“…U rm is the retained voltage of sag in bus m, U m pref , U f pref are the pre-fault voltage at bus m and f, the length of line is 1p.u., Z ii, Z jj are the driving-point impedances of buses i and j; Z mi , Z mj , Z ij are transfer impedances between the corresponding couples of buses; z ij the physical impedance of line k. Moreover, if an unbalanced fault occurs on line k, the voltage magnitude of bus m is shown in[9] in detail.Owing to the stochastic fault in system and distribution of voltage magnitude at bus m caused by line, probability density function of severity for voltage sag at bus m can be calculated by maximum entropy method proposed in[10…”