Signal processing system for overload relay or the like

In a circuit protection device for monitoring three-phase current in an electrical circuit, sensing means for sensing the current flow in the circuit and outputting periodic signals at a frequency f.sub.1 representing the magnitude of the composite current, comprising the combination of:

• a set of threee current transformers connected in a delta configuration;
• a set of three differential amplifiers, first inputs of said amplifiers being coupled to a point of reference potential, second inputs of said differential amplifiers each being coupled to a different one of the intersections between the current transformers;
• a comparator having an additive and a subtractive input;
• a signal generator for periodically providing a ramp signal coupled to the subtractive input of said comparator at said frequency f.sub.1 ;
• means coupling the outputs of said differential amplifiers to the additive input of said comparator;
• whereby said comparator adopts a first state when of sensed current signal provided by the current transformers is less than the magnitude of the ramp signal and a second state when the magnitude of the ramp signal is less than the sensed signal, the duration of the change of state of said comparator being an inverse function of the magnitude of sensed current flow.

In a control system for detecting overcurrent conditions in an electrical circuit and including sensing means for deriving a signal representative of current flow and converter means for periodically outputting sample signals representing current flow;

• adding means for algebraically adding a threshold value to said sample signals;
• accumulator means for accumulating the algebraic sums of said threshold values and sample signals;
• feedback means coupled to said accumulator for deriving a thermal correction signal which is a function of the contents of said accumulator means and applying said thermal correction signal to said adding means for adjusting said threshold value;
• and
• means for producing an overcurrent;
• indication when the contents of said accumulator attain a predetermined level.

In a contact system for detecting overcurrent conditions in an electric circuit and including sensing means for deriving a signal representative of current flow and converter means for periodically outputting sample signals representing current flow, the method of accounting for the heating effect of current flow comprising the steps of

• combining each ith current sample I.sub.i with a current threshold value I.sub.t to produce a net current sample value;
• accumulating said net current sample values in an accumulator,
• deriving a thermal correction signal from said accumulated signals to form at least a portion of said threshold value I.sub.t whereby net current values which are accumulated are caused to approach a maximum value which is not exceeded in the absence of an overcurrent condition;
• and
• producing an overcurrent indication when the accumulated net current values exceed said maximum value.

In a control sytem for responding to overcurrent conditions in an electrical circuit and including means for producing periodic digital signals representing current flow, the method of accounting for the heating effect of current flow comprising the steps of

• subtracting a thermal correction value from each of said periodic digital signals to produce periodic net current signals,
• accumulating said net current signals by adding successive ones of said signals to form an accumulated sum;
• dividing each sum so produced by a factor Q to provide a thermal correction value to be subtracted from the next successive periodic digital signal so that the value of the accumulated net current signals stabilizes below a trip threshold value in the absence of an overcurrent condition;
• and
• producing an overcurrent condition indication when the value of the accumulated net current signals exceed said trip threshold value.

In a control for monitoring current flow in an A.C. circuit and selectively actuating protective apparatus therefor, and comprising sensing means for sensing current flow in the circuit and outputting digital sample signals representative thereof at periodic internvals which occur at a no-harmonic of circuit current frequency,

• counter means for receiving said sample signals and responsive to signals representing a current value above a predetermined level to output an energizing signal having a fixed time duration in response thereto;
• and
• annunciator means coupled to said counter means and energizable by said energizing signal to provide a momentary indication of overcurrent,
• whereby said annunciator means is enabled progressively more often as the average current value approaches said predetermined level and is enabled substantially continuously after the average current attains said level.