Current Balance and Voltage Compensated Protection Systems Citation Applegate, Lindsay Moritz (1933) Current Balance and Voltage Compensated Protection Systems. Master's thesis, California Institute of Technology. doi:10.7907/d7a9-9d72. https://resolver.caltech.edu/CaltechTHESIS:09162024-173342826 Abstract No abstract. Item Type: Thesis (Master's thesis) Subject Keywords: (Electrical Engineering) Degree Grantor: California Institute of Technology Division: Engineering and Applied Science Major Option: Electrical Engineering Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Thesis Committee: Unknown, Unknown Defense Date: 1 January 1933 Record Number: CaltechTHESIS:09162024-173342826 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:09162024-173342826 DOI: 10.7907/d7a9-9d72 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16731 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 17 Sep 2024 18:24 Last Modified: 17 Sep 2024 18:24 Thesis Files PDF - Final Version See Usage Policy. 8MB Repository Staff Only: item control page

CURRENT BALANCE AND VOLTAGE COMPENSATED PROTECTIVE SYSTEMS by Lindsay M. Applegate A thesis presented for the degree of NJ.ASTER OF SCIEN CE I N ELECTRIC AL ENGINEERING CALIFORNIA IN STITUTE OF TECHNOLOGY 1933 CONTENT~ CURRENT BALANCE AND VOLTAGE COMPENSATED PROTECTIVE SYSTEMS APPENDIX : United States Patent No. 1,901,488 FIGURES FIGURE 1. Two-Phase Current Balance Pr otective System FIGURE 2. Three-Phase Current Balance Protective System FIGURE 3. Voltage Compens a tea: Protective System, Double Coil Relay FIGURE 4. Voltage Compensated Protective System, Single Coll Relay CURRENT BALAI~CE AND VOLTAGE COMPENSATED PROTECTIVE SYSTEMS Lindsay M. Applegate Occasionally the development of a new device in one field of engineering influences the trend of development in another field. An example is the copper oxide type of rectifier which has begun to influence the development of power circuit protective systems. Formerly the lack of convenient, compact, and economical rectifiers inhibited attempts to use rectification in many applications, and, as a result, there was relatively little interest in inventing schemes of measurement or of relay operation which would require rectifiers. The advent of the copper oxide rectifier with its advantages over earlier types has stimulated the invention and development of measuring and relaying systems using rectification. 1 Some new protective relay systems which make use of small rectifiers are described in this thesis. Most protective relay systems at present use induction relays or relays having a plurality of magnetic 1 British Patent No. 315,621, 1929, Australian Patent No. 20,014, 1929, and J.R. Beard of London England U.S. Patent No. 1,779,724, 1930 2 circuits acting together on a mechanical linkage which operates a set of contacts. The co mplication of the existing relay systems is due in part to the condition that in combining two or more alternating currents or magnetomotive forces one must take into account the phase relationship between them. For example, if the two currents of a two-phase system are carried by two equal coils on a single magnetic circuit there will be a resultant alternating magnetomotive force regardless of the relative polarity of the two coils and currents. If the two currents are rectified before entering the coils, the resultant magnetomotive force can be zero if the relative pol a rities of the coils and currents are correct. Thus the rectification of alternating currents permits their combination alg ebraically while if un-rectified they must be combined vedorially. The advantages of al g ebraic combination of uhidirectional. currents obtained by rectification from alternating currents will be seen in the systems to be described. There are still other advantages to be gained from the use of rectifiers in relay systems. can be very simple. oper a tion. The magnetic circuits The relays can be small and very fast in The Weston or other types of polarized relays and various kinds of differentially wound relays can be used. Most power circuits are for t wo- or three-phase alternating current. In general, the phases are balanced. While not absolutely exact, the current balance between phases is 3 usually such that the current in any one phase is not more than a few per cent greater or less than the current in any of the other phases. If such a system be subjected to a short circuit, or a ground, or a broken conductor, the current balance between phases is usually disturbed. A perfectly balanced polyphase short circuit is possible but very unlikely, and we may say that in polyphase circuits faults are characterized by unbalanced currents in the phases. Accor dingly, a protective system whi ch operates when an unbalanced condition exists should be effective for protecting any polyphase circuit. If current transformers are placed in the conductors of a polyphase system, the secondaries of the transformers will carry currents proportional to, and in definite phase relationship with, the currents in the conductors. If these secondary currents are rectified before being passed through the coils of a relay, their effects can be combined algebraically as pointed out above. This ls the underlying principle used in the protective systems shovm in Figures 1 and 2. 