Research On Soil Resistivity Measurement And Distribution Characteristics Of Electric Field In EHV Transmission Corridor

Grounding technology of power system is one of the most significant measures to ensure personal safety as well as equipments' normal working and also it plays a crucial role in maintaining safe and reliable running of power system. Requests for grounding get stricter and stricter with rapid expansion of powernet capacity which results in substantial increase of earth short circuit current and huge input of various monitoring devices plus with lightning hazard. Soil resitivity measurement is the first step to conduct grounding design which reflects the earth divergence characteristics when grounding body releasing fault current or lighting current whose equivalent frequency is high. Therefore, the soil response to high frequency current must be studied for the purpose of grounding design accurately. On the other hand, owning to the flourishing development of EHV and UHV transmission projects as well as people' increased awareness with the safety of electromagnetic environment, restricting the electromagnetic environment index within safety limits is an important issue encountered by designers when designing transmission line and power frequency electric field pivotal to evaluate the security of electromagnetic environment is one of that indexes.Based on Controlled Source Audio-frequency Magneto-telluric(CSAMT), this thesis firstly studied soil divergence characteristics under the action of high frequency current which showed that penetration depth of higher frequency current under lower soil resistivity was shallower while it was deeper when lower frequency current penetrating higher soil resistivity. Under the circumstance of soil resistivity being 100Ω·m and 300Ω·m, simulation measurement of soil resistivity surveyed by CSAMT method and Schlumberger method were realized respectively whose frequency range was both from 10 Hz to 105 Hz with the help of CDEGS software. The result indicated that measuring soil resistivity with CSAMT method was more accurate and more credible especially in high frequency section whose result was quite stable so as to reflect soil divergence characteristics adequately under high frequency current.For power frequency electric field of high voltage, catenary equation of transmission line that was unequal height suspension in a span was derived firstly which suggested that the shape of line sag was determined by conductor load on per-unit length and cross-sectional area as well as stress borne by the lowest point of conductor while it was irrelevant to the distance from lowest point to ground and also had nothing to do with the height difference of suspension points. The catenary model of two conductors' transmission line in rectangular space coordinate was found on above derivation. Combining image method and electric coefficients method, the curvilinear integral method in three-dimensional space was obtained to calculate equivalent charge on conductor surface, extension of which the general method of calculating equivalent charge on each sub-conductor surface for multi-conductor transmission system was deduced furthermore. By scalar decomposition of vectorial integral for space electric field, how to work out the effective value of each component and its composition of power frequency electric field under AC transmission line was given as well as its instantaneous value expression in time domain.The mathematical model that was taken conductor sag into consideration to calculate power frequency electric field in three-dimensional space was established based on design parameters of some 500kV EHV transmission line. According to three-dimensional curvilinear integral method, the distribution function of equivalent real charge and equivalent image charge on conductor surface was obtained respectively by means of Matlab numerical calculation, after which the electric distribution on groundsurface as well as on the plane of 1.5 high was got and compared with that in the center of span calculated under two-dimensional model, the accuracy and validity of three-dimensional model and curvilinear integral algorithm was verified. The transverse and longitudinal distribution characteristics of three-dimensional electric field as well as its variation regularity of each component were analyzed. The result showed that the X axis component of electric field on groundsurface in the center of span was zero and the electric field was determined by Y axis and Z axis components among which the proportion of Z axis component was larger. The electric field decreased gradually with the migration of transverse distance and the proportion of X axis component got larger and larger yet Z axis component was still the major factor to determine electric field. Electric field reached to maximum at the position that was about 3 meters far away from lateral conductors of the two side phases in the center of span and then reduced towards the two sides of tower symmetrically until it reached to minimum at the position of tower(neglecting the distortion effects caused by tower). In addition, electric field reduced sharply after the position that was about 10 meters away from the two side phases. The three-dimensional model is more comprehensive to reflect the variation regularity of each component of electric field under the line as well as its transverse and longitudinal distribution characteristics compared with two-dimensional model.