Iterative Distribution System State Estimation for Integrated Primary and Split-Phase Secondary Feeder Monitoring

The observability of split-phase low-voltage (LV) secondary distribution feeders continues to pose a challenge in distribution system state estimation, which has traditionally been modeled for medium-voltage (MV) primary feeders. This work aims to improve the observability of LV feeders using smart meter data to enable a joint primary (MV) and secondary (LV) feeder state estimation. Towards this goal, a two-stage MV-LV state estimation algorithm is proposed that is iteratively executed until convergence at the MV-LV boundary. Specifically, Stage-I estimates primary feeder (MV) states using approximated load demands at the MV buses obtained from Stage-II. Stage-II estimates the secondary feeder (LV) states and improved load demands at the MV level using the MV voltage estimates obtained from Stage-I and smart meter data. The algorithm terminates once the sequence of estimated variables from both stages at the MV-LV boundary bus, i.e., load demand (from Stage-II) and voltage (from Stage-I) have converged. The proposed algorithm is demonstrated using the IEEE 123 bus distribution test feeder modified to include secondary network extensions at 47 different nodes. The simulation results validate that the proposed approach facilitates the observability of the split-phase LV systems and enables the joint estimation of MV-LV states. Moreover, the accuracy of the primary feeder (MV) state estimation results is improved using the proposed approach to model and estimate split-phase LV feeders.