The cold weather and swirling winds gripping the northeastern United States have created the sort of winter scenario that Energy Secretary Rick Perry has cited as a reason to bolster the reliability of the grid by boosting coal and nuclear power plants. Perry said that only those power plants could assure reliability because only they could keep 90 days’ fuel supply on site. But so far in this windy two-week cold snap, the region’s electricity grid has responded with little disruption, and without any need to rev up aging coal plants, which supplied 6 percent of electricity in New England on Thursday. And the biggest failure Thursday came from a power line failure that forced Entergy Corp. to shut down its 688 megawatt Pilgrim nuclear power plant in eastern Massachusetts. No homes were affected, however, because the grid reserve was three times as big.