Control of voltage stability on EHV power systems

Collapses of power systems following severe disturbances leading to voltage stability are occurring with increasing frequency throughout the world. From an analysis of reported incidents, four distinct types of voltage instability have been identified: (1) Classic System Voltage Instability; (2) Transient System Voltage Instability; (3) Slow System Voltage Instability; (4) Radial Voltage Instability. The development of various measures for the control of each of the different types have followed an analysis of reactive power under static and dynamic conditions. Each of the different types of voltage instability has been analysed and a method of long-term dynamic simulation has been developed. The simulation has successfully replicated both the first slow and the second fast phases, that have occurred in reported incidents. This has been possible because substantial networks have been modelled which included all four components of system reactive power: (1) The reactive power component of the loads; (2) The series reactive power losses; (3) The charging of lines, cables and capacitors; (4) The reactive power outputs at the terminal of all rotating units. The simulation of post-disturbance events has uncovered distinctive changes of voltages and rotating unit reactive power outputs. These parameter changes can identify the first post-disturbance phase and actuate timely measures at the most affected locations. The important practical implication is that would avoid the need for complex computer computations in controlling system voltage instability after extremely severe system disturbances. Voltage instability and collapse has been found to occur when rotor overcurrent protection suddenly reduces rotating unit excitation. Rotor heating tests on 500MW and 660MW thermal generators have led to a mathematical model which can replicate the rotor's thermal response. This thermal model could, in future, be used with automatic measures for the control of system voltage stability