Dominant mode filter for enhancing power system stability

This paper proposes a new type of filter, that is, a dominant mode filter (DMF) which uses the rotor speed fluctuations for dynamic security assessment and stability enhancement of power systems. The proposed filter can be used in detecting the degradation of damping power of the system and in improving the stabilizing ability of a control device such as PSS. The basic concept of the DMF design is based on the inherent characteristics of the power system network and the application of principal component analysis to a covariance matrix of the rotor speed fluctuations. The coefficients of the DMF are easily computed from the eigenvector of the covariance matrix. The weak damping modes which are useful for generating the control signals of the stabilizing devices, are satisfactorily estimated from the rotor speed fluctuations by means of the DMF. In the computing process of the DMF coefficients the 'mode energy' and 'mode profile' are also obtained from the eigenvalue and the eigenvector of the covariance matrix respectively. The mode energy is the strength of each mode contained in the fluctuations. The mode profile expresses the distribution of the mode oscillation with the rotor speed responses of all machines. Since, in the DMF design, the on-line fluctuation data are used, the weak damping modes, the mode energy and the mode profile can be estimated adaptively according to the system situation. In this paper, applications of the DMF are also discussed. One application is to use the mode energy as an on-line stability index. If the damping power of the system becomes weak, a system operator can detect signs of an unstable condition by observing the time variation of the mode energy. Another is a disturbance identifier to find the location and severity of a disturbance. This identifier utilizes the mode profile and mode energy. The third application of the DMF is to supply the slower mode component as a global control signal for HPSS (Hybrid PSS)¹¹. The mode energy can be also utilized for adapting the parameters of the HPSS.