Modern protection relays require accurate and fast phasor estimation under harsh transient conditions, including a decaying DC component, harmonics, interharmonics, noise, and frequency instability. The original CCHDF (Cleaned Characteristic Harmonic Digital Filter) produced a harmonic cleaned signal using the Biunivocal Frequency Relationship of Phasors (BFRP) technique, but relied on DFT, Hanning windowing, and peak detection to identify interharmonic components. This paper replaces that spectral estimation block with the Modified Covariance Method (MCM) estimator, a high resolution autoregressive (AR) spectral estimator capable of superior frequency, magnitude, and phase estimation of non-harmonic components even with a short data window. The result is an improved filter named MCCCHDF (Modified Covariance CCHDF), preserving the original algorithmic pipeline, but achieving higher accuracy and faster convergence in the presence of closely spaced harmonics/interharmonics and noisy decaying DC conditions.
Alwan et al. (Thu,) studied this question.