New Calculation And Protection Method For Grounding Fault In Low-voltage Distribution Network

With the development of society and the improvement of living standards,the people’s requirements for the safety level of electricity are gradually increasing,and the negative impact of electric shock accidents is also increasing.However,our country’s electrical shock protection cannot play its due role.Personal electric shock accidents occur frequently in the rainy season every year,which seriously threatens the lives of users.In addition to poor site management and irregular construction,the main reason for the continuous occurrence of electric shock accidents is that the development of low-voltage distribution network technical is not perfect.Compared with the mature medium-voltage distribution network,the low-voltage distribution network grounding fault,especially the neutral line grounding fault,lacks a simplified fault calculation method like "symmetrical components method".Moreover,there are many problems in the field application of residual current protection of low-voltage distribution network,such as frequent misoperation or even failure to put into operation.So the residual current protection cannot play its due role,especially the general insurance and the middle insurance.In this Thesis,a new phase-to-mode conversion method is proposed for the three-phase four-wire system,which can convert the electrical quantity of the low-voltage distribution network system into independent mode 1 and mode 2,as well as mode 0 and mode R which still exist coupling.The phase-to-mode conversion method provides a theoretical tool for the analysis of the grounding fault in low-voltage distribution network.Secondly,according to the new phase-to-mode conversion method,the calculation of single-phase grounding fault and neutral grounding fault of the three-phase four-wire system is realized.Furthermore,for different grounding type low voltage power distribution network,the electrical quantity characteristics of the upstream of the fault point in the whole process of electric shock fault are deeply analyzed.On this basis,an electric shock protection method based on load current braking is proposed,which improves the sensitivity of residual current protection.Besides this method makes full use of the electric quantity information of the protected line and overcomes the influence of line switching,load fluctuation and other factors.So it can ensure the reliability of the protection action.and expand the application range of residual current protection.Finally,the correctness of the grounding fault analysis and the proposed shock protection algorithm for the three-phase four-wire system in the low-voltage distribution network are verified by using Matlab-Simulink simulation and test data of protection deviceThe phase-to-mode conversion calculation method proposed in this Thesis provides a new,accurate,theoretically supported and physical meaning calculation method for the three-phase four-wire system ground fault,especially for the calculation of the neutral line ground fault.And it provides theoretical tools for fault analysis of phase four-wire system.The electric shock protection method based on load current braking proposed in this Thesis improves the sensitivity and reliability of electric shock protection,and its popularization and application can better guarantee the personal safety of users.