Research Of The Automatic Single-phase Fault Line Selection In Non-Direct Grounded System

The neutral grounding mode widely used in the current distribution network can be divided into two categories:large current grounding mode and small current grounding mode. Due to its small fault current, easy arc-quenching, high reliability, the small current grounding system is more and more widely applied to low-voltage distribution network. However, the automatic line selection for small current grounding system after the fault has hampered its large-scale development, which is necessary to overcome in order to protect the safe operation of power.Firstly, three neutral grounding mode of the small current grounding system are systematically introduced. The steady-state characteristics after fault are analyzed in theory,as well as the transient fault analysis features such as transient capacitive current, transient inductor current and transient ground current. Subsequently, a more comprehensive introduction of the line selection methods based on various fault signals is made. Including the automatic line selection methods based on the steady-state information, the automatic line selection method based on the transient information and a comprehensive line selection based on various criteria.For the system whose neutral point is earthed by an arc extinguishing coil, the principle of small perturbation method and its apparatus is analyzed in detail. Theoretical model is also made on MATLAB/Simulink platform. The superiority of the small perturbation line selection method has been proved by simulation. Combined with examples, the shortcomings of traditional small perturbation method is also pointed out:In the case of high-impedance ground, the fault line selection is prone to happen. Moreover, when there is insufficient compensation capacity, it will lead the line not able be selected because of its over-reliance on the peterson coil device.This paper proposes an improved method of small perturbations line selection. On one hand, the detuning perturbation interval of the traditional small perturbation line selection method is optimized, which can ensure the maximum of ΔION,, thus making convenient for line selection. At the same time, the optimization method can also solve the problem that the fault line is hard to be selected when it is high-impedance ground. On the other hand, combining the arc suppression coil grounding system based on traditional small perturbation line selection with low resistance grounding system, makes improved small perturbation method has the advantage of both traditional small perturbation method, but also able to use the zero-sequence protection device of low resistance grounding system to make up for the lack of traditional small perturbation method, thus greatly improving the accuracy of selecting line. And through MATLAB/Simulink platform and actual case, it is carried out that the small perturbation improved method has a great theoretical superiority and practical value.