  We formulate an interpretation of the theory of physical superselection
sectors in terms of vertex operator algebra language. Using this formulation we
give a construction of simple current from a primary semisimple element of
weight one. We then prove that if a rational vertex operator algebra $V$ has a
simple current $M$ satisfying certain conditions, then $V\oplus M$ has a
natural rational vertex operator (super)algebra structure. Applying our results
to a vertex operator algebra associated to an affine Lie algebra, we construct
its simple currents and the extension by a simple current. We also present two
essentially equivalent constructions for twisted modules for an inner
automorphism from the adjoint module or any untwisted module.
