The loop closing operation of a distribution network may generate a rather large surge current affecting safe and stable distribution network operation. A method for calculating the surge current from loop closing in distribution networks is proposed

and the accurate analytic formulas for calculating the surge currents of non-closing branches are deduced. Based on the Thevenin's theorem and the power flows before and after loop closing operation at optimum frequency

the decay time constant is computed. Based on the decay time constant and the power flow results before and after loop closing operation at the rated frequency

the surge currents of loop closing branch and non-closing branches can be calculated. Finally

the computing results of different methods

including PSCAD/EMTDC transient simulation

are compared and the results show the practicality and validity of the proposed method. Moreover

the results also show that the surge currents of some non-closing branches are larger than that of the closing branches. Thus it is quite necessary to check the surge currents of non-closing branches.