US2276150A - Pilot wire supervision - Google Patents

Description

March 10, 1942- I M. A. BOSTWICK ET AL 2,276,150

PILOT WIRE SUPERVIS ION Filed June 15, 1959 49 P I I I 5 F/gZ. 5

l N s 62 INVENTORS Myron ffifioszw/ck a/n/ Ber! V. Hoard.

BYgZE ATTORNEY Patented Mar. 10, 1942 PILOT WIRE SUPERVISION Myron A. Bostwick, Budd Lake, and Bert V. Hoard, Millburn, N. 1., assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 13, 1939, Serial No. 278,845

7 Claims.

Our invention relates to the supervision of alternating-current pilot Wires for the purpose of detecting all kinds of faults therein, as well as protecting the operators against high voltages which may be induced by inductive interference from parallel power-lines. Pilot-wire faults may consist of short-circuits or crossed wires, open circuits, or grounded conductors. The alternating-current pilot wires to which our invention particularly relates are such a are utilized in protecting a transmission-line against faults.

An object of our invention is to provide novel and effective means for giving the complete pilot-wire supervision just described, or any portion thereof, and to provide pilot-fault-detectors which are utilized either to give an alarm or to short-circuit the pilot wires, either one or both.

A further object of our invention is to provide a pilot-wire supervisory system in which a directcurrent source is associated with only one end of the pilot wire, and a supervisory relay or relays are provided for detecting over-currents, under-currents, ground-currents, or any one or combination of these features.

A still further object of our invention is to utilize a special double-Winding relay in the supervision of pilot wires.

A still further object of our invention is to utilize a plug-mounted supervisory equipment, detachably connected to the insulating transformer at each end of the pilot wire to be supervised.

With the foregoing and other objects in view, our invention consists in the combinations, systems, methods, circuits, and apparatus hereinafter described and claimed, and illustrated in the accompanying drawing, wherein:

Figure l is a simplified diagrammatic view of circuits and apparatus illustrating our invention in a form of embodiment in which complete pro-- tection is provided against over-currents, undercurrents, ground-currents, and induced currents, in the pilot wires, with the electromagnetic relays illustrated in their deenergized positions, features which are unnecessary to an understanding of our invention bein omitted for the sake of clearness;

Fig. 2 is a similar view illustrating a modification, and including an illustration of a doublewinding supervisory relay which may be utilized in carrying out our invention; and

Fig. 3 is a similar view illustrating a still further modification, including an illustration of a plug-mounted detachable supervisory equipment at each end of the pilot wire.

Our invention is illustrated as being applicable to a circulating-current pilot-wire protective system for a three-phase transmission-line section 4, which is illustrated as terminating in circuit-breakers 5 and 6, having trip-coils 1 and 8 respectively.

Our invention is illustrated as being applied to the supervision of a double-wire pilot wire ID, the two ends of which are connected, through insulating transformers H and I2 respectively, to line-current-responsive voltage-producing means or networks, which we have indicated by rectangles marked HCB, at the respective ends of the protected transmission-line section 4, the two networks HCB bein energized from two sets of line-current transformers l5 and It at the respective ends of the line-section, so that each of the networks HCB receives polyphase line-currents and supplies single-phase voltages to an end of the pilot wire I0. There are a number of networks known for answering our foregoing description of our HCB networks, and our invention is not limited to any particular network, although our invention is particularly designed for use with the network which is described and claimed in an application of E. L. Harder, Serial No. 183,044, filed January 3, 1938, Patent No. 2,183,646, granted December 19, 1939, for Belaying apparatus, assigned to the Westinghouse Electric & Manufacturing Company.

The insulating transformers H and [2 are not an essential, but in most practical cases would be utilized, both for the purpose of insulating the terminal equipment from any possible induced voltages which may appear in the pilot wire, and also for the purpose of providing a low-voltage primary winding P and a high-voltage secondarywinding SI and S2, the latter being split into the two halves SI and S2 and being connected to the two wires of the pilot wire it, while the primary winding P is energized from the associated network HCB. As will subsequently be evident, the insulating transformers also provide a convenient and practical means for separating the a1ternating relaying current and the direct supervisory current which we utilize.

