US3160787A

US3160787A - Restraint operation relay employing zener diode

Info

Publication number: US3160787A
Application number: US852390A
Authority: US
Inventors: William K Sonnemann
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1959-11-12
Filing date: 1959-11-12
Publication date: 1964-12-08
Anticipated expiration: 1981-12-08
Also published as: CH387149A

Description

1964 w. K. SONNEMANN 3,

RESTRAINT OPERATION RELAY EMPLOYING ZENER DIODE Filed Nov. 12, 1959 Fig.l.

WITNESSES INVENTOR William K. Sonnemonn United States Patent 3,l6tl,7$7 RESTRAHNT tlPERATltih-l RELAY El'ViPLGYiNG ZENER DHGDE Vl iiliam K. .Gonnernann, Roselle Park, Ni, assignor to ilestinghouse Electric Corporation, East Pittsburgh, Pan, a corporation of Pennsylvania Filed Nov. 12, 1e59, Ser. No. 852,3)li 3 Claims. (til. Till-32) This invention relates to protective relays and it has particular reference to protective diilerential relays ernploying static components.

A dil ferential relay is employed for protecting an electrical system or electrical apparatus having terminals through which currents normally enter and leave the system or apparatus against internal faults. To this end the diiierential relay is designed to respond to the difierence between currents entering and leaving the system or apparatus to be protected through the terminals.

in some cases, a differential energization of the relay is obtained in the absence of an internal fault on the system or apparatus to be protected. For example, if the energization of the relay is derived from current transformers associated with the terminals on the system or apparatus to be protected different saturating characeristics of the transformers can result in an undesirable differential output even though the primary currents of the transformers are the same. To guard against improper operation of the relay under these circumstances, it is the practice to restrain operation of the relay to a degree dependent on the sum of the currents entering and leaving the system or apparatus to be protected through the terminals. A differential relay employing such restraint is known as a percentage differential relay. A further safeguard against improper operation of the differential relay is through the utilization of a flared characteristic as described in my Patent 2,240,677 which issued May 6, 1941.

in accordance with the invention, a diilerential relay is designed to respond to the difference between a first electrical quantity and a second electrical quantity when applied to the protection of alternating current systems or apparatus. The first and second electrical quantities are first and second direct voltages obtained by rectification. The difference between these direct voltages provides a third difference direct voltage which is employed for energizing suitable protective translating means. in one embodiment of the invention, the third direct voltage must exceed a predetermined minimum of threshold value in order to operate the protective translating means. In addition, a flare characteristic similar to that described in my aforesaid patent may be employed in the present invention. The utilization of direct voltages facilitates the provision of a differential relay employing static components.

In a preferred embodiment of the invention, the first and second electrical quantities are derived from transformers which are suitably loaded and which are energized respectively in accordance with the difference and sum of currents entering and leaving an alternating system or apparatus to be protected. Conveniently one of the transformers is designed to saturate to provide the flared characteristic, and a threshold value for the third difference direct voltage is established by a device such as a Zener diode which is energized by such voltage.

The flared characteristic also is obtained by including a non-linear resistor in one of the circuits energized by one of the aforesaid direct voltages.

It is, therefore, an object of the invention to provide an improved differential relay having static components.

it is another object of the invention to provide for an alternating current system or apparatus to be protected,

rectifier means for providing direct voltages representing dilierential and restraint quantities together with a translating protective device responsive to the difference between the direct voltages.

It is a further object of the invention to provide a system as defined in the preceding paragraph having a flared characteristic.

it is also an object of the invention to provide in a system as defined in each of the immediately preceding two paragraphs a minimum or threshold value of the difference between the direct voltages which is necessary to operate the translating protective device.

Other objects of the invention will be apparent from the following description, taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a schematic view of a system employing a percentage differential relay embodying the invention; and

FIG. 2 is a schematic View showing a modification of a portion of the system of FIG. 1.

As shown in FIG. 1, a system is provided for establishing a direct voltage across operating terminals represented by terminals of a load resistor 3 which is dependent on internal fault current of apparatus to be protected. This direct voltage is an operating voltage E which is employed for operating suitable protective means. The protective means is restrained against operation by a r straint voltage E which is a direct voltage appearing across restraining terminals represented by terminals of a load resistor 5. The two voltages E and E are connected in series opposition in an electrical loop which also includes a resistor 7. Consequently, the current flowing through the resistor 7 and the voltage appearing across the terminals of such resistor depend on the difference between the voltages across the resistors 3 and 5 and thus depend on the diiierence between the voltages E0 and Although the difference between the voltages E and E may be employed for controlling the energization of a suitable protective device, preferably a minimum or threshold value of this difference is required to produce such operation. To this end, a threshold device 9 is included in the loop circuit which contains the resistors 3, f5 and 7. This threshold device may take the form of any device, such as that known as a Zener diode, which breaks down when the desired minimum or threshold voltage is applied thereacross.

