## 1 Introduction  
Voltage instability has been regarded as one of the primary threats to the security of modern power network operation during the past few decades. Power system disturbances such as a continuous load increase and/or a major change in network topology can result in voltage collapse. The voltage collapse problem, which is characterized by the loss of voltage magnitude at certain locations of the power grid, has caused several severe blackout events worldwide [1; 2].  
A number of planning and operation technologies have been proposed to mitigate the risk of voltage collapse [3]. Among these technologies, phasor measurement unit (PMU) based schemes to secure power systems have become one of the enabling techniques which are under active investigations. Indeed, the use of PMUs in modern power systems becomes popular [4; 5].  
In order to provide a better voltage support in transmission networks, the coordinated voltage control has been developed. It is organized as a hierarchical structure with three levels: the primary, secondary, and tertiary voltage control. Significant attention has been given to the study of the secondary level, which is an automatic regulation of voltage and reactive power for power systems [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. The task of the secondary voltage control (SVC) is to regulate system voltage profile and to protect against potential voltage instability scenario at an early stage. The conventional approaches use only the voltage information at the observed buses (the so-called pilot nodes) as a triggering signal,but just monitoring voltage magnitudes alone may give the wrong indication in static voltage stability study [20]. In other words, voltage stability issues cannot be fully prevented by the traditional SVC methods; therefore, an additional accepted measure of static voltage stability is required for a more reliable SVC scheme.  
The voltage stability margin (VSM), which is defined to be the distance between the given operating point and the voltage-collapse point, can serve as the static voltage stability indicator. A wide variety of approaches have been proposed for static VSM evaluation [21; 22; 23; 24; 25; 26], wherein the measurement-based methods [25; 26] address such problem by using the impedance-matching concept, _i.e._, at the maximum loading condition, load impedance is equal to Thevenin equivalent impedance in magnitude. Due to the elegance and simplicity of the measurement-based method, it becomes an attractive alternative in static voltage stability monitoring. In this paper, a VSM index computed from synchrophasor data is utilized.  
This paper is concerned with designing an optimal voltage control scheme using synchrophasor measurements. In the proposed technique, the voltage magnitude and static voltage stability margin are considered as two key criteria for activating the proposed control strategy; therefore, a more efficient secondary voltage control can be accomplished. The rest of the paper is organized as follows: In Section 2, the proposed scheme based on the PMU technique is presented. Section 3 describes the fundamental theories and mathematical principles of the proposed scheme. Numerical simulations and test results are given and discussed in Section 4. Section 5 concludes the paper.