Fault Analysis And Relay Protection For Six-Phase Transmission Lines

Six-phase transmission belongs to multiphase transmission, and it can evidently improve power transmission density and utilize the right-of-way more efficiently. Many countries paid more attention to this technology, and made great progress in this area. In 1992, a three phase double circuit line between Goudey and Oakdale station was reconstructed to six-phase line and was operated commercially by New York state electric and gas (NYSEG). It is a milestone from theoretical research to practical application in the development of six-phase transmission.With the decrease of cultivated land and the environmental problem in china, it is an important factor to improve power transmission density in the construction of electric network, and the predominance of six-phase transmission is obvious in this aspect. The method of fault analysis for six-phase transmission is very complicated, and the research on the protective scheme for six-phase transmission lines is few at the present time, and it had badly confined the practical application of six-phase transmission. The paper expounds the power transmission characteristics and studies several relay protection scheme of six-phase transmission lines. The main contributions of the paper are as follows:(1) The paper expounds the characteristics of power transmission of EHV and UHV six-phase transmission lines. Researching results show that six-phase can utilize the overhead transmission right-of-way efficiently, and improve the density of power transmission. At the same time, the electric field on surface of conductor, the power frequency electric-magnetic field at ground level can be controlled within the range of permission for six-phase transmission lines.(2) According to the characteristics of fault current sequence component on six-phase transmission lines, the paper proposes a novel fault phase selection scheme. The scheme can accurately distinguish the faulted phases under various fault conditions, and can be free from the influences of the transition resistance and system parameters.(3) The paper proposes a new comprehensive scheme for directional comparison pilot protection based on half positive sequence, half zero sequence and half negative sequence fault component. The scheme can select proper criterion according to the property of faults and is unaffected by system impedance. By adding directional criterion based on positive sequence fault component, it can identify all faults on six-phase transmission line.(4) The paper proposes a new protective principle based on linear combination of half positive sequence, half zero sequence, and half negative sequence current component for phase differential protection on six-phase transmission line. The phase differential protection based on this principle can be free from the influence of system impedance, heavy load current and transition resistance.(5) The paper proposes a new distance protection scheme, and it consists of 6 ground impedance relay and 15 inter-phases impedance relay. The scheme use quadrilateral directional impedance characteristics and R-L algorithm to calculate the impedance of the faulted phase. The results of simulation show that the performance of distance protection proposed is reliable, and can be used as backup protection for six-phase transmission line.(6) The paper proposes a novel criterion to distinguish instantaneous faults from permanent faults, at the same time, an adaptive reclosing mode based on phase sequence is presented in order to judge the permanent inter-phase faults and symmetrical faults on the six-phase transmission line. The adaptive auto-reclosing scheme can avoid reclosing the fault lines to permanent fault nearby, and enhance the possibility of reclosing successful.