Research Of New Four-Phase Power Transmission System Theory

1Multi-phase power transmission is an important method to increase the power density transmitted, because the potential gradient of the transmission conductor surface can be obviously decreased. The research and field experiment operation have been carried out for more than twenty years, in the USA and other countries, and a lot of notable results has been obtained. Compared with the traditional three-phase transmission system, multi-phase transmission system has a lot of advantages. It has low phase-to-phase voltage, light and simple poles, compact overhead line space, great transmission capacity and low fault capacity for switchgear. It is easy for multi-phase transmission system to operate with the three-phase system. Because of multi-phase system with diagonal phases, the sum for odd harmonic currents for each phase is zero, this can reduce the interference for communication of harmonic currents. By further research and test, it is shown that the environmental indices in multi-phase systems such as audible noises, radio noises and electric and magnetic field at ground level are better than that in three-phase systems.Currently, the research for multi-phase transmission system is confined to the multi-times phases of 3, that is 6, 12 phases, and so on, because the transformation between 3 phase system and 3-times multi-phase system is easy by changing the connection of three-phase transformers. In current multi-phase transmission system research, in order to avoid the complex transpositions for multi-phase lines, the phase conductors have to be arranged in a symmetrical polygon, which makes it difficult to suspend the multi-phase conductors; furthermore the pole structure is complex, and the cost for transmission lines is expensive. With the increase of transmission line phases, the types of fault for multi-phase transmission increase rapidly, which makes the fault analysis, protection design and settings more difficult; the switchgears for multi-phase system complex, the over-voltage times between phases high. Because of the above disadvantages, six-phase and over six-phase transmission systems have not been used in practical transmission system. Four-phase transmission system has the list possible even phases. It has the advantages of multi-phase system, but overcomes the disadvantages of multi-phase system. Four-phase overhead transmission lines have special advantages in multi-phase transmission lines, because of the good symmetry and the simple structure of its lines and poles. Under the same condition of parameters, four-phase transmission lines can increase the transmission capacity by 33.3%, compared with three-phase lines. As the land width occupied by four-phase lines decreases, the capacity transmitted in unit land width by four-phase lines increases by 41.0~88.7%, and the economical and environmental benefits are much more notable. The reliability and transient stability of the transmission system may be improved, because two adjacent phase conductors of four-phase transmission lines can be operable. The particular characteristic of the four-phase transmission is: although the method is new, the technologies are roughly similar to that of the exiting thee-phase transmission system; technically there not many difficulties to apply it, and can therefore be developed and applied without delaying. The attention from the power industry is expected. The large key equipment to the application of four-phase power transmission systems is a three-phase to four-phase transformer and the inverse transformer. Principally, when the winding-connection scheme is adopted to add a set of auxiliary windings on the two-phase side of a balance transformer, we can get a three-phase to four-phase transformer. But in fact, the windings on each leg of a balance transformer are rather more, winding impedance relationships that the balance condition requires are strict, so it is difficult to design and make such transformers. If we adopt the connection scheme of dual windings, then the windings on single leg w...