US2710368A - Electronic differential relay systems - Google Patents

Description

June 7, 1955 D. 1.. WYLIE ETAL 2,710,368

ELECTRONIC DIFFERENTIAL RELAY SYSTEMS Original Filed May 25. 1950 \B+ 2 l 4/92: 8 \i g 4! I m l n E. %crsnu'in I 53 l as- 001% l Q1 9 l IN VEN TORS DONALD J. MARIHART BY 0. l.

5 DONALD L. WYLiE ited States Patent Donald L. Wylie, Portland, 0reg., and Donald J. Marihart, Carnation, Wash., assignors to the United States of America as represented by the Secretary of the Interior Continuation of application Serial No. 164,122, May 25,

1950. This application July 25, 1950, Serial No. 175,847

7 Claims. (Cl. 317-28) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment of any royalty thereon in accordance with the provisions of the act of March 3, 1883, (22 Stat. 625), as amended by the act of April 30, 1928 (45 Stat. 467, U. S. C., 1946 Ed. Sec.

This application is a continuation of the pending Wylie and Marihart application Serial No. 164,122, filed May 25, 1950, now abandoned.

This invention relates to the protection of high voltage power transmission lines. It is concerned particularly with protective relay arrangements of the kind in which conditions at the two ends of a line are compared to determine the existence of a fault in the line. This invention can be regarded as a variety of pilot Wire relay systems with the unusual feature of using a radio transmission system instead of a conventional pilot wire.

The principal object of this invention is to provide a fast and accurate protective relay system comparing current conditions at the ends of a line, and communicating between the ends of the line through a pilot connection channel. Another object is the production of a relay system which is faster than conventional electro-mechanical relay systems. Other objects are: to provide a system in which relay operation is definitely restrained and operated correctly in reference to line conditions; to provide in a relay system an arrangement of phased alternating and direct current circuits in which two sets of rectified waves overlap to avoid the momentary periodic decrease to zero of rectified pulses characteristic of the ordinary rectified output of a single-phase rectifier; to provide in a relay system, an arrangement whereby a plurality of restraints are present under normal conditions to avoid incorrect operation, in which actuating forces overbalance Y the restraints only under fault conditions; and to provide a system in which the magnitudes and phases of alternating currents inv the system control direct currents which can be directly combined and used simply for operation of protective circuits which are inherently fast in their response to changing conditions.

What constitutes my present invention is set forth, with reference to the accompanying drawing, in the following specification and succinctly defined in the appended claims.

In the drawing, Figure l is a circuit diagram of a preferred form of embodiment of my invention, and Figure 2 is a graphical representation of certain potential diiferences existing in the circuits of my invention under specified conditions.

In Figure l, a conventional polyphase electric power transmission line 1 terminating at substation busses 2 and 3 is equipped with conventional sets of current transformers 4 and 5 at busses 2 and 3, respectively. The usual circuit breakers, 6 and 7, are provided. Current transformer sets 4 and 5 are connected respectively to phase sequence networks 8 and 9 in ways known in the prior art. This 2,710,368 Patented June 7, 1955 invention compares the two single-phase output currents of networks 8 and 9 in novel ways to be described.

The output of network 8 is delivered through a transformer 11 to a radio transmitter 12 which sends a signal to a radio receiver 13. Receiver 13 delivers to input coil 14 of a transformer 15 a signal which is essentially in phase with the output of network 8. The two signals will generally not be in exact phase coincidence due to the finite time required to propagate the power and microwave radio energies between the two line terminals. The communicating channel, in this instance consisting of radio transmitter and receiver, could under suitable conditions consist of solid conductor, transmission line carrier current, or any other applicable inter-connection. The output of network 9 is delivered in a way analogous to that of network 8 to a coil 16 of a transformer 17. Transformers 15 and 17 are interconnected by output coils 18 and 19 and by coils 21 and 22. The resultant voltage of coils 18 and 19 is impressed on the grid of a thermionic triode 23. Similarly the resultant voltage of coils 21 and 22 is impressed on the grid of a triode 24.

Under normal conditions of line operation, the currents in a particular conductor of line 1 are in phase. The corresponding voltages in transformer coils 18 and 19 are phased so as to be additive. The voltages in coils 21 and 22 are phased to be subtractive. If a fault occurs outside line 1, the current therein will usually increase. This will cause the resultant voltage of coils 18 and 19 to increase. The resultant voltage of coils 21 and 22 is normally a small value. When a fault occurs outside the line 1, any increase in current in the line, being the same at both ends, still leaves the resultant voltage at a small value. Actually the resultant voltage of coils 21 and 22 is arranged to be practically zero under normal conditions, and it remains practically zero for faults outside the protected line 1.

