The understanding of harmonics propagation in power systems recently gained interest. In high-voltage (HV) networks, the current trend of undergrounding is modifying the network resonance frequencies. In low-voltage (LV) distribution systems, the increasing deployment of decentralised generation induces new harmonic pollution, which could interfere with power line communications. In this analysis, the most important elements of power systems (overhead lines, underground cables, power transformers, linear loads and inverters) are modelled in frequency and integrated into frequency scan and resonance mode analysis methods. Two network types are analysed to point out the specificities of each network level: the first one is an extra high voltage (EHV)/HV network (380, 220, 125 and 50 kV) and the second one is a part of a medium-voltage/LV distribution system (20 kV and 400 V). With this method, the network behaviour in a large frequency bandwidth (up to 5 kHz for transport grids and up to 200 kHz for the distribution system) can be predicted. Recent network impedance measurements contribute to the validation of this method by showing the same trends and resonance frequencies close to the ones visible in the simulation results.