Research On Individual Phase Parameter Decoupling And Circulating Unbalanced Current Of Overhead Line

Along with the constant expansion of the grid, the un-transposed double-circuittransmission line is used widely. The unbalanced problem of double-circuit transmission linewhich caused by un-transposition and complex electromagnetic coupling is increasinglyserious. Nowadays the research about single line unbalanced degree is mature and there areabundant of researches results about the through unbalanced degree of double-circuittransmission line. Actually besides through unbalanced current, circulating unbalancedcurrent of double-circuit transmission line, also has a significant effect on power system, relayprotection and so on. At the same time, the impact of the unbalanced degree is moreprominent in the short line because of circulating unbalanced degree will not change with thelength of transmission line. The difficulty in analyzing unbalanced degree of double-circuittransmission line is increasing by the complex coupling between double-circuit transmissionlines. There is also no clear conclusion has been reached about whether circulatingunbalanced degree changes with the variation of other factors so that we can’t analyze andreduce the unbalanced degree at the source. Therefore, it’s urgent to carry out the researchabout circulating unbalanced degree of un-transposed double-circuit transmission line.In this paper, an accurate method based on phase-mode transformation is proposed tocalculate unbalanced degree of double-circuit transmission line on the same tower. In theaccurate method, the influence of power source, load, ground wire and other relevant factorsof double-circuit transmission system on the same tower are taken into account. This paperfills in the gaps in individual phase modeling of double-circuit transmission line as well as themechanism of unbalanced degree, explains the rule that circulating unbalanced degree isbasically unchanged with the variation in length of transmission line and load theoretically.The accuracy and universality of this method is proved by comparisons of theoreticalarithmetic and simulation of220kV and500kV typical transmission line.Based on the accurate method of calculating unbalanced degree of double-circuittransmission line on the same tower, this paper analyzed a variety of possible factors which will affect the circulating unbalanced degree and simulates the variation of circulatingunbalanced under the affection of these factors. It’s found that ground wire, which is usuallyignored on the previous studies of unbalanced degree, has significant influence on unbalanceddegree and also is an important factor restraining circulating current unbalanced ofdouble-circuit transmission line in this paper. After comprehensively analyzing various factorswhich can affect unbalanced degree, the study found that phase sequence arrangement, thesplit form of conductor, space between wire, ground wire parameters and space betweenground wire have significant influence on circulating current unbalanced. In addition, lengthof line, load level, impedance of power source, conductor type, height of wire, earthresistivity and the impedance of the source have little influence on circulating currentunbalanced. Finally, the paper considering the on-site requirement and give some constructivemeasures to reduce circulating unbalanced degree.This work is supported by the National High Technology Research and DevelopmentProgram “(863”Program) of China (No.2012AA050209)，the National Science Foundationof China (51377059) and the Science Foundation of Guangdong Province (S2013010012885).Some of the research results have been applied to the operation of actual HVDC systems,obtaining great economic and social benefit.