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Publication numberUS3602776 A
Publication typeGrant
Publication dateAug 31, 1971
Filing dateApr 29, 1970
Priority dateApr 29, 1970
Publication numberUS 3602776 A, US 3602776A, US-A-3602776, US3602776 A, US3602776A
InventorsGonnam Russell W
Original AssigneeWestinghouse Electric Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Quick resetting apparatus
US 3602776 A
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Description  (OCR text may contain errors)

United States Patent Inventor RmsellW. Gmnam Morris Plains, NJ

Filed Apr. 29, 1970 Patented Aug. 31, 1971 Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

QUICK RESETTING APPARATUS 16 Claims, 5 Drawing Fig.

US. 317/33 SC, 317/22, 317/28, 317/36 TD, 317/142, 320/1 Int. Cl. ..H0lh 47/18,

H02h 3/08 Field ofSearch 317/36 TD,

33 R, 33 SC, 22; 320/1; 317/27, 28, 142

PRIMARY RELAY DEVICE PHASE SFSLITTER RECTIFIER PHASE SPLITTER AND RECTIFIER BUFFER [56] References Cited UNITED STATES PATENTS 3,295,019 12/1966 Altfather 317/36 TD 3,543,094 11/1970 South et a1 317/36 TD Primary Examiner-James D. Trammell 4ttomeysA. T. Stratton, C. L. Freedman and John L.

Stoughton ABSTRACT: A fault current magnitude actuated resettable sensing apparatus particularly for resetting the timer of a timer actuated backup fault responsive device which sensing a; paratus response to the steep slope of a decaying actuating signal resulting from the decay of the' current responsivesignal upon opening of the breaker to terminate the fault current whereby the time interval for the resetting of the timer by the sensing apparatus remains substantially constant irrespective of the time of the decay of the actuating signal resulting from the interruption of fault current of any expected magnitude.

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' sum 2 [IF 3:

PRIOR ART DEVICE NEW RESET DEVICE 1 2O 24 28 MULTIPLES OF MINIMUM PICKUP CURRENT F IG.2.

SGNOOHSI'IIIW NI EIWLL 1.3838

PATH-HEB AUBBI m CURRENT REDUCED FROM 2X PICKUP TO 0 AMPERES CURRENT REDUCED FROM lOX PICKUP TO 0 AMPERES CURRENT REDUCED FROM 40X PICKUP TO 0 AMPERES AMPERES AMPERES AMPERES SHEET 3 OF 3 [PRIMARY CURRENT VOLTAGE 79 -78 2 MILLISECONDS/DIVISION {PRIMARY CURRENT VOLTAGE 79-78 -I I I I I o 2 MlLLlSECONDS/DIVISION {PRIMARY CURRENT 2 MlLLISECONDS/DIVISION VOLTAGE IO VOLTS CM VOLTAGE IO VOLTS/CM VOLTAGE IO VOLTS /CM FIGBB.

FIG.3C.

QUICK RESETTING APPARATUS BRIEF SUMMARY OF THE INVENTION This invention is particularly intended for use in protective relaying in which backup operation is provided for tripping backup breakers in the event that the primary breaker fails to interrupt the fault current and provides a fast and constant reset time for the backup protection timer irrespective of the magnitude of the fault current.

In the prior art, such a current actuated unit the time for resetting typically would vary from 2 to 9 milliseconds depending upon the magnitude of the primary current at the time of breaker opening. Additionally, if the current is suddenly reduced below pickup but not to zero, then the reset time would be as high as 15 milliseconds. Present reset apparatus requires the use of a backup timer which provides a time delay sufficiently longer than the longest possible expected reset time. Unless this time is provided the backup timer could provide a false actuation of the backup breakers when in reality the primary breaker has succeeded in opening the primary current.

It is desirable that the resetting circuit have a substantially constant reset time independent of the magnitude of the primary current which actuates the same. This enables a minimum time setting of the backup protection timer and permits a quicker actuation of the backup protection should the primary breaker fail to interrupt the primary current. The present invention obtains this substantially constant interval by utilizing the slope of the rapidly decaying primary current to actuate the resetting unit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a backup protecting relaying network, partially in block form, embodying the current sensing quickly resetting circuit of the invention.

FIG. 2 is a curve comparing the times required to reset the prior art sensing circuit and to reset the sensing circuit of this invention.

