Impact of composite loads on dynamic loadability of emerging distribution systems

Integration of renewable energy resources has introduced a number of dynamic stability issues in distribution systems. One of the major issues is the dynamic loadability limit, which is caused by a pair of eigenvalues crossing the imaginary axis of a complex plane. Due to the influx of distributed generators and controllers, a possible negative interaction can limit the dynamic loadability of emerging distribution networks. Similar to generators, loads can also actively participate in system dynamics which may sometimes lead to instability. The load contribution depends on their type and composition. A suitable load model should be used for accurate stability assessment to reflect the real impact of load dynamics. In this paper, contribution of a load model to the dynamic loadability of emerging distribution system is examined in detail. Tracking of Hopf bifurcation phenomenon has been used for identification of the critical modes. Indices based on extended eigenvectors are utilized for load ranking. The approach is verified by comparing the results of composite load modeling of high and low ranked loads. The results of composite load modeling of multiple buses with varying composition of static and dynamic load are also presented. The results suggest that dynamic loads deteriorate the dynamic loadability of distribution systems.