Simultaneous tripping of the circuit breaker at both ends of a transmission line is achieved by

• A. none of these
• B. overcurrent protection
• C. distance protection
• D. pilot protection

Answer: (D)

Carrier current over the power line provides simultaneous tripping of circuit breakers at both ends of the line in one to three cycles.
Thereby high-speed fault clearing is obtained, which improves the power system stability.
The auto-reclosing simultaneous reclosing signal is sent thereby simultaneous (1 to 3 cycles) reclosing of the circuit breaker is obtained.
For simultaneous faults, carrier-current protection provides easy discrimination.

As per answer key "pilot protection" is true but there is mistake the right answer is "none".