Multivariable DG Impedance Modeling and Reshaping for Impedance Stabilization of Active Distribution Networks

Stability of inverter-based distributed generation units (DGs) is highly dependent on the grid impedance. This paper proposes an adaptive control algorithm for optimally reshaping DG output impedance so that the system dynamic performance is improved. Such adaptation is essential for managing variations in grid impedance and changes in DG operating conditions. The proposed algorithm is generic so that it can be applied for both grid-connected and islanded DGs. It involves mainly two design stages. First, the multivariable DG output impedance is derived mathematically and verified using a frequency sweep identification method. The grid impedance is also estimated using the refined instrumental variable for continuous-time system identification algorithm so that the impedance stability criteria can be formulated. In the second stage, multi-objective programming is formulated using the ϵ-constraint method in order to minimize the system response time while obtaining a robust damping performance. The proposed algorithm has been validated in both grid-connected and isolated distribution networks, with the use of OPAL real-time simulators via a hardware-in-the-loop application.