1 Figure 1 shows a current balance protective system for a two-phase three-wire circuit. In it there ls the usual two-phase three-wire bus, 4 circuit breaker, a direct current control bus, a three-wire line which may be a 1 See u.SJ?atent No . 1,901,488, March 14, 1933 4 A B 8 ,4 FIGURE 1. Two-Phase Current Balance Protective System L .M.APPLEGATE b feeder, for example, and the current transformers. In series with the secondarle s of the current transformers are some relays of conventional type such as ordinary induction relays. Also in series with the current transformer secondaries are two auxiliary current transformers which in turn deliver current to the current balance relay system. These transformers are not always necessary but they are sometimes desirable to permit using a current balance system rated a t 1/2 or 1 ampere with other relays rated at perhaps 5 amperes. The lower current is particularly advantageous for the economical design of the rectifiers. The lower the current rating of the rectifier, the easier in general it is to provide current carrying capacity for heavy overloads. The secondaries of the auxiliary current transformers are connected to the alternating current terminals of full wave rectifiers. The direct current terminals of the rectifiers are connected to a pair of co lls, one on each of as many relays as there are phase s. That ls, on each relay there ls one coil for each phase. In a two- phase system only one relay ls absolutely necessary, but two are shown in Figure 1. In Figure 1, the letters A and Bare used to refer to the two phases A and B respectively. In each relay, the coil A is connected so tbat its magnetomotlve force is normally opposed to that of the coll B. Thus, when the phase currents are equal the resultant magnetomotlve 6 .force in the relay is zero, but when the currents are unequal a resultant magnetomotive force proportional to the current difference causes operation of the relay. If the two phase s are to opera te normally with an unbalance, the currents in the relays can be adjusted accordingly by the use of a resistance R in parallel with the direct current terminals of the rectifier carrying the greater current. Adjustment in the relay coils themselves is more difficult and introduces undesir able complications in production. For small normal unbalance, adjustment in the relay itself to prevent operation before the magnetomotive force rises to a v alue consistent with fault conditions is tbe most satisfactory method. Although a fault in which the phase currents remain balanced is unlikely to occur, the possibility of such an occurrence should be anticipated by the use of other relays such as overload relays. A perfectly balanced fault, i f it did occur, would probably be of low resistance and high current, providing conditions favorable for the oper a tion of over-current relays. The use of induction overload relays in addition to any other relay system is consistent with usual conserv ative pr a ctice. Breaks i n conductors as well as resistance faults can oper a te the current balance relay system. In over- head lin es, where the vertical distance to the nearest conductors crossing underneath is more th an about fifteen feet, a line in which a break occurs may be expected to 7 be cleared before the broken conductor has fallen into the conductors below. The application of the current balance relay system to a three-phase system ls shown diagrammatically in Figure 2. The general features are as described for Figure 1. In the two-phas e system each relay has two equal coils. In the three-phase system, one of the coils in each relay has twice the number of turns of each of the other two coils. The polarities are arranged so that the magnetomotive force of the coll with the greate r number of turns ls opposed by the magnetomotive forces of both of the smaller coils. As long as the currents in all three coils are equal the resultant magnetomotive force is zero. The large coil of one relay is connected in series wl th a smaJ.l co 11 on each of the other two relays. Designating the three-phase currents as A, B, and C, the magnetomotive forces in the three relays are: 2 B =A+ C, and 20 =A+ B. 2A = B + C, These conditions make It certain that any current unbalance will be effective in one of the larger coils. The current balance system suggests the application of rectification to the co mparison of voltage s and currents. The voltages in an alternating current system are usually not in phase with the currents, under either normal or abnormal conditions. Fo r this reason voltage relays and current relays having a mechanical connection between the t wo relay armatures have been used. The usual scheme ls 8 01 .... -_J -} fJ ) c......=::..r:::::::::::::::p;:it--, C ("-r--+------1r+-il--t-, ,I. -T -) ) B~t::> A (-.