1 An Improved Static Voltage Stability Index using Synchrophasor Measurements for Early Detection of Impending Voltage Instability Ch V V S Bhaskara Reddy, Student Member, IEEE chbr@iitk.ac.in S C Srivastava, Senior Member, IEEE scs@iitk.ac.in Saikat Chakrabarti, Senior Member, IEEE saikatc@iitk.ac.in Department of Electrical Engineering Indian Institute of Technology Kanpur KANPUR, India Abstract— This paper presents the application of an Improved system-wide Synchrophasor based Voltage Instability Monitoring Index (ISVIMI) to effectively detect the onset of static voltage instability. To detect the voltage instability, the method requires only load bus voltage phasors. These can be obtained from Phasor Measurement Units (PMUs), which are being increasingly deployed in the power transmission networks. The effectiveness of the proposed method, for real time detection of the voltage instability has been established on Northern Region Power Grid (NRPG) 246-bus Indian system. Keywords- Voltage stability, Synchrophasor, Phasor measurement unit, Early detection. I. INTRODUCTION OWER system is possibly one of the most complex man- made dynamical systems. Its stability and control have always been challenging tasks. Instability in a power system may be analyzed in different ways depending upon the system configuration, operating conditions and disturbances. With the development of improved control and protective devices like static VAR compensators, generator fast speed governing systems and voltage regulators, transient stability limits of power systems have increased considerably. The improvement in the transient stability limits allow more real power transfer over longer distances. Also due to economical, geographical and environmental reasons, the transmission and generation networks of the power system are being operated close to their maximum loadability limit. These factors have resulted in the increased reactive power demand in the system, leading to the difficulty of voltage control. This has contributed to the increasing number of voltage instability incidents worldwide that had led to the system voltage collapse [1]. Voltage stability can be defined as the ability of a power system to maintain steady acceptable voltages at all the buses in the system after being subjected to a disturbance from a given initial operating condition [2]. The problem of voltage collapse may be caused by the inability of power system to supply the reactive power or by an excessive absorption of reactive power in the system. The nature of loads also plays an important role in deciding the final state of the system. Other factors that strongly influence voltage instability and collapse include transformer On-Load Tap Changer (OLTC) dynamics and generator exciter current limits. To prevent the system from going into the state of voltage instability, it is required to know the closeness of a particular operating point to the point of voltage instability or stability boundary. In most of the incidences of voltage instability, which occurred worldwide, voltage collapse occurred after several minutes of initiation of the disturbance. Hence, most of the studies have considered the voltage stability as a static phenomenon. A number of methods for voltage stability and voltage collapse prediction have been proposed in the literature. In [3], [4], voltage stability analysis using PV and QV curves was proposed and in [4], [5], voltage stability margin prediction using continuation power flow method is discussed. In the above two methods, a particular direction of change in load was assumed. But in practice, the change in load may not follow the assumed linear direction and the actual results may be different from those expected. Further, it is difficult to consider all possible load change directions. Voltage collapse indices based on load flow solution and system Y-bus were discussed in [6], [7], and those based on sensitivity of the load flow Jacobian were discussed in [8], [9]. The methods proposed in [6-9] need load flow solution for every load change and also need topology data to update Y-bus to incorporate the changes in the network configuration. It is difficult to implement the above methods online as they take significant time to execute. With the use of the synchronized phasor measurements using Phasor Measurement Units (PMUs), it has become possible to build Wide Area Monitoring Systems (WAMS) and Wide Area Control Systems (WACS). The phasor measurement based voltage instability monitoring methods can be classified broadly into two categories, a) Local phasor measurement based methods and b) Global phasor measurements based methods. P