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📘 3-Phase Fault at Intermediate Bus – Advanced Problem

Now fault occurs at transformer secondary bus before transmission line. Careful network reduction required.

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🔹 Problem Statement

Generator 1: 50 MVA, 11 kV, X1 = 0.2 pu Generator 2: 50 MVA, 11 kV, X2 = 0.25 pu Transformer: 50 MVA, 11/132 kV, Xt = 0.1 pu Transmission line: 0.2 pu Fault occurs at transformer secondary bus (before line). Find: 1) Fault current 2) Fault MVA

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🔹 Step 1 – Choose Base

S_base = 50 MVA Voltage base: Generator side = 11 kV Transformer secondary = 132 kV

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🔹 Step 2 – Parallel Generators

Both generators already on 50 MVA base. Parallel combination: 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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🔹 Step 3 – Add Transformer

Now from fault location looking back: Generator equivalent + transformer in series. Z_th = 0.111 + 0.1

Z_th = 0.211 pu

IMPORTANT: Transmission line NOT included Because fault is before line.

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🔹 Step 4 – Fault Current

I_fault = 1 / Z_th = 1 / 0.211 ≈ 4.74 pu

Fault current ≈ 4.74 pu

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🔹 Step 5 – Fault MVA

Fault MVA = 50 / 0.211 ≈ 237 MVA

Fault MVA ≈ 237 MVA

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🔹 Why Fault Level Increased?

Because: Earlier case included line reactance Now line removed Lower Z_th → Higher fault current

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🎯 Concept Understanding

• Always remove components beyond fault
• See network from fault location
• Parallel first, then series
• Transmission line excluded if fault before it

Fault Location Changes Z_th → Changes Fault Level