📘 Advanced Distance Protection – Multi-Source Fault & Infeed Effect
When multiple sources feed a fault, the apparent impedance seen by relay changes. This causes underreach or overreach.
🔹 System Description
Two generators G1 and G2 connected by transmission line AB. Relay located at bus A.
- Line AB impedance = 10 Ω
- Fault at 8 Ω from A
- Current contribution from A side = 1000 A
- Current contribution from B side (infeed) = 500 A
- Fault voltage at A = 8000 V
🔹 Step 1: True Fault Impedance
True impedance = 8 Ω
🔹 Step 2: Apparent Impedance Seen by Relay
Relay measures only current from A side.
Z_seen = V / I_A = 8000 / 1000 = 8 Ω
Now consider total current at fault:I_total = 1000 + 500 = 1500 A
Voltage drop along line depends on total current.
🔹 Step 3: Infeed Effect
Correct apparent impedance:
Z_apparent = Z_true × (I_total / I_A)
= 8 × (1500 / 1000) = 8 × 1.5 = 12 Ω
🔹 Step 4: Relay Decision
If Zone 1 setting = 8 Ω:Relay sees 12 Ω > 8 Ω Zone 1 does NOT operate (Underreach)
🔹 Engineering Understanding
- Infeed increases apparent impedance
- Relay may underreach
- Zone 2 usually clears such faults
- Modern numerical relays compensate infeed
🔹 Overreach Scenario
If remote end contribution decreases:- I_A large
- I_total small
- Apparent impedance reduces
- Relay may overreach
🎯 GATE & Interview Points
- Infeed causes underreach
- Outfeed causes overreach
- Zone 2 clears underreach faults
- Distance relay must consider multi-source system
Distance Protection in Multi-Source System Requires Careful Setting
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