What are the two measured parameters of a transmission line that will cause a distance relay to operate?

The two measured parameters of a transmission line that cause a distance relay to operate are voltage and current. Distance relays function based on the measurement of both these electrical quantities to determine the condition of the transmission line.

When a fault occurs on a transmission line, the relay measures the current flowing into and out of the protected section along with the voltage across the transmission line. By using these measurements in conjunction with the known characteristics of the line (like its impedance), the relay can calculate the impedance (or distance) to the fault. If this calculated impedance falls below a predetermined threshold, indicating that a fault is present within its protection zone, the distance relay will operate to isolate the fault by tripping the appropriate circuit breaker.

Considering the other options, power and voltage do not provide sufficient information to ascertain the distance to a fault, as they do not directly relate to the transmission line's impedance characteristics. Resistance and losses alone are insufficient because they do not reflect real-time current and voltage fluctuations at the time of a fault. The option regarding current in and current out lacks the critical voltage measurement necessary for the relay's operation. Thus, the combination of voltage and current is essential for the effective functioning of distance relays in transmission lines.