Dominant frequency characteristics analysis of reclosing transient overvoltage in 10 kV cable-overhead hybrid line

Studying the characteristics of reclosing overvoltage is of great importance for risk assessment of reclosing in the cable-overhead hybrid networks because transient overvoltage (TOV) at different dominant frequency can have quite different negative effect to cable insulation. In a hybrid line, when a cable connects with an overhead line (OHL), the surge propagation can become very complex. To clarify the frequency characteristics of the reclosing transient overvoltage in a 10 kV hybrid line, in this paper, the simulations are carried out using ATP-EMTP (Alternative Transients Program, Electro-Magnetic Transients Program). The traditional approach to estimate the dominant frequency is suitable for pure cable-based or overhead line-based networks. However, for a hybrid line, due to the difference in line parameters between the cable and OHL, the characteristics of dominant frequency is investigated based on the distributed-parameter theory with new boundary conditions at the junction. According to the variation of dominant frequency considering the source impedance and the wave propagation in the modal domain, the non-monotonous variation of TOV with cable proportion is analyzed. The frequency calculation and the TOV analysis are verified by ATP-EMTP simulation.