The continuing geographical expansion and interconnection of power systems over the last century has been motivated by a variety of technical, social and economic factors. For example early drivers included a sense of cultural progress associated with a connected grid. In recent years economies of exchange or opportunities for sale of electricity have been a key motivator for strengthening transmission interconnections or interties between regions. The main technical justifications for expansion of interconnection are threefold: economies of scale, improvement of the load factor, and enhancement of reliability by pooling generation reserves