Abstract— This paper deals with the important task of voltage control of the ultra-capacitor (UC) in an isolated grid-off wind turbine (WT), photovoltaic (PV) and fuel cell (FC) islanding. Renewable and distributed energy resources have recently been introduced as alternative energy source due to pollution-free operation and no fossil fuel consumption. Among various alternative sources, PV, WT, and FC are more desirable than the others. The frequency control of an isolated network is not exact as enough. Therefore, to improve the frequency control, using an UC bank is proposed. In this paper improved droop control is utilized for the electric power sharing between PV, FC, and UC. The output power of WT is controlled based on wind speed to capture the maximum energy from wind. If the frequency of grid is going to decrease, then the UC will boost the active power rapidly, to keep the power balance. Meanwhile, the PV, WT, and finally FC will increase the power generation, if possible, to preserve the UC charge. If the load demand is decreasing and extra energy source from wind, day-light, and hydrogen flue is available, then the electric power sources supply the load and generate exceed power to charge the UC. Supposing that the load is very light and the FC produces minimum power and the UC is fully charged. In this case, if WT and/or PV continue maximum power extraction then the UC starts to over change and an over- voltage will occur on the dc terminal voltage of the UC which may damage the insulation (di-electric) of the capacitor units. Therefore, a voltage control on UC terminal is unavoidable.

Keywords— Frequency Control, Wind Turbine (IGWT), Solid Oxide Fuel cell (SOFC), Ultra-Capacitor (UC), Photovoltaic (PV), Isolated Network, Fuzzy Controller

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