REAL-TIME ASSESSMENT OF SYSTEM VOLTAGE STABILITY

Not only are existing protections unable to prevent system voltage instability and maintain the integrity of the transmission grid, but the actions of transmission line or rotor overcurrent protections actually trigger the final collapse phase. Much effort is being devoted to computer modelling the power system by a central Energy Management System for controllling system voltage stability. In an effort to achieve realtime simplifying assumptions are made, but it is shown that even the assumed load-voltage models are incorrect for the slow first phase because of the influence automatic transformer tap changing an the reactive power demand. A much simpler approach uses voltage and reactive power output changes to both identify and guide countermeasures at affected loacations. Not only does this avoid the need for error producing assumptions but it offers a real-time response, as has been shown by simulation on a 92 bus transmission network. A significant improvement is possible by monitoring rotor current and voltage on each rotating unit to immediately assess if the disturbance could lead to excessive rotor heating. This intelligence can be used to provide a better timing of the measures, such as strategic load shedding, and so ensure the integrity of the transmission grid.