FIGS. 3A, 3B and 3C illustrate typical time-voltage curves showing the decrease in voltage with three different magnitudes offault current.

DETAILED DESCRIPTION Referring to the drawings by characters of reference, the numeral 1 designates generally a substation bus energized by three power supplying polyphase lines 2, 3, and 4 from suitable energy sources (not shown) through breakers 5, 6 and 7 respectively. As illustrated in FIG. I, the primary fault protection from the line 2 is the breaker 5. In the event the breaker should fail to open the circuit therethrough, the backup breakers 6 and 7 would be actuated at a time interval subsequent to attempt to actuate the breaker 5 which time interval is sufficient long to indicate a failure of the breaker 5 to en.

The primary relay device 8 for actuating the breaker may be similar to that found in U.S. Pat. to C. T. Altfather No. 3,295,019 dated Dec. 27, 1966 and is shown within the dotdash rectangle 4A of the said Altfather patent. The fault responsive signal is applied to the primary relaying device 8 by the current transformers 10, l l and 12 individually associated with the conductorsC, B and A respectively of the network 2. These current transformers are connected in Y circuit arrangement. The negative and positive sequence current is car ried by the conductors l4, l5 and 16 and the zero sequence current by the conductor 18.

The breaker failure or backup protecting apparatus 20 includes current transformers 22, 23 and 24 which have their primary windings in series in the conductors l6, l4 and 18 respectively. The output potential quantities of these transfor mers energize the phase splitter and rectifying networks 26, 27 and 28 respectively through setting circuits 30, 31 and 32 which comprise resistors adjustable to determine the current to voltage ratio in the usual manner. The rectified output quantities of the phase splitter and rectifying networks 26 and 27 are connected together in parallel and the resulting signal is supplied to a sensingcircuit and amplifier 34. The rectified output quantity of the phase splitter and rectifier network 28 is supplied to a sensing circuit and amplifier 36 of the same construction as the circuit and amplifier 34.

The output quantities of the sensing and amplifying circuits A 34 and 36 are supplied individually to the two inputs of an OR network 38; the output circuit of which is connected to a first input terminal 40 of an AND network 41. The output terminal 42 of the AND network 41 is connected to the input terminal 44 of a timer-amplifier 46. The output of the timer-amplifier is connected to a relaying device 48 which has two sets of contacts 49 and 50. These contacts are connected, as diagrammatically illustrated,-to trip the breakers 6 and 7 respectively in the event of the actuation of the relaying device 48 and closure of the contacts 49 and 50 when the backup protection timer 46 times out.

As discussed more fully in the said Altfather patent, the occurrence of a fault in the network 2 causes the primary relay device 8 to close its contacts 52. These contacts 52 correspond to the contacts 50a of the said Altfather patent. When contacts 52 close a circuit is completed from the positive power supply terminal plus through the closed contacts 52, the contacts 52a of the breaker 5 and the trip coil 54 to the negative power supply terminal. Energization of the trip coil 54, in normal operation, will disengage the latch 56 and the breaker contacts open to disconnect the network 2 from the bus 1. Closure of the contacts 52 also completes an energizing circuit through the winding of the relay 58, whereby this relay closes its contacts 59 and completes a circuit through the buffer 64 for energizing the first input terminal 60 of the 0R network 62. When so energized, the OR network 62 energizes the second input terminal 66 of the AND network 41, and the AND network 41 therefore will initiate the timing function of the timer-amplifier 46.

The timing interval of the timer-amplifier 46 is of sufficient duration so that it will not time out and actuate the relaying device 48 to trip the breakers 6 and 7 if the breaker 5 success fully terminates the current flow therethrough. If however, the breaker 5 fails to interrupt the fault current therethrough, the OR network 38 will not remove its operating signal from the input terminal 40 of the AND network 41 so that the timer will time out and energize the relaying device 48. As set forth above, the closure of contacts 49 and 50 trip the breakers 6 and 7 to deenergize the station bus 1 and thereby prevent the flow of fault current from the station bus into the supply network 2.

