  In order to protect smart distribution grids from intrusions, it is important
to understand possible risks and impacts of attacks. We study the worst-case
attack strategy of a power injection attack against the physical layer of a
smart distribution grid with a high penetration of photovoltaic resources. We
derive both the worst attack signal and worst attack location: The worst attack
signal is a step function which switches its sign at the final stage, and the
worst attack location is the node with the largest impedance to the grid
substation. Numerical examples on a European benchmark model verify the
developed results. Finally, both theoretical and numerical results are used to
discuss feasible defense strategies against power injection attacks.
