State Estimation of the Transmission System in the Presence of Unbalanced Load at the Transmission Distribution Boundary

The conventional transmission system state estimation assumes that all loads are balanced at the transmission distribution boundary. However, the loading of a distribution feeder is inherently unbalanced. The unbalanced loading can further increase with the inclusion of small-scale single-phase renewable energy sources in a large number in the modern distribution grid. This may result in an unbalanced load at the transmission distribution boundary, which can cause voltage variation at the boundary and adjacent nodes inside the transmission system. The traditional transmission system state estimation cannot deal with the unbalanced nodes. In this context, this paper proposes a method that can detect and identify unbalanced nodes of the transmission system. Furthermore, the proposed method quickly updates the states of the unbalanced nodes by using a quasi-Newton approach. The proposed approach has been successfully tested on a combination of IEEE transmission and distribution systems.