Love Field Airport Modernization Corp., the local government entity that oversees the development of the Dallas, Texas home of Southwest plans to sell $310 million of tax-exempt debt today to help finance an expansion project. The bonds are rated BBB by Standard & Poor’s, second-lowest of 10 investment grades, and are backed by “facilities payments” from discount airline Southwest Airlines Co. Underwriters led by Goldman Sachs Group Inc. will market the issue to investors.