Abstract
Recently, virtual synchronous generators (VSGS) are a hot topic in the area of microgrid control. However, the traditional fixed-parameter-based VSG control methods have an obvious disadvantage. Namely, if the damping value is set to be small, the amplitude of frequency deviations under external power disturbances is large, meaning that the frequency suppression capacity is insufficient, but if the damping value is large, the dynamics of the system will be greatly sacrificed. To solve the problem, taking the dynamic characteristics and the maximum allowable frequency deviation (MAFD) into account, in this paper an improved fuzzy adaptive damping-based VSG control strategy is proposed to simultaneously attenuate the microgrid frequency fluctuations and guarantee the system dynamics. Firstly, in order to address the necessity of using an adaptive damping-based VSG, the structure of a fixed-parameter VSG method that incorporates the f-p/Q-V droop controllers is introduced, based on which a small signal model is established to discuss the impacts of the virtual damping on the frequency response characteristics concerning the different penetration levels of power disturbances. Then, considering the dynamics and MAFD, a fuzzy adaptive controller is constructed relying on the well-designed membership functions, control rules and output scaling factors. The main feature of the improved fuzzy controller is that two alternative output scaling factors are employed to allow the system to be overdamped when the frequency deviation is large and undamped when the frequency deviation is small, balancing the frequency response dynamics and stability characteristics. To verify the effectiveness of the proposed fuzzy adaptive damping-based VSG technique, a computer simulation is conducted on a microgrid system in MATLAB/Simulink, and the obtained results are compared with the conventional droop control and fixed-parameter based VSGs. By using the proposed fuzzy adaptive damping-based VSG control method, the peak frequency deviations under the large power disturbances would become at least 8% lower compared to the traditional droop control and fixed-parameter VSG control, and meanwhile, the frequency response speed is fast when the disturbance stands at a low position. Consequently, it is valuable to promote the proposed techniques in engineering.
Highlights
Due to the advantages of low cost, flexibility and environmental friendliness, microgrid systems are gaining increasing attention in the area of energy generation and supply [1,2,3,4]
To verify the effectiveness of the proposed fuzzy adaptive damping-based VSG technique, a computer simulation is conducted on a microgrid system in MATLAB/Simulink, and the obtained results are compared with the conventional droop control and fixed-parameter based VSGs
By using the proposed fuzzy adaptive damping-based VSG control method, the peak frequency deviations under the large power disturbances would become at least 8% lower compared to the traditional droop control and fixed-parameter VSG control, and the frequency response speed is fast when the disturbance stands at a low position
Summary
Due to the advantages of low cost, flexibility and environmental friendliness, microgrid systems (e.g., diesel, wind and photovoltaic power generation systems, etc.) are gaining increasing attention in the area of energy generation and supply [1,2,3,4]. Energies 2020, 13, 4885 including grid-forming, grid-feeding and grid-supporting [5]. Grid-forming is to establish a local grid that is powered only by the microgrid systems; Grid-feeding means that the microgrids will inject specified power into the CG through the PCC; Grid-supporting aims to use the microgrid or distributed generation systems to back up the CG by regulating its voltage/frequency and sharing active/reactive power. In order to achieve these functionalities or connect a microgrid to the CG, VSIs together with particular control algorithms must be employed to transfer the DC voltage into the AC one with the rated frequency [6]. Without taking the frequency and voltage into account, only the active and reactive power injected into the grids are controlled to maintain at the reference levels.
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