Research On The Inception Mechanism And Modeling Method Of Lightning Upward Leader

Lightning strike, especially shielding failure, is one of the main reasons that cause faults on Extrahigh Voltage (EHV)/Ultrahigh Voltage (UHV) transmission lines. Due to frequent lightning strikes in China, the evaluation of lightning shielding performance and the design of lightning strike defending are hot issues which have attracted wide attention in the design of EHV/UHV transmission lines.The existing analytical models of lightning shielding of transmission lines include Regulation Model, Electrogeometric Model (EGM) and Leader Progression Model (LPM). LPM describes the developing process of lightning shielding failure reasonably, which is in accordance with the physical mechanism of the development of lightning and has gained much attention. This model involves key components such as downward leader channel model, upward leader inception criterion, relative motion of upward and downward leader, final breakdown criterion, etc. Among all the components, upward leader inception mechanism and its criterion has a significant impact to the final selection of lightning strike point and the evaluation lightning shielding performance of transmission lines. Many scholars have done researches about upward leader inception mechanism and put forward relative inception criterion, including Rizk criterion, Peek criterion, gap criterion and recent inception criterion based on leader discharge theory (Becerra criterion), which is more closely in accordance with microphysical mechanism of leader inception.Based on previous researches, the concept of "critical electric field strength of stable upward leader" EL was defined in this thesis from the consideration of electric field and a novel upward leader inception criterion, the critical electric field strength of stable upward leader criterion, was put forward, which is more applicable in lightning shielding analysis of transmission lines. On the basis of leader discharge theory, the programme did large amount of simulation calculations under different conditions, confirming that EL was mainly related to the conductor radius. Then the expression of "critical electric field strength of stable upward leader" EL was fit with respect to it. Considering the influence of environmental factors, humidity and altitude, the expression of it was modified. Then the criterion was applied in the upward leader inception of single conductor and compared with several previous upward leader inception criterions. It is proved that the result of the criterion put forward in this thesis is in accordance with that of Becerra criterion, which is more credible. However, the physical model of this criterion is simpler, which only considered the calculation of electric field and is more applicable in the complex transmission lines.Using the upward leader inception criterion put forward in this thesis, the improved LPM was established, based on which the calculation model of lightning shielding performance of transmission lines was built and applied to double circuit UHV Alternating Current (AC) transmission lines in Japan. By calculating trip-out rate and comparing with practical operation experience, the correctness of the upward leader inception criterion and analytical model of lightning shielding in this thesis was tested and verified. Then taking Yunnan-Guangdong±800kV UHV Direct Current (DC) transmission lines as an example, the lightning shielding performance of it was studied, followed by the analysis of influence of factors on the transmission lines such as shielding angle, tower height, ground obliquity, valley depth, altitude, etc, proposing corresponding measures that can decrease the shielding failure rate. The proposed research presents useful reference for the design and operation of UHV DC transmission lines.