Overvoltage Investigations of Cable Shieldings and Protection Methods

Due to the fact that there is a special high alpine situation in Austria, different earthing
systems and shielding concepts are in practical use. The aim of this paper is to review a current
project at the Institute of High Voltage Engineering and System Management which deals with
medium voltage cables that are used for the internal power supply of a substation, where the
handling of overvoltages at cable shieldings are researched. The investigation includes computer
simulations of several types of faults based on local earth resistance measurements. The focus was
set on single-phase faults as well as the transient behaviour of the cable shield. Therefore, a cable-
model consisting of RLC-elements had to be set up. The entire simulation has been carried out
using a numerical simulation software based on EMTP/ATP. Those numerical calculations include
the overvoltages occurring on the cables shield, the stray current and the energy absorption
capacity of the surge arresters. According to the simulation results, a strategy for the dimensioning
and application of several surge arresters was drawn up, in order to avoid contact and step
voltages. Additionally, the paper shows the influence of the propagation impedance as well as the
dependence on the fault current amplitude.