📘 Distance Protection – Zone Setting & Numerical Problems
Distance relay measures impedance (Z = V/I) and operates when measured impedance is less than preset zone impedance.
🔹 1️⃣ Basic Principle
Z = V / I
- Short circuit → Current increases
- Measured impedance decreases
- If Z_measured < Z_set → Relay trips
🔹 2️⃣ Zone Settings
- Zone 1 → 80% of protected line (Instantaneous)
- Zone 2 → 100% line + 50% next line (Time delay)
- Zone 3 → Backup (Further reach)
🔹 Problem 1: Zone 1 Setting
Line impedance = 0.5 + j4 Ω Length = 100 km Find Zone 1 setting.
Solution
Total impedance magnitude = √(0.5² + 4²) = √(0.25 + 16) = √16.25 = 4.03 Ω
Zone 1 = 80% × 4.03 = 3.22 Ω
🔹 Problem 2: Fault at 60 km
Fault occurs at 60 km on same line. Will Zone 1 operate?
Step 1: Fault Impedance
Impedance per km = 4.03 / 100 = 0.0403 Ω/km Fault impedance = 60 × 0.0403 = 2.418 Ω
Since 2.418 < 3.22 → Zone 1 trips instantly.
🔹 Problem 3: Zone 2 Setting
Adjacent line impedance = 3 Ω Calculate Zone 2 setting.
Solution
Zone 2 = Line impedance + 50% next line = 4.03 + (0.5 × 3) = 4.03 + 1.5 = 5.53 Ω
🔹 Problem 4: Measured Voltage & Current
Measured V = 11 kV Measured I = 1000 A Find measured impedance.
Solution
Z = V / I = 11000 / 1000 = 11 Ω
If Zone setting = 5 Ω:Since 11 > 5 → Relay does NOT operate.
📊 Types of Distance Relays
- Impedance Relay
- Reactance Relay
- Mho Relay (most common)
🎯 GATE & Interview Points
- Distance relay independent of fault current magnitude
- Zone 1 is instantaneous
- Zone 2 and 3 are time delayed
- Mho relay has circular characteristic
Distance Protection = Impedance Based Intelligent Protection
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