Modeling and Stability Analysis of 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 caused by the resonances (e.g. LC or LCL filter resonances of DERs) in ST LV side grid. This paper studies the impact of the grid synchronization of the grid-converter-based DER on the stability of the ST as well as the grid. The equivalent admittance of the grid converter considering the effect of synchronous reference frame phase-locked loop (SRF-PLL) is developed and merged with the ST voltage control. Based on the generalized Nyquist criterion, the stability analysis of a ST-fed grid is carried out, showing that the PLL bandwidth is a key factor that determines the system stability. To address the stability issue, a stabilization method based on virtual impedance is proposed and can be seamlessly integrated with the ST voltage control. Simulation and experimental results are provided to validate the effectiveness of the analysis and the proposed control strategy.