Revisiting Grid-Forming and Grid-Following Inverters: A Duality Theory

Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters.They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure.In this article, a new perspective based on duality is proposed to create new insights.It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form.Analysis shows that the grid-forming and grid-following inverters are duals of each other in several ways including a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltage-following current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) inner-loop controllers: output impedance shaping and output admittance shaping; and e) grid strength compatibility: stronggrid instability and weak-grid instability.The swing equations are also derived in dual form, which reveal the dynamic interaction between the grid strength, the synchronization controllers, and the inner-loop controllers.Insights are generated into cases of poor stability in both small-signal and transient/large-signal.The theoretical analysis and simulation results are used to illustrate cases for single-inverter systems, two-inverter systems, and multiinverter networks.