Research And Application On Working Characteristics Of Zero Sequence Current Transformer With Large Secondary Resistance

Most of the distribution networks in China are low-current grounded systems,70%of which are neutral-point ungrounded systems.Single-phase grounding faults are the most common faults in the distribution network.Boundary switches are effective equipment for single-phase grounding fault isolation and tripping.An imported boundary switch has a good single-phase grounding fault detection function.After disassembling it,it is found that the detection device is very small and simple.The key is that a large resistance is connected to the secondary side of the zero-sequence current transformer.In order to ensure the accuracy of current transformers,the secondary side impedance is reduced as much as possible in conventional applications,and there are few studies and reports on connecting large resistors on the secondary side.In order to clarify the technical know-how of foreign products,this article has carried out the following work:In order to grasp the transfer characteristics when the secondary side of the zero-sequence current transformer is connected with a large resistance,the mathematical expressions of the error,excitation current and load voltage when the secondary side of the current transformer is connected with a large resistance are derived theoretically,and given the vector relationship of each parameter of the transformer when the secondary side is connected with a large resistance.Based on the PSCAD/EMTDC simulation software,a current transformer simulation model is built,and the change rule of the secondary side voltage and error when the secondary side of the zero-sequence current transformer is connected to different resistances is studied,and the load voltage under different resistances is obtained.The relationship with the primary current clarifies the influencing factors that affect the characteristics of the secondary side of the current transformer when a large resistance is connected.Experiments verify the correctness of the theoretical analysis and simulation results.The results show that connecting a suitable large resistor on the secondary side of the current transformer can enhance the voltage signal of the secondary side.When the primary current is small,the secondary side voltage has an approximate linear relationship with the primary current,and when the primary current is large,the secondary side voltage signal will be distorted,but the relationship between the secondary side voltage and the primary current is still monotonic.On this basis,the parameter design problem of the large resistance connected to the zero-sequence current transformer used for single-phase ground fault detection is studied.The results show that the appropriate design resistance parameters and detection threshold can be applied to single-phase ground fault detection.And can provide convenience for high resistance grounding detection with small zero sequence current,and can directly use its secondary side voltage to realize user-side single-phase grounding protection and single-phase grounding line selection without the need to configure secondary current transformers.Make the device structure simpler.