-1----, C C-..--+------1-+-11-t-t-, ,.._..,, --:> 0 C '-;::. ~1--+----+-t--HI----' A t''-+---+---.8 c-r--+------ A FIGURE 2. B C Three-Phase Current Balance Protective System 9 to have the voltage force opposed to the current force, so that in case of a fault the operation of the relay is partly controlled by voltage. If t,rrn rectifiers are used, one producing a direct current proportional to the phase current and the other producing a direct current proportional to one of the circuit voltages, the two currents thus provided can be combined in a variety of ways. One arrangement is shown in a voltage-compensated protective system in Figure 3. For convenience in drawing, the system is single phase. The parts of the circuit indicated by the letter C are those carrying a current proportional to a line current. The letter V refers to those parts in which the current ls proportional to a line voltage. reguiooted by a resistor Rv• The latter current ls Coil C of the relay ls arranged so it tends to close the relay contacts while coil V tends to hold them open. Such an arrangement is accomplished most readily by the use of a polarized relay. The ratio of the ampere turns of coils C and V are such that the he4viest overload likely to occur at some predetermined voltage will not open the relay. The resistor R is adjusted depending upon local conditions to give the performance desired under actual conditions. It is not necessary in a voltage compensated system to use separate coils for the voltage and current. By adjusting the magnitudes of the currents in the current and voltage circuits to suitable values, they can be com- 10 bined in the same circuit, and a sing le coil can be used. Such an arrangement is shown in Figure 4. polarized. The relay is Resistors Rv and R0 are provided for making adjustments. The power required for operating one of the rectifier s and its accompanying relay in a circuit of the kind shown in Figures 1 and 2 is 5 or 10 volt-amperes. This requirement may be reduced, but it is usually preferable to use enough power to operate substantial and easily adjusted apparatus. For . circuits using polarized relays of the kind shown in Figures 3 and 4, the voltampere requirement may be of the order of magnitude of that required for the ope ration of indicating instruments. The speed of oper ati on of tbese relay systems is very high. Sys tems of the kind shown in Figure 2 operate apparently instantaneously on polyphase short circuits made to simulate actual conditions. Jfor appreciable unbalance between phase currents, the time of operati on of the relays is not more than one or t wo cycles in a sixty cycle sy.s tem. This is about the same as the maximum speed attained by the fastest alternating current relays. The scope of application and the general features of operation of the current balance protective systems show.a in Figur es 1 and 2 may be compared with those of the so-called ne gative phase sequence systems , in which lack of symmetry of the phase currents in a polyphase system can be detected i n alternating current relays. The 11 C ~rGURE 3. Voltage Compensated Protective System, Double Coil Relay L. M. APPLEGATE 12 C IGURE 4. Voltage Compensated Protective System, Single Coil Relay L.M. APPLE GATE 13 principal operating advantag e resulting from the use of rectifiers in the circuits is that the system is independent of phase relationships and changes in frequency in the polyphase circuit, and functions entirely on the basis of the relative magnitudes of the phase currents. The voltage compensated protective systems shovvn in Figures 3 and 4 have, in general, the characteristics of the alternating current systems classed as impedance relays. They are, however, like the current balance systems, independent of the frequency and phase relationa1ips in the power circuits, operating solely on the basis of relative maa.onitudes. AP P ENDI X 15 1,901,488 Patented Mar. 14, 1933 UNITED ST A TES PA TENT OFFICE LINDSAY M. APPLEGATE, OF SEATTLE, WASHINGTON ELECTRIC CIRCUIT PROTECTIVE SYSTEM Application filed April 5, 1930. Serial No. 441,962. This invention relates to the protection of Figure I is a diagrammatic view illuselectric circuits. Its purpose is to afford trative of my invention as applied to a threemethods of controlling the switching of al- phase circuit. ternating current circuits by means of inFigure II is a similar view, showing, for 5 strumentalities, the operation of which de- purposes of illustration, my invention as ap- 55 pends upon comparison of the magnitudes plied to a two-phase circuit. of currents in the circuit. Reference is hereinafter made to numerals In the operation of most polyphase elec- on the several figures. tric circuits it is usually considered that the A form of my invention as applied to a IO circuit is normal if the currents in the re- three-phase circuit is shown in Figure I. 60 spective conductors of the various phases Energy as three-phase alternating current is are substantially equal, and that inequality