零序电流互感器检测法
Zero sequence current transformer detection method
这是一种最常用的方法(见图1)，3根相线全部穿过零序电流互感器(CT)。
This is one of the most commonly used methods (see Fig. 1), and the 3 phase lines all pass through zero sequence current transformer (CT).
设零序电流互感器的变比为K，向量Ia为A相电流，向量Ib为B相电流，向量Ic为C相电流．则继保装置检测到的零序电流为：
The change ratio of the zero sequence current transformer is K, vector Ia is A phase current, vector Ib is B phase current and vector Ic is C phase current. The zero sequence current detected by the protection device is:
这种方法的优点是：直接检测三相电流的向量和(零序电流)，准确性和可靠性都比较高。零序电流互感器的变比K较小(常用的为30)，当允许的一次零序电流，一定时，进入继保装置的二次电流i=l/K较大(一般可以设到1A以上)这样其抗干扰的能力就强(这种干扰主要来自于电磁干扰造成的零飘)，误动作率就低。
The advantage of this method is that the vector and zero sequence current of three-phase current can be directly detected, and the accuracy and reliability are relatively high. The change ratio of the zero sequence current transformer (K) is smaller (usually 30). When a zero sequence current is allowed, the two current of the relay device is larger (usually above 1A) so its ability to resist interference is strong (this interference is mainly from zero drift caused by electromagnetic interference), and the misoperation rate is low.
这种方法的缺点是：受现场安装条件及制造工艺的限制，零序电流互感器的体积都比较小，一般只适用于单根电缆的零序电流检测。
The shortcoming of this method is that the volume of the zero sequence current transformer is small because of the field installation and manufacturing technology, and it is generally applicable only to the zero sequence current detection of single cable.