Evaluation of Discrete Wavelet Transform implementation for protective relaying

This paper critically assesses the performance of phasor estimation using the discrete wavelet transform (DWT) implemented in a digital relay and compares it to the discrete Fourier transform (DFT). With regards to convergence, using the full cycle implementation the DWT´s initial response was faster but reached the new value at the same time as the DFT. Using the half cycle implementation the effect is more pronounced with higher levels of recursion of the DWT showing a further improvement of its performance. With regards to the decaying DC offset, using the full cycle implementation the DWT was largely affected by the DC offset, whereas the DFT was able to filter the majority of it out and give an accurate phasor estimation. With the half cycle implementation both did poorly with much oscillation around the nominal value. A method for removing the decaying DC offset prior to the phasor estimation algorithm would be required.