| The current capacity of transmission line is an important index of power supply company.According to the current situation of the transmission line, on the premise that without changing the power network structure and moreover insure grid running safety and reliability,fully tapping the current capacity potential of transmission line will reduce the capacity pressure of transmission line during peak times or the transmission line fault. Also brings considerable economic benefits for power supply company.The main contents in this paper are as follows:Considering the practical significance of the restriction on span is the minimum distance between the transmission line and the spanning rivers or spanning buildings. On the basis of the traditional calculation model of the restriction on span of transmission line,the vertical distance between the spanning spot and a point’s tangent line on the transmission line is introduced into the space coordinate system as the restriction on span, so the improved mathematical model is obtained. And according the correspondence between the restriction on span and sag, the space curve parametric equations of the transmission line are obtained.Three lines of Guangdong power grid are selected as the calculated objects. Under the same weather conditions, the restriction on span is calculated using the improved calculation model and the traditional model respectively. Then calculate the restriction on span under different positions of the spanning spot, under different conductor types with the same span distance, under different span distances with the same conductor type separately. Then analyze the parameters’ s influence on restriction on span, such as the wind speed, the distance between the spanning spot and the the front pole, the span distance, the section’s size of conductor and so on.The conventional calculation formulas for the transmission line’s current capacity are introduced, including Morgan formula, corrected Morgan formula and ANSYS simulation,then analyze the principle and characteristic of every kind of calculation method. Then simulate the actual transmission line of Guangdong power grid by ANSYS under a certain current capacity, and the corresponding temperature value is obtained. And verify the results of ANSYS simulation by Morgan formula and corrected Morgan formula, and arrirm the accuracy of ANSYS simulation method.According to the minimum restriction on span between transmission line and spanning spot in the national standard, the sag is calculated by the corresponding calculation formula of restriction on span and sag. Then with combinations of the improved calculation model and the traditional modela and transmission line’s equation of state, the restriction on span and the current capacity of transmission line are calculated under same conductor operating temperature. As a consequence, the restriction on span between transmission line and thespanning spot still meet the national standard under the maximum allowable temperature of conductor, and the current capacity increase obviously than rated value. Tapping the potential of overload operation of transmission line, it provides certain reference for the design of the transmission line.The software development application platform is introduced briefly in this paper,including the user interface, characteristics and the program flow chart of the main algorithm of “The expert system to assess the overload capability of overhead lines”. And according to the overload test results of three transmission lines of Guangdong power grid, verify the expert system and arrirm the accuracy of the calculation model of expert system. Then assess the typical lines in Dongguan by expert system. The expert system provides a reliable data to support the overload operation of transmission lines, and offers the possibility of short-term overload operation of transmission lines during peak times. |