The voltage appearing across the resistor 7 is employed for operating a suitable protective device such as the trip coil ilT of a circuit breaker 11. Although various types of amplifiers may be interposed between the resitsor 7 and the trip coil 111, an amplifier employing transistors has been found to be particularly suitable for the invention. As shown in FIG. 1, the voltage across the resistor 7 is applied through a resistor 13 across the base and emitter of a transistor 15. Although the transistor may be of the PNP type, an NPN transistor is shown in FIG. 1. An output resistor 17 is connected between the collector of the transistor 15 and the upper terminal of the resistors 3 and 5. Consequently, when the transistor 15 conducts, an output voltage appears across the resistor 17 which is applied to the trip coil 11T through a suitable amplifier 1Q.

The voltages E and E have polarities which are represented in FlG. 1

markings.

The operation of the portion of FIG. 1 which thus far has been described may now be set forth. If the restraint voltage E exceeds the operating voltage E a current flows through the resistor 7 in a direction such that the transistor 15 remains nonconducting. Under these circumstances, the circuit breaker 11 remains closed. In

by the positive (-1-) and negative (in order to protect the transistor, a rectifier 21 is connected across the emitter and base of the transistor. Consequently, for the assumed voltage relationship, current flows through the rectifier 21 without building up an excessive voltage across the input circuit of the transistor 15.

Let it be assumed next that the operating voltage E exceeds the restraint voltage E Until the excess exceeds the breakdown voltage of the Zener diode 9 no current flows through the resistor '7. Consequently, the circuit breaker 11 remains closed.

When the difference between the operating voltage E and the restraint voltage E becomes sufficient to break. down the Zener diode 9, current flows through the resistor '7 and current now fiows through the input circuit of the transistor 15. Since current flows through the collector of the transistor 15, an output voltage appears across the resistor 17 and through the amplifier 19, the circuit breaker T1 is tripped.

Tripping of the circuit breaker T1 is employed for protecting suitable electrical apparatus or an electrical system. For the purpose of discussion it will be assumed that the electrical apparatus to be protected takes the form of an electrical generator 23 which is a three-phase generator having

phase windings

25, 25B and 25C. It will be assumed that the generator operates at a frequency of 60 cycles per second.

The left-hand terminals of the three-

phase windings

25, 25B and 250 are connected through externally accessible leads 27, 27B and 27C to ground. The right-hand terminals of the phase windings are connected through externally accessible leads 29, ZfiB and 29C to an electrical distribution system through the circuit breaker 11.

Information concerning the condition of the generator 23 is obtained from

current transformers

31 and 33 which are associated respectively with the

leads

27 and 29 for the phase A of the three-phase generator. For the present the discussion will be confined to circuits associated with phase A. The secondary windings of the two-

current transformers

31 and 33 are connected in the same direction in a series circuit which includes the primary winding of a restraint transformer 35. The terminals of the secondary windings which are connected directly to each other are also connected to ground. The primary winding of an operating transformer 37 is connected between a center tap on the primary winding of the transformer 35 and ground.

With these connections a current flows through the primary winding of the restraint transformer 35 which is dependent on the sum of the currents entering and leaving the phase winding 25 through the leads 2'7 and 29. The current flowing in the primary winding of the operating transformer 37 is dependent on the difference between the currents entering and leaving the phase winding 25 through the leads 2'7 and 29. Consequently, the primary current of the transformer 37 represents fault current for a fault within the generator 23.

The secondary winding of the operating transformer 37 is connected across an adjustable load resistor 39. Consequently, an alternating voltage appears across the resistor 39 which is dependent on internal fault current affecting the phase winding 25. For any value of fault current, the voltage appearing across the resistor 39 may be varied by adjustment of the resistor. The voltage appearing across the resistor 39 is applied to the input terminals of a suitable rectifier 4i and the rectifier output of this rectifier is applied across the resistor 3 to produce the direct operating voltage E The rectifier 41 may be of any desired construction. For present purposes, the rectifier 41 is assumed to be a full-wave bridge-type rectifier. The rectifier 41 may include suitable filtering means if so desired.

For present purposes, a separate filter 43 is illustrated. This filter may be of any desired construction. Preferably, however, the filter has a slight time delay inherent therein. This time delay is desirable for the reason that it assists in preventing undesired relay operation in response to an external fault involving a direct current transient when one of the

transformers

31 or 33 saturates slightly before the other of the two transformers. The specific filter 43 illustrated in FIG. 1 includes

capacitors

55 and 47 on opposite sides of a resistor 49.

