New quick-convergence invariant digital filter for phasor estimation

Digital filters are used, among other purposes, for estimating the harmonic components of a signal. it is important that this estimation should take into account transient periods generated when a change in signal state occurs, e.g. when a fault occurs in an electrical system. Phasor estimation in electrical systems faces several problems that need to be solved. One of these problems is signal distortion occurring under short-circuit conditions. The resulting signal may contain, along with the fundamental component, a number of harmonics and a decaying dc-offset. Also, to allow this estimation to be performed in real time, some algorithms are needed to keel) a proper balance between the accuracy of results and the computational load required for its application. This paper presents the design of a new digital filter, CharmDF (Characteristic Harmonic Digital Filter), incorporating a treatment of samples prior to applying the Discrete Fourier Transform (DFT) filter. The application of the method proposed remains the same in the face of any signal change. Moreover, the convergence of the response provided by the algorithm developed when a change in signal state occurs is significantly improved. For this reason, the method proposed is suitable for use in real time estimations, such as on electrical protection devices. (C) 2008 Elsevier B.V. All rights reserved.