Closed-Form Expressions for the Estimation of the Minimum Backflashover Current of Overhead Transmission Lines

The minimum backflashover current, I-BF, of typical overhead transmission lines is estimated through ATP-EMTP simulations considering the tower ground resistance and AC voltage phase angle as parameters. Several single- and double-circuit lines are investigated with operating voltage from 66 kV to 765 kV and Basic Insulation Level from 325 kV to 1950 kV. The backflashover rate, BFR, of the investigated lines is obtained using an expression that accurately considers lightning incidence to the line for prospective currents higher than I-BF; the relevant CIGRE expression underestimates BFR. BFR can be easily estimated using solely the phase angle values corresponding to zero AC voltage of the phase conductors. Based on the I-BF and BFR results, simple expressions are introduced for the calculation of an equivalent minimum backflashover current as a function of insulator length and tower ground resistance or impulse impedance. These expressions enable straightforward estimation of BFR without the need for time-consuming simulations or laborious analytical methods adopting simplifying assumptions.