Which type of protection is most suitable for high-voltage transmission lines?

Distance protection is most suitable for high-voltage transmission lines.

Distance protection

• Distance protection operates by measuring the impedance (ratio of voltage to current) of a transmission line.
• If a fault occurs, the impedance seen by the relay decreases, and the relay trips the circuit breaker accordingly.

Reasons for Distance Protection Suitable for Transmission Lines:

• Independent of Communication Links: Unlike differential protection, it does not require communication between substations.
• Fast and Reliable: It provides quick fault clearance with different zones of protection.
• Selective Protection: It ensures tripping for faults within the set distance and avoids unnecessary operations for external faults.
• Adaptable to Long Transmission Lines: Overcurrent protection is not effective for long lines due to varying fault currents, whereas distance protection works based on line impedance.
   

Additional Information

• Merz-Price Voltage Balance Protection: This is a differential protection scheme primarily used for transformers and generators, not for long transmission lines.
• Differential pilot wire protection: Used for short transmission lines due to the requirement of pilot wires for end-to-end communication, which is impractical for long-distance high-voltage lines.
• Time-Graded Overcurrent Protection: Effective for radial distribution networks but not suitable for long high-voltage transmission lines because of variable fault current levels.