Why can't the current non-stop protection protect the whole line?

In the three-stage current protection, the current quick-break protection and time-limited current quick-break protection are the main protection of the line, and the overcurrent protection is the near backup protection. As the main protection, current quick-break protection is simple, reliable and quick-acting, so it is widely used. But it can't protect the whole line, and the protection scope is directly affected by the change of system operation mode. Because the selective current quick break can't protect the whole length of this line, the time-limited current quick break protection is added to remove the faults outside the scope of this line quick break protection, and it is also used as a backup for quick break. Overcurrent protection usually refers to a protection device that sets the starting current according to the maximum load current. It should not be started during normal operation, but it can act in response to the increase of current when the power grid fails. In general, it can not only protect the whole length of this line, but also protect the whole length of adjacent lines to play the role of backup protection.

The requirements of secondary protection are: it can protect the whole line length, and it should also have the shortest possible action time limit, which is the basis of time-limited current rapid protection. To protect the whole line, the protection zone will inevitably extend to a part of the next line, and its sensitivity check is based on the end of the line. According to the regulations, this sensitivity coefficient should not be lower than 1.25.

It is easier to understand with schematic diagram and calculation formula.