Current three-level protection

Friend, I don't quite understand your question!

I understand it as protection setting in power system, from instantaneous protection at the end to short delay protection at the next level, and then to long delay protection at the next level.

The other is the segmentation in the protection characteristic curve of the protection switch. The protection switch can meet the requirements of selective protection and has the characteristics of two-level protection and three-level protection. Three-stage current protection generally consists of non-time limit current quick-break protection (section I), time limit current quick-break protection (section II) and time limit overcurrent protection (section III). Among them, section I and section II are used as the main protection of the line, and section III is used as the near backup protection of this line and the far backup protection of adjacent lines. In some cases, only two or even one paragraph can be loaded.

In order to realize the action selectivity of overcurrent protection, the action time of each protection is generally set according to the step principle. That is, the action time of adjacent protection increases step by step from load to power supply, and the action time of each set of protection remains unchanged.

Independent of short-circuit current. Over-current protection with this time-limit characteristic is called fixed-time over-current protection.

Inverse time overcurrent protection refers to the protection that the action time automatically decreases with the increase of short-circuit current. Inverse time overcurrent protection used in transmission lines can cut off the fault at the head end of the protected line more quickly.