Low frequency signal injection for grid impedance estimation in three phase systems

This paper deals with the estimation and decoupling of grid impedance and LCL filter parameters variation using signal injection techniques and Luenberger type observer. When integrating a power converter in the AC grid as an interface for any Distributed Generation Systems (DGS) or other grid quality compensator like Active Power Filters (APF) or STATCOM, inner control loop normally requires current control. Current controller performance is greatly affected by the filter and grid impedance values. Although normally the filter impedance dominates the dynamics of the current controller, in weak networks the impedance of the grid can not be neglected. Additionally, other often required functions, as islanding detection, also rely on the estimation of the grid impedance. For this paper, a Luenberger based observer is proposed for controlling the grid current when a LCL filter is used. The proposed method will rely on measuring the converter side current and the grid voltage and will cope with parameter variation at the filter transfer function. For variations at the grid impedance, the control action deliver by the observer feedback path will be used for triggering an injection mechanism. A Low Frequency Signal Injection (LFSI) approach is proposed for online estimating the grid impedance using an RLS algorithm. The proposed estimation technique is well suited to be incorporated into an adaptive current controller scheme.