State Estimation for Medium- and Low-Voltage Distribution Systems Considering Measurement Characteristics and Grounded Slack Bus
Zhengmei Lu, Wei Yan, Dezhi Huang, Junjie Tang, Ferdinanda Ponci, Antonello Monti
Abstract
The medium voltage (MV) and low voltage (LV) distribution systems are usually built in a mixed three-phase three-wire and three-phase four-wire pattern, in which the conventional measurements are obtained through the two-wattmeter method (TWM) and three-wattmeter method (HWM). The existing three-phase state estimation (SE) models only involve the measurement characteristics of the HWM. Instead, a set of new and improved measurement equations are formulated in this paper for such buses measured by TWM, and the measurements include line-to-line complex power and virtual zero-sequence current. Along with the increasing underground cable, the neutral point of MV feeder head is normally grounded through a small resistance, and its voltage magnitude is not equal to zero. Accordingly, an accurate model for the slack bus together with its measurement equations is proposed herein, based on the conventional measurements and neutral point virtual measurement. Moreover, this work presents a three-phase SE model for MV and LV distribution systems that incorporates the constraints of zero-sequence current and zero injection, in combination with the exponentially weighted least squares method. The test results of the IEEE 34 bus system and IEEE 123 bus system verify that the proposed formulation takes full advantage of the measurements and structural characteristics of practical distribution networks, and simultaneously boosts the estimation accuracy.