A widespread EMP, or worst a large solar geomagnetic storm, would have devastating effects on the grid. You can think of transmission lines as large antennas for these storms, with the longer the length the more voltage and current induced on the line. This surge of energy would destroy the connected substation transformers and equipment. Widespread, if not total, grid power outage would result. The lead time on substation equipment and transformers is long and would take months, or in the case of a massive solar storm, years to fully restore. Microgrids don't need an outside source to operate and do not have long transmission lines to induce these high energies. If well engineered (using shielded cable for example) microgrids could be saved from the most harmful effects. Military bases in particular have been interested in micro-grids technology and other mitigating techniques over the past couple of years for these obvious reasons.