Phasor estimation, as performed by the discrete Fourier transform (DFT), is adversely affected by the decaying dc offset which occurs following a fault. This letter presents an improved algorithm based on the DFT, hereafter named IDFT, which effectively mitigates the decaying dc component. The IDFT uses the even and odd samples of the estimated phasor. The method is based on the fact that, in the phasor domain, the even and odd samples in a purely sinusoidal signal produce the same results for the amplitude estimation. However, in an exponential signal, especially at the beginning of the fault due to a higher slope, they are different. The IDFT technique is compared with the conventional full-cycle DFT algorithm. The results show that IDFT is more accurate and has faster convergence than the desired value compared to the conventional DFT algorithm.Index Terms-Decaying DC offset, discrete Fourier transform (DFT), phasor estimation.

This paper presents a fault classification technique for transmission lines based on the fault sequence components, for fast and reliable operation of protective relays. First, symmetrical components of fault current and voltage signals are extracted. Next, the fault type is classified using the zero and negative sequences. To realize the faulted phases in ground faults, a criterion index based on the zero and negative sequences is defined. The imaginary part of the defined criterion index is used. This index is maximum in the faulted phase in single-phase-to-ground faults, and it is minimum in one of the faulted phases in phase-to-phaseto-ground faults. In addition, to identify ungrounded faults, a different criterion index using the positive and negative sequences is defined. The results show that the scheme provides a fast fault classification, without the need for a threshold to operate.Index Terms-Fault detection, protective relay, symmetrical components. 0093-9994

No abstract

SUMMARY This paper presents a novel algorithm for power transformer differential protection which differentiates internal faults from magnetizing inrush currents and saturation of current transformers using wavelet transform. The technique is based on pattern recognition of the instantaneous differential currents using the high‐frequency components generated during the disturbance. These components are extracted using a wavelet‐based processor stage. In this method, an appropriate criterion at a suitable frequency range is developed. Using the developed criterion for three phases, internal faults are correctly detected in about 1/8th cycle after occurrence of the disturbance. The proposed algorithm is evaluated using generated data from simulation of a model power system using Electromagnetic Transients Program software. The performance of the proposed algorithm is tested in many different cases and promising results are obtained. Copyright © 2011 John Wiley & Sons, Ltd.

No abstract