A microgrid (MG) can operate in both grid-tied and autonomous mode. Without the support from the public utility, the control of an autonomous MG is more complex due to its poor system inertia. Though energy storage system (ESS) can act as a main power source to maintain system frequency and voltage stability, traditional droop control is usually invalid in practice due to the resistive line of low/medium voltage MG. Virtual impedance control can be a solution to decouple the active and reactive power allocations among ESSs. However, the control bandwidth is reduced since it requires low-pass filters with reduced bandwidth to calculate the average active and reactive power. In this paper, a novel ESSs control method is proposed with V/f droop control (VFDC) and P/Q droop control (PQDC) combined. It can distribute the active and reactive power precisely since the interference of line parameters uncertainty is prevented and system stability is enhanced. The comparison between traditional droop and the hybrid VFDC/PQDC is analyzed based on equivalent circuits. A hybrid VFDC/PQDC-based MG control scheme is proposed and its small-signal stability is analyzed. The proposed method is verified through experimental test on a MG platform with two 100 kVA ESS prototypes.Index Terms-Energy storage system (ESS), microgrid (MG), P/Q droop, small-signal model, V/f droop.

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Abstract:The impact of the rapid development of large-scale centralized wind power farms on the power system is drawing more and more attention. Some topics about grid-connected wind power are discussed from the view of complex network theory in this paper. Firstly, a complex network cascading failure model is established, combined with dynamic AC power flow (DACPF). Then, the IEEE 30 bus system is used to analyze its validity using the simulations of nodes removal, wind power integration, as well as the change of current and voltage boundaries. Furthermore, the influences of wind power before and after smoothing are investigated. Also, different wind power coupling locations are studied. Finally, some significant conclusions are obtained to provide references for large-scale wind power integration.