Voltage stability of radial power links

The general phenomenon of voltage collapse or instability at the load end of transmission links is introduced with reference to previous work, and the induction motor is seen to be the critical constituent of system loads. The power/voltage and reactive-power/voltage characteristics of power-system loads are predicted and accurately represented by polynomial expressions which are used in the analysis of radial transmission links fed from infinite busbars. Graphs summarising the critical load voltages and the corresponding short-circuit levels for various transformer tap ratios are given and are general in application; these are based on the dV/dZ criterion for voltage stability. It is shown that, although tap changing to raise the load voltage increases the critical length of the link, it also reduces the margin between normal operating voltage and the voltage at the onset of instability. The effects of the injection of reactive power by static capacitors are discussed, together with the effect of the induction-motor content of the loads. The use of the dEs/dVr criterion as advocated in the Russian literature is discussed.