Study On The Internal Overvoltage And Its Influencing Factors Of±800kV DC And500kV AC Transmission Line Built On The Same Tower

With the rapid development of electric power industry in China, UHVDCtransmission, with its advantages over long distance and high capacity transmission, isbecoming a promising developing direction in the UHV transmissiontechnology.Several EHV and UHV DC transmission project have been put intooperation in China and the power grid in China is forming a pattern of parallel operationof large-capacity AC/DC transmission. Facing the reality of corridor shortage in someregions and environmental restriction, siting of new transmission corridors has becomeincreasingly difficult, especially in southeast China. Adopting AC/DC hybridtransmission line provides an attractive solution to the problem of restrictedright-of-way，so it is predictable that AC/DC hybrid transmission lines will occur inChina.Thus far, no EHVAC and UHVDC hybrid line built on the same tower exists inChina and abroad. Very few hybrid AC/DC transmission lines have been constructedabroad, but the voltage and load level are relatively low compare with UHVDC/EHVAC hybrid line, so the coupling effect between the AC/DC conductors is lessserious. Research of AC/DC coupling in China mainly focus on AC/DC line built alongthe same right-of way, but UHVDC/EHVAC lines built on the same tower have closeproximity between conductors, which makes the coupling effect more serious. As such,a computer simulation of the AC and DC coupling effect of lines built on the sametower can serve as a reference for future designs.This thesis discussed the method of calculating electromagnetic coupling betweenDC and AC transmission lines,analyzed the formation mechanism of capacitive andinductive component of the fundamental coupling and its influencing factors,thecomputing method of AC/DC coupling with AC transposition is also put forward.Withthe analysis and results, acording to the parameters of planning Ximeng-ShanghaiEHVAC/UHVDC hybrid transmission project, the detailed simulating model of DCconverter station and hybrid transmission line with double circuits AC500kV and DC±800kV transmission line are presented.Base on the simulating model ofUHVDC/EHVAC hybrid transmission line, the study are carried out in the followingaspects:1) The fundamental frequency component of DC line with different AC/DCoperation conditions;2) AC/DC switching overvoltage and their influence on hybrid line insulation;3) Induced current and voltage during outage maintenance and secondary arccurrent during DC restart, their restricting methods are also analyzed.Through simulation and analysis, the following conclusions are obtained：1)Normally operating AC line can produce serious fundamental frequency component onDC line,Three-segment-transposition of AC line within the hybrid line can reduce theDC line induced component effectively；2) With AC asymmetric fault, there is norestricting role of AC line transposition on power frequency coupling component of DCline, which should be considered in the design of the converter transformers；3) ACpower frenquency overvoltage and AC/DC switching overvoltage within the hybrid linedoesn’t pose a threat to the insulation of adjacent circuits；4) Compare with thesecondary arc current on AC line, secondary arc current on DC line is very small, whichdoesn’t affect the DC line restart obviously,we may extend the first restarting delayappropriately to avoid DC restart failure.