Very in-depth article on why power from a wind turbine farm in northern Vermont had to be curtailed (ie, its output deliberately reduced) to ensure stability of the local grid during a recent heat wave. Transmission line inadequacies and not having all the needed interconnection equipment were to two causes. FYI, it starts out from the perspective of Vermonters who wonder why these turbines were not used but diesel-powered generators were, then ends with a clear explanation from ISO NE.
I recommend reading this piece if you are involved in utility-scale renewable energy projects anywhere.
PS–smaller renewable energy projects (like PV panels on homes or businesses) on their own do not usually cause these types of grid stability problems. Another reason to push for more distributed energy in our future electrical system.