  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:
strong-grid 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 simple single-inverter-infinite-bus systems and a
multi-inverter power network.
