Distance protection uses the characteristics of simultaneous change of voltage and current during short circuit to measure the ratio of voltage and current, reflecting the distance between the fault point and the protection installation place.

According to the requirements of several protection options, the distance protection installed at both ends of the line should only act immediately when the internal fault of the line is there, tripping the corresponding circuit breake.

When the positive direction outside this line is short-circuited, the protection device shall not operate. In order to ensure that the protection does not malfunction when short-circuited occurs at the next line, we need to set a protection range that is less than the full length of the line.

When a fault occurs in the system, the distance protection will first determine the direction of the fault. If the fault is in the positive direction of the protection zone, the protection will measure the distance from the fault point to the protection installation. Then it will compare the measrued distance with the zone setting. Only when measured fault distance is less than the zone setting we can say the fault occurs within the protection range and the protection system should act immediately and trip the corresponding circuit breaker.