In the Danish transmission system, the impact on the harmonic distortion on 150 and 400 kV from the upcoming undergrounding of 324 km lines at the 150 kV is unknown. The analyses made in this report are based on the PowerFactory model: "Power System Model for Resonance Studies" which is based on a grid made by Oscar Lennerhag. The model contains the voltage levels 130 and 400 kV. Therefore, this report contains a study of the changes in harmonic propagation, when 130 kV OHLs are replaced by UGCs in both a radial and meshed system in PowerFactory. A literature study regarding the state of the art of propagation of harmonics is described and used as a basis for studies conducted in this report.

Analyses are made in a radial system in order to obtain knowledge about the harmonic propagation in a simple system, when line types are changed.
In the radial system analyses, the system is analysed with and without a transformer. The analysis with a transformer is done to analyse the impact of having different voltage levels in a system when analysing the harmonic propagation. From this analysis it is found that when a power system consists of multiple voltage levels, the impedance is transformed from one side to the other due to the per unit conversion factor. Hence, observed from the HV-side, the impedance of the LV-side is multiplied by the conversion factor. Oppositely, observed from the LV-side, the impedance of the HV-side is divided by the conversion factor. Additionally, it is shown that the propagation of harmonic voltage can be obtained from the off-diagonal impedance between two busses from the frequency scan. The harmonic voltage propagation from one voltage level to an other can either be amplified or attenuated depending on the system characteristic and harmonic order.

In the meshed system, analyses are performed where the line types are changed. Here it is found that line type changes made in a system primarily affect the harmonic impedance at the voltage level where the changes are made. Additionally, line type changes made at higher voltage levels has a larger effect on the lower voltage level impedance than opposite. These findings are explained based on the transformer impedance and the conversion factor and is supported by additional case studies. Furthermore, the harmonic propagation between voltage levels is analysed utilising the off-diagonal impedance, when line types are changed on the 130 kV level. Here it is shown that when changing the line types, the changes in harmonic distortions mainly appears at the 130 kV level. Lastly, an exploratory study of mitigation of harmonic distortions are analysed. From the specific case analysed, it was found that the optimal filter location was not always at the location of the harmonic injection, but at the bus with the largest impedance characteristic at the harmonic order to be decreased.