US3243656A - Circuit interrupter control responsive to wave form - Google Patents

Description

March 29, 1966 1. BAUDE CIRCUIT INTERRUPTER CONTROL RESPONSIVE TO WAVE FORM 3 SheetS-S'neet l Filed Sept. 9 1963 March 29, 1966 J. BAUDE 3,243,656

CIRCUIT INTERRUPTER CONTROL RESPONSIVE TO WAVE FORM Filed Sept. 9, 1963 2 Sheets-Sheet 2 United States Patent Office Patented Mair. 29, 1966 3,243,656 CIRCUIT INTERRUPTER CQNTROL RESPONSIVE T WAVE FORM John Bande, Milwaukee, Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis. Filed Sept. 9, 1963, Ser. No. 307,566 2 Claims. (Cl. 317-11) This invention relates generally to circuit interrupters; more specically to a control for actuating a circuit interrupter to open at a preselected point in the current or voltage wave form of the electrical system that the circuit interrupter is connected in.

In the conventional circuit interrupters, interruption takes place in two steps. First the contacts separate and form an arc of hot ionized metal from the contacts and gases from the dielectric medium. This ionized medium maintains conduction between the separated contacts until it cools and deionizes sufficiently to withstand the voltage between the contacts. In the second step of conventionally interrupting the circuit, devices such as arc chutes operate on the arc medium to restore its dielectric strength. In an alternating current circuit the arc stops conducting at each current zero, and circuit interrupters are usually designed to interrupt the circuit by preventing reconduction after some current zero. If the contacts happen to be separated at a current zero, an arc would not form and the circuit would simply not start conduction on the following half cycle.

A general object of this invention is to provide a new and improved control for opening a circuit interrupter at a predetermined point in the current or voltage wave form where arcing should be prevented. Although the terms zero crossing and current zero will be used to describe this point, the control can be set to open the breaker at some other point that may be appropriate for a particular circuit interrupter or for a particular electrical system. Opening the circuit interrupter only at a current zero gives several advantages. It reduces contact erosion and thereby simplifies maintenance of the circuit interrupter. It could also permit using smaller circuit interrupters than before. Furthermore, there are advantages in the electrical system in addition to the advantages to the circuit interrupter. Interrupting only at the current zero reduces the transients on the system that are caused by conventional switching. Zero current switching is faster than conventional switching which requires several half cycles to complete. It could also simplify the problems of coordinating a succession of switches in a system.

One of the problems of actuating a circuit interrupter to open at a current zero is that the control must allow for the delay between the time that the interrupter is signaled to open and the time that the contacts separate. Some present day circuit breakers can be made to open in about one millisecond, a rather short time for mechanical devices, but still about 21 in the sinusoid of a sixty cycle per second electrical power system. For a power system that has a sixty cycle pure sinusoidal current wave form, circuits have been proposed that use phase Shifters to provide a sinusoidal tripping signal that crosses zero appropriately ahead of the expected current zero in the line. After rst receiving an actuating signal, and then receiving the phase shifted current zero signal, the control trips the switch with the proper lead time to open the breaker at current zero.

If the line current wave form is not a pure sinusoid, the control described in the preceding paragraph will trip the breaker too soon or too late. In fact, a circuit breaker is most likely to be opened during a fault condition when the line current wave form becomes nonsinusoidal, the frequency changes and the next zero crossing cannot be predicted in the circuit just described. A more specific object of this invention is to provide a new and improved control that responds to the slope of the current wave form to produce the proper tripping signal for the adverse conditions that usually exist during a fault.

The control of this invention responds to a first predetermined magnitude on the peak going slope of the wave form to indicate that the maximum allowable instantaneous current has been exceeded; when this magnitude is reached, circuits are set up to sample the slope magnitude on the zero going slope and predict the next zero crossing. The control responds to second and third preset current values on the zero going slope of the wave form and generates a count value proportional to the time between these points. The control waits a related time (which may be substantially zero) and then produces a tripping signal for the circuit interrupter of the line with the approaching current zero.

An important feature of the control is that for a three phase system a fault sensing and slope sampling control is provided for each phase; the three controls are interconnected so that a peak going current that passes the first point on any phase signals the controls for all three phases to start sampling the slope between the second and third points. Since one other phase will usually cross zero before the phase where the fault was first detected, this interconnection improves the speed of response of the control. After the rst line to reach zero is opened, the controls for the other two lines open their circuit interrupters at current zeros. (These two lines may be tripped together or at different times, depending on whether there are ground currents.)