delivered to a transmission circuit composed of the magnitudes of the currents indicates of conductors 1, 2, and 3, by a bus 4 connectabnormal · conditions, in which the flo,v of ed with a source of energy 5. Conductors 1, rn current in the circuit concerned should be 2, and 3 receive current from the bus 4 65 interrupted. My invention provides for the through a circuit breaker 6, which is arautomatic detection of inequality of the ranged to open when a trip coil 7 is enermagnitudes of the phase currents in circuits gized. Trip coil 7 receives energy for opof this kind, and the operation of appro- eration from a control bus 8, through con20 priate switches by means of relays respon- tacts of relays 31, 32, and 33. In connection 70 sive to the inequality of the current magni- with conductors 1, 2, and 3, current transformers 11, 12, and 13 deliver alternating tudes. What constitutes my invention will be dis- current to rectifiers 21, 22, and 23. closed in the specification following and will Rectifiers 21, 22, and 23 deliver rectified 23 be succinctly defined in the appended claims. unidirectional currents to coils of the relays '15 The features of my invention common to 31, 32, and 33. Rectifier 21 delivers unidirecall its applications are those involved in ob- tional current to coil 41 of relay 31, coil 51 taining alternating currents substantially of relay 32, and coil 61 of relay 33. Rectifier proportional to the circuit currents, recti- 22 delivers current to coils 42, 52 and 62 30 fying them, comparing their magnitudes, of relays 32, 31, and 33, respectiveiy. Rec- 80 and rendering them effective for the control tifier 23 delivers current to coils 43, 53, and of switching. The comparison of alternat- 63 of relays 33, 31, and 32, respectively. The ing currents could, perhaps, be done by coils of relay 31 are arranged so that the efmethods other than those set forth in this fect of the magnetomoti ve force of coil 41 is 35 specification, but the comparison is greatly equal and opposite to the combined effects of 85 facilitated by rectifying the currents to be coils 52 and 53 when the currents in the coils compared and comparing the unidirectional are equal. Likewise, in relay 32 the effect of currents resulting from the rectification. coil 42 is equal to the combined effects of coils The addition and subtraction of alternating 51 and 63, and in relay 33 coil 43 is equal to 90 40 voltages, currents, and fields differing in coils 61 and 62. phase cannot be performed arithmetically, In addition to the coils connected to the but must be done vectorially. But unidirecrectifiers, each of the relays 31, 32, and 33 tional voltages, currents, and fields derived may have a short circuited coil 70. These therefrom by rectificat ion can be added or 45 subtracted arithmetically or directly in the assist in maintaining uniform magnetic flux 95 same ways as unidirectional voltages, cur- conditions in the relays by compensating for rents, or fields originating from other the pulsating character of the unidirectional currents from the rectifiers. The coils 70 sources. In the accompanying drawings which may be omitted when the inductance of the circuits through the coils connected to the 100 50 constitute a part of this specification, 16 2 1,901,488 rectifiers is sufficient to effect satisfactory opI£ abnormal conditions result in a short eration without them. circuit between two conductors, for example In operation the circuit conductors 1, 2, 1 and 2, the currents in conductors 1 and 2 and 3 carry substantially equal currents. will both be increased, while the current in 5 Current transformers 11, 12, and 13 associat- conductor 3 usually will not be increased, 70 ed with conductors 1, 2 and 3, respectively, but will remain about normal or, possibly, deliver secondary currents proportional to will decrease. The result will be that the their respective line or primary currents in currents in relay coils 41, 51, 61, 42, 52, an<l the usual way. The secondary currents, rec- 62 will be increased and the current in coils 10 tified by the use 0£ any convenient form of 43, 53, a11d 63 will remain at norn1al value or 75 rectifiers 21, 22, and 23, preferably of the less. dry metallic oxide form, because unidirecIn relay 33, with the coils 61 and 62 each tional currents which, in the normal operat- carrying more current than coil 43, the coming ranges 0£ current and voltage, are sub- bined magnetic effect of coils 61 and 62 will 15 stantially equal in effective values to the al- be greater than that 0£ coil 43, so relay 33 80 t~rnating currents from which they were de- will operate and close its contacts. In rela,y nved. In each relay the current of one rec- 31 the current in coils 41 and 52 will be intifier is compared with the currents of the creased, but since co:l 41 has twice as many other two. For example, in relay 31, the turns as coil 52 the increase in the magnetic l!o current 0£ rectifier 21 corresponding to the effect of coil 41 will be greater than that of 85 current of conductor 1 is compared with the coil 52, causing unbalance to exist between currents 0£ rectifiers 22 and 23. coils 41, 52, and 53, thus closing the relay Since the rectifier currents are equal, the contacts. The action in relay 32 will be simcoils must be arranged so that one coil has ilar to that of relay 31. '25 twice as many conductor turns in it as either In the event of a symmetrical or balanced ·IJO 0£ the other two in order for one current to three-phase overload on conductors 1, 2, and equal the combined effects 0£ the two others. 