CA1136727A

CA1136727A - Transformer arrangement for coupling a communication signal to a three-phase power line

Info

Publication number: CA1136727A
Application number: CA000333816A
Authority: CA
Inventors: William C. Perkins
Original assignee: Rockwell International Corp
Current assignee: Boeing North American Inc
Priority date: 1978-08-17
Filing date: 1979-08-15
Publication date: 1982-11-30
Also published as: DE2933473A1; FR2433863B1; ATA556979A; GB2028065A; SE7906860L; AU4981579A; FR2433863A1; NZ191226A; AU525935B2; IL58085A; GB2028065B; AT366213B; US4188619A; DK340479A

Abstract

PATENT APPLICATION

ON
TRANSFORMER ARRANGEMENT FOR COUPLING A COMMUNICATION
SIGNAL TO A THREE-PHASE POWER LINE

BY

WILLIAM C. PERKINS

Abstract of the Disclosure A communication signal is coupled simultaneously to all three phases of a neutral-wye three-phase line through a transformer arrangement consisting of a low voltage set of three serially connected windings across which the signal is applied to produce the same current in all three windings and a high voltage set of three windings which are connected in a neutral-wye configuration to the power line with a winding of each set being mag-netically coupled to a different winding of the other set such that the currents induced in all three windings of the high voltage set flow in the same direction with respect to the neutral connection.

Description

~3~'72~

TRANSFORMER ARRANGEMENT FOR COUPLING A COMMUMICATION
SIGNAL TO A T~IREE-PHASE POWER LINE

BACKGROUND OF THE INVENTION

The present invention pertains generally ~o transmitting communication signals over three-phase power lines and particularly to a transformer arrangement for coupling the communica~ion signals to the power line.

15 Although the transmission of communication signals over three-phase power lines, for example, to monitor and control the electric power system itself, has been suc-cessfully addressed heretofore, the need by electric power companies for monitoring and controlling directly 20 customer loads to combat increasingly intractable energy problems has created a need for new and different types of communication techniques. When the power line, it-self, is used as a communications medium as an alter-native to telephone circuits sr radio waves for trans-25 mitting data to and from utility customer sites, all ofthe customer loads served by a three-phase power distri-bution feeder are normally moni~ored and controlled from a central site via the distribution substation which ; supplies the feeder. Since these customer loads are individualLy connected between one of the phase con-ductors and the neutral conductor in a three phase neutral-wye system or between two of the phases in a --2~

three wire delta system, the transceiver at the su~-station for transmitting command signals to the customer loads and receiving monitor signals therefrom must be capable of coupling the signal onto all three phases since the loads are always distributed among the three phases in order to afford a balanced three phase load condition. This coupling may be accomplished by the use of three transceivers, there being one for coupling signals of a much higher frequency than the 60 hertz power frequency to each of the three-phase conductors or, alternatively, a signal transceiver which is sequen-tially connected to the three-phase conductors so that all of the loads respectively connected thereto can be connected to the transceiver for communication pur-poses. The former approach is not cost effective since it constitutes equipment redundancy while the latter approach entails a switching mechanism which not only adds to the cost and diminishes the reliability of the equipment, but also increases time required for broad-cast commands and adversely affects the signal wave 25 which is propagated down the power line. A third alter-native is to couple the communication signal to all three phases simultaneously such as described in U.S.
Patent No. 4,065,763, which issued to ~hyte, et al. The transformer arrangement depicted therein of a single 30 phase transformer with a multiple tap high voltage wind-ing has notable disadvantages in that unbalanced phase conditions or tap settings result in circulating com-munication signal currents in the transformer which de-tracts from efficiency and the operation of the trans-35 ceiver which is already burdened with translatingsignals in a difficult environment.

In view of the foregoing, it is a primary object of the present invention to provide a new and improved trans-40 former arrangement for coupling a communication signal ~3~

simultaneously onto all three phase conductors of athree-phase power line.