In accordance with this invention, the sensing circuits 34 and 36 remove the operating signals to the OR network 38 in a substantially constant time interval which is substantially independent of the magnitude of the fault current. Both of the sensing circuits 34 and 36 are identical and only the details of the circuit 34 are illustrated. The output winding of the 'current transformer 22 has its end terminals connected to the end terminals of a variable resistor 68 and to the input terminals 69 and 70 of the phase splitter and rectifier network 26. The secondary winding of the current transformer 22 has its center tap connected to an input terminal 71 of the network 26. In order to maintain a minimum time delay and yet provide a fairly even output the rectified alternating quantity supplied by the transformer 22, at its output terminals 72 and 73 of the phase splitter and rectifier 26, the single phase input supplied by the transformer 22 is split into two phase displaced quantities by the capacitor 74 and resistor 76. Such a phase splitting is well known to the art and will not be further described. The three phase displaced quantities are rectified in a full wave three phase rectifying network connected to energize the output terminals 72 and 73,. The terminals 72 and 73 are connected across a pair of conductors in busses 78 and 79 of the sensing circuit 34.

The conductor 79 is connected through a resistor 80, a Zener diode 82 to the base electrode 84 of a transistor 85. The emitter of the transistor 85 is directly connected to the conductor 78 whereby the transistor 85 is rendered conductive upon energization of the busses 78 and 79 to a potential greater than the breakover voltage of the Zener diode 82. A second transistor 86 has its emitter connected to a source of positive potential +and its collector connected through a resistor 88 to the negative terminal of the potential supply which terminal is also connected to the conductor 78. The base of the transistor 86 is connected to the collector of the transistor 85 through a resistor 85A. Therefore, upon energization of the busses 78 and 79 to the breakover voltage of the Zener diode 82, the transistor 85 will conduct and base current will flow through the transistor 86 which thereupon conducts to raise the potential of the output terminal 90 which is connected to one input terminal of the OR network 38.

When the transistor 86 is not conducting the potential of the output terminal 90 is that of the negative power supply.

The conductor 79 is also connected to the base 84 through a timing device or network 92. This network includes a capacitor 94 shunted by a resistor 95 with one terminal 94A of the capacitor 94 and resistor 95 connected to the base 84 of transistor 85. The other terminal 943 of the capacitor 94 and resistor 95 is connected through a Zener diode 96 and a resistor 98 to the conductor 79. The common terminal 94A is connected to the conductor 78 through a resistor 99 shunted by a capacitor 100 and by a solid state rectifying diode 102 all in parallel with the base-emitter circuit of the transistor 85. The diode 102 is connected to limit the reverse voltage which can appear between the emitter and base of the transistor 85. The maximum voltage to which the capacitor 94 may be charged is limited by a pair of Zener diodes which are connected between the conductor 78 and the common connection of the Zener diode 96 and the resistor 98.

It is believed that the remainder of the description may best be set forth by a description of operation of the apparatus which is as follows:

In the event of the occurrence of a fault in the network 2 which causes current IA of fault magnitude to flow in the conductor A associated with the current transformer 12, the current transformer 22 will be energized and provide an alternating potential between the terminals 69, 70 and 71 of a magnitude with respect to the magnitude of the current flowing through the conductor A as determined by the setting of the variable resistor 68. This alternating quantity is rectified by the three-phase full wave rectifying network of the phase splitter and rectifier circuit 26. Assuming that the current IA in the A conductor is a number of multiplier of the normal rated or pickup current and greater than the current 1C in the C conductor, the rectified potential of a magnitude proportional to the IA current will appear between the output terminals 72 and 73 and between the conductors 78 and 79. The variable resistor 68 is set so that when the current flowing through the conductor A rises to approximately 1% times the pickup or normal maximum value of current in the line A, the voltage between conductors 79 and 78 will be sufficiently high to cause the Zener diode 82 to break over. Therefore with a value of current greater than the set pickup, the Zener diode 82 will conduct base current from the bus 79 through the transistor 85. This will allow base current of transistor 86 to flow through a resistor 85A and the collector emitter circuit of transistor 85 to conductor 78. The conducting transistor 86 raises the potential of the output terminal 90 to substantially that of the positive bus. This potential supplies a base drive current to the transistor 104 of the OR network 38. When transistor 104 conducts, the potential of the input terminal 40 of the AND network 41 is reduced to substantially the negative source potential and one actuating signal is applied to the AND network.

