There are some factors making the power system network unbalanced; i.e., UHV transmission lines in which three phase transmission lines are not transposed, unbalanced transformer, unbalanced load as well as sustained unbalanced faults. On the other hand, the number of variable speed generators used in pumping-up power stations is recently increasing in Japan. This paper presents a new method of unbalanced power flow calculation of power system which contains variable speed pumping-up generators. The new method is based on the phase coordinate method, because a power system which has elements with unsymmetrical impedance can be easily analyzed by using the phase coordinate method.
In this paper, a phase coordinate models of the variable speed generators and its secondary exciting circuit composed of GTO converter/inverter are derived first. And then, procedures of power flow calculation of unbalanced power system are presented.