The record for the portion of total electric supply provided by wind keeps rising. We are now just learning that wind provided 60.5% of the electricity consumed for a one-hour period by the customers of a Colorado utility--XCEL. It happened on May 24, 2013 from 1 to 2 a.m, and the lights stayed on!
Wind penetration records are set by a combination of strong winds and low-system demand. Spring and Fall seasons are when the conditions are most likely for the setting of wind penetration records.
Finally, as the wind penetration record set by XCEL shows, grid systems can reliably operate with large percentages and quantities of wind.