In the particular embodiments of our invention shown in the drawing, the terminal equipment at each end of the pilot wire 10 includes a line-protecting relay 2| and 22 at the respective ends, said relays having operating coils which are connected across the primary winding of the associated insulating transformer H or [2, as the case may be, and having make- contacts 23 and 24, respectively, which are utilized to energize the trip-coils 'I and 3, respectively, of the circuit breakers 5 and 8. While we have thus illustrated a simple over-voltage tripping relay 2| and 22 for responding linearly to the magnitudes of the voltages appearing across the respective primary windings P of the insulating transformers I and i2, it is to be distinctly understood that our invention is not limited to this particular sort of tripping relay, and that it may be applied to any known or subsequently invented forms of tripping relays, including those which are illustrated in the aforesaid Harder application.

The illustrated relaying system. is a so-called circulating-current system in which currents are normally circulated in the pilot wire HI, under normal fault-free line-conditions, when the current which enters the line 4 at one end is the same as the current which leaves theline 4 at the other end. In this sort of system, the effective impedance of the pilot wire I'O must be small as compared to the effective internal impedances of the voltage-producing networks HOB which apply voltages to'the ends of the pilot wire in response to the -line-current at'the respective ends of the line 4, so that, under the normal fault-free line-operating conditions, thepilotwire voltage-drop, which appears in the respective primary windings P of the insulating transformers and I2, will be-too small to effectively energize the operating coils of the respective tripping relays 2| and 22. Sucha system becomes inoperative in the event of crossedwires in the pilot-wire channel, because such crossed wires will prevent the primary winding P from ever having sufficient voltage to operate the tripping relays 2| or 22. An open circuit in the pilotwire channel of such asystem, accompanied by a current-flow above the settingof'the relays 2| and 22,- will' inevitably result in a tripping operation, un1ess,-as will subsequently be described, the open pilot-wire channel can be shortcircuited,- as by our supervisory means, within a time which is less than the'time required for the operation of the respective tripping relays 2| and 22.

It is the particular object of our present invention to provide a supervisory system such as will detect any one of a number of different kinds of faults in the pilot-wire channel, so as to give an alarm that the pilot-wire protective system is no longer operative, and in some instances, to also operate to short-circuit the pilot wire so that tripping is no longer possible.

In the particular embodiment of our invention shown in Fig. we also provide protection against induced currents in the pilot wires, by adding equipment, at each end of the pilot wire it, comprising a neutralizing transformer 26, which serves as an equalizing reactor having two windings which are substantially non-inductive to equal currents flowing in opposite directions in the two wires of the pilot-wire channel ll], while imposing considerable impedance to the flower induced currents, which would be flowing in the same direction in each of the two wires. In order to provide a path togl'ound, to make possible the'flow of a small amount of these induced currents, sufficient to supply the magnetizing current necessary to cause the neutralizing transformer-'26 to develop the necessary neutralizing or back-voltage, without providing a low-impedance by-pass or drainage path for the circulating alternating current in the pilot wires, and without draining or grounding the supervisory direct currents which will subsequently be split secondary windings SI and S2 of each of the insulating transformers H and I2 are joined by a relatively low-impedance alternating-current circuit including a capacitor 30 which is relatively large, so that its impedance is sufficiently small to keep the total effective pilotwire impedance small, relative to the internal network-impedance, as previously described. In apractical embodiment of our invention, capacitors 3i) of 10 microfarads capacity have been satisfactorily utilized, one at each end of the pilot wire, although'we are notlimited to any particularcapacity.

In the preferred embodiments of our invention, the supervisory equipments at the two ends of the pilot wire Ill are different from each other, although our invention is not altogether limited to this feature, in some of its more generic aspects.

