Equivalent circuits for power-flow studies

NETWORK ANALYZER STUDIES of power-flow conditions on large interconnected systems frequently require preliminary simplification and reduction of the network because of the limited number of analyzer circuits and sources available. Even in smaller systems which do not exceed the capacity of the analyzer being used, it is sometimes desirable to represent in detail only a portion of the system, simplifying the remainder by use of equivalent circuits for the purpose of speeding up studies on the portion of primary interest. Simple equivalent circuits can be devised for extensive power networks by adopting the viewpoint that in power-flow studies the system generation and loads approach, as an approximation, constant-current quantities. The network being simplified may contain numerous loads and generators, but the equivalent circuit derived by this method retains the identity of only the tie-line terminals or points of interconnection with the detailed setup. The line network of the reduced system is replaced by an equivalent mesh network between retained tie-line terminals. If m is the number of such terminals, this mesh network in general will consist of m (m — 1)/2 branches. The effects of the suppressed generators and loads are reproduced by an equivalent generator or load at each tie-line terminal. An equivalent circuit of this type can be derived for a given system either by analytical methods or by simple tests on an analyzer setup of the complete network which is to be reduced. The equivalent circuit is flexible in that any schedule of internal generation and loads can be reproduced readily without the necessity of repeating the reduction or of making additional tests.