  The progressive displacing of conventional generation in favour of renewable
energy sources requires restoring an adequate capacity of regulating power to
ensure reliable operation of power systems. Battery Energy Storage Systems
(BESSs) are considered to be promising assets to restore suitable frequency
regulation capacity levels. BESSs are typically connected to the grid with
power-converters, able to operate in either grid-forming or grid-following
modes. This paper quantitatively assesses the impact on the local distribution
grid of BESSs providing frequency regulation to bulk power systems. Specific
metrics are proposed to compare the performance of grid-forming and
grid-following control. Experimental results are obtained taking advantage of a
720 kVA/500 kWh BESS connected to the 20 kV distribution grid of the EPFL
campus. The quantitative evaluation based on suitably proposed metrics confirms
the superior performance of the grid-forming strategy, compared to the
grid-following one.