It is a further object of the present invention to pro-vide such a new and improved transformer arrangement which is relatively inexpensive and which optimizes the transceiver performance.

~he foregoing objects, as well as others, and the m~ans by which they are achieved through the invention herein, may best be appreciated by referring to the Detailed Description Of The Invention which follows hereinafter, together with the accompanying drawing.

BRIEF DESCRIPTION OF THE INVENTION
In accordance with the foregoing stated objects, the present invention entails simultaneously coupling a com-munication signal to all three phases of a neutral-wye three-phase power line through a transformer arrangement which consists of a low voltage set of three serially connected windings across which the signal is applied to develop the same current in all three windings and a high voltage set of three windings connected in a neutral-wye configuration to the power line, with each winding of one set being magnetically coupled to a different winding of the other set so that currents in-duced in the high voltage windings flow in the same direction with respect to the neutral connection. This arrangement minimizes transformer circulating currents and assures that the AC three-phase distributlon volt-ages applied across the high voltage windings are not applied to the transceiver equipment by transformer action via the low voltage windings since the three voltages induced therein by the AC distribution voltages are cancelled upon vector addition. A second embodiment of the invention which affords coupling a communication ~fj7~7 01 -~-03 signal simultaneously to all three-phase conductors of 04 three-phasP delta system entails -the use of a three-p~ase ope~
05 delta tra~sformer with the co~nmunicatior signal being applied 06 across the low voltage pair of wi~dings.

08 More generally, the inve~tion is a signaling system comprising 09 three-phase power li~e comprising three phase conductors and a neutral conductor, an AC sig~aling device having a pair of 11 terminals through which the signal i9 translated, and a 12 transformer having a low voltage set of three like windings 13 serially conr~ected, with one end of each of two of the wi~di~gs 14 being connected to a different one of the terminals. A high voltage set of three like windings is con~ected in a wye-neutral 16 co~figuration to the power line, and each winding of one set is 17 magnetically coupled with a differe~t winding of the other set 18 so that a curre~t through the low voltage set of windi~gs causes 19 currents to flow through the high voltage windi~gs in the same directio~ with respect to the ~eutral connection. A single 21 phase distribution transformer has its primary winding connected 22 between oDe o the three phase conductors and the neutral 23 conductor of the power line and has its secondary windin 24 providing a low voltage for customer utilization. A low voltage distribution circuit is connected across -the second winding of 26 the single phase transformer, and a transponder is con~ecte 27 across the distribution circuit for receiving and responding to 28 signals from the AC signaling device.

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32 Figure 1 depicts a first embodiment of the invention designed 33 for use with a neutral-wye three-phase power Line.

Figure 2 depicts a second ernbodiment oE the inventior~ designed 36 for u~e with a delta three-phase power line.

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1~1L3t~Z7 01 -4a-03 DET~ILED DESCRIPTION OF THE INVENTION

05 As symbolically shown in Figure 1, a three-phase power 06 distribution line designated generally by the reference numeral 07 10 consists of three phase conductors ~-C and a neutral 08 conductor ~. The distribution line 10, which most 09 conventionally would assume a nominal voltage of 12 KV across any two phases thereof, is connected to the output terminals 12 11 of a three-phase step-down transformer 14 having low voltage 12 windings 16 conventionally arranged in a wye configuration and 13 high voltage windings 18 connected in a delta configuration.
14 High voltage ter~inals 20 connect the transformer 1~ to the high voltage transmission line represented generally by reference 16 numeral 22.

18 A transceiver 24 is located at the substation for generating and 19 sending command signals to be transmitted via the distribution line 10 to utility customer sites for controlling the loads and 21 functions thereat, as well as for receiving signals from the 22 customer sites in order to monitor the loads. The transceiver 23 24, which -r~
;~.

