Secondary arc extinction in AC/DC overhead lines
The structural evolution of the electricity sector, in particular the separation between production, transmission and distribution, and the presence of more and more Renewable Energy Sources (RES) requiresamore flexibleelectrical grid. DC has the meansto provide this flexibility. Therefore, the transformation of the AC grid into a hybrid AC / DC grid is an alternative, which is considered seriously by Transmission System Operators (TSO). Considering that the construction of new transmission overhead lines is almost impossible because of worldwide public reluctance, the transformation of existing AC overheadtransmission lines into DC or hybridAC / DC overhead transmissionlines is an attractive option. This paperfocuses on a specific issue related tothis transformation: the extinction of the secondary arc current of a faultedDC pole in an AC / DC hybrid overhead line.Dueto pollution or lightning, aDC pole-to-ground faultappears along the line. This fault leads to an opening of breakers to interrupt the short-circuit current. If the line is long, after this opening, a small AC current (a few 10ths Amps) keeps on circulating in the fault due to the inductive and capacitive coupling between the AC circuit and the faultedpole. This current is named the secondary arc current. It may disappear after some time. However, if the pole is reclosed before its extinction, the fault starts again because the electrical channel created by the initial fault has not disappeared and this channel is still a path for the DC short-circuit current.From an electrical system point of view, this is critical to have the faultedpole operational as soon as possible (in particular to reduce the risk of stability issues), but the pole cannot be reclosed before a minimum timebecause of the concerns referred to previously. The same concern exists for AC lines.This interruption timedepends,among others,on the structure and rated voltage of the line, the intensity and duration of the short-circuit current. In AC, various solutions to the problem do exist and are mainly applied to long Ultra High Voltage lines. The paperis devoted to the studyof extinction conditions and interruptiontime reduction ofa secondary arc following a DC pole-to-groundfaultalong an AC / DC overhead line.Calculations conducted with an EMTP-like program show that the transposition of the AC circuit can significantly reduce the AC secondary arc current and then the dead time following a DC fault.
Myriam RATAJCZYK, D. Hart, A. Bertinato, M. Beley, C. Delgado Díaz de León, G. Yin, l. Zheng, E. Sellin, A. Xemard
Published in Cigre Science and Engineering