Analysis of Harmonic Propagation in Meshed Power Systems using Standing Waves

It is necessary to restructure and expand the Danish transmission system with significant changes as a part of the green transition. The Danish Transmission System Operator, Energinet, considers the use of underground transmission cables at 400 kV and 150 kV level instead of overhead lines whenever new
transmission lines as well as reinvestment in existing transmission lines are planned in the meshed grid. The extensive use of underground cables is known to lead to measurable changes in the propagation of harmonic voltages in the grid and we have no proper explanation of why grid changes lead to destined
variations of harmonic levels in meshed power systems today.

This paper presents observations of harmonic voltage propagation in a simple meshed system representing the fundamental topologies of transmission systems having series and parallel branches and it explains the basic behaviour of the propagation in the meshed system when it undergoes topological
changes, in the form of replacement of overhead lines with underground cables in both series and parallel connections and when the meshed system topology changes. As a contribution, the explanation of propagation phenomena of harmonics is based on standing wave theory and verified using harmonic
load flow calculations.

It is shown that both a series and parallel connection with underground cables greatly affect the propagation of harmonic voltages in the system and that the phenomena can be explained by means of standing voltage waves, determined solely by the terminal condition of the lines. It is found that a series connection of overhead lines and underground cables greatly increases the harmonic voltage in the system, whereas a new parallel connection significantly lower the harmonic voltage regardless the type of line. It is also shown that these phenomena can be foreseen using the determined reflection
coefficients for the system.