# INTRODUCTION n recent years greater demands have placed on the transmission network, with this increased demands on transmission lines, hence it is the responsibility of the power suppliers to supply safe and economical electric power to customers with the existing transmission line efficiently. "Voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance" [1]. In power system environment voltage stability plays major role, it is integral part of the power system stability. In general Voltage stability problems occur more frequently in a heavily loaded system. The change in voltage is directly proportional to change in load and hence voltage stability is sometimes termed as load stability. Author ? : Asst.professor, Dept. of EEE, Srikalahasteeswara Institute of Technology, Srikalahasti, A.P, India. Mobile: 09618443660, E-mail : rssr305@gmail.com Author ? : Associate Professor & Guide, Dept. of EEE, S V University College of Engineering, Tirupati, A.P, India. Mobile: 09030459483 , Email : rssrskit@gmail.com Voltage stability is a part of power system stability and hence is a subset of overall power system stability and is a dynamic problem. Thus voltage instability and collapse cannot be separated from the general problem of system stability. The reactive power compensation close to the load centres as well as at critical buses in the network is essential for overcoming voltage instability. The location, size and speed of control have to be selected properly to have maximum benefits. The SVC and STATCOM provide fast control and help improve system stability [2]. The suitable location of FACTS devices, under contingencies is more important than consideration of normal state of system. Now a day's many literatures are proposed various intelligent techniques to control FACTS devices in optimal manor for enhancing voltage stability which intern enhanced the power system stability. # II. VOLTAGE STABILITY PHENOMENON IN POWER SYSTEM In recent years, voltage instability has been responsible for several major network collapses in New York, Florida, French, Northern Belgium, Swedish, Japanese, Mississippi, Srilanka, North America, Pakistan and Tokyo etc. [1][3]. # a) Major reasons for voltage stability problems in power system There are some reasons for voltage stability problems in power system as follows It is define as the ability of the power system to maintain stable voltages for large disturbances such as such as system faults, loss of load, or loss of generation. Large disturbance voltage stability may be further subdivided into two types a) Transient stability b) Long term stability ii. Smalldisturbance (Small signal) voltage stability Small disturbance voltage stability is concerned with a system's ability to control voltages following small perturbations, such as gradual change in load, this types of stability can be studied with steady-state approaches that use linearization of the system dynamic equations at a given operating point. # c) Factors Affecting voltage instability and collapse The main factor causing instability is the inability of the power system to meet the demand for reactive power. # i. Transient voltage instability Under low voltage condition the electrical torque of an induction motor is not adequate to meet the required mechanical torque due to this effect the induction motor may not regain the original speed and continue to decelerate leading to stalling of motors which intern aggravates the low voltage problem. This phenomenon is called transient voltage instability. Transient voltage instability is also associated with HVDC links, particularly inverter terminals connected to AC systems with low short circuit capacity [2] [5] [6]. # ii. Long term voltage instability On-load tap-changing transformers and distribution voltage regulations act within a time frame of tens of seconds to tens of minutes to regulate the load a voltage is termed as long term voltage instability. An important factor in long term voltage stability is the current limiting generator [2] [7]. # d) Typical scenario of voltage collapse When a power system is subjected to a sudden increase of reactive power demand following a system contingency, the additional demand is met by the reactive reserves carried by the generators and compensators. Voltage instability may occur in several different ways. In its simple form it can be illustrated by considering the two terminal network of fig. 1 it consists of a constant voltage source (Es) supplying a load (ZLD) through a series impedance (ZLN). This is representative of a simple radial feed to load or a load area served by a large system through a transmission line. The control of voltage levels is accomplished by controlling the production, absorption, and flow of reactive power at all levels in the system. The devices used for this purpose may be classified as follows ii. Series capacitors ? Series capacitors are self-regulating. ? The reactive power supplied by series capacitors is proportional to square of the line current and is independent of the bus voltages. ? This has favourable effect on voltage stability. The present trend is to operate the existing transmission system more close to their stability and thermal limits with reliable and optimal. Power electronics based Flexible AC transmission system (FACTS) gives efficient solution for optimal utilization of transmission systems with minimal installation and operational cost [1][4]. # III. # OVERVIEW OF MAJOR FACTS CONTROLLERS The development of FACTS-devices has started with the growing capability of power electronics components. Devices for high power levels have been made available in converters for higher and even highest voltage levels. Several FACTS have been introduced for various applications worldwide. a) Basic Types of FACTS controllers ? Shunt controllers ? Series controllers ? Combined shunt-series controllers ? Combined series-series controllers The shunt controllers are applied to control voltage at and around the operating point by injecting reactive current. Series controllers are applied to improving voltage profile in a cost effective way ware voltage fluctuations are large. However the series controllers are several times more powerful than the shunt controllers. The combined controllers provide the best of both i.e. an effective power/current flow and line voltage control. FACTS-devices provide a better adaption to varying operational conditions and improve the usage of The ability of FACTS controllers to control the interrelated parameters that govern the operation of transmission systems including series impedance, shunt impedance, current, voltage, phase angle and the damping of oscillations at various frequencies below the rated frequency. These constraints cannot be overcome, while maintaining the required system reliability, by mechanical means without lowering the useable transmission capacity. There are a number of stability issues that limit the transmission capacity these include transient, dynamic, steady state stabilities, frequency collapse , sub synchronous resonance and voltage collapse. The FACTS technology can certainly be used to overcome any of the stability limits. An over-view of problems occurring in the grid and which FACTS to be used to solve these problems are given in the table blow. The application of these devices depends on the problem which has to be solved, below table shows various problems in the grid and which FACTS device to be used to solve these problems [12][16]. # CONCLUSION This paper gives a summary of voltage stability analysis, importance of voltage stability & voltage instability in power system, and various reasons for voltage instability, methods of preventing voltage instability, characteristics of reactive power compensating devices (shunt & Series) and also explains the importance of FACTS controllers in power system environment enhancing voltage stability which intern enhance the power system stability. # Global Journal of Researches in Engineering ![Journals Inc. (US) b) Classifications of voltage stability The voltage stability may be broadly classified in to two categories: -i. Large-disturbance voltage stability](image-2.png "") 1![Fig.1 : A simple radial system for illustration of voltage stability phenomenon. Voltage stability, in facts, depends on the relationships between P, Q and V. The traditional forms displaying these relationships are shown in fig.2.The V R -P R relationship for different values of load power factor. The locus of critical operating points is shown by the dotted line in the fig.Normally; only the operating points above the critical points represent satisfactory operating conditions. sudden reduction in the power factor(increase in QR) can thus cause the system to change from a stable operating condition to an unsatisfactory, and possibly unstable, operating condition represented by the lower part of a V-P curve [1][2][8][9].](image-3.png "Fig. 1 :") 2![Fig.2 : V R -P R characteristics of the system of fig.1 with different load-PF e) Characteristics of voltage collapse ? The initiating event may be due to a variety of decay of voltage. ? The collapse generally manifests itself as a slow decay of voltage. ? The duration of voltage collapse dynamics in some situations may be much shorter, being on the order of a few seconds. ? The time frame of this class of voltage instability is the same as that of rotor angle instability [1][10].](image-4.png "Fig. 2 :") ![existing installations b) Applications of FACTS controllers ? Control of power flow in a transmission line ? Increase the loading capability of line to their thermal limit. ? Control of voltage in a line ? Control of reactive power in a power line ? Improvement of system stability, security & reliability ? power quality improvement in a line ? Provide greater flexibility insisting new generation ? Upgrade of lines ? Reduce reactive power flows ? Increase utilization of lowest cost generation ? Rapid dynamic response ? Ability for frequent variation in output ? Smooth adjustable output ? Minimized transmission losses. Journals Inc. (US) The voltage and stability limits shall be shifted with the means of the several different FACTS-devices shown in below fig.[11][13]-[16].](image-5.png "") 3![Fig. 3 : Operational limits of transmission lines for different voltage levels](image-6.png "Fig. 3 :") 1 © 2012 Global Journals Inc. (US) * Power system stability and control Prabha Kundur EPRI power system Engineering Series McGraw-Hill,inc 1994 * KPadiyar power system dynamics stability and control 2002 2 Edition B S publications * RBindeshwar International journal of engineering science and technology 2 5 2010 John J.Paserba is with Mitsubishi. electric power products * LAzimoh KFolly SChowdhury electrical power &Energy conference 978-1-4244-4509-7/09/s25.00@2009 IEEE * study of voltage collapse converter bus in asynchronous MTDC-AC systems KPadiyar VKalyanramana Int.J.of elc.power and energy system Syst 15 1 Feb 1993 * dynamic analysi of voltage instability in AC-DC systems KRPadiyar SSuresh Rao Int.J.of elc.power and energy system Syst To appear in * Voltage stability analysis using static and dynamic approaches GMorrison BGao PKundur IEEE Trans.on Power systems 8 3 August 1993 * References Références Referencias * Planning against voltage collapse CIGRE Task force 38-02-10 march 1987 Electra * Voltage collapse in EHV power systems WRLachs January 28-February 3 1978 New York paper No.A78057-2, presented at the 1978 IEEE PEs winter meeting * Power system voltage stability CTaylor 1993 McGraw-Hill * Flexible Ac transmission systems(facts)".IEE power series 30. 11. 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