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📘 Symmetrical Fault at Different Buses – Comparison Study

Same power system, different fault locations. Observe how fault level changes.

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🔹 System Data

Generator 1: X1 = 0.2 pu Generator 2: X2 = 0.25 pu Transformer: Xt = 0.1 pu Transmission Line: Xline = 0.2 pu Base MVA = 50 MVA

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🔹 Step 1 – Generator Parallel Equivalent

1/Xeq = 1/0.2 + 1/0.25 = 5 + 4 = 9 X_gen_eq = 1/9

X_gen_eq = 0.111 pu

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📍 Case 1 – Fault at Generator Terminal

Here only generator reactances act. Z_th = 0.111 pu Fault current: I_fault = 1 / 0.111 ≈ 9 pu Fault MVA: = 50 / 0.111 ≈ 450 MVA

Highest fault level because no transformer or line reactance added.

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📍 Case 2 – Fault at Transformer Secondary Bus

Z_th = 0.111 + 0.1 = 0.211 pu Fault current: I_fault = 1 / 0.211 ≈ 4.74 pu Fault MVA: = 50 / 0.211 ≈ 237 MVA

Fault reduced because transformer reactance added.

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📍 Case 3 – Fault at Line End

Z_th = 0.111 + 0.1 + 0.2 = 0.411 pu Fault current: I_fault = 1 / 0.411 ≈ 2.43 pu Fault MVA: = 50 / 0.411 ≈ 121 MVA

Lowest fault level because line reactance also included.

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📊 Final Comparison

Fault Location	Z_th (pu)	Fault Current (pu)	Fault MVA
Generator Terminal	0.111	9.0	450 MVA
Transformer Bus	0.211	4.74	237 MVA
Line End	0.411	2.43	121 MVA

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🎯 Important Conclusions

• Fault level decreases as distance from source increases.
• Every added reactance reduces short circuit current.
• Protection rating must consider worst-case (maximum fault).
• Circuit breaker at generator side must handle highest current.

Fault Current ∝ 1 / Total Reactance