3, current in all three conductors will be inFor example, in relay 31, co.il 41 carries creased uniformly, and balanced conditions the current of rectifier 21. Coils 52 and 53 in relays 31, 32, and 33 will be maintained. So carry the currents of rectifiers 22 and 23, Under actual working conditions, most cir- 95 respectively. The polarities of the coils are cuit faults result in relatively large inequalarranged so that the magnetomotive forces ity in the magnitudes of the phase currents of of coils 52 and 53 are additive, and are op- the part;cular circuit concerned, whereas, in posed by the magnetomotive :force of coil 41. circuits connected thereto but not in fault 35 Since each coil carries the same value 0£ cur- themselves, there is some inequality of phase 100 rent, the number 0£ turns in coil 41 must be currents, but it is usually much less. For equal to the total number 0£ turns in coils this rea?on, it is desirable to design relays 52 and 53 or twice the number 0£ either one 31, 32, and 33 so that their contacts will not alone. Similarly, in relay 32, coil 42 is op- be closed for very small degrees of current 0 posed by coils 51 and 63, and in relay 33 coil inequality, but will be closed for predeter- 105 43 is opposed by coils 61 and 62. When the mined degrees of inequality found from obcurrents in all the relay coils are 0£ the same servation to be immediately associated with magnitude, the resultant magnetomotive circuit faults. This method ot protection can be used adforces in all three relays are zero a,nd the revantageously in conjunction with other 110 ~5 lays' contacts will be open. In case of abnormal conditions in the con- methods of protection as now practiced in ductors 1, 2, and 3, :for example, a fault to the art. In using my invention with others, ground on conductor 1, the current in con- the limitations of design make desirable arductor 1 will increase ( a,ssuming that current rangements such as the one illustrated in 'lio is flowing from the bus 4), while the cur- Figure II. Figure II shows my invention 115 rents in conductors 2 a,nd 3 will remain applied, :for purposes of illustration, to a twoabout normal. The current delivered by cur- phase circuit composed of conductors 81, 82, rent transformer 11 and rectified by rectifier and 83 connected with a source of energy 85 21 will be increased correspondingly. The through a bus 84 and a circuit breaker 86, 55 current through coils 41, 51, and 61 will be which is arranged to open when a trip coil 120 increased, while the currents in the other 87 is energized from a control bus 88 through coils will remain about normal. The result the contacts of relays 131 and 132. The genwill be that in relay 31 the magnetomotive eral operating features of the arrangement force of coil 41 will exceed the combined shown in Figure II are similar to those de.i60 :forces of coils 52 and 53, and the relay will scribed for that of Figure I, except that in 125 close its contacts. In the relays 32 and 33 Figure II a two-phase circnit is nsed, an<l the magnetomoti ve forces will be unbalanced only two of the conductots 'are concerned due to the increased current in coils 51 and in the current comparison arrangeme'nt. It 61, respectively, although by .a smaller is considered that nnder h◊rmal conditions •65 amount than in relay 31. • the cunent in Nmductor 81 is equal 1/o that 130 17 1, 90 1,488 3 in conductor 82, and that the magnetomofr,e For a three-phase arrangement the number :force of coil 141 will equal that of coil 142 of turns in the larger coil is n and in the in relay 131, and similarly coil 151 will equal smaller coils coil 152 in relay 132. n n n 70 Ii In connection with conductors 81 an<l 82, p-1 =3- 1 =2· two current transformers, 91 and 92, are used to deliver secondary alternating currents pro- The same equation can, of course, be apportional to the conductor currents. In con- plied for any number of phases, as long as nection w'th transformers 91 an<l 02, two one coil is provided in each relay for each 10 protective instrumentalities 101 and 10~ are phase, which is the preferable arrangement. 