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In the islanded operating condition, the microgrid has to maintain the power balance independently of a main grid. Because of the specific characteristics of the microgrid, such as the resistive lines and the high degree of power-electronically interfaced generators, new power control methods for the generators have been introduced. For the active power control in this paper, a variant of the conventional droop P/f control strategy is used, namely the voltage-droop controller. However, because of the small size of the microgrid and the high share of renewables with an intermittent character, new means of flexibility in power balancing are required to ensure stable operation. Therefore, a novel active load control strategy is presented in this paper. The aim is to render a proof of concept for this control strategy in an islanded microgrid. The active load control is triggered by the microgrid voltage level. The latter is enabled by using the voltage-droop control strategy and its specific properties. It is concluded that the combination of the voltage-droop control strategy with the presented demand dispatch allows reliable power supply without interunit communication for the primary control, leads to a more efficient usage of the renewable energy and can even lead to an increased share of renewables in the islanded microgrid.
Date of Publication: March 2011