Distance protection of multiple‐circuit shared tower transmission lines with different voltages – Part II: fault loop impedance

Abstract

Multiple-circuit transmission lines combining different voltage levels in one tower present extra challenges when setting a protection philosophy, as faults between voltage levels are possible. In this study, the fault loop impedance of combined faults is compared with the fault loop impedance of single-phase-to-ground faults and it is demonstrated that they are similar for high short-circuit powers; however, the fault loop impedance of a combined fault may increase substantially as the short-circuit power of the system decreases, a behaviour that is less noticeable for single-phase-to-ground faults. It is also demonstrated that the fault loop impedance of combined faults is more resistive, when compared with equivalent single-phase-to-ground faults. It is concluded that the settings used to protect a line against single-phase-to-ground faults are capable of protecting the line against combined faults, being advised to increase the resistive limit of the protection zone, if the network has lower short-circuit power. It is recommended to assure that the fault can only happen for cases where the faulted phase from the higher voltage level leads the faulted phase from the lower voltage level, if the length of the line at lower voltage level is smaller than of the line at higher voltage level.