Numerical Analysis and Comparison of Magnetic Fields Caused by Constant and Time Varying Currents in Medium Voltage Underground Cables

Abstract

The magnetic fields formed around the conductors carrying current alternating current in the power system adversely affect human health. Especially in medium voltage systems, where energy is mostly carried by underground cables for electrical safety, due to the high current levels and the installation of cables in areas where people live, studies in this field have gained importance. In literature, many studies have been carried out on the calculation of magnetic fields caused by underground cables using numerical methods and their comparison with limit values. In some of these studies, the boundary conditions for phase currents are defined as constant. However, alternating current, which is the source of the magnetic field, is a time-varying vector quantity. For this reason, it is important for the accuracy of the analysis to take into account the direction of the current, the time-dependent change of the current and the phase difference while calculating the magnetic flux density. In this study, the magnetic flux density values caused by a sample medium voltage underground cable system at the reference plane one meter above the ground surface are determined using Comsol Multiphysics. Analyzes are performed both in the stationary domain where the current is constant and in the time domain when it changes depending on time, and the results are discussed and compared. According to the results, it is determined that the maximum magnetic flux density exceeded the limit value of 0.2 mT, while the phase current values are constant. However, it is seen that the magnetic flux density obtained in time-dependent analyzes remains within the safe limit. In addition, it has been determined that the results obtained in the stationary domain are considerably higher than the results obtained in the time domain. As a result, it has been revealed that the time-dependent variation of the current must be taken into account in order to accurately determine the magnetic flux density in the magnetic field analyzes to be performed for underground cables or overhead lines carrying alternating current.