mu-synthesis-based Generalized Robust Framework for Grid-following and Grid-forming Inverters

Abstract: Grid-following and grid-forming inverters are integral components of microgrids and for integration of renewable energy sources with the grid. For grid following inverters, which need to emulate controllable current sources, a significant challenge is to address the large uncertainty of the grid impedance. For grid forming inverters, which need to emulate a controllable voltage source, large uncertainty due to varying loads has to be addressed. In this article, a mu-synthesis-based robust control design methodology, where performance under quantified uncertainty is guaranteed, is developed under a unified approach for both grid-following and grid-forming inverters. The control objectives, while designing the proposed optimal controllers, are: i) reference tracking, disturbance rejection, harmonic compensation capability with sufficient LCL resonance damping under large variations of grid impedance uncertainty for grid-following inverters; ii) reference tracking, disturbance rejection, harmonic compensation capability with enhanced dynamic response under large variations of equivalent loading uncertainty for grid-forming inverters. A combined system-in-the-loop (SIL), controller hardware-in-the-loop (CHIL) and power hardware-in-the-loop (PHIL) based experimental validation on 10 kVA microgrid system with two physical inverter systems is conducted in order to evaluate the efficacy and viability of the proposed controllers.