State estimation of observable and unobservable power systems

State estimation is a key Energy Management System (EMS) function, responsible for estimating the best state of the power system. As the power system goes larger and complex in structure, state estimation plays a vital role in system control, which demands accurate estimate of the system state. This paper presents a bird´s eye view on four methodologies namely weighted least squares, decoupled weighted least squares, regularized least squares and decoupled regularized least squares, based on our comprehensive survey of the available literature and the case studies.