Study On Simultaneous AC/DC Power Transmission Technology

With the high-speed development of national economy, there is a rapid growth of the demand for electric power. But the distributions of energy sources are not corresponding to those of load center in china, namely, the energy sources is often not available at the load center but at remote locations, forming a long-distance transmission of electric power from north to south and from west to east. At present, the shortage of transmission corridor restricts the construction of new transmission lines, and there are the limited transmission capacity, low transmission capacity, and low utilization for the present transmission lines. These lead to requirement for reviewing the traditional power transmission theory and practice, on the basis of new concepts that allow full utilization of existing transmission facilities without decreasing system availability and security. Therefore, effective utilization of the present power transmission capacity has been an urgent problem in the development of power industry. On the other hand, AC/DC transmission technology is a new way of transmitting electric power. By means of this way, it may be expected that the transmission capacity of the lines can be close to their thermal limits.In this dissertation, based on the tentative plan of the simultaneous AC/DC transmission technology, the scheme of realizing this technology is introduced, and the corresponding power transmission theory and the electromagnetic environment around the simultaneous AC/DC transmission lines are analyzed.Firstly, the basic principle of the simultaneous AC/DC transmission lines are described in detail, the two types of circuit topology are presented, and the correlative primary devices are analyzed.Secondly, for the simultaneous AC/DC transmission operation system based on the improvement of the AC transmission lines with double circuits, its transmission theory is derived from the traditional one, and PSCAD/EMTDC simulation model is established to examine its reliability. Also, the influences of power angle, reactance of lines, and voltage classes on the power transmission of the simultaneous AC/DC transmission lines are analyzed, and the following results are obtained:(1) For the same voltage level, the relationship between power transmission and power angles is close to cosine function, but larger power angle can not be taken because of the influence of system stability;(2) For the same voltage level, the longer the line length, or the greater reactance, the improvement of the power transmission ability due to the simultaneous AC/DC transmission lines is more obvious;(3) The higher the voltage classes, the weaker the simultaneous AC/DC transmission system’s ability for improving the line power transmission. Due to this, the economy assessment for its rationality is required.Finally, the electromagnetic environment for the simultaneous AC/DC transmission lines is analyzed, and the emphasis is placed on the analyses for electric fields around it. The numerical method of calculating the electric fields around the simultaneous AC/DC transmission lines is presented. Using this method, the differences between the electric fields around the present transmission lines and around the simultaneous AC/DC transmission lines are analyzed. The simulation results show the following. For the simultaneous AC/DC transmission line resulting from the improvement of the present AC transmission line, for the power frequency electric field, its distribution at1m under the lines is changed, the symmetrical maximum is shown, and its intensity is also increased, which enlarge the public areas which is not suitable for long-term living. Therefore, it is necessary to determine whether the electromagnetic environment conforms to the limit standard for it, when taking into account the effective utilization of the potential power transmission capacity. If no, corresponding measures must be adopted to improve it.