  In this work, fundamental control limitations for rotor angle stability are
considered. Limitations are identified by characterizing open-loop transfer
function zeros for input-output combinations of certain power system
configurations. Of particular interest are non-minimum phase (NMP) zeros that
limit the achievable performance of the closed-loop system. By studying a
single-machine infinite bus power system model, analytic conditions for the
presence of NMP zeros are derived. They are shown to be closely linked to the
destabilizing effect of automatic voltage regulators (AVRs). Depending on the
control loop, it is found that NMP zeros may persist in the system even if the
closed-loop system is stabilized through feedback control. A simulation study
shows that NMP zeros introduced by AVR limit the achievable performance and
stabilization using feedback control.
