Question

A relay used on long transmission lines is ________.

• A. Mho’s relay
• B. Reactance relay
• C. Impedance relay
• D. No relay is used

Hint:

Long transmission line = Mho relay → directional, stable under power swings.

Answer 1

The Correct Option is A

Step 1: Types of Distance Relays
Distance relays operate based on impedance between the relay location and the fault. Common types:
- Impedance relay: circular characteristic.
- Reactance relay: straight-line characteristic, independent of arc resistance.
- Mho relay: circular characteristic, directional, suitable for long lines.
Step 2: Why Mho Relay for Long Lines?
Mho relay is inherently directional and stable for long transmission lines where load impedance is small and fault impedance dominates. Its operating characteristic is more selective and less prone to load encroachment.
Step 3: Other Relays?
- Reactance relays are better for short lines (arc resistance independence).
- Impedance relays can be over-sensitive to power swings and load encroachment.
Conclusion:
Mho relay is best suited for long lines due to its directional and stable operation.