The detailed description of the invention will suggest other problems in zero current switching and the associated advantages and objects of this invention.

The drawing FIG. 1 is a single line schematic diagram of a three phase electrical power system, a circuit interrupter, and the control of this invention;

FIG. 2 shows a current wave form of the electrical system of FIG. l and illustrates how the control operates to predict a zero crossing; and

FIG. 3 shows two wave forms of the control of FIG. 1.

The three phases of the electrical system of the drawing are designated A, B and C. The control for phase A is shown in detail and similar controls for phases B and C are shown by boxes with the interconnections to the electrical system and between the phase controls. Where components that are identical except for their association with a particular phase are shown in the drawing, they are distinguished by the letter suix of the associated phase.

The electrical system and circuit nterrupter The electrical system of FIG. 1 has three lines 10A, 10B and 10C, and a circuit interrupter 11 with three independently operable switch units 11A, B and C. Each unit 11A, B and C responds to a tripping signal 14A, 14B, 14C to open after a time delay that is inherent in the switch. Preferably, switch units 11A, B, C are quick opening; for example, they may be vacuum switches.

The control of this invention produces tripping signal 14A, B, C ahead of a predicted zero crossing by the inherent delay of circuit interrupter operating mechanism for the contacts to open at the predicted current zero. In the embodiment of the invention that is shown in the drawing, the control generates its own fault indicating signal in response to overcurrent in any line 10A, B or C.

Later the description will explain how the control can be actuated with a manual trip signal and with trip signals generated by independent fault sensing devices.

Predicting a current zero FIG. 2 shows a wave form 19 for one of the lines 10 that illustrates the principles that the control of this invention uses in predicting a current zero. FIG. 2 will be easier to understand if it is first thought of as describing -a single phase system. During a fault, wave form 19 may be aperiodic and nonsinusoidal and it may have a D.C. component. A succession of points 20 through 27 on wave form 19 illustrates the operation of the control. The corresponding times along the horizontal axis are identified by the letter t with the subscript of the associated point, and the voltages along the vertical axis are identified by the letter V with the subscript of the associated point, The vertical axis is labeled voltage because the components of the control that will be described later are voltage sensitive and the control transforms the current wave form of lines A, B, C into a corresponding voltage wave form.

At the first zero crossing 20 in FIG. 2 the electrical system is operating normally and the wave form is undistorted. Shortly after time t20, a fault occurs that causes the current magnitude to rise above its normal peak value into the region above point 21. Point 21 is a selected magnitude where it is desirable for the control to begin operating to open the breaker. Point 22 on the zero going slope of the curve corresponds in magnitude to point 21.

Point 27 is the next zero crossing where the contacts might be advantageously separated. Point 26 corresponds to time 126 which is ahead of time t27 by the delay in operating the circuit breaker 11. The object of the control is to establish the relationship between point 26 and one of the preceding points (specifically point 25). In the region between points 24 and 25 the control samples the slope of the wave form. Point 24 corresponds to a predetermined voltage V2.2 and point 25 corresponds to a predetermined voltage V25. The corresponding times t24 and t25 are unknown in relation to each other or to the other points, but time 125 can be thought of as a reference time in the sequence of operating the control. Point 23 is located just ahead of the sampling region and is defined by a predetermined voltage V23. Thus, points 24, 25 and 27 are related by known voltage magnitudes and points 26 and 27 are related by a known time interval.

As FIG. 2 shows, the wave form 19 is approximately a straight line between points 24 and 27 and the voltages and times in this region are related by similar triangles; the sides of these triangles are known except for V26 and the intervals f2.2 to t25 and t25 to f2s. As the detailed description of the control will explain, the control measures the interval t24 to f2s and in effect determines the required interval t25 to t26. Notice that in this analysis the half period t20 to t2, has been undefined and the peak voltage has been undefined; thus the simple sine wave of FIG. 2 illustrates the operation of the control with complex current wave forms.

