# Analyzing Six Indices for Online Short-Term Voltage Stability Monitoring in Power Systems
## 1 Introduction
These indices have been used for quantifying the short-term voltage stability and the related risk level to optimal placement of dynamic VAR support against short-term voltage instability [15; 16; 17; 18]. In [19], a practical SVS index based on voltage curves has been developed which gives continuous assessment results on the SVS and can be used in the optimization of the offline preventive control for improving the SVS of power systems. Nowadays, power system operators around the world recommend suggesting new voltage stability indices that can help with improving the overall stability and security in modern power systems. In [15], two voltage severity indices based on NERC/WECC N-1 contingency criteria have been developed. These severity indices utilize PMU data and have been adopted for optimal placement of VAR support in modern power systems. Authors in [17; 18] have defined and used a transient voltage severity index to quantify the transient voltage performance of the system buses following the clearance of disturbances. In [16], the stability prediction has been done using the definition of lower and upper reference boundaries for voltage trajectory and calculation trajectory violation integral. In [12], voltage stability index (VSRI) has been used to assess voltage stability. However, the suggested instruction for VSRI needs time intervals of more than 10 seconds so is not suitable for SVS monitoring. It is clear that the aforementioned voltage stability indices suggested in literature are not suitable for short-term voltage stability monitoring due to technical issues limiting their feasibility. Therefore, a research gap in the field of defining suitable short-term voltage stability indices exists that needs to be fulfilled by either applying suitable modifications to the existing long-term voltage stability indices or by suggesting new indices. The main aim of this research is to solve the above-mentioned research gap by modifying the existing indices and by proposing a novel practical index suitable for SVS monitoring. This paper proposes a novel index for monitoring short-term voltage stability in modern power systems. Likewise, the paper reviews the existing short-term voltage stability indices and compares them with each-other and with the proposed novel index for determining the most suitable and feasible index for implementation in reality. The main aim of this work is to gather and provide appropriate voltage indices which are used in literature for different applications (other than SVS applications) and then to investigate their ability in SVS monitoring and assessment. The paper focuses on the simply computed SVS indices that can be adopted in reality and to avoid any complexities and unpractical techniques. In addition, with applying appropriate modifications and changes, six simple computing voltage indices for SVS monitoring using online measurement PMU data are introduced. Most of these indices are used in literature for reactive power planning and one of them is used as a long-term voltage index, but they have never been introduced for SVS monitoring and assessment. By applying these indices for six scenarios related to SVS phenomena that are obtained from simulations on a 39-bus test system, simple instructions are developed for using these indices as proper indices for SVS monitoring. To compare the reliability and speed of proposed indices, a suitable database is generated by simulations, and indices' abilities in short-term voltage instability detection are obtained and compared with each other. Results show that proposed SVS indices have good performance in SVS monitoring and assessment. The main contributions of this paper are:  
* The proposal of novel data-based short-term voltage stability method for modern power systems. * The ability assessment of reactive power stability compensation and long-term stability indices for SVS monitoring in modern power systems. * Modifying traditional voltage stability indices to be able for assessing SVS in smart grids.