Appropriate signal selection for damping multi-modal oscillations using low order controllers

This paper highlights the importance of considering the phase angle of the residues (residue angle) in addition to their magnitude while selecting appropriate feedback signals for damping control design. Residue angle is closely related to the phase compensation requirement at each modal frequency and hence should not differ significantly over adjacent frequencies. This is especially true when the required phase compensation has to be provided by a limited number of lead-lag blocks which cannot change the phase appreciably over a narrow frequency range. Moreover, the signals should be selected such that the residue angles do not vary much over different operating conditions. Use of such phase angle criterion results in a set of signals which yield better performance compared to signals selected based only on the magnitude of residues. This is illustrated through simulation on a test system.