In an analogous manner, the secondary winding of the restraint transformer 35 is connected across an adjustable load resistor 51 to produce an alternating voltage thereacross which is dependent on the sum of the currents in the leads 27 and 2.9. The voltage across resistor Sit is applied across the input terminals of a rectifier 53 which is similar to the rectifier ill. The output terminals of the rectifier 53 are connected across the resistor 5 through a manually-operated switch 55. By opening a manuallyoperated switch on, a Zener diode may be connected in the output circuit of the rectifier. For the present the switch as is assumed to be closed.

If desired, the rectifie 53 may include a suitable filter, however, for the present purposes it will be assumed that suificient filtering for the output of the rectifier 53 is provided by a capacitor 57 which is connected across the output terminals of the rectifier.

The performance of the system of PEG. 1 now may be considered for a fault occurring external to the generator 23 and for a fault occurring internally of the generator. Let it be assumed first that a fault Fl to ground appears on phase A to the right of the circuit breaker Tl. Since this is an external fault, substantially equal currents flow through the

transformers

31 and 33 and a substantial restraint alternating voltage appears across the resistor 51. This alternating voltage is rectified by the rectifier 53 and applied after rectification across the resistor 5.

At the same time, virtually no current flows through the primary winding of the transformer 37 and consequently substantially no voltage is applied through the rectifier d1 across the resistor 3. Since a large restraint voltage E is present and a small or zero operating voltage E is present at this time, the circuit breaker it remains closed.

Let it be assumed next that a fault to ground F2 appears between the phase winding 25 and ground inside the generator 25. A restraint voltage E appears across the resistor 5 in the manner previously discussed. Hov ever, the current flowing through the primary windings of the

transformer

31 and 33 no longer are equal and a substantial difference current consequently flows through the primary winding of the transformer 37. This difference current produces a substantial alternating voltage across the resistor 39 and a substantial direct voltage E across the resistor 3. The operating voltage E exceeds the restraint voltage E by an amount sufficient to break down the Zener diode 9 and a substantial current consequently flows through the resistor 7. The flow of current through the resistor 7 is in the correct direction to produce conduction within the transistor 15 and the resultant output of the transistor trips the circuit breaker 11.

From the brief analysis, it is clear that the relay system of PEG. 1 properly discriminates between external and internal faults. if protection is desired only for the phase A, or for a single phase installation, the components associated with the phase A which thus far have been discussed suffice. However, FIG. 1 shows protection for all three phases of the generator 23. Corresponding components for the three phases are identified by the same reference characters except that the sufiixes B and C are added to define components associated respectively with the phases B and C respectively. For example, the rectifiers 41, 41B and 41C identify corresponding rectifiers associated with respectively the phases A, B and C in analogous manners. As a further example, the

rectifiers

53, 53B and 53C denote corresponding rectifiers which are associated in analogous manners with the phases A, B and C.

By inspection of FIG. 1, it will be noted that the rectifiers at, are and 41C are connected in parallel across the capacitor 45. Consequently, the operating voltage E corresponds to the largest voltage output of the three associated rectifiers.

In a similar manner the

rectifiers

53, 53B and 53C have their outputs connected in parallel across the capacitor 57. For this reason, the restraining voltage E appearing across the resistor corresponds to the largest of the output voltages of the three associated rectifiers. With the system thus far described, complete differential protection is provided for the generator 23.

As previously pointed out, the differential relay of FIG. 1 may be provided with a flared percentage differential characteristic. To this end, the operating

transformers

37, 37B and 37C may be designed to saturate in response to energization thereof at high levels of their primary currents. Such saturating transformers provide a flared characteristic in the manner discussed in my aforesaid patent.

Other alternatives are available for altering the sensitivity of the relay under desired conditions. For example, let it be assumed that the switch 55 is opened for the purpose of introducing a varistor 59 in series with the resistor 5. The varistor 59 has a non-linear resistance characteristic. As the voltage across the

resistors

59 and 5 in series increases, the resistance value of the varistor 59 decreases and a larger proportion of the voltage drop in the series arrangement of the varistor and the resistor is present across the resistor 5. Consequently, the restraint voltage E increases at a rate faster than the increase of the voltage appearing across the capacitor 57 and the relay thus has a flared characteristic.

As a further alternative, let it be assumed that the resistor 3 is a varistor. The voltage across the resistor 3 then increases at a rate smaller than the rate of increase of voltage across the capacitor 45 and the relay again presents a flared characteristic.