When a fault occurs in line 1, some current flows to the fault from both ends, thus producing components of current degrees out of phase between the two ends. These out-of-phase components, when they appear in coils 18 and 19, add to the voltage of one coil and subtract from the other. The resultant voltage of the two coils is thereby decreased in comparison with that voltage which would exist without the presence of outof-phase components. The out-of-phase components, when they appear in coils 21 and 22 increase the resultant voltage from the normal resultant of practical zero. These conditions presuppose proportionality between the output of phase sequence network 8 and the output of radio receiver 13.

The combination of networks 8 and 9, and coils 21 and 22, in the normal condition of practically zero resultant voltage together with the departure under fault condiiCC tions of the resultant voltage provides a reliable method of fault detection by comparing conditions at the two ends of line 1. The additional combination of coils 18 and 19 in which the resultant voltage decreases under fault conditions contributes to additional reliability through differential effect between the two triodes 23 and 24.

There are some fault conditions in which extreme unbalance of current magnitudes at the two ends of the line will cause an increase in the resultant voltage of coils 18 and 19 instead of a decrease as can be expected under ordinary fault conditions. It is necessary therefore to provide an adjustment of ratio between the outputs of triodes 23 and 24 to establish an optimum relationship between their relative effects. A potentiometer 33 connected between the cathodes of triodes 23 and 24 is grounded at the tap 34. Adjustment of the tap 34 changes the ratio of the cathode to ground resistance for the two triodes. Shifting the tap 34 toward triode 23 decreases 2 w the grid bias voltage on triode 23 relative to that of triode The output of triode 24 is delivered to .a transformer 3L which delivers to a full wave rectifier the alternating component of the plate output of triode 24. The output of'rectifier 35 is impressed on a resistor 36 and the grid of a gas-filled triode 37, such as a thyratron. A series resistor 33 and a by-pass condenser 39 are conventional parts of the thyratron circuit. Triode 37 operates a re lay 41, referred to as a trip relay. The contact terminals 42 of relay 4 1 control the actual operation of circuit breaker 7. The inter-connection between terminals 42 and the mechanism of circuit breaker 7 being conventional are omitted from the drawing.

The alternating current component of the output of triode 23 is delivered by transformer 25 through coil 27 to a rectifier 43. The output of rectifier 43 is impressed on a resistor to develop a voltage which is algebraically added to the voltage developed by resistor 36. The resultant voltage across resistors 36 and 44 is impressed on the grid of gas triode 37 the voltages of resistors 36 and 44 are opposite each other. The voltage of resistor 36 referred to as an operating voltage, is in the direction to excite tube 37 to conduct and to operate relay 41. The opposing voltage appearing on resistor 44, referred to as a restraining voltage, always tends to hold the grid of tube 37 negative and to prevent firing.

The output of coil 28 also produces a voltage that tends to restrain tube 37. Coil 28 is connected to the grid of a triode 45 through a resistor 58 and a bias bat- 3T tery 57. The operation of this part of the system is illustrated in Figure 2. The voltage produced by coil 28 is E1. This is a wave sinusoidal in general form composed of the fundamental frequency of the line and some harmonies. This voltage, added to the fixed unidirectional voltage of battery 47 produces a voltage E2 which is impressed on the grid of triode 45. This voltage is composed mainly of a -cycle component if the transmission line 1 is operating, for example, at the usual frequency of 60 cycles per second.

The grid of triode 45 being excited at 60 cycles per second is adjusted to be overdriven so as to produce a rectangular wave output E3 as shown in Figure 2. This wave is impressed on a low pass filter 46 which extracts the 60-cycle component from the constant amplitude wave E3, thus producing a constant amplitude 60-cycle wave E The output of filter 46 is impressed on a rectifier 47 which produces a unidirectional wave E5 composed of rectified sinusoidal pulses per second. E5 is impressed on the grid of a triode 43. This rectified sinusoidal signal drives the grid below cut off so that plate current flows for only a portion of each pulse. By adjusting the cathode selfbias voltage on tube 48 the width of output pulses may be varied as desired.

The output E6 of triode 48 is a series of separated, peaked, triangular-shaped pulses, each having a width at the base of about 45 electrical degrees as shown in Figure 2. These pulses are separated accordingly by spaces about electrical degrees. These pulses are delivered to the grid of a triode 49. Voltage E6 is produced initially across a grounded resistor 51. A condenser 52 transmits pulses of current from resistor 51 to a grounded resister 53 and the cathode of triode 54 connected to act as a rectifying diode.