, ~L~L3~7 may consist of a separate transmitter and recelver or a single unit integrating both unctions, is well known in the art and need not be de~ailed herein. ~he trans-ceiver 24 block is intended to denote all of the equip-ment necessary for generating and modulating a carrier signal of suitable frequency, e.g., 3~10 kHz, for trans-mission to a customer site over the dis~ribution line 10, demodulating and detecting a carrier signal trans-mitted from the customer site over the power line 10 and further includes the attendant power supply. The com-munication signal is coupled to or from the distributionline 10 via low voltage terminals 25 through a trans-former arrangement 26 consisting of a set of three like low voltage windings 28 serially connected (in split delta fashion) across which the communication signal is applied or developed so that the same current IL flows through all three windings 28. A set of three like high voltage windings 30 is connected in a wye configuration to the phases A-C of power line 10 via high voltage ter-minals 31 with its neutral terminal connected to the neutral conductor N of power line 10 via terminal 33.
Each of the windings 30 is magnetically coupled to a different one of the low voltage windings 28 (by virtue of a single, three-phase transformer or three single-phase transformers) so that, as denoted by the conventional dots of Figure 1, the currents IH induced in high voltage windings 30 by the current IL flowing through windings 28 all flow in the same direction with respect to the neutral connection. It is to be noted that, since all three currents, IH, must be equal to one another since they are all proportional to the same current IL by the ~ame turns ratio (the windings of each set being alike), no circulating currents ar0 developed in the transformer 26 as the pos~ible result of unbalanced phase conditions in the distrlbution line 10, thus avoiding any signal degradation which would otherwise be caused thereby. Moreover, the low voltage '72~7 5 winding split delta arrangement assures that the 60 hertz AC voltages induced in the low voltage windings 28 from the high voltage windings 30 are cancelled when vectorially summed across terminals 25 so that no 60 hertz voltage is applied to transceiver 24 except that 10 due to residual unbalance. This permits the use of low voltage solid state equipment without the need for artificially blocking the 60 hertz AC such as through a large capacitor.

15 The utility customer loads are supplied from single phase feeders, generally designated as 32 from the dis-tribution line 10, which are connected to different phases thereof so as to impose on the line 10 an approximately balanced load condition. After the dis-20 tribution voltage i~ stepped down to customer utiliza-tion voltage, nominally 120/240 volts through dis-tribu~ion transformers 34, the electricity is routed to the various customer sites through low voltage dis-tribution circuits 36. Although the individual customer 25 loads are not pictorially represented in Figure 1, they would be connected to the low voltage distribution cir-cuits 36 at the same points as are transponders 38, there being an individual transponder 38 provided for each customer site. The transponder 38, which is well 30 known in the art and need not be described in detail herein, responds to the command signals received from transceiver 24 by either effecting some load control ` function or transmitting back to the transceiver 24 some load information which is being monitored. Accordingly, 35 each transponder 38 is understood to contain a trans-ceiver, itself. Individual transponder control is effectuated by encoding in the command signals the address of the transponder 38 to which the command signal is directed, there being a unique address for 40 each transponder 38. Since the command signals are applied to all three phases A-C of the distribution line ~3~t727 lO, it makes no difference which phase the addressed transponder 38 is connected to since it will always receive the signal over one of the three phases. Like-wise, since the transceiver 24 receives response signals over all three phases of the distribution line 10, it lO makes no difference which transponder 38 is transmitting since the signal will be received via the appropriate phase conductor.

In comparison measurements between single-phase and split delta coupling techniques using standard distribution transformers at the substation and mea-suring between typical substation and residential loca-tions, test data shows the split delta configuration provides 3 dB to 6 dB advantage in transfer impedance in the 3 kHz to 10 kHz ranges. That is, the split delta configuration requires 3dB to 6 dB less transmitted cur-rent to produce the same received signal level in either direction. Thus, the signal levels required for com-munication are materially reducPd through use of the 25 split delta coupling configuration.

