当线路发生短路故障时，重要特征之一是线路的电流急剧增大，当电流流过某一

When a short-circuit fault occurs on the line, one of the important features is that the current of the line increases sharply. When the current flows through a predetermined value, the protection device that acts in response to the current increase is called overcurrent protection and the three-stage current protection has As all the functions of main protection and backup protection, it is widely used in practice. The result of the combination of instantaneous current quick-break and time-limit current quick-break protection not only can protect the full length of the protection device, but also meet the requirements of fast action However, this kind of time-limit quick-break protection can only make up for the shortcomings of the main protection, but it can not meet the requirements of backup protection because its protection range cannot reach the end of the next line. Therefore, in order to provide backup protection for this line and the next line, a set of overcurrent protection must also be installed. In this way, the overall protection device that combines instantaneous current quick-break time limit current quick-break and time-limit overcurrent protection is formed. Said three-stage current protection.

当线路发生短路故障时，重要特征之一是线路的电流急剧增大，当电流流过某一预定值时，反应于电流升高而动作的保护装置叫过流保护而三段式电流保护具有作为主保护和后备保护的全部功能，在实际中得到广泛应用.瞬时电流速断和时限电流速断保护配合使用的结果，不仅可使保护装置保护线路的全长，而且尽量地满足了快速动作的要求但是，这种时限速断保护只能弥补主保护的不足，却不能满足后备保护的要求因为它的保护范围达不到下一线路的末端。所以，为了对本线路和下一线路起后备保护作用，还须装设一组过流保护这样就形成了由瞬时电流速断时限电流速断和定时限过电流保护互相配合的保护装置整体，就是通常所说的三段式电流保护。

When a short circuit fault occurs in a line, one of the important characteristics is that the current of the line increases rapidly. When the current flows through a predetermined value, the protection device that acts in response to the current rise is called overcurrent protection. Three section current protection has all the functions of main protection and backup protection, and is widely used in practice. Instantaneous current quick break protection and time limit current quick break protection are used together As a result, the protection device can not only protect the whole length of the line, but also meet the requirements of fast action as much as possible. However, this kind of time limit quick break protection can only make up for the shortcomings of the main protection, but can not meet the requirements of the backup protection because its protection range can not reach the end of the next line. Therefore, in order to play a backup protection role for this line and the next line, a group of over-current protection must also be installed, so as to form a whole protection device which is composed of instantaneous current quick break time limit current quick break and time limit over-current protection, which is commonly referred to as three-stage current protection.<br>