Among multi-phase power transmission,six-phase transmission is the most feasible one.For the situation of 120 kinds of faults,six-sequence components method,which includes same positive sequence,same negative sequence,same zero sequence,opposite positive sequence,opposite negative sequence and opposite zero sequence,is applied in the fault analysis of six-phase power transmission system,and the transferred sequence components have clearly physical meanings.The same sequences in six-phase transmission power correspond to opposite sequences in parallel transmission lines,and the opposite sequences in six-phase transmission power correspond to the same sequences in parallel transmission lines.Based on six-sequence networks and boundary conditions of various faults,composite sequence networks have been established and the characteristics of each sequence component have been found out when fault happens. In a six-phase transmission system,these characteristics provide the fundamental principle for protection setting.Based on this methodology,a kind of fault phase selection scheme of six-phase transmission system is proposed.

In order to utilize right-of-way efficiently of six-phase transmission lines,the characteristics of fault current sequence component on six-phase transmission lines were analyzed.According to the amplitude and phase angle of fault current sequence components,a phase selector for six-phase transmission lines was proposed.The results of EMTDC simulation show that the fault phase selector can be free from the influences of the transition resistance and the unbalance of system parameters,and the method can accurately distinguish the faulted phases under various fault conditions.

In typical six-phase transmission system, half positive sequence, half zero sequence and half negative sequence fault components are independent of the variance of system impedance. A new comprehensive scheme, i.e. directional comparison pilot protection for six-phase transmission lines is presented based on the above mentioned three fault sequence components. It can automatically select the proper criterion according to the characteristics of faults, and reflect all faults except 6-phase fault on a transmission line. By adding positive sequence fault components of voltage and current, it can identify the direction of 6-phase fault. The results of EMTDC simulation confirm that the proposed scheme is unaffected by system impedance and transition resistance. Hence it is simple, reliable and rapid.

Six-phase transmission belongs to high phase order transmission, and it can improve the power transmission density efficiently. Comparing with transmission lines on the same tower, the advantage of six-phase transmission line on pole structure and using overhead transmission right of way was discussed in detail. Additionally, the electric field on surface of conductor, the distributions of audible noise and power frequency electric-magnetic field at ground level, and lines current unbalance were discussed. The result demonstrates the feasibility of UHV six-phase transmission.