State Estimation on Unbalanced Three-Phase Distribution System Using Hybrid Measurements

Abstract

This paper proposes a method to estimate the magnitude and phase angle of bus voltages including transformer tap position in an unbalanced three-phase power distribution system. The method uses a branch-current state model that sets slack bus voltage and branch currents of the system to be estimated. Estimating transformer tap position, the tap variables were added to the state using state vector augmentation method. The state estimation uses a weighted least squares estimator with Gauss-Newton method. This approach assumed the distribution system to be fully observable with several metering instruments. It is assumed to obtain hybrid measurements such as voltage, current, active power flow, and reactive power flow, from Supervisory Control And Data Acquisition and phasor measurement unit that represent a realistic condition of the modern power system. The proposed branch-current state estimation was verified to be effective in accuracy and convergence on IEEE 13 and 123 test feeder. The method showed high accuracy and fast convergence as the conventional methods, even including estimation of tap ratios. The estimated states can provide information for real-time control and optimal operation of distribution systems.