Voltage stability as affected by static load characteristics

Voltage stability is analysed as a dynamic phenomenon associated with the excitation field dynamics of synchronous machines. It is shown that, for constant impedance loads, a single machine powersystem is voltage stable for all reasonable operating conditions. For constant power loads, however, a simple system has a voltage controllability limit that can be close to nominal voltage. The voltage stability region as a function of real and reactive load dependence on voltage is calculated for a single machine system with and without automatic voltage regulation. The analysis is then extended to the multimachine case, for which approximate voltage stability criteria are developed using the extended linearisation coefficient matrices. The linearised model analysis is found to be absolutely consistent with the results obtained by nonlinear simulation of a multimachine power system