Development of Optimal Power Flow Algorithm for Combined AC-DC System

Optimal power flow (OPF) is to minimize a certain objective function such as generation cost or transmission loss while satisfying various constraints posed from system operations and hence can be a powerful tool for making up reasonable operation plans. Since the advent of DC transmissions in mid 1950's, it has been used as a useful substitute of an ac transmission in cases of long distance transmission, transmission through marine cables, frequency changers, ac-dc parallel transmission, etc. Therefore, it is highly needed to develop an efficient OPF algorithm into which dc transmission systems can be integrated without significant modifications of an OPF program for ac systems. This paper presents an extension of the current Newton OPF program, which directly solves the Kuhn-Tucker conditions and is regarded as most efficient and robust at present, so that the OPF can incorporate dc transmission systems. The control schemes of a dcsystems have been satisfactorily integrated into the developed OPF algorithm. Numerical experiments on a test system have verified the effectiveness of the proposed algorithm.