Impact of electromagnetic coupling on common 110 KV/400 KV overhead lines on a 110 KV grid

It is sometimes unavoidable to run high and extra-high voltage circuits on common overhead line towers. The spatial proximity between the conductors inevitably leads to inductive and capacitive coupling between both voltage levels. However, it is not necessary to entirely eliminate coupling, provided it doesn’t have a detrimental effect on the operation of the overhead line or on the other components of the grids involved (including compliance with the relevant limit values for voltage and current asymmetries). This paper describes the basic principles and the modelling of the electromagnetic coupling of a 400 kV and a parallel 110 kV (low-impedance neutral point grounding) system. Based on their geometry, critical configurations are shown and the dimensions for induced voltages and influenced currents are given. Measures to avoid ferroresonance are presented.