State estimation in smart power grids
Date of Issue2016
School of Electrical and Electronic Engineering
State Estimation (SE), first proposed by F. C. Schweppe in 1970s, is an essential module in Energy Management System (EMS). The phasor voltage of each bus in the system is estimated based on the measurements, such as power flows, power injection, voltage magnitudes and etc. The estimated results are later utilized to identify bad measurements/inconsistent topology and evaluate the system original condition. It can be claimed that power system State Estimator constitutes the core module for the on-line security analysis. Smart power grids refer to the modern power systems with the advanced technologies in communication, system control and operation. In contrast to the traditional electric power systems, the smart power grids are advantageous by equipping with computer-based remote control and automation, intelligent devices supported by two-way communication technologies, active consumers and renewable energy resources. High efficiency of energy utilization can be expected in smart power grids. Traditional monitoring and control techniques over power systems will not be suitable for the smart power grids, both in the transmission and the distribution levels. In this thesis, conventional SE methods are improved to cater for the needs of smart power grids. Phasor Measurement Units (PMUs) are firstly introduced and their application in SE is reviewed. Prior to utilizing PMU measurements or synchrophasor measurements in SE, the placement strategy on PMUs is studied over various test systems. Case study results verify that the measurement redundancy can be improved via the proposed techniques. Next, SE in smart transmission systems is conducted. In this work, the adaptive underfrequency load shedding (UFLS) procedure is considered as the pseudo measurements to improve the SE results. By correcting the power injection measurements at the unobservable buses where load shedding takes place, it enables the State Estimator to track the system states more accurately. Case studies on the IEEE test systems are conducted to verify the results. For SE in smart distribution systems, the study has been carried out on the newly emerging hybrid AC/DC microgrids. A decentralized technique is proposed for the estimation by modelling the AC and DC microgrids separately and utilizing the power flow measurements obtained from the connecting line of the two networks as an equality constraint. By virtue of PMUs, SE in the AC network is formulated in a three-phase linear manner and the unbalanced problem can be investigated. SE in the DC network is built in a linear manner as well, based on the voltage-current relationship. The decentralized SE can perform comparatively with the centralized technique regarding the accuracy, computational speed and bad data detection, while the shared measurement information is reduced and data privacy can thus be improved.