Triode 54 has low impedance to potential difierences which would make the grid of triode 49 negative and accordingly tends to prevent a negative bias thereon. While a triode tube connected as a diode is shown, this function may be performed by any one of a number of rectification devices Well known to the art. This action is characteristic of arrangements called clamping circuits in radar technique. The voltage Be is made sufliciently large to produce what is referred to as clipping in triode 49 thus producing E7 in the form of flat-topped waves as shown in Figure 2. The plate current of triode 49 prothrough resistor 38. The polarities of duces flat-topped pulses across a resistor 55 to which is .connecteda bias battery 56.

The polarity of triode 49 .in respect to bias battery 56 is such that the potential difference across resistor 55 adds to the bias voltage E9 of battery 56 to produce an increased bias voltage on gas triode 37. The potential difference E3 is of the same polarity as Er which appears across resistor 44 and is also referred to as a restraining voltage. Es, as shown .in Figure 2 fills in the time during which Er becomes momentarily zero, thus producing a voltage E1-+Ea, which under normal conditions always has a value sulficient to prevent tube 37 from firing. This voltage exists to prevent tripping except under fault conditions.

Voltage Es, also by itself, is sufficient to prevent tube 37 from firing under normal conditions. The added voltages Er and E8 together with E9 produce a bias voltage, restraining tube 37, so that tube 37 cannot fire unless there is an overpowering positive bias voltage E11 produced across resistor 36. Under conditions of fault in line 1. E11 increases and Er and E8 decrease. Under these conditions, if E11 becomes sufiiciently large, Er, Es and E9 are all exceeded so that the negative bias voltage on the grid of tube 37 is decreased to the level indicated by the broken line marked tripping voltage in Figure 2, thus permitting tube 37 to become conducting and to operate relay 41.

If, under abnormal but not faulty conditions, triodes 23 and 24 both lose their voltages, Er and Es will vanish.

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lint under these conditions E11 also will vanish. This still leaves E9 to bias the grid of tube 37 and to prevent firing. This is the situation that would exist in case of loss of voltage on line 1 or of failure of radio transmitter 12 or receiver 13.

Figure 1 shows the installation for operating circuit breaker 7 only. The circuit shown in Figure 1 opens circuit breaker 7 when relay 41 closes. A complete protective relay installation of this kind includes a similar arrangement at the other end of line 1 for operating circuit breaker 6. The communication channel, to take care of both ends, needs to be a two-way circuit in Figure 2, the angular values 45 degrees and 135 degrees shown for B6 are illustrative, but not limiting. Thevalue 45 degrees can be any value in a range of about 30 to 60 degrees for the angular width of the pulses and from 120 to degrees for the angular spacing between pulses. in the system described, it will be apparent that the filling-in effect of E8 at the zero points of Er could be accomplished, per se, by a condenser connected across resistor 44, but this would not be operationally equivalent to "the arrangement shown. The condenser if present would hold the voltage under fault, as well as normal conditions for a time of the order of a cycle and this would make the operation of the system slower than is desired. The restraining effect of Er and E3 can be dispensed with if desired, but with some sacrifice of flexibility of control and certainty of operation.

We claim:

1. In a transmission line relay system, the combination of a first phase sequence network at one end of a line, a signal transmission system for producing at said one end of said line a voltage proportional to the output of a second phase sequence network located at the other end of said line, transformers receiving respectively said voltage and the output of said first network, coil connections in said transformers such that a sum voltage proportional to the sum of the inputs of the two transformers, and a difference voltage proportional to the ditference between said inputs of the transformers is produced, two amplifiers responsive'respectively to said sum and difference voltages, rectifiers receiving respectively the alternating components of the outputs of said amplifiers, a series of resistors to which are connected, in opposite polarity, the outputs of said rectifiers, means for adding to said series of resistors a third constant voltage additive to the voltage produced in said series of resistors by the rectifier connected to the amplifier responsive to said sum voltage, and means for operating a relay when the ratio of the voltage derived in said resistors from said sum and additive voltages to the voltage derived 1! said resistors from said difference voltage exceeds a predetermined value.

2. In a transmission line relay system, the combination of means for producing a sum voltage proportional to the sum outputs of phase sequence networks at the two ends of a line, means for producing a difference voltage proportional to the difference between the said outputs, two channels respectively under the control of said sum and difference voltages, each channel containing in sequence, an amplifier, a transformer, and a rectifier, means connecting the outputs of the two rectifiers in opposite polarity to two resistors in series, means for adding to the resistors in series a constant voltage and means for operating a relay when the ratio of voltage produced in said series by said channel under the control of said difference voltage to the sum of the other voltages in said series exceeds a predetermined value.