In Fig. l, the supervisory equipment which is associated with the insulating transformer H comprises a direct-current circuit 32 of relatively high impedance, shunting the capacitor 30 which is connected between the two inner ends of the split secondary windings Si and S2. This directcurrent circuit'32 comprises the ope 'ating coil of a very sensitive under-voltage direct-current supervisory relay 33, the operating coil of a sensitive over-voltage direct-current supervisory relay 34, and a diagonal of a rectifier-bridge 35, the other diagonal of which is energized from the secondary winding 36 of a transformer 31, the midpoint of the secondary winding 36 being grounded through a high-voltage fuse 38 and the operating coil of a sensitive over-voltage direct-current supervisory relay 39. The undervoltage supervisory relay 33 has a back-contact 4|, while the over-voltage supervisory relays 34 and 39 have front-contacts 42 and 43, respectively, and these relays are arranged so that their contacts 4|, 42 and 43 are normally open, during normal pilot-wire conditions when there is no fault on the pilot-wire channel. The three relaycontacts 4|, 42 and'43 are utilized to energize fault-detector or'alarm-circuits, or to perform any other function which can be controlled by the closing of the relay-contacts.

In Fig. l, the supelyisory equipment at the other end of the pilot wire In is illustrated as comprising a relatively high-resistance directcurrent circuit in the form of a resistor 45 shunting the capacity 39 which is connected between the inner ends of the split secondary windings SI and S2 of the insulating transformer l2.

As a result of our use of a high resistance 45 in the direct-current circuit at the receiving end of the supervisory circuit, we obtain a definite increase in the direct-current signal when the pilot wire is shorted at said receiving end. Although our invention is obviously not limited to any particular size of the resistor 45, other than that its resistance should preferably be quite large compared to the impedance of the pilot wire, we

mention, by way of illustration. that we have successfully utilized a resistor 45 having a resistance of 8000 ohms, in conjunction with an over-voltage supervisory relay 34 which picks up, and closes its contact 42, at a current of something like 8 or 9 milliamperes, an under-voltage supervisory relay 33 which is normally energized by the normally circulating direct current of something like 6 milliamperes in the pilot wires I0, said under-voltage relay 33 dropping out at from 2 to milliamperes so as to close its back-contact 4i, and a pilot-wire ground detector relay 39 which is similar to the over-voltage relay 34. It will thus be seen that there is another reason why the alternating-current impedance of the capacitors 30 should be small, in addition to avoiding the introduction of a high alternating-current impedance in the pilot wire 10, because the alternatingcurrent voltage-drop across the capacity 30 must be low enough not to come anywhere near operating either one of the direct-current supervisory relays 33 or 34.

In operation, it will readily be observed from the foregoing description, that our supervisory system which is shown in Fig. 1, will give protection against short-circuits or crossed wires in the pilot channel ID, by the picking up of the over-voltage supervisory relay 34, which responds to the increased flow of operating direct current in the pilot wires, under these conditions,

said relay being sufficiently sensitive to detect shorts of 2000 ohms or less. In the event of a breakage of the pilot wire I 0, resulting in an open-circuitcondition in the pilot-channel, the under-voltage supervisory relay 33 drops out and gives an indication by the closure of its backcontact 4i. Ground-faults on either one of the pilot wires result in the flow of current through the pilot-ground detector 39, giving an indication by the closure of its front-contact 43.

The high-voltage fuse 38, which is included in the direct-current grounding circuit in Fig. 1, is utilized to guard against the remote possibility of encountering simultaneously a pilot-wire ground-fault and considerable induction from the transmission-line 4, or from some other power-circuit running parallel to the pilot wire l0. Such a coincidence would subject the supervisory relays 33 and 39 or 34 and 39 to excessive ground-currents which might destroy the Fig. 1, is merely illustrative, and that our invention is not confined, in its entirety, to any particular form of embodiment, except with respect to the most detailed claims.

In Fig. 2 we show a modification of our invention in which the means for protecting against high induced voltage takes the form of a gasfilled lightning-arrester tube 46, at each end of the pilot wire I0, said tube being illustrated as a three-electrode discharge-gap device of the type described and claimed in the patent of J. L. McCoy, No. 1,649,035, granted November 15, 1927, and assigned to the Westinghouse Electric 8: Manufacturing Company. Two of the electrodes 41 and 48 are connected across the capacitor 30, While the third electrode 49 is grounded. These tubes have a breakdown voltage of 500 volts, so that they will effectively protect the equipment against any induced high voltages in the pilot wire l0.

