Abstract
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.
Highlights
The smart transformer (ST) is a solid-state transformer adopted as intelligent substation with control and communication functionalities
A stabilization method based on the virtual impedance is proposed to address the issue
This paper studies the modeling of a ST-fed grid and the effect of phase-locked loop (PLL)-based synchronization on the system stability
Summary
The smart transformer (ST) is a solid-state transformer adopted as intelligent substation with control and communication functionalities. In [15], the design-oriented transient stability analysis of PLL-synchronized converter has been studied during the grid faults conditions From these previous researches, it is seen that an asymmetric frame model could be obtained since the phase detection is realized through the q-axis channel of the PLL. With appropriate control and parameters, the ST LV converter can stabilize all local converters equipped with PLLs. Comparing to the conventional distribution grids, the reshaping of the equivalent impedance is expected to be an indispensable service provided to all the PLL-synchronized converters. As reported in [13], asymmetrical cross couplings between d- and q-axis of the overall system can be introduced due to the PLL For this reason, a multi-input multi-output (MIMO) dynamic model of the LV ST-fed considering the effect of PLL of the connected converters will be developed in this paper.
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