3. An improved protective relay system for an electric power transmission line comprising means for obtaining two signals responsive to the current at opposite ends of said electric power transmission line; means for comparing the two signals and obtaining sum and difference signals; means for amplifying the said sum and difference signals: an actuating thermionic tube with at least one grid; means for positively biasing the grid of said actuating thermionic tube in proportion to the said difference signal; means for negatively biasing the grid of said relay actuating thermionic tube a predetermined amount; means for full wave rectifying the said sum signal; means for further negatively biasing the grid of said actuating thermionic tube with the full wave rectified sum signal; means for still further negatively biasing the grid of said relay actuating thermionic tube with a signal of roughly square wave form and of frequency twice the fundamental frequency of the current in the said electric power transmission line and of constant amplitude whenever magnitude of said sum signal exceeds a given bias value; means responsive to the piate current in said actuating thermionic tube for opening the said electric power transmission line.

4. In an electrical system of the type comprising relay tube means having a control electrode and a full wave rectifier for rectifying an alternating current and applying the rectified current as bias to said control electrode to control the conductivity of said tube means, a circuit for instantaneously preventing said bias from approaching zero, said circuit comprising means to amplify and limit the magnitude of said alternating current, an electrical filter to extract from said amplified and limited current a constant magnitude fundamental alternating current frequency, a second vacuum tube amplifier, a full wave rectifier to rectify said fundamental frequency and apply negative half waves, equal in number to the number of half waves of the fundamental alternating current frequency, to the grid circuit of said second vacuum tube amplifier properly biased to produce positive peaked unidirectional pulses of approximately 45 electrical degrees duration, and a third vacuum tube amplifier operating in conjunction with a diode clamping circuit to amplify, invert and limit the peaked pulses thereby producing negative, unidirectional, constant magnitude, fiat topped narrow pulses of approximately 45 electrical degrees duration so phased as to overlap in time the zero points of said rectified signal and means for applying said pulses in series with said rectified signal to the control electrode of said vacuum tube to instantaneously prevent the bias thereon from approaching zero.

5. Apparatus for protecting a polyphase transmission line carrying current of a given frequency, comprising, in combination, means at each end of the line for generating counterparts of the three phase currents on the line and for combining and deriving a single phase current from said three phase counterparts, a radio link for transmitting the derived single phase current from one end of said line to the other, means at said other end of said line for deriving from the two single phase currents a sum voltage and a difference voltage proportional respectively to the sum and difference between said two single phase currents, means for rectifying said difference voltage to obtain a positive biasing voltage, means for deriving from said sum voltage a train of unidirectional rectangular pulses and a train of unidirectional sinusoidal pulses, means for phasing one of said trains to overlap the rectangular pulses with the zero points of said sinusoidal pulses, means for combining the so overlapped rectangular and sinusoidal pulses into a negative biasing voltage, a relay, relay tube means connected to actuate said relay, said relay tube means comprising a control electrode, means for connecting said positive biasing voltage and said negative biasing voltage to said control electrode, and means for adjusting the magnitudes of said positive and negative biasing voltages to a point at which they hold said relay tube means non-conducting under normal line conditions and render said relay tube means conducting under line fault conditions.

6. In an electrical system of the type comprising relay tube means having a control electrode and a full wave rectifier for rectifying an alternating current and applying the rectified current as bias to said control electrode to control the conductivity of said tube means, a circuit for preventing said grid bias from approaching zero as said rectified full wave approaches zero, said circuit comprising amplifying and limiting means for deriving from said full wave a constant amplitude alternating current component, means for deriving from said constant amplitude component a series of unidirectional pulses phased to overlap the zero points of said constant amplitude component, and means for combining said unidirectional pulses in overlapping relation to the zero points of said bias and with like polarity thereto and for supplying the same to said control electrode to supply biasing potential thereto during the intervals when the bias afforded by said rectified full wave approaches Zero.

7. Apparatus according to claim 6, in which said means for deriving unidirectional pulses comprises means for full wave rectifying said constant amplitude component, a circuit containing a source of current, said circuit comprising means responsive to said rectified constant amplitude component to interrupt flow of current from said source except when said rectified component approaches zero value, and means for deriving said series of unidirectional pulses from said interrupted current.

References Cited in the file of this patent UNITED STATES PATENTS Re. 19,034 Fitzgerald Dec. 26, 1933 2,027,214 Nideroe Jan. 7, 1936 2,183,646 Harder Dec. 19, 1939 2,213,294 Ward Sept. 3, 1940 2,275,941 Bostwick Mar. 10, 1942 2,399,322 Carlin Apr. 30, 1946 2,406,616 Lensner Aug. 27, 1946 2,406,617 Lensner Aug. 27, 1946 2,408,868 Mehring et a1. Aug. 27, 1946 2,462,799 Young et a1. Feb. 22, 1949 2,523,813 Chevallier Sept. 26, 1950

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