Inclusion of Wire Twisting Effects in Cable Impedance Calculations

Cable-series impedance modeling is widely applied in electromagnetic transients simulations and power-loss calculations. The calculations are usually performed in a 2-D frame using the finite-element method or alternative approaches, such as MoM-SO, thereby losing the 3-D effects imposed by twisting wire screens and armors. One of the implications of using 2-D modeling for three-core and closely packed single-core cables is that currents will always circulate among the individual wires of each wire screen or armor. However, in the case of twisted screens/armors where the wires are insulated from each other, such current circulation will in reality not exist. As a result, the calculated impedances become incorrect as well as the induced currents and losses on individual conductors. A procedure is introduced for preventing such false current circulations in a 2-D calculation frame by simple manipulation of the system impedance matrix. The approach is demonstrated for the modeling of single-core and three-core cables, with and without external armor. It is shown that the representation of the wire screen/armor armor with respect to current circulations can substantially influence the calculated result, both for the 50/60 Hz impedance and the cable transient behavior. The use of tubular screen representations is also investigated.