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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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The project DC-bus for microgrid at HES-SO Valais-Wallis is a full scale demonstrator where the main electrical components in a modern building, i.e. photovoltaic power sources, various consumers and distributed storage systems are directly connected to a common DC distribution system. This paper describes the specifically designed DC/DC converters with MPPT for individual PV panels and the bidirectional AC/DC interface to the LV AC distribution grid. A model of the controller in the frequency domain of the AC/DC inverter has been used to optimise the control coefficients, with focus on both DC bus stability and reduction of the current harmonics on the AC grid side. Realisation, a simulation model and tests results are presented.