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The program outputs the hierarchical system equipment tree with short circuit MVA calculated at each node. Short circuits contributed by upstream (red) and downstream (blue) equipment are listed across each node ( see sample output ). For a three phase fault, use the accompanying SC MVA to KA converter (see below) or simply divide total SC MVA value by 1.73 * kVLL to get short circuit current values in kA.
Three phase faults are unique in that they are balanced ( symmetrical) in the three phases, and can be calculated from the single one line diagram.The resulting short circuit current values in kA are suitable for arcflash calculations, or any other study requiring symmetrical three phase bolted fault current analysis.
The protection specialist is interested in a wider variety of faults than just three phase fault. The most common fault is a single phase to earth fault which in low voltage systems can produce a higher fault current than a three phase fault. Experience has shown that between 70 to 80 percent of transmission line faults are single line to ground faults, which arise from the flashover of only one line to the tower and ground. Similarly, because protection is expected to operate correctly for all types of fault, it may be necessary to consider the fault currents due to many different types of fault. Since the three-phase fault is unique in being a balanced fault, a method of analysis that is applicable to unbalanced faults is required. It can be shown that, by applying the principle of superposition, any general three-phase system of vectors may be replaced by three sets of balanced (symmetrical) vectors; two sets are three-phase but having opposite phase rotation and one set is co-phasal. These vector sets are described as the positive, negative and zero sequence sets respectively. By performing short circuit MVA analysis for all the three sequences, unsymmetrical fault currents can be resolved by substituting respective SC MVA (1, 2 and 0) and X/R into the form below. When filled in, the form will calculate 3 phase, phase to ground and phase to phase faults.
Impedance Ohmic value and x/r are required for faults through impedance.
VLL, SC MVA and X/R should be numerical above 0. Leave SC MVA2, 0, X/R2, 0 blank if unbalanced fault currents are of no interest.
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