Under conditions outlined above, the nominal 120-volt split delta termination presents to the communication transmitter and receiver a power frequency fundamental (60Hz) level of less than two volts rms as compared to the full 120 volts rms with the single-phase ter-mination. The level of the power frequency third har-monic (180 Hz) is dom;nant in the split delta ter-mination at typically less than five volts rms. From 3 35 kHz to lO kHz, power frequency harmonic levels to the two configurations are comparable and less than one millivolt. Thus, without apparent operational penalty, the problem of blocking and protecting the transmltter and receiver from the power frequency is notably 40 mitigated by the split delta coupling configuration.

~ 2 ~

An additional benefit of the aforedescribed arrangement is the reduced likelihood that the communication signal applied to the distribution line 10 will interfere with the receiption of the communication signal applied to the distribution lines via the high voltage transmission system including transformer 14. This is attributable to equal currents IH being applied ~o phase conductors - lOA-lOC, so that any equal components thereof which pass through the low voltage windings 16 of transformer 14 rather than out onto the line 10 merely induce a cir-culating current in the high voltage windings 18 which does not get coupled onto transmission line 22 For transmission to other distribution points. Only in the event of an unbalanced impedance condition might some portion of the communication signal be so coupled.
A test at a distribution station showed that under like conditions applying the communica~ion signal to all three phase conductors rather than a single phase there-of resulted in a 3 dB increase in signal attenuation, and consequently reduced cross-talk, as measured at sur-rounding distribution stations.

It will be readily recognized that the transformer arrangement depicted in Figure 1 is not suitable for a 30 delta three-phase system. The problem of transmitting communication signals over this type of system is obviated by the transformer arrangement depicted in Figure 2 wherein a three-phase open delta transformer arrangement 40 consists of two high voltage windings 42 40 connected in open delta fashion to high voltage ter-minals 44 while the two low voltage windings 46 are ser-ially connected to low voltage terminals 48 across which the communication ~ignal is applied or developed.
Since, in this arrangement, the 60 hertz AC voltages, 45 induced in the low voltage windings are not cancelled , , _9_ out, a capacitor 50 interconnects the transceiver 24 with the transformer arrangement 40 to block the appli-cation of high 60 hertz voltage to transceiver 24 As the foregoing demonstrates, the transformer arrange-ment of the subject invention allows communication signals to be coupled to and from a power distribution line using transceiving equipment which is of conventional state of the art design, thereby affording cost effectiveness and simplified operation. Since, undoubtedly, modifications to the foregoing embodiments can be made by those skilled in the art without depart-ing from the scope and spirit of the invention, the de-tailed description herein is intended to be merely ex-emplary and not circumscriptive of the invention, which will now be claimed hereinbelow.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A signaling system, comprising:
a three-phase power line comprising three phase conductors and a neutral conductor;
an AC signaling device having a pair of terminals through which the signal is translated;
a transformer having a low voltage set of three like windings serially connected, with one end of each of two of said windings being connected to a different one of said terminals, a high voltage set of three like windings connected in a wye-neutral configuration to the power line and means for magnetically coupling each winding of one set with a different winding of the other set so that a current through said low voltage set of windings causes currents to flow through said high voltage windings in the same direction with respect to the neutral connection;
a single phase distribution transformer whose primary winding is connected between one of the three phase conductors and the neutral conductor of said power line and whose secondary winding provides a low voltage for customer utilization;
a low voltage distribution circuit connected across the second winding of said single phase transformer; and a transponder connected across said distribution circuit for receiving and responding to signals from said AC
signaling device.

2. The system of claim 1 including a second single phase distribution transformer whose primary winding is connected between one of the other three phase conductors and the neutral conductor of said power line and whose secondary winding provides low voltage for customer utilization;
a second low voltage distribution circuit connected across the secondary winding of said second distribution transformer, and a second transponder connected across said distribution circuit for receiving and responding to signals from said AC
signaling device.