In the form of our invention shown in Fig. 2,

the supervisory equipment which is associated with the insulating transformer I2 is shown as comprising the operating coil of a sensitive under-voltage direct-current supervisory relay 5!, which may be utilized either in place of, or in addition to, the high resistance 45 of Fig. 1. We have illustrated the supervisory relay Si, in Fig. 2, as having its operating coil connected in series with a resistance 52 which can be adjusted for the purpose of regulating the normal supervisory current which flows through the pilot-channel W.

In Fig, 2, the supervisory relay 5! is provided with a back-contact 53 which is utilized to enerr gize the operating coil of an auxiliary relay 54 which operates sluggishly, as by means of a dashpot 55 or other retarding mechanism, to close its make-contacts 55. The make-contacts 56 are connected across the primary winding P of the associated insulating transformer l2, thus thereafter blocking a tripping operation of the pilot-relay 2|. On important transmission lines, it is often desirable to block pilot-relay tripping when through faults occur on the transmission line, and to rely upon the usual back-up relays (not shown) to clear internal faults. In some cases, however, the blocking of pilot-wire tripping, by means of the short-circuiting contacts 56, may be a disadvantage, as when the supervisory source 3'l35 fails.

In Fig. 2, we also show a modification of the supervisory equipment which is utilized at the other end of the pilot wire l 0, in association with the insulating transformer II, the modification consisting in replacing the short-circuit detector 34, the open-circuit detector 33, and the pilotground detector 39 with a single polarized relay El, which is polarized by means of a permanent magnet 62, having north and south poles as indicated by the letters N and S. The polarized supervisory relay 6! has two operating windings 63 and 64 which are disposed on a common intermediate core-member 65. The polarized supervisory relay 6| is provided with a back-contact 66 and a make-contact 61.

The two energizing coils 63 and 64 of the polarized supervisory relay 5| are connected across the capacitor which is associated with the insulating transformer II, with said operating coils 63 and 64 connected in series with each other, with one diagonal of the rectifier-bridge connected between them, the polarity of the coils 83 and 64 being such that, when current is entering at one coil and leaving at the other, as in the normal circulation of direct current in the pilot-circuit ID, the magnetomotive forces of the two coils add together and partially energize the relay Bl to an intermediate position, such that both its back-contacts E5 and its front-contacts 6'! are open.

The two relay-contacts and 57 are connected in parallel and are utilized to energize the operating coil of an auxiliary relay '74, which operates sluggishly, as by means of a dashpot 15, to close two make-contacts l0 and 11. As in the case of the auxiliary relay 54 at the other end of the supervised pilot-channel It], the relay-contact 10 is utilized to short-circuit the associated primary winding P, and hence the associated tripping relay 2|. The relay-contact Tl may be utilized to energize an alarm-circuit, which, in general, is the primary object of the supervisory circuit.

In the normal operation of the system shown in Fig. 2, the supervisory relays 5i and 6!, at the respective ends of the pilot-channel it, are both energized so as to assume such a position that their relay-contacts are all open. The relay will drop out, and close its bacl -contact 53, in response to either a short-circuit or an opencircuit on the pilot-wires iii, but it will not re spend to a single ground thereon. At the other end, the supervisory relay 6i Jill respond by closing its maize-contact iii in the event of either a short-circuit or a ground on the pilot-channel i0, and it will respond by closing its back-contact 65 in response to an open-circuit or break in the pilot-channel Hi.

The result of a response of either one of the supervisory relaysfii and 8! is to energize the associated auxiliary relay 5 1 or M, as the case may be, thereby short-circuiting the associated primary winding P and short-circuiting the tripping relay ti or 22 as the case may be. The time-delay means, which is conventionally represented by the dashpots and I5, is for the purpose of ensuring that the tripping relays 2| and 22 will operate faster than the auxiliary relays Hi and 5 1, in order to make it possible to secure a tripping operation in response to a fault occurring within the length of the protected linesection 4 at thesaine time when there occurs a light short-circuit or ground on the pilot-channel iii. It is, of course, understood that, when the auxiliary relays 5t and "M respond by closing their respective contacts 55 and It, it will be impossible thereafter to effectively energize the tripping relays 2i or 22.

