Research On High-resistance Fault Detection And Line Selection In Small Ground Current Systems

The detection of high resistance grounding fault,which is related to the safety of personal electric shock,is still in a technical blank in China.At present,the consensus of distribution network experts is that the highest level of fault detection is about 2-3kΩ,and in practical application,it is much higher than 1-2kΩ,up to 10kΩ under most grounding conditions.Even the grounding fault of 20kΩ transition resistance is not uncommon,but the traditional protection can not detect this kind of high resistance sensitively,which leads to catastrophic events such as personal electric shock casualties and contact fire.The rapid disposal of single-phase grounding fault has become a consensus in the distribution network.The existing disposal methods are mainly divided into three categories,which are the installation of full compensa tion device at the neutral point,the neutral point grounding through small resistance and fault line selection protection.This paper mainly studies the line selection protection.The existing line selection methods have some disadvantages for high resistance.In order to detect high resistance fault sensitively and reliably,this paper reduces the threshold value of start-up and proposes an integrated algorithm of start-up and line selection.At the same time,a fault protection method based on similarity principle is proposed.Firstly,the paper analyzes the transmission characteristics of voltage and current transformers,and shows that the two transformers have their own amplitude frequency response and phase frequency response.The rational function me thod is used to model the transformer,and the T-type equivalent circuit of the transformer is established,which can theoretically eliminate the changes of amplitude frequency characteristics and phase frequency characteristics caused by the changes of frequency.But they measure different electrical quantities,and there will be measurement error.Secondly,considering the boundary of nonlinear high resistance grounding fault under different grounding modes,it is concluded that the existing methods can not detect the signal sensitively when the transition resistance is above 1kΩ under the premise of considering nonlinearity.Thirdly,in order to reduce the starting threshold of zero sequence voltage reliably,the influence of various disturbances and faults on zero sequence voltage is analyzed,and the variation of power frequency voltage is proposed as the starting criterion,at the same time,the method of standard unit capacitance is used to filter out the influence of voltage transformer,and the similarity between measured current and calculated current is used for line selection,which is verified by actual data.Finally,the influence of the fault on the similarity is analyzed when the insulation performance of the tower insulator in the distribution network declines.It is confirmed that the similarity of protection action is less than-40% in steady-state electric quantity.Similarly,when the similarity is more than 80%,it is judged that there is no fault in the line,and the action criterion can make the protection device remove 5kΩ high resistance fault.