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Hybrid ac/dc power networks are recently emerged in distribution generation (DG) systems with widespread acceptance under the smart grids environment. However, system-level dynamic interactions might be yielded due to the active control nature and tight regulation of power converters to meet load/generation requirements. This paper presents an assessment and mitigation strategies of such interactions in hybrid networks. A typical and comprehensive hybrid network composed of a DG power park, dc microgrid, islanded ac microgrid interfaced by voltage-source converter (VSC), and a grid-connected VSC is considered. Mathematical modeling and analysis of the input/output admittances of these entities are provided to evaluate the overall system stability based on the Nyquist admittance ratio criterion. It can be shown that the tight regulation of VSCs introduces incremental negative admittances reflected to the common dc link which significantly degrades the system stability. Therefore, active compensators are proposed to actively reshape the input dc-side admittance of the VSCs so that the Nyquist criterion is satisfied. Time- domain -large signal model of a typical hybrid network is implemented to validate the analytical results.
Date of Publication: Sept. 2012