The fault also actuates the primary relay device 8 to close its contacts 52 to energize the relay 58, thereby closing contacts 59 to supply a positive potential to the buffer circuit 64 which will actuate input terminal 60 of the OR network 62. This positive potential causes base current for transistor 106 to flow and make it conduct whereby the potential of the second input terminal 66 of the AND network 41 is substantially that of the negative supply. This is the remaining actuating signal for the AND network 41. With both terminals 40 and 66 at negative potential, transistor 108 will stop conducting and cause the potential of the output terminal 42 of the AND network 41 and of the input terminal 44 of the timer amplifier 46 to increase sufficiently to provide base current for the transistor 110 which conducts to cause base current in the transistor 111. Charging current flows through the transistor 111 and the variable timing resistor 114 and the main timing capacitors 1 15 and 116 charge at a rate determined by the resistance value of the timing resistor 114.

Assuming that the breaker 5 did not terminate fault current, the primary relay device 8 will maintain the relay 58 energized. Under these conditions both of the input terminals of the AND network 41 will remain energized by the actuating signals and a positive potential will be held at its terminal 42 to maintain the timer in its timing condition with the capacitors 115 and 116 charging. At the end of the timing interval, the critical potential necessary to fire the thyristor 118 will be reached and the thyristor 118 thereupon conducts to render the transistors 119 and 120 conducting. Conduction of the transistor 120 energizes the relaying device 48 which actuates its output relay 122 and the contacts 49 and 50 close, to open the breakers 6 and 7 to interrupt the current flowing in the line 2. This opening of the breakers also deenergizes the substation busses and the loads (not shown) supplied thereby.

The timing interval of the timer amplifier 46 should be as short a time as possible in the event of the failure of the breaker 5 in order to alleviate damage to the power systems and yet at the same time the delay must be large enough so that the timer will not time out until the breaker 5 is given time enough to interrupt the fault current therethrough. Under the prior art resetting or timing terminating network, the interval required to do this increased as the magnitude of the fault current increased. Therefore, the timer was adjusted to provide a delay sufficient to accommodate the longest expected resetting or terminating time which as indicated in FIG. 2 could exceed 8 milliseconds and as explained above could under the circumstances where the current did not decrease to zero be as long as upwards of 15 milliseconds. With the resetting network operating as described herein, the resetting or terminating time is rendered from a practical viewpoint independent of the pickup or fault current magnitude which initiated the timing operation. In the instance shown and with the values to be given below this interval will remain between 1 and 2 milliseconds for a multiple of pickup current between 8 and 40 with a maximum time of only 3.2 milliseconds at a fault current two times pickup. This permits a great reduction of the timing interval which must be provided by the timeramplifier 46 and thereby greatly lessens the danger of substantial damage to the power systems.

Assume an interruption of the fault current by the breaker 5 for current IA through the line A. The output voltage of the phase splitter and rectifier 26 will decrease and eventually reach a zero potential. However because of the reactance in the circuits for energizing the conductors 78 and 79, the time required for this voltage to reach a zero magnitude will be dependent upon the magnitude of the current flowing through the conductor A at the time that the breaker 5 terminated this current flow. This is illustrated in FIGS. 3A, 3B and 3C which show the time for the voltage between the conductors 78 and 79 to reach zero potential for multiples of pickup current FIG. 3A, 10 multiples of pickup current, FIG. 3B, and 30 multiples of pickup current, FIG. 3C. The zero point of time being of course when the primary current reached its zero value time t in each of the FIGS. 3A, 3B and 3C. The sensing circuit of this invention does not, upon the potential between the conductors 78 and 79 reaching a .value less than the breakover of Zener diode 82 remove the positive output potential applied by the output terminal 90 to the R network 38 and thereby reset or terminate further timing of the timer 46, but rather utilizes the initial rapidly decaying portion of the voltage curve to terminate the conduction of the transistors 85 and 86 and the removal of the positive output potential to the OR network 38.

During the interval in which the conductors 78 and 79 were energized, the capacitor 94 was charged through the resistor 98, Zener diode 96 and base to emitter of the transistor 85. The magnitude of the resistance of the resistor 98 is small and the capacitor 94 therefore assumes, with a minimum time delay, a charged condition in which the potential thereacross is substantially equal to the potential between the conductors 78 and 79 minus the drop across the'Zener diode 96 (which may for example be 9.1 volts as compared to the 6.8 volt drop across the Zener diode 82). Any small decreases in the potential between the busses 78 and 79 will be followed by a comparable discharge of the capacitor 94 through the shunt connected resistor 95. Small increases will, or course, be taken care of in the same manner in which the capacitor 94 is originally charged.