As the wave form of FIG. 2 has been described for a single phase system, the control would sense the overcurrent condition at time t21 and then trip the breaker at time t26. In a three phase system, at least one of the other lines will usually cross zero in this interval; the control responds to the overcurrent condition at time t21 to cause any one of the three lines that goes through the sequence of points 23 through 27 to trip at its zero crossing. In other words, for the three phase system the rising slope of FIG. 2 can be thought of as describing the line where the fault is first detected and the descending slope can be thought of as the line that first goes through zero.

It would give a false reading to begin a slope sample within the interval f2.2 to t25. As the sequence of points 20 through 27 has been described for a single phase system (or three independently controlled phases), wave form 19 would not enter the sampling period f2.2 to t25 unless it had already crossed point 22; but this is not necessarily true when the controls are interconnected, and point 21 on one line may occur between points 24 and 25 of the first line to go through zero. To prevent starting a slope sample in the middle of the sampling period, the control first senses the point 23 just ahead of the sampling period. Crossing point 23 sets up the circuitry for the sampling operation.

In the control that will be described next, voltage V25 is made high enough so that in the shortest operating time between points 25 and 26 the highest current amplitude that the control is intended to respond to will cross the line of V25 ahead of time x26 sufficiently to produce the tripping signal. Voltage V24 is made as close as possible to voltage V25 within the limitations of the accuracy of the control in distinguishing small signals. Point 23 may be made to coincide with point 24 but is preferably somewhat ahead of point 24 to allow for delays within the control; it may be made as early as point 21 but this slows the speed of response of the control; preferably point 23 occurs later than point 22. Point 22 is shown only as the zero going slope counterpart of point 21; none of the components that will be described respond specifically to point 22.

The control The control includes some digital devices; with exceptions mentioned later a zero or negative voltage is considered to be a 0 and a predetermined positive value is considered to be a l.

The control includes a current transformer 30A, B, C for each line of the system. The control for phase A has an amplifier 31 connected to receive the output 32A of current transformer 30A and to produce at its output 33 a voltage that corresponds to the current wave form. The wave form at output 33 contains any D.C. term and both the positive and negative polarity portions of the current wave form. A full wave rectifier 34 is connected to receive the voltage wave form 33 and to produce a full wave rectified wave form at its output 35. By full wave rectifying the -wave form at output 33, the peak going slopes of FIG. 2 are made positive going and the zero going slopes are made negative going. The control components that will be described later respond to the slope polarity of the rectified wave form and thereby respond to the peak going and zero going slopes of the wave form of FIG. 2. In other respects the wave form at output 35 is equivalent to the Wave form 19 of FIG. 2 and the two wave forms will be described as though they were identical.

The control for phase A also includes a fault detector i 36. The fault detector is illustrated as part of the control connected to rectifier output 35 rbut it may be an entirely separate unit. Fault detector 36 is illustrated as a voltage responsive gate comprising a diode 37 having its anode connected to receive the positive potential of rectifier output 35, a resistor 38 connecting the cathode of diode 37 to a reverse biasing potential point 39 having the positive potential of V21, and a capacitor 40 that is connected to block the potential of point 39 from the output 41A of fault detector 36. A positive going voltage change at output 41 means that the current in line 10A has exceeded the fault level defined by voltage V21.

An OR gate 42 is connected to receive the outputs 41A, 42B and 41C of the fault detectors 36 for each of the three phases. When a fault occurs on any phase, OR gate 42 energizes its output 43 and sets a flip-flop 44. When Hip-flop 44 is set, it energizes its one output 45 in the proper polarity (negative) to signal each of the three slope sampling circuits to begin sampling the slope at the appropriate time to operate the switch units. When Hipop 44 is set, it closes a switch 47 in the control for phase A, and switch 47 conducts the full wave rectied wave form 35 to its output 49. Switch 47 is illustrated somewhat schematically as a transistor having its emitter and collector terminals connected to conduct between points 45 and 49 and having its base terminal connected to flipiiop 44 to receive the proper polarity with respect to the emitter terminal to be turned on when flip-Hop 44 is set.

