Study On Online Loadability Of Transmission Lines

The limiting factors in transmission line capability can be divided into subjective class and objective class. The subjective class is thermal limits, and the objective class includes voltage limits and steady-state stability margins. The load carrying capability considering all the limits of a line is known as the line loadablility. Due to the voltage level and the steady state stability of a line is related to system environment, the line loadability of a line is time varying, as a result, it is helpful to estimate line loadability online, the result can be used as the basis of power system dispatch as well as to improve transmission line capability.Currently the swift growth of power demand and the limited power transmission resources causes power systems to work near their operating limitations. Under this background, it is necessary to consider line voltage limits and the stady-state stability in power system operating. Aiming at this problem, this thesis proposes a new concept named online line loadability, that is to calculate the line loadability online with system environment. On this concept, this thesis studies the method of online line loadability evaluation.Firstly, based on the operating circle and limiting circle, this thesis analysis the dialectical relation of line voltage limits, stability limits and thermal limits. Then proposes the mathematical version of line loadability, discuss the influence of system operating state to line loadability, and point out the necessity online line loadabilty.Secondly, based on state estimation in power system, establish equivalent model and the corresponding line estimation algorithm for any given transmission which need to be analysis. The model is able to effectively deal with the parallel flow problems around the transmission line, bring improvement in accuracy and some progress in this area. Two Solutions to the model equivalent parameter estimation are proposed, one based on the three-phase short-circuit capacity, this method works with good computational speed, but accuracy is not high; Second, under the assumption that equivalent parameters do not change a lot in the limited extension of time, make use of more than 3 state estimation result in consecutive times to estimate model parameters online. This method follow the real time variation, can get a more accurate result. A test shows that the model and algorithm are effective.Finally, on the basis of these studies, a online transmission line loadability analysis system is established, and used in the real power system of Yantai, Shandong province. then verify the theory of this thesis by a practical test of the system in Yantai, Shandong province.Above all, online transmission line loadability estimation is helpful to improve the safety and reliability of power systems, while let the systems operating more economically. Of course, there is still many problems needed to be solved, but it is believed that the online transmission line loadability estimation will face a good prospect.