零序电流互感器作用原理
Principle of action of zero sequence current transformer
零序电流保护一般适合使用于TN接地系统。因为当发生一相接地时，对TN-S系统Id回路阻抗包括相线阻抗Z1，PE线阻抗ZPE和接触阻抗Zf，即Zs=Z1+ZPE+Zf；对于TN-C系统，Id回路阻抗包括相线阻抗Z1，PEN线阻抗ZPEN和接触电阻Zf，即ZS=Z1+ZPEN+Zf；对于TN-C-S系统，Id回路阻抗包括相线阻抗Z1，PEN线阻抗ZPEN，PE线阻抗ZPE和接触电阻Zf，即ZS=Z1+ZPEN+ZPE+Zf，产生的单相接地故障电流Id=220/ZS，明显大于无故障时的三相不平衡电流，只要整定合适，就可检测出发生接地故障时的零序电流，以切断故障回路。而对IT系统，一般均是使用对供电可靠性要求较高、对单相接地不必要立即切断供电回路、但需发出绝缘破坏监察信号、以维持继续供电一段时间的工矿企业内的不配出中性线的三相三线配电线路。
Zero sequence current protection is generally suitable for TN grounding system. Because when a phase is connected, the Id circuit impedance of the TN-S system includes the phase line impedance Z1, the PE line impedance ZPE and the contact impedance Zf, that is, Zs=Z1+ZPE+Zf. For TN-C systems, the Id loop impedance includes the phase impedance Z1, the PEN line impedance ZPEN and contact resistance. Z1, PEN line impedance ZPEN, PE line impedance ZPE and contact resistance Zf, that is, single phase grounding fault current Id=220/ZS produced by ZS=Z1+ZPEN+ZPE+Zf, is obviously larger than the three-phase unbalanced current without fault. As long as the setting is appropriate, the zero sequence current of grounding fault can be detected and the fault circuit can be cut off. For IT system, the three phase three line wiring road, which is not matched with the neutral line in the industrial and mining enterprises, is usually used for the high demand for the power supply reliability and the unneed of the single-phase grounding unnecessary to cut off the power supply loop immediately, but the insulation damage monitoring signal is needed to maintain the continuous supply of power supply for a period of time.
当单相接地时，该故障线路上流过的零序电流是全系统非故障系统电容电流之和，因而容易检测出接地故障电流，故可用零序电流保护装置来监察相对地第一次接地故障。
When the single phase grounding is grounded, the zero sequence current over the fault line is the sum of the system capacitance current of the whole system, so it is easy to detect the grounding fault current, so the zero sequence current protection device can be used to monitor the relative ground fault.
TT接地系统常应用于工农业、民用建筑的照明、动力混合供电的三相四线配电系统中，常发现三相不平衡电流较大，当发生一相接地时，Id回路阻抗包括相线阻抗Z1，PE线阻抗ZPE，负载侧接地电阻RA和电源侧接地电阻RB，接触阻抗Zf，即ZS=Z1+ZPE+RA+RB+Zf，接地故障电流Id=220/ZS，由于RA+RB>>Z1+ZPE+Zf，且RA+RB数值一般均较大，很明显TT系统的故障环路阻抗大，产生的单接故障电流Id，远远小于不平衡电流，很难检测出故障电流，故不适用于TT接地系统。
The TT grounding system is often used in the three-phase four wire distribution system of lighting and power hybrid power supply for industrial and agricultural buildings and civil buildings. It is often found that the three-phase unbalanced current is large. When a phase connection occurs, the impedance of the Id loop includes the phase impedance Z1, the PE line impedance ZPE, the load side grounding resistance RA and the power side grounding resistance RB, and the contact impedance Zf. ZS=Z1+ZPE+RA+RB+Zf, the grounding fault current Id=220/ZS, because of RA+RB>>Z1+ZPE+Zf, and the RA+RB value is generally large, it is obvious that the fault loop impedance of the TT system is large and the single fault current Id is produced, which is far less than the unbalanced current, so it is difficult to detect the fault current, so it is not suitable for the TT grounding system.