Study On Power Frequency Electric Field Distribution And Environmental Factors Of High-voltage AC Transmission Lines

Along with the rapid development of power grid, power frequency electromagnetic radiation environmental problem which is caused by high-voltage power transmission and transformation project has brought more and more attention to the people.There are three typical sections of high-voltage AC transmission lines of110kV,220kV and500kV which are most widely erected at present in this study. By measuring the lines once per month for one year and comparing to the calculated values obtained by software simulation, distribution and changes of power frequency electric field of one year are found, which will help to give degree of attenuation when it is at traverse leg or protection distance of the lines. Analysis are made on the impact of some environmental factors, including temperature, relative humidity and underlying surface etc, on power-frequency electric field, which will help to put forward measures to reduce the power frequency electromagnetic radiation environment problem.(1)According to a whole year’s monitoring data of high-voltage transmission lines of110kV,220kV and500kV, it can be found that the changing trends of power frequency electric field intensity in calculation which is near the ground keep in step with measured values. The changing amplitude of calculated values between different months is smaller than that of measured values. It is mainly because that there are some ideal assume and simplification in calculation while the actual situation is more complicated. On the basis of the compare of the maximal calculated and measured values of power frequency electric field intensity which is near the ground, it can be inferred that the underlying surface is main influencing factor which make difference of calculated and measured values. Meanwhile, according to the attenuation of power frequency electric field intensity of every voltage level, advices of setting safety protection distance are given.(2)The changing trends of relative humidity is consistent with that of the maximum of power frequency electric field intensity which is near the ground, and it is the opposite trend to the minimum sag height of the conductor. The variation range of the minimum sag height of the conductor of110kV,220kV and500kV on average all over the year respectively is94.85%～105.15%,91.86%～108.14%,95.10%～104.20%. The corresponding power-frequency electric field fluctuate in an amplitude range through the year. So we should take the amplitude of power-frequency electric field into consider in the work like circuit design and environmental impact assessment of transmission lines in the future, adjusting the maximum of power-frequency electric field as the safety standard according to the temperature.Relative humidity have some limitation in site monitoring of power-frequency electric field. We should make sure that it is between5%and80%during site monitoring. And it relates to point discharge. The lower relative humidity is, the more liable to point discharge. It can be ignored because the impact of relative humidity to power-frequency electric field is very weak. What we should make sure is keeping a reasonable scope of the relative humidity during site monitoring.The underlying surface of the transmission lines has an impact on power-frequency electric field near the ground. And this kind of impact can not be ignored. Taking underlying surface of flat farmland as the object of the research, a new concept, virtual ground, is put forward in this study. Virtual height of conductor h’<actual height of conductor h, and when the vegetation is higher, the difference between the virtual and actual height of conductor is bigger, which is the same as the difference between the calculated and measured values.(3)According to the study on power frequency electric field distribution of high-voltage AC transmission lines, technical and management measures to reduce the power frequency electromagnetic radiation environmental problem which is near high-voltage power transmission lines are put forward, which will surely provide references to circuit design and environmental impact assessment of high-voltage transmission lines in the future.