phase wire and single bundled neutral conductor in the air. Each transmission line is made up of sections (i.e., spans) which are all of the same length. Each span is terminated by a ground shunt connected to the neutral bundle. This represents the ground impedance of the transmission line towers connected to the neutral lead. Each section of the neutral has a series impedance and mutual impedance with respect to the phase wire.
  • FCDIST computes the series impedance and mutual impedance of a neutral conductor in the air with respect to the phase wire based on their cross sectional location in the right-of-way, their conductor characteristics and a uniform soil resistivity.
  • With impedance input mode, FCDIST can also model transmission line networks interconnected via buried power cables.
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