Samples from a first and second cycle of a system signal are used to generate discrete Fourier transforms for the respective first and second cycle. Multiplication of the discrete Fourier transform of the second cycle by the complex conjugate of the discrete Fourier transform of the first cycle produces a frequency deviation phasor whose angle with respect to the real axis is representative of the frequency deviation of the system signal from a predetermined reference frequency. Actual system fr...

A generalized real-time thermal model of an induction motor produces values indicative of the transient and steady state temperature condition of the motor. Each of these values is compared with respective predetermined limits and power to the motor is removed when any value exceeds its respective limit in order to prevent damage to the motor.

A method for detecting excessive current in a system and interrupting the delivery of further current thereto comprises determining actual system current, obtaining a value of a predetermined system parameter indicative of the time the actual system current can safely exist in the system, creating a dynamic history of the predetermined system parameter by repetitively modifying the predetermined system parameter in accordance with actual system current, and interrupting power to the system if th...

A high speed relay for detecting multiphase faults using operating and polarizing signals from static transmission line protective relays provides an instantaneous estimate of the relative phase angle between relay signals without any loss of relay security.

A commutation circuit for a high voltage DC circuit breaker incorporates a resistor capacitor combination and a charging circuit connected to the main breaker, such that a commutating capacitor is discharged in opposition to the load current to force the current in an arc after breaker opening to zero to facilitate arc interruption. In a particular embodiment, a normally open commutating circuit is connected across the contacts of a main DC circuit breaker to absorb the inductive system energy t...

A system for rapidly detecting short circuits on power transmission lines in electrical power systems is provided to protect the power network from the effects of short circuits within the system. The instant invention provides a system for detecting multi-phase faults and causing three-phase tripping when required and also detecting single-phase faults and providing a control signal to trip the single-phase that is faulted. Signals proportional to system voltages are applied to a memory circuit...

A method for determining transient errors in an unsynchronized, digital current differential transmission line protection system includes receiving a first current phasor measurement at a time, t, and receiving a second current phasor measurement at time t. The second current phasor is adjusted so as to account for a determined offset between a first sampling clock associated with the first current phasor measurement and a second sampling clock associated with the second current phasor measureme...

Method and apparatus are disclosed for providing a sampling signal having a frequency which is maintained in constant proportion to the frequency of an input signal. The input signal is sampled at instants determined by the sampling signal to provide a plurality of data signals associated with one cycle of the input signal. A signal processor is adapted to receive first and second pluralities of data signals associated with first and second cycles of the input signal and to perform two discrete ...

A digital frequency locked loop is disclosed for use with programmable signal processing apparatus wherein frequency locking is desired from staggered data samples. Sampling means are provided for sampling an input signal, at instants determined by the sampling signal, to provide N data signals from M cycles of the input signal wherein each data signal represents the magnitude of the input signal at the sampling instant. Signal processing means are provided for performing a unique approximation ...

Disclosed is a method for overcoming one of the limiting factors in applying digital techniques to measuring analog signals over a wide dynamic range, namely, the uncertainty introduced into the measurement by noise and A/D conversion, particularly at low signal levels. A central feature of the technique is the filtering of the signal with a variable time constant digital filter, the time constant being a function of the measured signal in such way as to increase the effective resolution of smal...