By Tim McLaughlin Jan 22 (Reuters) - Winter electricity demand on the largest U.S. power grid - the PJM Interconnection - is expected to set an all-time record next week as large swaths of the United States brace for an Arctic blast of cold weather. PJM's seven-day forecast on Thursday morning predicted 144,465 megawatts of demand on January 27. That would top PJM's all-time winter high of