Implementation of Real-Time Impedance-Based Stability Assessment of Grid-Connected Systems Using MIMO-Identification Techniques

Grid impedance has a major effect on the operation of inverter-connected systems, such as renewable energy sources. Stability of such system depends on the ratio of the inverter output impedance and the grid impedance at the point of common coupling. Because the grid impedance varies over time with many parameters, online grid-impedance measurement acquired in real time is most preferred method for observing the stability. Recent studies have presented methods based on multiple-input-multiple-output (MIMO) identification techniques, where the stability of grid-connected system is rapidly assessed in the dq domain. In the methods, orthogonal injections are used with Fourier techniques, and the grid impedance d and q components are measured. The Nyquist stability criterion is then applied to assess the stability. This paper extends previous studies, and presents a real-time implementation for the online stability analysis using MIMO-identification methods. The practical implementation is discussed in detail and experimental results based on a grid-connected three-phase inverter are provided to demonstrate the effectiveness of the methods.