In both of the previously described embodiments of our invention, the supervisory equipments at the respective ends of the pilot-channel H3 may be mounted either in the substation on the switchboard which is attended by a station-attendant, or they may be mounted directly on the insulating transformers H or l2 with which they are associated, which would place the supervisory equipment out of the sub-station away from the places to which the station-attendant has ready access. -Where the switchboard mounting is utilized, it is usually necessary to provide some means for protecting the station-attendants or operators from induced voltages, as illustrated at 232i il8 and 46, in Figs. 1 and 2 respectively.

In Fig. 3, we show a form of embodiment of our invention which is adapted for utilizing the principle or a direct mounting of the supervisory equipment on the insulating transformer, and We do this by putting each supervisory terminalequipment in an insulated box, 8! and 82 respectively, which is insulatedly mounted on the associated insulating transformer H or l2, as the case may be, and which is electrically connected to the four terminals of the split secondary windings SI and S2 by plug-connections BS-and 84, respectively. The plug- connections 83 and 84 insure easy and safe access to the supervisory equipment-boxes 8i and 82 for inspection and test.

In the equipment illustrated in Fig. 3, the lowimpedanoe alternating-current circuit, comprising the capacitor 35, is permanently connected between the inner terminals of the transformersecondaries SI and S2 at each end of the pilot wire, these capacitors 3i] being preferably placed outside of the supervisory terminals-boxes 8| and 32. At the so-called sending end, the supervisory terminal-box 8| contains a direct-current circuit which is connected in shunt to the capacitor 30, and which includes the operating coil of a n sensitive supervisory direct-current relay 86, a make-contact 81 of thesupervisory relay 86, and a diagonal of the rectifier-bridge 35. The other diagonal of the rectifier bridge 35 is energized, in this instance, from a supply-transformer 88 having well-insulated primary and secondary windings 89 and 93, respectively, for confining the highinduced pilot-Wire voltages to the circuit of the secondary winding 90. The supervisory relay is provided with two back-contacts 9| and 92, the former being, utilized to shortcircuit the pilot wire In, and the latter being utilized as an alarmcircuit. The supervisory relay is also provided with an insulating'handle 93, extending outside of the box 8|, for the purpose of initially actuating the relay 86, by hand, so as to cause the relay to be thereafter held in its'actuated position through the circulating direct current which flows through its operating coil and its make-contact 81.

In Fig. 3, the supervisory terminal equipment in the box 82 at the other end of the pilot wire it consists of a direct-current circuit in the form of the operating coil of a sensitive under-voltage direct-current supervisory relay 95, having a back-contact 96 which is utilized to short-circuit that end of the pilot wire l6.

. In the operation of the system shown in Fig. 3,

the supervisory relay responds, by dropping out and closing its back-contact 96, whenever the circulated direct current decreases as a result of either a short-circuit or an open-circuit in the pilot-channel l0, while the other supervisory relay 86, at the other end of the pilot-channel, responds only to open-circuits which result in a decrease in the circulated direct current in the pilot-channel Hi, this response of the supervisory relay 86 resulting in short-circuiting the pilotchannel, through the back-contact 9|, and energizing an alarm circuit, through the back-contact'92.

In the event of an open-circuit in the pilot- ,ohannel l0, if the short-circuiting of the two ends of the pilot channel, as a result of the closure of the relay-contacts 9i and 96, in Fig. 3, is sufiiciently fast, in coordination with the operating time of the respective tripping relays 2| and 22, it may be possible, in some instances at least, to prevent the operation of these tripping relays 2| and 22 as a result of the open-circuit condition in the pilot-channel 10, in installations in which the settings of the pilot-relays 2| and 22 are below full-load conditions on the protected transmission line 4. It will be understood that an open-circuit in the pilot-channel would ordinarily cause a faulty tripping operation, by impressing on each of the tripping relays 2| and 22, a voltage corresponding approximately to the entire internally generated voltage in the respective I-ICB networks, said voltage being normally shortcircuited by the pilot wire It), so that said voltage is normally almost entirely consumed in the internal impedances of the I-ICB networks.

