Rethinking Grid-Forming and Grid-Following Inverters: The Duality Theory

Grid-forming and grid-following inverters are recognized as two main types of power electronic converters for integrating renewable energy resources into power systems. They hold certain similarities, but more differences, which makes their relationship quite subtle and ambiguous. In this article, a new perspective called duality is proposed to overcome this problem, which successfully unifies the grid interfacing characteristics and grid synchronization of them in a very symmetric and graceful form. As analyzed, the grid-forming and grid-following inverters are essentially dual to each other, which is further based on the duality of a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltagefollowing current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) controllers: output impedance shaping and output admittance shaping; e) grid strength: stronggrid instability and weak-grid instability; etc. The detailed swing characteristics are also derived in dual form for both inverters, which reveals the dynamic interaction among grid strength, synchronization controllers, and inner-loop controllers, as well as their effects on stability. Typical cases, namely single-inverterinfinite-bus systems and multi-inverter power systems, are studied and simulated to validate the theoretical analysis.