The understanding of harmonics propagation in power systems recently gained interest. In HV networks, the current trend of undergrounding is modifying the network resonance frequencies. In LV distribution systems, the increasing deployment of decentralized generation induces new harmonic pollution, which could interfere with power line communications (PLC). 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 EHV/HV network (380 kV, 220 kV, 125 kV and 50 kV) and the second one is a part of a MV/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.

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Presently there is a great interest in underground cable transmission lines in extra high and high voltage level networks. The undergrounding of the transmission system can lead to the decrease of the resonance frequencies resulting in high overvoltages and possible power system component damages. It is therefore necessary to analyze possible resonance problems in the mixed power transmission system. In this work a frequency dependent model for transmission systems is implemented and the frequency scan results are presented. In order to gain a better understanding of the nature and extent of the resonance, a resonance mode analysis is also executed. The proposed procedure is tested on two examples: a ten-node power system and the Swiss extra high voltage (EHV) power transmission system. Power systems comprising solely overhead lines and different variations of mixed power system are examined.

Medium to long-distance underground AC lines and both short-and long-distance HVDC are frequently integrated into network expansion plans. In conjunction with growing numbers of power electronic converters in the grid there is a rising risk related to the effects of harmonic currents. Resonance analysis should therefore be part of power system expansion planning. In this paper, network harmonic modeling and analysis are discussed, in particular power transformer and power converter frequency models are proposed. Frequency scanning and resonance mode analysis are compared and discussed. The changes in resonance response due to transmission network expansion are drawn. The methods are applied to the investigation of the impact of one link reinforcement, both AC and DC solutions, as well as the impact of the power transformer disconnection. The performed analysis allows to find the resonance location at each studied case. The frequency analysis methods presented in this paper enables to foresee network frequency behavior that is valuable for both network analysis and network expansion planning.

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