| The half wavelength AC transmission(HWACT) technology is attracting more and more attentions home and abroad by its unique economic and technological advantages. In China, the energy resources is against the productive forces development, the base of energy in the west is about 3000km away from load centers in the east, power energy can not be transmitted fast and efficient, so the transmission systems with long distance and large capacity should be established, and the HWACT system is a good choice.The HWACT technology is a newly arisen technical topic, and the studies of it at home and abroad is just beginning.The paper starts by describing the history, research status and the advantages of the HWACT technology, and got conclusion that it was meaningful to study the relay protection of HWACT lines. Based on the studies of 1000kV UHV transmission lines, by analyzing the research status and relay protection of them, got a preliminary cognition of the relay protection of UHV transmission lines. The next, sums up the line parameters, electric transportation properties, power transmission properties and steady state characteristic of HWACT lines. Then, a mass of simulation experiments base on the software PSCAD/EMTDC have been done and the results showed the correctness of the study and the transient state characteristics of HWACT lines. The fault analysis shows that the distributed-parameter characteristics of lines are notable, and its distributed capacitance is large. The over-voltage caused by capacitance effect is a big problem. Because of that, the traditional transmission line protection strategies can not be directly used in this kind of lines. At last, after knowing the fault characteristics, based on the Bergeron model which can directly consider the effect of distributed capacitance a kind of protection strategy suitable for HWACT lines is proposed. Also use PSCAD/EMTDC to examine the correctness of the new protection principle.
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