Studies About Condition Monitoring And Earth Fault Of Small Current Grounding System

In China, small current grounding system has been used in most of the medium voltage power distribution networks. Therefore, to monitor the conditions of the small current grounding system and detect the possible failurs in the distribution lines is of great significance to ensure a reliable electricity supply for industrial and domestic customers.This paper has mainly focused on the characteristics of the small current grounding system and its condition monitoring. By means of theoretical analysis, simulations and practical field data validations, researches have been carried out on some related topics as: online inspection of insultaion conditions, grounding-fault line screening and detection, and algorithmes for using temperature to intepret the conditions of electric devices.This paper has studied how to monitor the insulation conditons of a power system at online circumstances. It suggests a new short-time disturbance method for state monitoring of the power transmission and distribution lines using electromagnetic field and wavelet transform theories. The method has the characteristics of being able to realize online monitoring of possible insulation problem. By injection through the bifurcate points using voltage transient clamping technology, it can achieve independent monitoring of all branch lines.This paper gives a new screening method based on the ratio of current increment at the zero sequence. Because the current change ratio of the zero sequence in the fault line is greater than other normal lines, the fault line can be easily detected. The advantage of the method is that it is suitable for all neutral point grounding systems. Furthermore, through theoretical analysis and derivation, a multi-algorithm based fuzzy screening system is constructed for the detection of fault lines. A verification example shows that the system can effectively improve the reliability of the result than each individual algorithm does.In this paper, the changes of major electrical parameters in the two-phase grounding case are discussed. It also suggests a negative sequence current detection method to discover the fault in the case of one single-phase ground and another phase with high impedance grounding problem (current leakage). Based on the analysis how temperature is transmitted within time domain, a reference model in time domain is suggested to monitor the electrical contact resistance changes in a power device. This method uses experience data to select the reference model. It only responds to rapid changes in the temperature of the device but is not so sensitive to the slow variations in the environment temperature.This paper has also studied temperature transmission in the frequency domain. A frequency phase expression is deduced to describe the time delay of temperature transmission in the frequency domain. This paper has made further study about using Daubechies wavelets method to extract temperature changes from both time and frequency domains. The method shows to be very useful for assessing the internal loss resistance of the electric devices.