Abstract: An accurate slip calculation for providing monitoring and protection to an electric motor. The slip calculation is made using a minimum value of stator resistance as the initial stator resistance, where the minimum value of stator resistance is the minimum value of stator resistance calculated during an initiation period of the motor. The initiation period may be a predetermined time period or a predetermined number of cycles during the motor startup. The initiation period may start after a predetermined settling time or after a predetermined condition is met.

Abstract: An accurate slip calculation for providing monitoring and protection to an electric motor. The slip calculation is made using a minimum value of stator resistance as the initial stator resistance, where the minimum value of stator resistance is the minimum value of stator resistance calculated during an initiation period of the motor. The initiation period may be a predetermined time period or a predetermined number of cycles during the motor startup. The initiation period may start after a predetermined settling time or after a predetermined condition is met.

Abstract: The protective relay for an induction motor includes a first program function which establishes a first thermal threshold value for a start condition of the motor, using a start condition thermal model. The thermal condition of the motor is determined by measuring the motor's thermal response to stimulus. A comparator compares the start condition thermal representation with the first thermal threshold value and monitors whether the first thermal threshold value is exceeded by the start condition thermal representation. A second thermal threshold value is established for a run condition of the motor, including a selected time constant which results in the time-current curves of the start and run conditions being substantially continuous. A representation of the thermal condition of the motor is then developed in response to stimulus current. A comparator then compares the run condition thermal representation with the second thermal threshold value.

Abstract: A system used in a protective relay for motors for transitioning the value of the trip threshold to a motor start (stall) condition. The calculated transition trip threshold value follows I2t, specifically, the current to the motor. The transition trip threshold value includes a small fixed initial (offset) value, so that the increase in the trip threshold during transition to the actual start condition threshold value is always slightly ahead of the increase in the I2t value, preventing a trip until the input current exceeds the final trip threshold for the start condition.

Abstract: A system used in a protective relay for motors for transitioning the value of the trip threshold to a motor start (stall) condition. The calculated transition trip threshold value follows I2t, specifically, the current to the motor. The transition trip threshold value includes a small fixed initial (offset) value, so that the increase in the trip threshold during transition to the actual start condition threshold value is always slightly ahead of the increase in the I2t value, preventing a trip until the input current exceeds the final trip threshold for the start condition.

Abstract: The protective relay for an induction motor includes a first program function which establishes a first thermal threshold value for a start condition of the motor, using a start condition thermal model. The thermal condition of the motor is determined by means of a thermal model representation in response to current provided to the motor. A comparing element then compares the start condition thermal representation with the first thermal threshold value and produces an output signal when the first thermal threshold value is exceeded by the start condition thermal representation. A second thermal threshold value is established for a run condition of the motor, the second threshold value being different from the first threshold value and including a selected time constant which results in the time-current curves of the start and run conditions being substantially continuous.

Abstract: The instantaneous overcurrent element, used in a microprocessor-based protective relay for a power system, includes a finite impulse response filter which generally is a cosine filter and is responsive to the current waveform from the current transformer for fault determination unless the distortion in the current reaches a preselected threshold, at which point a peak detector circuit is used to provide the current magnitude values for fault determination.

Abstract: The instantaneous overcurrent element, used in a microprocessor-based protective relay for a power system, includes a finite impulse response filter which generally is a cosine filter and is responsive to the current waveform from the current transformer for fault determination unless the distortion in the current reaches a preselected threshold, at which point a peak detector circuit is used to provide the current magnitude values for fault determination.

Abstract: Current transformer (CT) secondary currents for the windings of a power transformer are used to produce a differential current. An operating current value is obtained from the differential current value. A restraining current value is obtained from processed winding current values. Second and fourth harmonic values of the differential current are obtained and are summed with a restraining current quantity which is a result of the restraining current multiplied by a slope characteristic factor. If the operating current value is greater than the sum of the restraining current quantity and the second and fourth harmonic values, an output signal is produced which may be used as a trip signal unless it is blocked by selected blocking signals generated by another portion of the system.

Abstract: A microprocessor-based multifunction overcurrent relay for protecting power systems has a plurality of overcurrent elements implemented in a software program. Sufficient overcurrent elements are provided to accomplish a selected one of three independent applications, including feeder protection, motor protection, or breaker failure protection. The relay includes elements for selecting one application by the user from the three possible applications and produces output signals when an overcurrent condition is determined.

Abstract: The motor relay includes thermal models representing the start and run conditions of a motor. The thermal models are implemented using measured current values and certain known operating parameters available from the manufacturer, specifically full load current, locked rotor thermal time and rated service factor. The thermal model information is compared with threshold thermal model values to determine when an overload condition has been reached.

Abstract: A microprocessorized thermal protection system and accompanying method measures input voltage and current of an induction motor and derives therefrom continuously updated estimates of the slip dependent rotor resistance during acceleration of the motor. The measurements and calculations of the varying rotor resistance enable determination of the heat generated within the motor based on a thermal model thereof. Phase shift distortions induced by the measurement system are accounted for by determining the value of the stator resistance at start up. In addition, during a brief period following start up, sampling of the voltage/current inputs is suspended to avoid the effects of the initially occurring DC-offset resulting from the application of the voltage input to the inductive motor load.

Abstract: A timing circuit for use in an overcurrent relay is disclosed. A plurality of non-interacting R-C circuits whose outputs are summed is provided. The signal representing the summed outputs of the R-C circuits represents one term of the inverse time-current relation it is desired to emulate. Each time the summed signal reaches a predetermined level, a pulse is generated. The frequency of the pulses determines the length of a time delay before the protected circuit is interrupted. When it is necessary to take the second term of the time-current relation into account, this can be done by providing a multivibrator that emits pulses of an adjustable duration responsive to the summed signal and that itself triggers the generation of the pulse train constituting the timing circuit output.

Abstract: A solid state relay for interrupting current flow in a power distribution circuit is disclosed. The relay includes a long time delay circuit which generates a first trip signal when the magnitude of the peak instantaneous line current rises above a pickup value I1 for a time period which varies as an inverse function of the magnitude of the peak instantaneous time current and which is determined by a first predetermined time current characteristic curve. The first predetermined time current characteristic curve defines the desired operating characteristic of the relay over a first current range extending between the pickup current I1 and the higher current I2. The predetermined time current characteristic curve is determined by a multi-node RC element in the feed back loop of an operational amplifier which forms part of the long time delay circuit.

Abstract: A transformer differential relay monitors the current flowing through the primary and secondary windings of a power transformer and disconnects the transformer from a power source feeding the transformer whenever the magnitudes of these currents indicate a fault condition in the transformer. First and second current transformer generate primary and secondary winding voltage signals representative of the magnitude of the current flowing through the power transformer primary and secondary windings, respectively. A solid state restraint signal generator is responsive to the primary and secondary winding voltage signals and generates a restraint signal representative of the magnitude of the larger of the primary and secondary winding currents, respectively, at each measurement instant.