Development of Unbalanced Load Flow Program for Large Power Systems

Abstract

The idea of unbalanced load flow calculation was proposed many years ago. At that time, however, the needs for such techniques was not an urgent one. Modern power system networks are comprised of long untransposed transmission lines. Therefore, for some kinds of analysis, it is now almost impossible to treat a system as though it were a symmetrical network. The aims of most previous studies were oriented to solve voltage/current imbalance in local or small systems, as local imbalance was a serious concern. This is still an important issue, but more recently our needs have become concentrated on practical bulk power systems, since principal EHV lines are entirely untransposed. In this paper, the development of practical unbalanced load program and practical experience with it are reported. This program was developed for steady state analysis of large-scale practical networks under many possible unbalanced condition. The Newton-Raphson method in polar coordinates is used, since fast and moderate convergent characteristics are suitable for solving solutions. The program was applied to practical case studies. The models contain all of the 500 kV and most of the 275 kV overhead transmission lines in TEPCO. Since the density of the Jacobian matrix is higher than in single phase load flows, the number of non zero elements is extremely large. Convergent characteristics are excellent in all cases. The fear of numerical instability has been allayed.