分析小电流系统单相接地时的运行状态，其不同于正常运行状态的信息主要有2点：故障线路流过的零序电流是全系统的电容电流减去自身的电容电流，而非故障线路流过的零序电流仅仅是该线路的电容电流。故障线路的零序电流是从线路流向母线，而非故障线路的零序电流是从母线流向线路，两者方向相反，或者说两者反相。从小电流系统单相接地时与正常运行时，状态信息的不同看，故障线路的判定似乎非常容易，然而事实并非如此，其原因主要有以下四点：
Analyzing the operation status of single-phase grounding in small current system, there are two main points of information different from the normal operation status: the zero-sequence current flowing through fault line is the capacitive current of the whole system minus its own capacitive current, while the zero-sequence current flowing through non-fault line is only the capacitive current of the line. The zero-sequence current of the fault line flows from the line to the bus, while the zero-sequence current of the non-fault line flows from the bus to the line. The two directions are opposite, or they are opposite. From the point of view of the difference of state information between single-phase grounding and normal operation of small current system, it seems very easy to determine the fault line. However, this is not the case. The main reasons are as follows:
1、电流信号太小
1. Current signal is too small
小电流系统单相接地时产生的零序电流是系统电容电流，其大小与系统规模大小和线路类型(电缆或架空线)有关，数值甚小，经中性点接入消弧线圈补偿后，其数值更小，且消弧线圈的补偿状态(过补偿、欠补偿、完全补偿)不同，接地基波电容电流的特点与无消弧线圈补偿时相反或相同，对于有消弧线圈的小电流系统采用5次谐波电流或零序电流有功功率方向检测，而5次谐波电流比零序电流又要小20～50倍。
The zero-sequence current generated by single-phase grounding in small current system is the system capacitive current, which is related to the size of the system and the type of line (cable or overhead line). The value is very small. After the neutral point is connected to the arc suppression coil to compensate, the value is smaller, and the compensation state of the arc suppression coil (over-compensation, under-compensation, complete compensation) is different. The characteristics of the ground fundamental wave capacitive current are different from that of the arc suppression line. For small current system with arc suppression coil, the direction of active power of 5th harmonic current or zero sequence current is detected, and 5th harmonic current is 20-50 times less than zero sequence current.
2、干扰大、信噪比小
2. High interference and low signal-to-noise ratio
小电流系统中的干扰主要包括2方面：一是在变电站和发电厂的小电流系统单相接地保护装置的装设地点，电磁干扰大；二是由于负荷电流不平衡造成的零序电流和谐波电流较大，特别是当系统较小，对地电容电流较小时，接地回路的零序电流和谐波电流甚至小于非接地回路的对应电流。
The interference in the small current system mainly includes two aspects: one is that the electromagnetic interference is large at the installation site of the single-phase grounding protection device in the small current system of substation and power plant; the other is that the zero-sequence current and harmonic current caused by unbalanced load current are large, especially when the system is small and the capacitive current to the ground is small, the zero-sequence current and harmonic current in the grounding circuit are even smaller than those in the non-grounding circuit. The corresponding current of grounding circuit.
3、随机因素影响的不确定
3. Uncertainty of the influence of random factors
我国配电网一般都是小电流系统，其运行方式改变频繁，造成变电站出线的长度和数量频繁改变，其电容电流和谐波电流也频繁改变；此外，母线电压水平的高低，负荷电流的大小总在不断地变化；故障点的接地电阻不确定等等。这些都造成了零序故障电容电流和零序谐波电流的不稳定。

Distribution networks in China are generally small current systems, whose operation mode changes frehengshui.75ix.cnpaishuigou.37ix.com.cnyamodai.37ix.com.cnmojuljs.37ix.com.cnzaoxingcz.37ix.com.c

nwww.baodingjiaju.cnquently, resulting in frequent changes in the length and number of substation outgoing lines, and frequent changes in capacitive current and harmonic current; in addition, the bus voltage level, the magnitude of load current is always changing; the grounding resistance of fault points is uncertain, and so on. These results in the instability of zero-sequence fault capacitive current and zero-sequence harmonic current.

4、电容电流波形的不稳定
4. The instability of capacitive current waveform
小电流系统的单相接地故障，常常是间歇性的不稳定弧光接地，因而电容电流波形不稳定，对应的谐波电流大小随时在变化。
The single-phase grounding fault of small current system is often intermittent unstable arc grounding, so the waveform of capacitive current is unstable, and the corresponding harmonic current changes at any time.