Westinghouse Electric Corporation

Abb Power Td & Company

Kung C Chow

Shan C Sun

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.

Sub-cycle digital distance relay

A predetermined unsymmetry of a bipolar waveform is detected by apparatus which includes a circuit which generates a pulse for each zero crossing of the waveform. Each zero crossing pulse triggers a first retriggerable one-shot for a first interval T.sub.1. Timing out of the first one-shot triggers a second retriggerable one-shot for a second interval T.sub.2. If both the first and second one-shots have timed out a third one-shot is triggered for a third interval T.sub.3. The intervals are selected such that twice T.sub.1 plus T.sub.2 is equal to the period of one cycle of the bipolar waverform so that triggering of the third one-shot is an indication of a predetermined unsymmetry determined by relative values of T.sub.1 and T.sub.2 of the bipolar waveform. T.sub.3 is selected such that two symmetrical cycles of the bipolar waveform are required to clear the unsymmetry indication. The apparatus is applied to blocking a false trip signal from a reverse current relay in response to the in-rush current when a transformer is energized with little or no load.

Apparatus for detecting unsymmetrical bipolar waveforms

An electronic protective relay which offers a very sensitive detection of a reverse power condition of a three-phase alternating current generator for the protection against generator motoring is disclosed. A current signal is generated from a current transformer coupled to one phase of the ac generator; and a voltage signal is generated from a potential transformer coupled across the one phase and another phase of the generator. A current input circuit is operative to maintain the phase angle between the voltage and current signals over a wide variation of input current, e.g. from one milliamp to 75 amperes. In addition, a phase-rectification type multiplier affords a relatively simple, but effective method for detecting the power flow direction of the generator. Once the power flow is determined to be in the reverse direction and above a preselected magnitude, a signal indicative of a potential generator motoring condition is geneated. Also included in the relay is an undervoltage detection circuit which operates to disable the generator motoring indication if the generator's voltage has not yet reached or has dropped below an adjustable undervoltage setting. Still further, the relay circuit includes timers in both the undervoltage and reverse power detection areas to insure that their respective conditions persist for a preset time duration in each case before an action is initiated.

Electronic reverse power relay

A solid state overcurrent relay which receives both an information signal and operating power from a single current transformer. A power supply, which couples the current transformer and relay, is constructed to present a constant impedance load on the current transformer, enabling the current transformer to simultaneously provide an accurate information signal.

Overcurrent relay

A filter, energized by a polyphase power source, is adapted to detect positive or negative sequence components present in the source. Means are provided for summing a first phase quantity with a second phase quantity which has been shifted by 180 electrical degrees. Additional means are utilized to phase shift the summed resultant itself by 120 electrical degrees and to sum the phase shifted resultant with the phase shifted second quantity and a third quantity. The second summation yields an output quantity proportional to only one of the sequence components, if any, present in the polyphase source.

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Coral Springs, FL, United States of America

Kung C Chow