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Enabling maximum power extraction for distributed energy resources after frequency droop characteristic Wang, Mingjun


This research focuses on two promising control strategies for integrating power-electronic-based renewable energy generation into power grids. The first strategy involves controlling the power-electronic converters as virtual synchronous generators (VSGs) to emulate the classic synchronous generators (SGs) with their droop characteristic, inertia and damping to regulate dynamic response. The second strategy is the virtual oscillator (VO) based controller, which governs power-electronic converters to mimic the conventional droop characteristic while automatically synchronizing with the grid without a traditional phase-locked loop (PLL). In both control schemes, the droop characteristic refers to the output power of the converter responding to frequency disturbances on the grid. However, most converter-interfaced renewable energy resources typically operate in maximum power point tracking (MPPT) mode for economic reasons. Thus, new control strategies with external control loops that allow the energy resource to smoothly restore its power dispatch level after a frequency disturbance are proposed for the VSG- and the VO-based distributed energy resources (DERs). The proposed modified droop controller for VSGs can regulate the converter frequency after disturbances for maximum power harnessing. For VO-based DERs, two modified controllers are proposed: one with an external power-control loop to regulate the commanded power reference; and the other with an outer frequency-control loop to update the natural frequency of the VO. The proposed methods are evaluated through simulation studies in MATLAB/Simulink. It is demonstrated that the proposed controllers improve the capability of the VSGs and VOs and enable a smooth transition between the droop behaviour and the MPPT mode after a frequency disturbance in the grid. This research highlights the potential of proposed control strategies for integrating renewable energy generation into power grids.

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