While we have illustrated our invention in three different forms of embodiment, which are at present preferred by us, we wish these illustrations to be taken in the sense of illustrations, rather than as limitations on our invention, as it will be obvious that many changes may be made, within the skill of the skilled workers, without departing from the essential principles of our invention. We desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

We claim as our invention:

1. In combination, a protected alternatingcurrent line-section, a pilot wire between the ends thereof, line-current-responsive voltage-producing means at the respective ends of the pilot wire for circulating alternating current through the pilot wire during normal fault-free line-conditions, a line-protecting relay at each end of the pilot wire, each line-protecting relay having an operating coil in shunt-circuit relation to the pilot wire, a capacitor in series-circuit relation to the pilot wire at each end thereof, the effective impedances of the pilot wire and the capacitors being sufiiciently small to produce no operation of the line-protecting relays during normal faultfree line-conditions, a shunting circuit around each capacitor, each shunting circuit having a resistance which is larger than the impedance of the pilot wire, a direct-current source serially included in only one of said shunting circuits, said one of the shunting circuits also including the operating means of a sensitive direct-current supervisory relaying-means, said supervisory relaying-means having pilot-fault-responsive backcontacts which are open during normal pilot-wire conditions and which are closed in response to a predetermined decrease in the direct current flowing in the pilot wire, and means responsive to a closure of said back-contacts for connecting a circuit across that end of the pilot wire.

2. In combination, a protected alternatingcurrent line-section, a pilot wire between the ends thereof, line-current-responsive voltage-producing means at the respective ends of the pilot wire for circulating alternating current through the pilot wire during normal fault-free line-conditions, a line-protecting relay at each end of the pilot wire, each line-protecting relay having an operating coil in shunt-circuit relation to the pilot wire, a capacitor in series-circuit relation to the pilot wire at each end thereof, the effective impedances of the pilot wire and the capacitors being sufficiently small to produce no operation of the line-protecting relays during normal faultfree line-conditions, a shunting circuit around each capacitor, each shunting circuit including the operating coil of a direct-current supervisory relay having pilot-fault-responsive back-contacts which are open during normal pilot-wire conditions and which are closed in response to a predetermined decrease in the direct current flowing in the pilot wire, each shunting circuit having a resistance which is larger than the impedance of the pilot wire, a direct-current source serially included in only one of said shunting circuits, and means responsive to a closure of said backcontacts for connecting a circuit across that end of the pilot wire.

3. In combination, a protected alternating-current line-section, a pilot wire between the ends thereof, line-current-responsive voltage-producing means at the respective ends of the pilot wire for circulating alternating current through the pilot wire during normal fault-free line-conditions, a line-protecting relay at each end of the pilot-wire, each line-protecting relay having an operating coil in shunt-circuit relation to the pilot wire, a capacitor in series-circuit relation to the pilot wire at each end thereof, the efiective impedances of the pilot wire and the capacitors being sufficiently small to produce no operation of the line-protecting relays during normal fault-free line-conditions, a shunting circuit around each capacitor, each shunting circuit having a resistance which is larger than the imsupervisory relaying-means having pilot-faultresponsive front-contacts which are open during normal pilot-wire conditions and which are closed in response to a predetermined increase in the direct current flowing in the pilot wire, and means responsive to a closure of said front-contacts for connecting a circuit across that end of the pilot wire.

