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A new filtering technique to eliminate decaying DC and harmonics for power system phasor estimation

During faults, the voltage and current signals available to the relay are affected by the decaying DC component and harmonics. To make proper decisions, most of the relaying algorithms require the fundamental frequency phasor information immune to decaying DC effect and harmonics. Conventional Fourier and LES phasor estimation algorithms are affected by the presence of decaying-exponential transient in the fault signal. This paper presents modified Fourier algorithm, which effectively eliminates the decaying DC component and the harmonics present in the fault signal. The decaying DC parameters are estimated by means of an out-of-band filtering technique. The decaying DC offset and harmonics are removed by means of a simple computational procedure which involves designing of two sets of orthogonal digital DFT filters tuned at different frequencies and by creating three off-line look-up tables. The technique was tested for different decay rates of the decaying DC component. The proposed technique is compared with the conventional mimic plus full cycle DFT algorithm. The results show that the proposed technique has a faster convergence to the desired value compared to the conventional mimic plus DFT algorithms over a wide range of decay rates. In all the cases the convergence to the desired value is achieved within one cycle of the power system frequency