Assume now an opening of the breaker and an interruption in the current therethrough. The potential between the busses 78 and 79 will decrease sharply (see FIGS. 3A, 3B and 3C). The Zener diode 96 has a small forward drop as for example six-tenths of a volt so that as soon as the potential of the conductor 79 drops to six-tenths of a volt below the potential of the terminal 94B, the capacitor 64 commences to discharge through the Zener diode 96, the resistor 98, a resistor 93 connected across the busses 78 and 79 and the resistor 99 and/or the diode 102 to the other terminal 94A of the capacitor 94. This current flow through the resistor 99 and/or the diode 102 results in the reversal of the potential between the base and emitter and the transistor 86 and a termination of the conduction thereof. It will now be apparent that the OR network 38 will be actuated to remove its signal to the AND network 41 to terminate further timing of and reset the timer 46 without the necessity of the quantity supplied to the busses 78 and 79 returning to zero or even substantially zero.

If the current through the line A reduced slightly rather than terminated, then the capacitor 94 would discharge through the resistor 95 at a similar rate to that of the rectified voltage and the sensing circuit and amplifier 34 would not terminate its signal to the 0R network 38 and no resetting of the timer 46 would result. The timer 48 would continue to time out its interval and trip the breakers 6 and 7.

Assuming that the circuit constants are so taken that a voltage of approximately 5 volts represents a single multiple of pickup current flow in the line A until limited by a Zener clipper across the secondary of 22, suitable circuit constants for the sensing circuit would be as follows:

Resistor 80=l 0K ohms Resistor 93=82K ohms Resistor 95=33K ohms Resistor 98.=2.2K ohms Resistor 99=l 0K ohms Zener diode 82=6.8 breakover voltage Zener diode 97=9.l breakover voltage Capacitor 94=.0.5 microfarads Capacitor l00=0.5 microfarads.

When the values as indicated are utilized, the time required for the sensing circuit to reset subsequent to opening of the breaker would, as indicated in FIG. 2, vary from a maximum of 3.2 milliseconds at two multiples of pickup current to a minimum of L2 milliseconds at 40 multiples of pickup current.

What is claimed and is desired to be secured by United States Letters Patent is as follows:

1. A sensing apparatus comprising a pair of input terminals adapted to be energized with a variable magnitude electrical quantity, a pair of output terminals, a first energizing network for said output terminals and including switching means having a control circuit connected to said input terminals and operable in response to the occurrence of a predetermined minimum magnitude of said quantity to render said energizing network effective to energize said output terminals at a first relative polarity, a chargeable device, a charging circuit for said chargeable device, said charging circuit being effective to charge said chargeable device when the magnitude of said quantity applied to said input terminals is not less than said predetermined minimum magnitude, a first discharge circuit connected to discharge said chargeable device, and means connecting said discharge circuit to said output terminals for energization thereof at a second relative polarity as a consequence of the discharging of said chargeable device.

2. The combination of claim 1 in which said quantity is of unidirectional potential, said discharge circuit includes an impedance energized by said quantity in a polarity to oppose the discharge of said chargeable device, and in which there is provided a second discharge circuit'for said chargeable device, said second discharge circuit discharging said chargeable device at a rate which is independent of the rate of reduction of the magnitude of said quantity.

3. The combination-0f claim 1 in which said energizing network is connected across said input terminals and includes an asymmetric current conducting device connected to conduct current between said output terminals in a first direction, said network further including a potential dropping device which limits the current flow through said asymmetric device, said asymmetric device having a less impedance to current flow in said first direction than in a second direction opposite to said first direction, said impedance to current flow in said second direction being effective to establish said second relative polarity.

4. The combination to claim 3 in which said asymmetric device includes the control element of a current actuated switch device.

5. The combination of claim 4 in which said switch device is a semiconductor solid state triode device having a control element and a pair of electrodes and which conducts between its electrodes as a consequence of the current flow through said control element.

6. The combination of claim 5 in which said triode device is a transistor having a base and collector and an emitter, said control element is said base and in which said base is connected to one of said output terminals and said emitter is connected to the other of said output terminals.

7. The combination of claim 6 in which said switching means is a Zener diode, said potential dropping device is a resistor, and an asymmetrically conducting diode is connected in shunt with said base and said emitter in a polarity to conduct through its lesser impedance in a direction opposite to the lesser impedance to conduction between said base and emitter of said transistor.

