Research On Leakage Fault Detection Methods For TN-C-S System

Leakage fault detection technology for low-voltage distribution networks plays an important role in preventing personal injury caused by electrical fires and ground faults.Traditional leakage fault detection methods only take residual current as the characteristic quantity,and judge whether leakage fault occurs based on the amplitude or phase of residual current.Traditional leakage fault detection methods have significant effects in TT and TN-S systems,but are not suitable for TN-C-S systems widely used in cities.In order to prevent personal electric shock caused by zero break fault,in addition to grounding the neutral point of the transformer,the TN-C-S system is also equipped with at least one repetitive grounding point on the PEN line at the user side,which allowing partial three-phase unbalanced current to return to the neutral point through the repetitive grounding point and reflect as residual current.The value of residual current can reach several amperes,far exceeding the 500 m A leakage protection action setting specified in national standards.Traditional leakage fault detection methods cannot distinguish between residual current changes caused by load switching and single-phase ground faults,which making the lower limit of fault detection threshold higher than the upper limit of protection action sensitivity requirements.If sensitivity is guaranteed with a lower action threshold,the protection maloperation rate is high.If reliability is guaranteed with a higher action threshold,the protection sensitivity does not meet the requirements.Therefore,a large number of TN-C-S systems cannot be put into operation under main and middle protection,The absence of line leakage protection before repeated grounding points may result in indirect personal electric shock and electrical fire hazards caused by single phase grounding faults.In this paper,aiming at the problem of difficulty in putting TN-C-S system into operation under main and middle insurance,a leakage fault detection method suitable for TN-C-S system is studied.The main research contents are as follows:(1)Modeling and analysis of single-phase grounding fault in TN-C-S system.According to the branch situation and the number of repeated grounding points,the TN-C-S system is sorted out and divided into a single branch single point grounding system,a single branch multipoint repeated grounding system,and a multi branch multipoint repeated grounding system.Starting with a single branch single point repeatedly grounded system,a simplified equivalent circuit for ground fault in a TN-C-S single point repeatedly grounded system is established;By analyzing the parameters of single branch multi point repetitive grounding system and multi branch multi point repetitive grounding system,the equivalent circuit model of single branch single point repetitive grounding system is modified,and the general equivalent circuit of single-phase grounding fault in TN-C-S system is constructed.Based on the equivalent circuit,the quantitative relationships between the fault current variation,residual current sudden variation,unbalanced current sudden variation and N-line impedance,total ground impedance are analyzed,and the reasons why the main/middle leakage protection cannot be put into operation are analyzed.(2)A double mutation vector current separation leakage fault detection method is proposed.Based on residual current and unbalanced current measurements,the ratio of N-line impedance to the total system ground impedance is calculated using the current mutation vector,and then the residual current introduced by the unbalanced current which generated by load switching is calculated to achieve the separation of fault currents.Using the separated fault current amplitude exceeding the limit as a fault criterion,which is not affected by the residual current changes caused by load switching,can reduce the protection action threshold and improve detection sensitivity while ensuring reliability.(3)Simulation and test verification.Based on MATLAB/Simulink software,the simulation models of single branch single point grounding and multi point grounding,and multi branch system of TN-C-S system are established.The multi branch system is divided into two situations according to the difference in the total grounding impedance of the branch.Simulation results show that the proposed method can effectively detect 30 m A and 150 m A single-phase ground faults in single branch and multi branch systems,respectively,and meets the sensitivity requirements of middle and main protection.Ground fault tests have been conducted in a laboratory closed test system and an actual operating system.The results show that the traditional residual current based detection method has a success rate of less than 1%.The transition resistance ability of the method proposed in this article reaches 3.3 kΩ,and the detection power for a single phase ground fault of70 m A is 100% higher,further verifying the feasibility of the proposed method.