Feasibility Study On Closed-Loop In Distribution Networks

In recent years, with the development of society and economy, the demand for distribution network reliability is rising. The "closed-loop design, open-loop operation" mode, which has been used for a long time for domestic distribution networks, cannot meet the stringent requirements on power supply reliability for some areas. Since the power supply reliability can be further improved by the closing-loop operation or closed-loop operation of distribution networks, some related problems are studied in this paper.In the aspect of loop-closing operation, a method of calculating the surge current from closing a loop is proposed; meanwhilea relatively accurate analytic formula for calculating the surge current of non-closing branches is deduced. With Thevenin’s theorem and the power flow analyses before and after closing a loop being employed, the decay time constant is computed through the optimum frequency method. Based on the decay time constant and the power flow analyses under the rated frequency, the surge currents of closing loop branch and non-closing loop branches can be calculated.The proposed method and the method in reference [23] are used to calculate the surge currents for two examples. The results are compared with those from PSCAD/EMTDC transient simulation with the proposed method’s practicality and validity being shown. The results also show that the surge currents of some non-closing loop branches are larger than the current of closing loop branch, and thus it is very necessary to check the surge currents of non-closing loop branches. Based on the proposed method, an aided decision making software of closing a loop in a distribution network is developed. The software is applied to the closing loop current calculation and analysis in an actual distribution network, and is of good practical value and guiding significance.In the aspect of closed-loop operation, the domestic and abroad closed-loop operation status quo of medium voltage distribution networks are introduced, as well as some features of closing loop modes, short-circuit current, relay protection and reliability. With both of reliability and economy being considered, the construction, operation and outage costs of closed-loop operation in a distribution network is included in the optimization objective, and a net profit model of upgrading a distribution network from opened-loop operation to closed-loop operation is established; at the same time, a simplified SAIDI evaluation model of medium voltage cable feeders is modified with the System Average Interruption Duration Index(SAIDI) being regarded as a main index. The difficulties and applicable scopes of closed-loop operation are discussed.Regarding a cable’s upgrading from the opened-loop operation to closed-loop operation, the corresponding qualitative analysis and quantitative calculation of reliability and economy are performed, and the sensitivities of some related parameters are also analyzed. The study result shows: for the areas of developed economy, large load density, distributed generation and high reliability requirement, the closed-loop operation mode(mainly the one of two feeders from the same bus) can be considered for use with the conditions being satisfied; although the cost of a power supply company may become greater due to the using of closed-loop operation mode, the power supply reliability can be improved with the social and economic benefits being ensured.