In the control for phase A, three gates 51, 52, 53 are connected to receive output 49 and to produ-ce a positive pulse at their outputs 55, 56, 57 in response respectively to the three points 23, 24 and 25 on the wave form. Each gate is similar to the fault detector 36 except that gates 52 and 53 for points 24 and 25 each include a phase reverser 58 that makes the wave form negative and causes gates 52 and 53 to indicate when the magnitude of wave form 19 is less than voltage V24 and V25 respectively. Gate 51 produces a positive pulse at lits output 55 when rectied Wave form 19 is higher in magnitude than voltage V23 and switch 47 is turned on. Output 55 is connected to set a ip-op 60 when the voltage magnitude of wave form 19 reaches point 23. When ip-op '60 is set, it energizes its l output to indicate that the control for phase A is ready to begin sampling the slope, or conversely that the fault was not detected by the control for phase B or C within the sampling interval, t24 to t25 of the phase A wave form. When the voltage magnitude of wave form 19 becomes as low as V24, gate 52 opens and produces a positive going pulse at its output 56, and AND gate 61 is connected to respond to the presence of a signal at the 1 output of Hip-flop 60 and a 1 output of gate 52 to energize its output 62; a 1 at output 62 indicates that the wave form is entering the sample zone r2.2 to t25. Output 62 is connected -to set a flip-nop 64 which energizes its 1 output. Energizing the l output of nip-flop 64 indicates that the wave form is in the sample zone and that the control should begin counting the time between 124 and f2s.

At time 125 the voltage magnitude V25 at output 49 causes 'gate 53 to open and produce a positive going pulse at its output 57. Output 57 is connected to reset flip-Hop 64 to indicate that the count period is over and that the reverse count interval has started.

A counter 70 is connected to be counted up during the sampling period and to be counted down after the sampling period. As the drawing illustrates the counter, it comprises a capacitor 71, a source 72 for charging the capacitor, an AND gate 73 operable in response to the 1 output of ilip-iiop 64 to connect capacitor 71 to he changed, and a switch 74 that is operable in response to the 0 output of ip-op 64 to connect capacitor 71 in a circuit to be discharged. Source 72 is illustrated as a voltage source 75 and a resistor 76- that make up approximately a current source for charging capacitor 71 linearly; this is the simplest circuit to explain but the capacitor may be charged nonlinearly as will be explained later. j

The discharge circuit is illustrated as a variable resistor 79 that switch 74 operates to connect across the terminals of the capacitor. Source terminal 80, capacitor terminal 81, and resistor 79 term-inal 82 are reference potential points in counter 70.

Since the voltage across capacitor 71 is proportional to its char-ge, the integral of current with respect to time, the capacitor voltage is a function of the time interval that the capacitor is charged. In the example of charging capacitor 71 linearly, the voltage is proportional to time. Similarly the capacitor voltage during discharge is some function of the interval remaining until the capacitor will have discharged to zero or to some other reference value. In this control the reference value corresponds to zero crossing point 27. This relation between voltage and time depends on the dis-charge rate, which can :be set by adjusting variable resistor 79, and on the reference voltage which can be adjusted as will be explained later. Thus, as will be explained next, a preselected voltage can be made to correspond to point 26 where the breaker should be tripped.

An adjustable gate is connected to receive the output of the counter 70 when switch 74 is closed and to respond to the preselected voltage level that corresponds to the time delay of operating the breaker (t26 to t27). Gate 90 is similar to gate 36 which was described in detail with the phase reverser 58 of gates 52 and 53 except that its biasing potential is adjustable by means of a potentiometer 92 so that the time delay voltage V93 in FIGS. l and 3 and the corresponding time interval f2s to t27 can be set individually for each switch unit. When capacitor 71 has discharged to time delay voltage V93, gate 90 produces a positive going pulse at its output 95 that trips the breaker 11A for line 10A.

In the description of counting down counter 70 in the interval to 125, it was assumed that the capacitor discharged linearly. For the simple counter of FIG. l, time delay voltage V93 is made rather close to the capacitor voltage so that time is measured on a fairly linear part of the discharge curve of the capacitor. Equivalently the capacitor can be made to discharge into a current source (such as source 72).

So far in this description it has been assumed that the curve of wave form 19 is a straight line between points 24 and 27. This assumption is valid for most current wave forms on electrical transmission systems; however, it is more accurate for a short interval (high current overload) than for a longer interval (moderate current overload). The circuit as it has been described so far can be thought of as generating the first two terms in a Taylors series; that is the control generates the curve of FIG. 3, capacitor voltage as a function of time, which corresponds in magnitude and slope to the wave form between points 24 and 25 but it does not necessarily have the same curvature. For more accuracy, additional terms of the series can be generated by the slope sampling techniques already described, or by making the count up or count down charge rate appropriately nonlinear, or by adjusting .the timing voltage value V93 as a function of current overload magnitude. Generally the simpler control that FIG. 1 illustrates is preferable.