8. The combination of claim 7 in which said chargeable device is a capacitor, and said second discharge circuit includes a resistor connected in shunt with said capacitor.

9. The combination of claim 1 in which said charging circuit includes an asymmetric conducting device connected to charge said chargeable device through its greater impedance, said first discharge circuit also including said asymmetric device connected to discharge said chargeable device through its lesser impedance.

10. The combination of claim 9 in which said first discharge circuit includes a first impedance element connected to be energized with a potential proportional to the magnitude of said quantity and polarized to oppose the discharge of said chargeable device and further includes a second impedance element connected in shunt with said output terminals.

11. The combination of claim 10 including a triode semiconductor switch having a pair of main electrodes and a control electrode, said control electrode being connected to a first of said output terminals, one of said main electrodes being connected to the second of said output terminals, said first energizing network including current control means to limit the current flow between said control electrode and said one main electrode, and a third impedance element connected in shunt with said chargeable device.

12. The combination of claim 11 in which said switching means comprises a first Zener diode, said asymmetric conducting device comprises a second Zener diode, said current control means comprises a fourth impedance element, said triode switch includes a junction permitting substantially unimpeded flow of current through said energizing network and a high impedance to the flow of current through said first charge circuit.

13. The combination of claim 12 in which said first Zener diode breaks over at said minimum magnitude of said quantity and said second Zener diode breaks over at a magnitude substantially greater than said predetermined magnitude.

14, A quick resetting circuit comprising first and second conductors, a plurality of resistors, a plurality of Zener diodes, a transistor having a base and a collector and an emitter, a first control circuit connected between said first conductor and said base and including in series circuit a first of said resistors and a first of said Zener diodes, means connecting said emitter to said second conductor, a capacitor, a charging circuit connecting said capacitor between said conductors and including in series circuit a second of said resistors and a second of said Zener diodes, means connecting a third of said resistors between said conductors, means connecting a fourth of said resistors between said base and said emitter, a first discharge circuit connected across said capacitor and including in series circuit said second Zener diode and said second resistor and said third resistor and said fourth resistor, and means connecting a fifth of said resistors in shunt with said capacitor.

15. The combination of claim 14 including an alternating current network connected to energize said conductors through a rectifying device whereby said conductors are energized with a rectified electrical quantity proportional to the alternating electrical quantity flowing in said network, said network including reactance which delays the collapse of said quantity upon deenergization of said network, the magnitude of said rectified quantity normally being not greater than a first predetermined magnitude but being subject to excursions which are many times said first magnitude, said first Zener diode being effective to break over at magnitudes of said rectified quantity not less than said first predetermined magnitude, said second Zener diode being effective to break over only at magnitudes which exceed a second predetermined magnitude of said rectified quantity, said second predetermined magnitude being greater than said first predetermined magnitude, the magnitude of said fifth resistor being less than the magnitude of said third resistor and greater than the magnitude of said second resistor, first potential limiting means connected in said charging circuit for limiting the maximum potential to which said capacitor may be charged, and second potential limiting means connected between said emitter and said base and effective to limit the potential therebetween due to current flow in said first discharge circuit,

16. The combination of claim 15 in which said first predetermined magnitude is about 6.8 volts, said second predetermined magnitude is about 9.1 volts, the magnitude of the resistance of said first resistor is about 10K ohms and of said second resistor is about 2.2K ohms and of said third resistor is about 82K ohms and of said fourth resistor is about 10K ohms and of said fifth resistor is about 33K ohms, the capacitance of said capacitor is about 0.5 mfd., the potential limit ofsaid first limiting means is about 40 volts and of second limiting means is about 1 volt.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3707651 *Jun 29, 1971Dec 26, 1972Westinghouse Electric CorpFrequency shifted modulated carrier protective relaying system with amplitude modulated voice communication
US4006387 *Sep 22, 1975Feb 1, 1977The United States Of America As Represented By The Secretary Of The Air ForceLow power solid state three-phase overcurrent/undercurrent protection circuit
US4056839 *Jan 19, 1976Nov 1, 1977Tokyo Shibaura Denki Kabushiki KaishaProtective relay devices
US6058353 *Jan 23, 1997May 2, 2000Commonwealth Edison CompanySystem and method for providing backup protection for circuit breaker failure
Classifications
U.S. Classification361/75, 361/71
International ClassificationH02H3/093
Cooperative ClassificationH02H3/093
European ClassificationH02H3/093