Grid Identification and Adaptive Voltage Control in a Smart Transformer-Fed Grid

The interaction of the smart transformer (ST) with the grid-converter-based distributed energy resources (DERs) could trigger instability. Recent research efforts have been made to study the stability issues of the DERs in an ST-fed grid and the corresponding stabilization methods. Differently from the previous investigation focused on the DERs' stability, this paper studies the impacts of the increasing penetration of grid-converter-based devices on the ST low voltage side converter, in particular the stability margin degradation in the low-frequency range. To improve the stability margin, a lead element filter (LEF)-based voltage control offers a simple and flexible solution, which is able to alleviate resonance and achieve phase compensation. Considering the variation of grid characteristics, the resonant frequency is online identified and used for the parameter tuning of LEF. Moreover, the grid characteristics in frequency domain can be thereby estimated using the vector fitting method based on the frequency sweeping data. The robustness and sensitivity analysis are carried out, showing that the proposed voltage control is insensitive to the changes of system parameters of local grid converters. Simulation and experimental results are provided to validate the effectiveness of the proposed control strategy.