The control has been described in terms of rather specific components, and a variety of functionally equivalent components may be substituted in the control. A digital counter may be substituted for the analog counter 70 and a source of pulses substituted for current source 72. A manual input toOR gate 42 may be provided to signal the control to open circuit interrupter 11. The specific logic network of FIG'. l can be transformed into different equivalent networks by well known design techniques. Those skilled in the art will recognize variations and modifications within the spirit of the invention and the scope of the claims.

Having now particularly described and ascertained the nature of my said invention and the manner in which it is to be performed, I declare that what I claim is:

1. A control for a circuit interrupter having a predetermined delay between receiving a tripping signal and separating its contacts, comprising,

means connected to receive a measure of a current wave form of the electrical system in which the circuit interrupter is connected and responsive to a first point of predetermined magnitude on the wave form to produce and maintain a signal indicating that a fault has occur-red in the electrical system, derivative means responsive to said fault indicating signal and responsive to said wave form in a region between second and third points of predetermined magnitude on the wave form to produce a signal value that is a function of the slope of the wave form in said region, delay means including a capactor connected to receive said signal value to charge said capacitor to a level determined by the magnitude of said signal produced by said derivative 3,243,656 7 8 means, and means for Ydischarging said capacitor References Cited by the Examiner at a predetermined rate, means responsive to the discharge of said capacitor to produce a trip signal delayed from said third point 1,967,849 7/1934 WderOe' by a time that is a functionv of said signal before 5 1,967,850 7/1934 Wlderofi' signal derivative, the magnitude of said third point, 2,531,443 11/1950 Kesselrmg and the circuit interrupter delay toseparate the conl tacts at a predicted point of predetermined magnitude SAMUEL BERNSTEIN Pnma'y Exammer' in the wave form. R. V. LUPO, Assistant Examiner. 2. A control according to claim 1 in which said pre- 10 dicted point is a zero crossing of the current wave form.

Claims (1)

1. A CONTROL FOR A CIRCUIT INTERRUPTER HAVING A PREDETERMINED DELAY BETWEEN RECEIVING A TRIPPING SIGNAL AND SEPARATING ITS CONTACTS, COMPRISING, MEANS CONNECTED TO RECEIVE A MEASURE OF A CURRENT WAVE FORM OF THE ELECTRICAL SYSTEM IN WHICH THE CIRCUIT INTERRUPTER IS CONNECTED AND RESPONSIVE TO A FIRST POINT OF PREDETERMINED MAGNITUDE ON THE WAVE FORM TO PRODUCE AND MAINTAIN A SIGNAL INDICATING THAT A FAULT HAS OCCURRED IN THE ELECTRICAL SYSTEM, DERIVATIVE MEANS RESPONSIVE TO SAID FAULT INDICATING SIGNAL AND RESPONSIVE TO SAID WAVE FORM IN A REGION BETWEEN SECOND AND THIRD POINTS OF PREDETERMINED MAGNITUDE ON THE WAVE FORM TO PRODUCE A SIGNAL VALUE THAT IS A FUNCTION OF THE SLOPE OF THE WAVE FORM IN SAID REGION, DELAY MEANS INCLUDING A CAPACITOR CONNECTED TO RECEIVE SAID SIGNAL VALUE TO CHARGE SAID CAPACITOR TO A LEVEL DETERMINED BY THE MAGNITUDE OF SAID SIGNAL PRODUCED BY SAID DERIVATIVE MEANS, AND MEANS FOR DISCHARGING SAID CAPACITOR AT A PREDETERMINED RATE, AND MEANS RESPONSIVE TO THE DISCHARGE OF SAID CAPACITOR TO PRODUCE A TRIP SIGNAL DELAYED FROM SAID THIRD POINT BY A TIME THAT IS A FUNCTION OF SAID SIGNAL BEFORE SIGNAL DERIVATIVE, THE MAGNITUDE OF SAID THIRD POINT, AND THE CIRCUIT INTERRUPTER DELAY TO SEPARATE THE CONTACTS AT A PREDICTED POINT OF PREDETERMINED MAGNITUDE IN THE WAVE FORM.

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