Trends in Voltage Stability – The present day transmission networks are getting more and more stressed due to economic and environmental constraints. The trend is to operate the existing networks optimally close to their loadability limit. This consequently means that the system operation is also near voltage stability limit (nose point) and there is increased possibility of voltage instability and even collapse.
Off-line and on-line techniques of determining state of voltage stability and when it enters the unstable state, provide the tools for system planning and real time control. Energy management system (EMS) provide a variety of measured and computer processed data. This is helpful to system operators in taking critical decisions inter alia reactive power management and control. In this regard automation and specialized software relieve the operator of good part of the burden of system management but it does add to the complexity of the system operation.
Voltage ‘stability analysis and techniques have been pushed forward by several researchers and several of these are in commercial use as outlined in this chapter. As it is still hot topic, considerable research effort is being devoted to it.
Pai et al.  considered an exponential type voltage dependent load model and a new index called condition number for static voltage stability prediction. Eigenvalue analyses has been used to find critical group of buses responsible for voltage collapse. Some researchers have also investigated aspects of bifurcations (local, Hopf, global) and chaos and their implications on power system voltage stability. FACTS devices can be effectively used for controlling the occurrence of dynamic bifurcations and chaos by proper choice of error signal and controller gains.
Tokyo Electric Power Co. has developed a 1.tP-based controller for coordinated control of capacitor bank switching and network transformer tap changing. HVDC power control is used to improve stability.
More systematic approach is still required for optimal siting and sizing of FACTS devices. The availability of FACTS controllers allow operation close to the thermal limit of the lines without jeopardizing security. The reactive power compensation close to the load centres and at the critical buses is essential for overcoming voltage instability. Better and probabilistic load modelling should be tried. It will be worthwhile developing techniques and models for study of non-linear dynamics of large size systems. This may require exploring new methods to obtain network equivalents suitable for the voltage stability analysis. AI is another approach to centralized reactive power and voltage control. An expert system  could assist operators in applying C-banks so that generators operate near upf. The design of suitable protective measures in the event of voltage instability is necessary.
So far, computed PV curves are the most widely used method of estimating voltage security, providing MW margin type indices. Post-disturbance MW or MVAr margins should be translated to predisturbance operating limits that operators can monitor. Both control centre and power plant operators should be trained in the basics of voltage stability. For operator training simulator a real-time dynamic model of the power system that interfaces with EMS controls such as AGC is of great help.
Voltage stability is likely to challenge utility planners and operators for the foreseable future. As load grows and as new transmission and load area generation become increasingly difficult to build, more and more utilities will face the voltage stability challenge. Fortunately, many creative researchers and planners are working on new analysis methods and an innovative solutions to the voltage stability challenge.