  Power grids are nowadays experiencing a transformation due to the
introduction of Distributed Generation based on Renewable Sources. At
difference with classical Distributed Generation, where local power sources
mitigate anomalous user consumption peaks, Renewable Sources introduce in the
grid intrinsically erratic power inputs. By introducing a simple schematic (but
realistic) model for power grids with stochastic distributed generation, we
study the effects of erratic sources on the robustness of several IEEE power
grid test networks with up to 2000 buses. We find that increasing the
penetration of erratic sources causes the grid to fail with a sharp transition.
We compare such results with the case of failures caused by the natural
increasing power demand.
