STUDY ON PV-PQ BUS TYPE SWITCHING LOGIC IN POWER FLOW COMPUTATION

A comprehensive investigation on bus PV-PQ type switching logic in power flow computation is carried out in this paper. Besides the power mismatches blow up, the frequent switching of some bus type is another kind of divergence to suffer. To remove this type of oscillation occurring on some buses in a power flow calculation, several complementary logics were used in references, but this kind of method may sometimes make the power flow converging to an unstable solution. Researches done by this paper show that failure in bus type identification may result in a kind of voltage instability corresponding to the limit induced bifurcation. Some numerical simulations on IEEE 118 bus test system and a practical power system have been carried out and the results support to explain this phenomenon. Based on a theoretical analysis, a set of inequality constraints describing stability conditions are defined as the necessary condition for a stable power flow solution. This paper gives an example to show that some wrong stability margins are obtained when an incorrect bus PV-PQ type switching logic is used in Continuation Power Flow computation for static voltage stability assessment.