The Researches Of Simultaneous AC-DC Transmission Technology

The situation that energy distributes unevenly and economic developsunbalanced determine the high-voltage transmission grid of our country will becomelarge-scale and distant. With the development of the socio-economic, the existingtransmission lines can not meet the increasing load demand for electricity. On theother hand, our country is vigorously develop clean energy，which will also bringsmany problems for the city power grid, such as insufficient power supply capacity,excessive short-circuit current, and DC load growth, etc. While in the construction ofpower, economic and space become the limiting factors of the new grid construction,so how to improve the transmission capacity of existing transmission lines hasbecome a key issue. The existing transmission mode includes AC transmission andDC transmission. When the short-circuit ratio is constant in both ends of the system,the transmission capacity of conventional AC transmission is mainly affected by theangle stability and reactive power control problem. While HVDC transmissioncapacity is mainly affected by the access point at the system short-circuit ratio andexchange system mechanical inertia, which has no angle stability problem. Thoughthe modulation of DC systems and the same line of AC and DC transmission can notonly improve the stability of the transmission system, but also can make full use ofthe existing transmission lines.The maximum transmission power of high voltage long distance transmission ismainly subjected to the static stability. Through analyzing the relationship betweenthe transmission power and power angle, we find that using the fast control of DCcomponents to respond to load disturbances can improve the stability margin oftransmission system and expand the transmission capacity indirectly. In addition, therelationship between the effective short circuit ratio and stability of both sides of theAC system has also been discussed. It is found that there is no need to change theexisting current circuit structure if DC components satisfy certain superposition rules.Consequently, the investment and operating cost have been drastically saved by thesuperposition of AC-DC transmission. Finally, MATLAB and PSASP simulations arecarried out, which verifies the conclusions.