Grid-forming (GFM) control has been considered as a promising solution for
accommodating large-scale power electronics converters into modern power grids
thanks to its grid-friendly dynamics, in particular, voltage source behavior on
the AC side. The voltage source behavior of GFM converters can provide voltage
support for the power grid, and therefore enhance the power grid (voltage)
strength. However, grid-following (GFL) converters can also perform constant AC
voltage magnitude control by properly regulating its reactive current, which
may also behave like a voltage source. Currently, it still remains unclear what
are the essential difference between the voltage source behaviors of GFL and
GFM converters, and which type of voltage source behavior can enhance the power
grid strength. In this paper, we will demonstrate that only GFM converters can
provide effective voltage source behavior and enhance the power grid strength
in terms of small signal dynamics. Based on our analysis, we further study the
problem of how to configure GFM converters in the grid and how many GFM
converters we will need. We investigate how the capacity ratio between GFM and
GFL converters affects the equivalent power grid strength and thus the small
signal stability of the system. We give guidelines on how to choose this ratio
to achieve a desired stability margin. We validate our analysis using
high-fidelity simulations