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Four alternators, each rated at 5 MVA, 11 kV with 20% reactance are working in parallel ┬╖ The short-circuit level at bus bars is
6.25 MVA
20 MVA
25 MVA
100 MVA
100 MVA
Given: Four alternators, each with SratedтАЛ = 5 MVA and reactance X = 20% (0.20 p.u.).
Four alternators, each with SratedтАЛ = 5 MVA and reactance X = 20% (0.20 p.u.).
MVAscтАЛ=XeqтАЛMVAbaseтАЛтАЛ
Calculate the total rated capacity
Since there are 4 identical alternators connected in parallel, the total rated capacity is the sum of the individual ratings.
StotalтАЛ=4├Ч5┬аMVA=20┬аMVA
Calculate the equivalent reactance in parallel
With 4 alternators each having 20% reactance in parallel, the equivalent per-unit reactance on the system base is XeqтАЛ=nXтАЛ=40.20тАЛ=0.05 p.u.
XeqтАЛ=40.20тАЛ=0.05┬аp.u.
Calculate the short-circuit level
The short-circuit MVA is determined by the ratio of total MVA to the equivalent per-unit reactance.
MVAscтАЛ=0.0520┬аMVAтАЛ=100┬аMVA
D is correct because the short-circuit level is calculated as the ratio of total rated capacity to the combined per-unit reactance, resulting in 100 MVA.
In power system fault analysis, adding more generators in parallel reduces the equivalent impedance, thereby significantly increasing the short-circuit current level at the bus bars.