A further variant is obtained by connecting a Zener diode in the output circuit of each of the rectifiers 53, 53B and 53L. Thus, by opening the switch 6% the Zener diode 602 is connected in the output circuit of the rectifier 53 which now must develop a voltage sufficient to break down the Zener diode before the rectifier is effective to produce restraint.

The transformers illustrated in FIG. 1 are iron core transformers. If the core of the transformer is provided with an air gap, the resistor 51 may be omitted.

The portion of FIG. 1 which is enclosed in the broken line rectangle 61 may be replaced by the modification shown in FIG. 2. The modification of FIG. 2 is identical to that shown in FIG. 1 with the exception of alterations which now will be discussed. The Zener diode 9 of FIG. 1 may be replaced by a resistor, but in the modification of FIG. 2, the Zener diode 9 is not employed. With this modification, the circuit breaker 11 may be designed to trip whenever the operating voltage E exceeds the restraint voltage E Alternatively, the amplifier 19 may be biased to require a threshold value of difference between the two voltages E and E before tripping occurs.

A threshold value of difference between the voltages E and E also may be required by introducing a rectifier 62. in the emitter circuit of the transistor. The rectifier is poled to pass current in the proper direction to produce an output from the transistor. The difference between the voltages E and E must now have the threshold value required to produce a current flow in'the forward direction through the transistor base-to-emitter junction and the rectifier 62.

In FIG. 1 the output circuit of the transistor 15 is.

connected across the resistor 3. in the modification of PEG. 2, the output circuit of the transistor 15 is connected across a battery 63. Consequently, the battery now supplies energy required by the output circuit of the transistor. In addition, the battery may be employed to supply energy for the amplifier i9.

As previously pointed out, a slight delay is introduced by the filter 43. A delay also may be effectively introduced by connecting a capacitor 65 across the rectifier 21 as shown in FIG. 2.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications falling within the spirit and scope of the invention are possible.

I claim as my invention:

1. In a differential protective arrangement for an alternating current system having terminals through which alternating current normally enters and leaves the system, operating rectifying means having a direct operating output dependent on a function of the difference between alternatingcurrent entering and leaving the system through said terminals, restraint rectifying means having a direct restraint output dependent on a function of the sum of current entering and leaving the system through the terminals, non-linear means energized from the output of one of said rectifying means for providing a modified direct output which is non-linear relative to the output of the associated rectifying means, and protective translating means responsive to the difference between the modiled direct output of the last-named rectifying means and the output of the other of the two rectifying means, said non-linear means requiring a minimum difference between the operating and restraint outputs to which the translating means responds which is greater for a large value than for a small value of one of said outputs, said non-linear device comprising a diode which is substantially non-conductive for voltages thereacross in a predetermined direction until the voltage reaches a predetermined value, said diode for values above said predetermined value having a non-destructive breakover which materially reduces the resistance or" the diode to current flow.

2. In a protective arrangement for an alternating current system having a plurality of pairs of terminals, each of said pairs having a circuit therebetween through which alternating current flows, entering the circuit through one of the associated terminals and leaving the circuitthrough the other of the associated terminals, during normal operation of the system, a separate current transformer associated with each of the terminals in each of said pa ts, each of the transformers having a primary connected for primary energization in accordance with current passing through the associated terminal, a separate adjustable restraint load impedance coupled effectively in series with the secondaries of the current transformers of each of said pairs for energization in accordance with the sum of currents passing through the primaries of said current transformers to develop a restrain voltage thereacross, a separate adjustable operating load impedance coupled effectively in parallel with the secondaries of the current transformers of each of said pairs for energization in accordance with the difference between currents passing through the primaries of said current transformers to develop an operating voltage thereacross, separate restraint rectifier means energized by each of said restraint voltages to develop a direct restraint voltage, separate operating rectifier means energized by each of the operating voltages to develop a direct operating voltage, circuit means connecting the outputs of the restraint rectifier means in parallel to produce a resultant restraint voltage dependent on the maximum of the direct restraint voltages, circuit means connecting the outputs of the operating rectifier means in parallel to produce a resultant operating voltage dependent on the maximum of the direct operating voltages, and protective translating means responsive to the difference in said resultant voltages,

said protective translating means comprising a diode connected for energization by the difference between said resultant voltages, said diode being poled to block flow of current when the resultant operating voltage exceeds the resultant restraint voltage below a predetermined amount, said diode having a breakover characteristic arenas? permitting current flow when the resultant operating voltage exceeds the resultant restraint voltage by at least said predetermined amount, and a protective device responsive to current flow through said diode.

