📘 Distance Relays – Types, Working Principle & Applications
Distance relays are used for transmission line protection. They operate based on the impedance measured between relay location and fault.
🔹 1️⃣ Basic Principle of Distance Relay
Distance relay measures impedance:
Zseen = V / I
Relay operates when:
Zseen ≤ Zsetting
Since line impedance is proportional to distance, relay indirectly measures distance to fault.
🔹 2️⃣ Impedance Relay
Working Principle
It operates when measured impedance is less than a preset value. Characteristic is a circle centered at origin in R-X plane.
Advantages
- Simple construction
- Easy to implement
Disadvantages
- Not directional
- Affected by power swings
- Less stable for long lines
Where Used?
- Short transmission lines
- Backup protection
- Simple radial systems
🔹 3️⃣ Reactance Relay
Working Principle
Operates when measured reactance is less than setting. Characteristic is a vertical line in R-X plane.
Advantages
- Not affected by fault resistance
- Good for high resistance faults
Disadvantages
- Not inherently directional
- Requires directional unit
Where Used?
- Short and medium transmission lines
- Systems where fault resistance is high
🔹 4️⃣ Mho Relay (Admittance Relay)
Working Principle
Characteristic is a circle passing through origin in R-X plane. It is inherently directional.
Advantages
- Stable during power swings
- Inherently directional
- Best for long transmission lines
Disadvantages
- Slightly complex
Where Used?
- Long EHV transmission lines
- 132 kV, 220 kV, 400 kV systems
- Modern numerical relays
🔹 5️⃣ Zone Protection Concept
Distance relays are set in multiple zones:
- Zone 1: 80–90% of line (Instantaneous)
- Zone 2: 120–150% (Time delayed)
- Zone 3: Backup protection (Further delayed)
📊 Comparison Summary
| Relay Type | Characteristic | Directional? | Best Application |
|---|---|---|---|
| Impedance | Circle at origin | No | Short lines |
| Reactance | Vertical line | No | High resistance faults |
| Mho | Circle through origin | Yes | Long EHV lines |
🎯 GATE & Interview Points
- Distance relay measures impedance (V/I)
- Mho relay preferred for long lines
- Zone protection ensures selectivity
- Reactance relay immune to fault resistance
Distance Relay = Intelligent Transmission Line Protection
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