On the Role of Phasor Measurement Units for Distribution System State Estimation

The Phasor Measurement Units (PMUs) are currently considered among the most useful instruments for future smart grid monitoring, due to their ability to measure with great accuracy both magnitude and phase of voltages or currents synchronized to the Coordinated Universal Time (UTC). In this paper, the role of PMUs and their impact on distribution system state estimation (DSSE) are reviewed and analyzed in a meaningful case study via numerical simulations. The reported results confirm that a limited number of PMUs can considerably improve the accuracy of state estimation based on the Weighted Least Squares (WLS) approach. However, this improvement seems to plateau when more than about 1/3 of grid nodes or branches are instrumented with PMUs, regardless of the placement sequence. When some state variables (i.e. voltage magnitudes and phases) are measured directly, the overall average estimation uncertainty is approximately a linear function of PMU accuracy, with a trend which is quite independent on the number of instruments. A more in-depth analysis of this behavior could be exploited to minimize the overall deployment expenses for a given state estimation uncertainty, once the relationship between PMU unit cost and accuracy is available.