| Live working method is widely adopted in the maintenance of extra-high voltage(EHV)and ultra-high voltage(UHV)transmission lines for the sake of improving power supply reliability and ensuring stable operation of power grid.Determining the minimum approach distance and complex gap through the discharge characteristics of live working gaps is the prerequisite for the implementation of live working.And live working gap discharge characteristics should be also taken into consideration when designing transmission towers.Since the live working gaps between the dynamic workers and various towers are different from the conventional air gaps,and the mechanism of the influence of atmospheric parameters on the gap discharge characteristics under high altitude conditions is not clear,at present the discharge characteristics of live working gaps are mainly obtained by repeated full-scale tests,which are very time-consuming and costly.Also,it is difficult to simulate the complicated working conditions in actual live working operation by tests.Therefore,it is of great significance to explore a simulation method to analyze the discharge characteristics of live working gaps under complex atmospheric environment,so as to provide guidance for live working implementation and reduce test work tasks.The conductor arrangement and irregular tower structures,different worker’s postures and the variety of atmospheric parameters caused by altitude increase will affect the characteristics of live working gaps.Based on the above background,considering the typical equipotential conditions and ground potential conditions in actual live working,the impact factors of discharge characteristics of live working gaps are fully studied.Firstly,for the change of electrode shapes and gap structures caused by worker’s postures and positions,considering the influence of bundle conductors and tower structures on leader inception characteristics,the discharge model of live working gaps under standard atmospheric conditions is established,which is applicable for complex tower structures,and the influence rules of the worker on the gap discharge characteristics are obtained.Then based on the discharge physical process,the characterization methods for the influence of atmospheric parameters on corona inception,streamer propagation and leader inception process are proposed,and the discharge model for live working gaps in high altitude environment is established.The validity of the model is verified by full-scale tests carried out in high altitude areas,which will provide theoretical references and practical supports for the implementation of live working in high altitude areas.The main contents and achievements of this thesis are as follows:(1)Based on continuous leader inception criterion of long air gaps,the leader inception mathematical model of bundle conductor-tower leg gap is derived.The structure difference coefficient is proposed to characterize the influence of streamer space charge on gap discharge characteristics with various tower shapes and tower widths,and the discharge model of bundle conductor-tower gaps is established based on finite element simulation,which is applicable for complex tower structures in actual transmission lines.The validity of the model and its generalization performance for application to different tower structures are verified by the test data from 750 k V and 1000 k V typical tower air gaps.(2)Taking the live working gaps under standard atmospheric conditions as the research object,the discharge voltage calculation method with different worker’s postures is proposed.And the discharge models for equipotential worker-tower gaps and bundle conductor-ground potential worker gaps are established respectively based on the corona inception criterion under impulse voltage and leader inception physical mechanism.The effectiveness of the established models is verified by the discharge test data of live working gaps.The discharge voltage calculation values are in good agreement with the test data,and the obtained influence rules of the worker on the discharge voltage according to the analysis of calculation results basically coincide with the test results,which validate the accuracy and rationality of the model.(3)Taking the 750kV double-circuit tower as research object,the discharge characteristics test of live working gaps under switching impulse are carried out in areas with altitudes of 23 m,2000 m,3000 m and 4300 m,respectively.The 50% discharge voltages under different equipotential and ground potential conditions are obtained,and the influence of altitude,worker’s postures and tower structures on the discharge characteristics of live working gaps are analyzed.According to the test results,the applicability of the existing discharge voltage correction methods in areas of which the altitude is above 3000 m are compared and analyzed.An improved m-parameter correction method based on altitude and an improved g-parameter correction method based on atmospheric parameters are proposed respectively.The research results can provide reference for discharge voltage correction of live working gaps in high altitude areas.(4)Taking the live working gaps under high altitude conditions as the research object,the relationships between the atmospheric parameters,such as air density and humidity,and the discharge characteristic parameters of corona inception,streamer propagation and leader inception are established.By coupling the space charge electric field and background geometry electric field,the discharge model of live working gaps for high altitude environment is built on the basic of discharge model under standard atmospheric conditions.The proposed model is verified by live working gap discharge voltage test data from different high altitude areas.The errors between the calculated values and the experimental values are within acceptable range,which proves the feasibility of the model in high altitude conditions.This model can solve the problem that the traditional discharge voltage correction method still depends on the test data. |
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