Voltage Stability Monitoring, Instability Detection and Control

Voltage stability refers to the ability of a power system to maintain steady voltages at all buses in the system after being subjected to a disturbance from a given initial operating condition [1]. Principal causes of voltage instability are [1-5]: heavy load system operation conditions, long distances between generation and load, low source voltages, and insufficient reactive power compensation. The degree of a system voltage stability (the proximity to voltage instability) is strongly influenced by system conditions and its characteristics such as: generation system characteristics (in particular excitation systems), load characteristics, reactive power compensation devices characteristics, voltage control devices, presence of under load tap changing (ULTC) transformers , etc.[4]. Usually, in a voltage unstable situation, the voltage magnitudes in some system buses undergo, generally monotonic, decrease in the seconds or minutes following a disturbance. When pronounced, this decrease might endanger the system integrity mainly due to protecting devices that trip generation, transmission, or load equipment leading eventually to a blackout in the form of a voltage collapse [2], [3]. Voltage collapse is defined as the process by which the sequence of events accompanying voltage instability leads to a blackout or abnormally low voltages in a significant part of the power system [1]. Voltage instability is considered as a mayor treat for secure power system operation in many power systems throughout the world. Taking advantages of existing technological solutions, such as synchronized phasor measurements, powerful computational facilities, networking infrastructure and communications, in order to monitor stability conditions and control detected instability are of paramount importance. Wide area monitoring, protection, and control systems open perspectives for effective solution of this problem [6]