75 provided, such, for e~ample, as _the_ordmary If the number of coils in each relay is difoverload relays used m many circmts. Two ferent from the number of phases, p in 1.he rectifiers 121 and 122, instead of being con- above equation should be the number of coils nected directly to transformers 91 and 92, in each relay. so 15 receive energy through two additional curIn some applications o:f my invention the rent transformers 111 and 112. normal phase currents may not be equal, or These transformers are used because the individual pieces of apparatus may not perdesign of the rect~fiers 121 an1122 is usually form equally on equal currents. In such inimproved by havmg the rectifier current of stances an adjustable resistor 161 in parallel 20 a magnitude ~ifferent fro~ ~hat of the cur- with some of the relay coils will permit the 85 rent used for mstrumentahties 101 and 102. necessary adjustment. For example, the ordinary overload relay is What I claim is : designed to operate on a current of five am1. An electric system comprising a polyperes, while most rectifiers suitable for use phase circuit, means :for obtaining a unidi25 in my invention can be designed more eco- rectional current proportional to each of the 90 nomically to operate on about 0.5 ampere. phase currents o:f said polyphase circuit, Under such conditions the current trans- and relays each having a single magnetic formers 111 and 112 should have a ratio of circuit and a plurality of coils operatively approximately 10 to 1. arranged on said ingle magnetic circuit, the In the operat;on of the arrangement shown system being arranged so that the operation 05 30 in Figure II, a fault between conductors 81 of said rela-vs will be a function o:f the ineand 82 or 81 and 83, or between all three, qualities of "said phase currents. that results in the currents in conductors 81 2. An electric system comprising a contact and 82 being different will cause_ the currents operating relay having a single magnetic cir35 in rect~fiers 121 and 122 to be ~1:fferent, and, cuit, and a plurality of coils, each o:f said 100 accordingly, the ~agnet?motive forces of coils being a part of a circuit containing an coils 141 and 151 will be different from those asymmetric cell connected to one phase of of coils 142 and 152, and hence will cause the a polyphase electric cir?uit, the system ~erelays to close their contacts, energizing tr:p in(l' arran<Ted so that said relay operates its 105 40 coil 87 and opening circuit breaker 86. co;tacts ~hen the currents of said polyIt will be observed that in Figures I and phase circuit become unequal. II one relay has been supplied for each 3. A protective relay system for polyphase phase. The number of coils on each relay alternating current systems which consists is like,vi se the same as the number of phases. of means :for causing the flow o:f unidire'c- llO 45 In each instance, one coil of each relay is tional currents which are substantially proopposed by the other coils of the relay. In portional to the respective phase curre:nts a two-phase system, each relay has two equal in said A . C. system, and means for causmg and opposed coils. In a three-phase system: said unidirectional currents to coact in coneach relay has one large coil opposed by two tact operating relays arranged to opera~e u 5 50 smaller coils, each with half the number of their contacts when the phase currents m said turns of the larger coil. The relationship A. C. system become unequal. of the number of turns in the large coil to 4. An electric protective system comprising the number 0£ turns in each of the smaller contact operating relays each having a sincoils is found by dividing the number o-f gle magnetic circuit and a plurality of coils,_ 1:20 65 turns in the larger coil by the n~m~ber of smaller coils. Since there 1s one coil m each said coils being connected to asymmetric cells relay for each phase the number of s~aller receiving current :from a polyphase circuit, coils in each relay is p-1, where p is the said system bein<T arranged so that said co1ls number of phases. The number of turns in carrv·unidirectiinal currents which are sub60 the smaller coils is therefore the number of stantially proportional to the currents of said J 2 ;; turns, n, in the larger coil divided by p-1. polyphase circuit, said relays operating their For a two-phase arrangement, the number contacts when the currents o:f said polyphase of turns in each coil is circuit become materially unbalanced. 5. An electric protective system which 1 ip._ connection with a polyphase system, consists 130 18 4 1,901,488 of means for obtaining alternating currents from each phase of said polyphase system proportional to the respective phase currents, means for rectifying said alternating cur.., rents, and relays having single magnetic circuit for comparing said rectified currellts with each other, said relays being capable of causing inequality of said rectified currents to control switching in said polyphase sys- v tern. In testimony whereof, I have hereunto set my hand. LINDSAY M. APPLEGATE. zo 30 35 40 45 50 55 60 61 19 March 14, 1933. 1,901,488 L. M. APPLEGATE ELECTRIC CI~CUIT PROTECTIVE SYSTEM 1 I Filed April 5, 1930 2 Sheets-Sheet l 5 6 2 21 F\GURE I INVENTOR: LINDSAY M. APPLEGATE; sv~Y~ ATTORNE'(S, 20 March 14, 1933. 1,901,488 L. M. APPLEGATE ELECTRIC CIRCUIT PROTECTIVE SYSTEM 2 Sheets-Sheet 2 Filed April 5, 1930 8 86 z f'IGURE JI INVENT Oft: L\NDSAY M. APPLEGATE1 8Y J/f:A,;_ ¥- ~ ~ ATTORNEV.S,