Generation and propagation of External-Common-Mode fast transients in the power mains

Fast transients can be excited in the low-voltage distribution lines with current running in parallel along the three wires of the line (phase, neutral and safety ground, in a typical system) as an “external common mode” or “antenna mode” propagation. Significant current values can be found in real-world situations. The ordinary differential-mode or common-mode protections do not work in this bulk-current case. An analysis is needed of the properties of propagation of this type of disturbance and the influence of the medium external to the wires. The excitation mechanisms are also of interest. A theoretical and experimental study is performed in the frequency domain analyzing the impedance, propagation constant and phase velocity. Propagation is seen to be supported by the wires and the building walls. A typical generation mechanism observed is mode conversion of originally differential-mode transients at lumped discontinuities. A good agreement between experiments in the frequency domain and theoretical predictions using data obtained by independent measurements of the complex permittivity of the walls is obtained. Time-domain analysis is also performed with good agreement between computer simulations and measurements of the transient voltages and currents excited by load switching in the mains line.