  Transient frequency dips that follow sudden power imbalances --frequency
Nadir-- represent a big challenge for frequency stability of low-inertia power
systems. Since low inertia is identified as one of the causes for deep
frequency Nadir, virtual inertia, which is provided by energy storage units, is
said to be one of the solutions to the problem. In the present paper, we
propose a new method for frequency control with energy storage systems (ESS),
called dynamic droop control (iDroop), that can completely eliminate frequency
Nadir during transients. Nadir elimination allows us to perform frequency
stability assessment without the need for direct numerical simulations of
system dynamics. We make a direct comparison of our developed strategy with the
usual control approaches --virtual inertia (VI) and droop control (DC)-- and
show that iDroop is more effective than both in eliminating the Nadir. More
precisely, iDroop achieves the Nadir elimination under significantly lower
gains than virtual inertia and requires almost $40\%$ less storage power
capacity to implement the control. Moreover, we show that rather unrealistic
control gains are required for virtual inertia in order to achieve Nadir
elimination.
