  As the share of renewable sources is increasing the need for multiple storage
units appropriately sized and located is essential to achieve better inertial
response. This work focuses on the question of "how to distribute constant
number of storage units in the gird under transient events such that the
inertial response of the maximum frequency deviation is minimized?". To answer
this question, we provide a comprehensive modeling framework for energy storage
units placement and size for frequency stability under spatial effects. The
distributed storage units are modeled as grid supporting inverters and the
total storage capacity in the grid is bounded based on the allowed steady-state
frequency deviation after disturbances. The problem of finding the optimal
distributions can be considered as combinatorial problem which consists of high
dimensional solutions. In this light, we develop two numeric approaches based
on Brute-force search and adaptation of the Cross-entropy method for finding
the best distribution and examined it on a case study of the future Israeli
grid. The results on the case study provide a new insight---the storage units
should be placed around the area of the disturbances, including in sites with
high inertia in accordance to the network topology.
