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An isolated synchronous generator with transient reactance equal to 0.1 pu on a 100 MVA base is connected to the high voltage bus through a step up transformer of reactance 0.1 pu on 100 MVA base ┬╖ The fault level at the bus is
1000 MVA
500 MVA
50 MVA
80 MVA
500 MVA
The fault level (Short Circuit MVA) is determined by dividing the base MVA by the total per-unit reactance between the source and the fault point ┬╖ In this system, the generator transient reactance and the transformer reactance are in series, resulting in a total reactance of $0.1 + 0.1 = 0.2$ pu.
The fault level (Short Circuit MVA) is determined by dividing the base MVA by the total per-unit reactance between the source and the fault point ┬╖ In this system, the generator transient reactance and the transformer reactance are in series, resulting in a total reactance of $0.1 + 0.1 = 0.2$ pu.
FaultMVA=Xtotal(pu)тАЛBaseMVAтАЛ тАФ Calculating fault capacity
XtotalтАЛ=XgenтАЛ+XtransтАЛ тАФ Total series impedance
When a symmetrical fault occurs at the high voltage bus, the fault current is limited only by the source internal transient impedance and the series connected transformer impedance ┬╖ The fault MVA is calculated as Fault┬аMVA=Xtotal(pu)тАЛBase┬аMVAтАЛ.
Fault MVA is inversely proportional to the per-unit impedance.
Reactances in series add up directly when calculated on the same base MVA.
Transient reactance determines the initial fault current sub-transient period.
Simple analytical approach for protection coordination
Uses per-unit system to simplify multi-voltage network calculations
Assumes constant voltage source behind transient reactance
Ignores resistance, which may lead to conservative results
Switchgear sizing and selection
Relay settings and protection studies
Calculation: Fault MVA = 100 / (0.1 + 0.1) = 100 / 0.2 = 500 MVA.
The fault level at the bus represents the maximum power that can flow into a bolted short circuit.
B is correct тАФ The total per-unit reactance is 0.2, and dividing the 100 MVA base by 0.2 results in a fault level of 500 MVA.
Always verify that all impedances are converted to the same MVA base before adding them in series or parallel.