Research Of Adaptive Line Protection For Middle And Low Ungrounded System

In the medium-low voltage neutral point ungrounded three-phase system,the current quick-break protection must calculate the setting value according to the short-circuit current when the three-phase short circuit occurs at the end of the line under the maximum operating state of the power system,andverify the sensitivity of protection according to the short-circuit current when the two-phase short circuit occurs at the end of the line under the minimum operating state of the power system.However,this method will cause the protection range change vary with the type of failure of the system and the occurrence of the fault.so that the protection device cannot protect the entire line.When the fault point is far from the protection device,the protection device will not act.The sensitivity of the protection device does not meet the requirements of relay protection.The setting value of the adaptive current protection can be automatically adjusted according to the state and the fault of the system at this time,thereby increasing the sensitivity of the protection device,so that it can meet the requirements of relay protection.Therefore,it is of great significance to research on adaptive current protection in improving the stability and safety of power system operation.This paper discusses the development status of adaptive protection at home and abroad,and lists the methods commonly used by experts and scholars to seek for systems and fault selection.Then compare the advantages and disadvantages of the traditional current fast-break protection principle and the adaptive current protection principle.thus reflecting the superiority of adaptive current fast-break protection.The adaptive protection can automatically adjust the protection setting according to the operating mode and fault type of the system.The method of calculating the impedance in this paper is obtained by using the sudden change of voltage and current when the fault occurs to obtain the equivalent impedance of the system.Analyze and research the method of fault phase selection that experts and scholars have proposed.Based on this,it is put forward to use the current difference value flowing through the two phases to determine which phase or phases the fault occurs in.In the end,the MATLAB simulation model can be built to analyse the characteristics of the current difference flowing two phases when the two-phase short circuit occurs in two different phases,Then calculate the equivalent impedance of the system.Thus the reliability and feasibility of the methods calculating impedance of system and selecting the fault phase are proved by the simulation result.