Research On Electric-thermal Overload Current Measurement Method And Overload Protection Algorithms

Overload protection technology is an important part of low-voltage electrical intelligent technology,which is widely used in low-voltage circuit breakers,overload relays and other products to ensure the safe operation of electric equipment and distribution lines.In recent years,intelligent protection technologies such as electronic protectors have become the development trend of overload protection technology,but electronic protectors often require a device for measuring current,namely current transformer,which is not suitable for miniature circuit breakers because of its large size.At present,the intelligentization of miniature circuit breakers has not made great progress due to factors such as technology and volume.Therefore,how to adopt new technologies and new ideas to study the current detection method with small size and electrical isolation is of great significance for the intelligentization of miniature circuit breakers.In this thesis,an electrothermal current detection method is proposed to solve the problem of current measurement in the intelligent process of miniature circuit breakers.The overload protection algorithm is developed to provide more rapid and accurate protection technology for overload protection of power equipment and distribution lines.This topic comes from the National Natural Science Foundation of China: Key Issues in New Thermal Electromagnetic Hybrid Release and Networked Selective Protection of Circuit Breakers(Project No.51777129).First of all,this thesis initially establishes a current detection model for miniature circuit breakers.An electrothermal current sensing method is proposed.A relatively closed measuring cavity is designed on the wire,and the measuring cavity is regarded as a system.According to the principle of thermal balance,the relationship between temperature rise and current of current-carrying conductor is established by analyzing the temperature characteristics of current in the measurement chamber,so as to achieve the purpose of current detection of current-carrying conductor.Solidworks modeling is carried out.Through the selection and setting of materials,establish a multi-physics environment,load current load,divide the grid and finally calculate by Comsol finite element software,get the temperature environment inside the measurement cavity,change the size of the load current and read accordingly.The node data determines theunknown parameters in the mathematical model according to the simulation data,so as to verify the correctness of the mathematical model.In fact,it is difficult to simulate the actual temperature environment,so there may be some errors in the simulation.This thesis builds a current and temperature measurement system based on LabVIEW.The system is mainly composed of current transformer,thermal resistance,temperature transmitter,data acquisition card and LabVIEW.Experiments were carried out by loading different currents on the conductor,and the temperature experimental data of the Pt100 measurement points were read to correct the accuracy of the mathematical model verification.Finally,based on the heat balance equation,this thesis establishes a mathematical model of dynamic overload protection with temperature rise as a variable,and proposes an overload protection discretization algorithm.The theory of wavelet analysis is introduced,and the current amplitude is calculated by Morlet complex wavelet algorithm to reduce the calculation error.The Simulink model is built to simulate the algorithm,and the overload protection discretization algorithm is compared with the commonly used inverse time overload protection to verify the feasibility of the model overload protection method.