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Abstract

A sub-cycle digital distance relay which uses only the first N/2 samples after a fault to calculate the distance to the fault using a Full Cycle Fourier Algorithm which operates on N samples, where N is the number of samples taken in a cycle of the power signal. The DC offset in the first N/2 samples taken after detection of the fault is cancelled using the N/2+1 sample taken after detection of the fault. The resulting DC adjusted N/2 samples are then multiplied by -1 in order to take advantage of the symmetry of the sine wave of the power signal in estimating the remaining N/2 samples of the first cycle of the power signal after detection of the fault. By so calculating N/2 artificial samples from the first N/2 actual samples taken after the fault, the digital distance relay of the invention can estimate the distance to the fault and thus provide a reliable trip signal in less than one cycle of the power signal. The technique of the invention is used to implement a high-speed tripping mode for use in conjunction with conventional tripping schemes to provide a more reliable tripping signal in a minimum amount of time after the detection of the fault, thereby providing improved fault protection for the power transmission components.

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First Claim