3. In a protective arrangement for an alternating cur rent system having a plurality of pairs of terminals, each of said pairs having a circuit therebetween through which alternatin current flows, entering the circuit through one of the associated terminals and leaving the circuit through the other of the associated terminals, during normal operation of the system, a separate current transformer associated with each of the terminals in each of said pairs, each of the transformers having a primary connected for primary energization in accordance with current passing through the associated terminal, a separate adjustable restraint load impedance coupled eiiectively in series with the secondaries of the current transformers of each of said pairs for energization in accordance with the sum of currents passing through the primaries of said current transformers to develop a restraint voltage thereacross, a separate adjustable operating load impedance coupled eiiectively in parallel with the secondaries of the current transformers of each of said pairs for energization in accordance with the diiference between currents passing through the primaries of said current transformers to develop an operating voltage thercacross, separate restraint rectifier means energized by each of said restraint voltages to develop a direct restraint voltage, separate operating rectifier means energized by each of the operating voltages to develop a direct operating voltage, circuit means connecting the outputs of the restraint rectifier means in parallel to produce a resultant restraint voltage dependent on the maximum of the direct restraint voltages, circuit means conn cting the outputs of the operating rectifier means in parallel to produce a resultant operating voltage dependent on the maximum of the direct operating voltages, said circuit means including non-linear means for providing a ratio of the difference between the resultant operating and restraint voltages to the restraint voltage which decreases with an increase in the restraint voltage, and protective translating means responsive to the difference in said resultant voltages, said non-linear means comprising a diode connected to block current therethrough until the voltage across the diode exceeds a predetermined value, said diode having a non-destructive breakover responsive to increase of said diode voltage above said predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 2,240,677 Sonnemann May 6, 1941 2,529,723 Chevallier Nov. 14, 1950 2,735,962 Ellis Feb. 21, 1956 2,775,725 Ellis Dec. 25, 1956 2,863,100 Rice Dec. 2, 1958 2,866,106 Schuh Dec. 23, 1958 2,920,242 Koss I an. 5, 1960 2,933,652 Cuttino Apr. 19, 1960 FOREIGN PATENTS 284,915 Switzerland JQC. 1, 1952 307,865 Switzerland Aug. 16, 1955

Claims

1. IN A DIFFERENTIAL PROTECTIVE ARRANGEMENT FOR AN ALTERNATING CURRENT SYSTEM HAVING TERMINALS THROUGH WHICH ALTERNATING CURRENT NORMALLY ENTERS AND LEAVES THE SYSTEM, OPERATING RECTIFYING MEANS HAVING A DIRECT OPERATING OUTPUT DEPENDENT ON A FUNCTION OF THE DIFFERENCE BETWEEN ALTERNATING CURRENT ENTERING AND LEAVING THE SYSTEM THROUGH SAID TERMINALS, RESTRAINT RECTIFYING MEANS HAVING A DIRECT RESTRAINT OUTPUT DEPENDENT ON A FUNCTION OF THE SUM OF CURRENT ENTERING AND LEAVING THE SYSTEM THROUGH THE TERMINALS, NON-LINEAR MEANS ENERGIZED FROM THE OUTPUT OF ONE OF SAID RECTIFYING MEANS FOR PROVIDING A MODIFIED DIRECT OUTPUT WHICH IS NON-LINEAR RELATIVE TO THE OUTPUT OF THE ASSOCIATED RECTIFYING MEANS, AND PROTECTIVE TRANSLATING MEANS RESPONSIVE TO THE DIFFERENCE BETWEEN THE MODIFIED DIRECT OUTPUT OF THE LAST-NAMED RECTIFYING MEANS AND THE OUTPUT OF THE OTHER OF THE TWO RECTIFYING MEANS, SAID NON-LINEAR MEANS REQUIRING A MINIMUM DIFFERENCE BETWEEN THE OPERATING AND RESTRAINT OUTPUTS TO WHICH THE TRANSLATING MEANS RESPONDS WHICH IS GREATER FOR A LARGE VALUE THAN FOR A SMALL VALUE OF ONE OF SAID OUTPUTS, SAID NON-LINEAR DEVICE COMPRISING A DIODE WHICH IS SUBSTANTIALLY NON-CONDUCTIVE FOR VOLTAGES THEREACROSS IN A PREDETERMINED DIRECTION UNTIL THE VOLTAGE REACHES A PREDETERMINED VALUE, SAID DIODE FOR VALUES ABOVE SAID PREDETERMINED VALUE HAVING A NON-DESTRUCTIVE BREAKOVER WHICH MATERIALLY REDUCES THE RESISTANCE OF THE DIODE TO CURRENT FLOW.