4. In combination, a protected alternatingcurrent line-section, a pilot wire between the ends thereof, line-current-responsive voltage-producing means at the respective ends of the pilot wire for circulating alternating current through the pilot wire during normal fault-free line-conditions, a line-protecting relay at each end of the pilot wire, each line-protecting relay having an operating coil in shunt-circuit relation to the pilot wire, a capacitor in series-circuit relation to the pilot wire at each end thereof, the effective impedances of the pilot wire and the capacitors being sufficiently small to produce no operation of the line-protecting relays during normal faultfree line-conditions, and a shunting circuit around each capacitor, each shunting circuit having a resistance which is larger than the impedance of the pilot wire, a direct-current source serially included in only one of said shunting circuits, said one of the shunting circuits also including the operating coil of a sensitive directcurrent supervisory relay having pilot-fault-responsive front-contacts which are open during normal pilot-wire conditions and which are closed in response to a predetermined increase in the direct current flowing in the pilot wire, said supervisory relay being slower, in its operation, than said line-protecting relay, and means responsive to a closure of said front-contacts for connecting a circuit across that end of the pilot wire.

5. In combination, a protected alternatingcurrent line-section, a pilot wire between the ends thereof, line-current-responsive voltage-producing means at the respective ends of the pilot wire for circulating alternating current through the pilot wire during normal fault-free line-conditions, a line-protecting relay at each end of the pilot wire, each line-protecting relay having an operating coil in shunt-circuit relation to the pilot wire, a capacitor in series-circuit relation to the pilot wire at each end thereof, the effective impedances of the pilot wire and the capacitors being sufficiently small to produce no operation of the line-protecting relays during normal faultfree line-conditions, and a shunting circuit around each capacitor, each shunting circuit having a resistance which is larger than the impedance of the pilot wire, a direct-current source serially included in only one of said shunting circuits, the other shunting circuit including the operating coil of a sensitive direct-current supervisory relay having pilot-fault-responsive backcontacts which are open during normal pilotwire conditions and which are closed in response to a predetermined decrease in the direct current flowing in the pilot wire, said supervisory relay being slower, in its operation, than said line-protecting relay, and means responsive to a closure of said back-contacts for connecting a circuit across that end of the pilot wire.

6. In combination, a protected alternating current line-section, a pilot wire between the ends thereof, line-current-responsive voltage-producing means at the respective ends of the pilot wire, insulatingtransformers at the respective ends of the pilot wire, each insulating transformer having a primary winding connected across the associated voltage-producing means and a split secondary winding connected across the associated end of the pilot wire, for circulating alternating current through the pilot wire during normal fault-free line-conditions, a lineprotecting relay at each end of the pilot wire, each line-protecting relay having an operating coil connected across the primary winding of the associated insulating transformer, a capacitor connected in series between the split portions of said secondary Winding at each end of the pilot wire, the effective impedances of the pilot wire and the capacitors being sufficiently small to produce no operation of the line-protecting relays during normal fault-free line-conditions, and a shunting circuit around each capacitor, each shunting circuit having a resistance which is larger than the impedance of the pilot wire, a direct-current source serially included in only one of said shunting circuits, supervisory means associated with at least one of said shunting circuits for detecting at least one kind of pilot-wire fault, and means responsive to a response of said supervisory means for connecting a circuit across that end of the pilot wire.

'7. In combination, a protected alternatingcurrent line-section, a pilot wire between the ends thereof, line-current-responsive voltage-producing means at the respective ends of the pilot wire, insulating transformers at the respective ends of the pilot wire, each insulating transformer having a primary winding connected across the associated voltage-producingmeans and a split secondary winding connected across the associated end of the pilot wire; for circulating alternating current through the "pilot wire during'normal fault-freeline-conditions, a line-protecting relay at each end of'the pilot wire, each line-protecting relay having-anoperating coil connected across the primary winding ofthe associated insulating transformer, a capacitor connected in series between the split portions of said secondary winding at each end of the pilot wire, the effective impedances of'the pilot wire and the capacitors being sufficiently small to produce no operation of the line protecting relays duringnormal fault-free line-conditions, and a shunting circuit around each capacitor, each shunting circuit having a resistance which is larger than the impedance ol the pilot wire;a sensitive direct-current supervisoryrelay havingtwo operating coils on a common magnetic circuit, a direct-current source, and a grounding means, one of said capacitorshunting circuits comprising said two operating coils witlrsaid direct-current source and said grounding means connected between them.

MYRON A. BOSTWICK. BERT V. HOARD.

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