CA1058089A

CA1058089A - Power line carrier communication system for signaling customer locations through ground wire conductors

Info

Publication number: CA1058089A
Application number: CA267,467A
Authority: CA
Inventors: Leonard C. Vercellotti; Ian A. Whyte
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1976-01-16
Filing date: 1976-12-08
Publication date: 1979-07-10
Also published as: US4016429A; JPS52109815A

Abstract

A POWER LINE CARRIER COMMUNICATION
SYSTEM FOR SIGNALING CUSTOMER LOCATIONS
THROUGH GROUND WIRE CONDUCTORS

ABSTRACT OF THE DISCLOSURE

Carrier current signals are impressed to and detected from grounding conductors connected to the power system and to real earth ground. The communication links provided by the grounded neutral conductors of the power system and by the real earth ground return path transmit the carrier communication signals between a substation communication terminal and remote communication terminals at the premises of utility company customers. The communication signals are coupled to the ground conductors by transformer-type couplers in which the grounding conductor constitutes one winding of the transformer.

Description

BACKGROUND OF THE INIENTION
Field OI' the Invei~.t.~ on.
This invention relates, .in genera:, co powe.r line carrier communica-tion 3-y3tems and3 mor speciflcally3 tG distribution network power line communica.tlon systems which transmit carrler current communlcation signals throu~h commonly grounded neutral llne conductors, Description of the Prlor Art:
It has been found desirable to improve techniques for establishing communicatlon links through the power conductors of a power distribution system. These communicatlon links provide communication data and/or control information between the customers OI an electrical utility company and a central communication termlnalO

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44,042 When the central communication terminal is located at a substation site, communication signals can be transmitted through the power line conductors of a distribution network to the remote terminals at individual customer premises. This avoids expensive and alternate communication transmission lines and/or radio link systemsO
Some conventional systems include transmitter and receiver circuits which impress and detect high-frequency carrier signals between pairs of power lineconductors of a distribution network. Frequency translating and signal reconditioning repeaters are used to amplify and retransmit the power line communication signal through common pairs of power line conductors forming the communication linksO
It has been recognized that the neutral line conductors of a distribution network provide a continuous unbroken metallic conduction path between a substation and subdistribution points, including distribution transformers, and to the secondary distribution portions serving .~ -:
individual utility company customers. These neutral line conductors are connected to real earth ground at multiple grounding points by grounding conductors located at the customer premises, the distribution points, and at various other equipment locations associated with the distribution ;-network, as well as at the substations which feed the distribution networks. ~ ;
A power line carrier communication system is ~ ~/~ 7 described in U.S. Patent No. 3,702,460, ~ e~-*~ember-~
o ~: ' -~7-~-b~ J. B~ Bloose, ln which communication signals are

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L~4,0L12 impressed onto a neutral conductor and a phase conductor.
The phase conductor is also referred to as a line or hot conductor as distinguished from the neutral conductor which is connected to the system ground distribution network. The communication links for the communication signals are established between points where the neutral conductor is isolated from the true earth grounding points at the communication signal frequency being usedO This isolation is provided by a separate circuit networkO Thus, 10 the conductors, including the neutral line which provides -the transmission link for communication signals~ are isolated from the true earth ground. This requires expensive isolating circuit arrangements to afford the communication links desired between remote terminals, such ;
as provided at customer premises and at a substation site~
Therefore, it is desirable, and it is an obJect of this invention, to provide a communication system for ~ , power line networks which uses only the grounded~neutral ,-conductor of the power distribution network to provide the metallic conductor portion of the communication link.

SUMMARY OF THE INVENTION

There is disclosed herein a new and useful distribution network power line carrier communication system for transmitting hi~h-frequency carrier current ~ .
signals over grounded~neutral conductors of a distribution network and for affording a signal return path through real earth ground. The carrier current communication signals are coupled to grounding conductors connected between the neutral line conductors and real earth ground.
Current transformers having ferrite magnetic cores couple 44,0L12 ~35~3~8~

the carrier current signals to the grounding conductors.
A central or interrogation communication terminal is coupled at the grounding conductor located at a sub-station site and a remote response communication terminal is coupled to a grounding conductor located at a customer premise7 , Low-impedance current transformer circuits are used to couple the carrier current signals to and from ~ the neutral line conductor in ~hich theAturns ratio of 10 the transformer primary and secondary windings may be ,:
sec~
adjusted to provide the proper~impedance at the frequency utilized for the carrier current communication signals.
Coupling at the substation is provided at a separate grounding conductor distinguished from the grounding conductor normally associated with the substation equlp-ment so as to avoid large currents which ~low may in the ,~
regular substation grounding conductor.
The carrier current signals are confined to multiple loops, including each o~ the grounding means or ; ;~;
conductors which connect the neutral lines of the distri~

bution network to real earth ground. Each loop dissipates ~.~ .. . .
some of the carrier signal. However, by using ~ulat~
methods which provide good communication reliability with relatively high signal-to-noise ratios, sufficient signal is available at remote customer premises to interrogate ~` -an energy meter or to control the power delivered to the load.

The distribution network of this invention may be provided with a repeater arrangement so as to increase the range of carrier signals transmitted between the sub-,. . ~ . . ........................................ . ..
.
, ', , : , '. ' ',. :, ~'' 44,0~2 station and the customer premise. Short circuits occurring in the distribution network have negligible additional effect upon the communication links since multiple points of the neutral line conductor are already connected to real earth ground through the multiple grounding conductors included therein. ~ , Low cost magnetic core transformers are utilized to couple the communication signal to the ;
ground wire conductor and to detect communication signals.
The magnetic core transformers are simple and easily applied to the grounding wire by completing the core around the ground wire conductor. The low-impedance circuits of the current transformers are particularly advantageous since they do not interfere with the normal and proper use of the grounding wire conductor at the power current frequencies, typically in the order of 50 to 60 Hz. The magnetic core current transformers may include resonant circuits to change the circuit impedance to desired values and to aid in increasing the amount of communication signal uvàilable for enhancing the efficiency of the transmission system. ~ ;
It is a general feature of this invention to provide a ground wire distribution network power line carrier communication system in which communication signals are coupled to a grounding wire conductor f'or transmission through a common neutral conductor of a power network and utilize earth ground as a communication signal return path so as to avoid the use of the line or hot conductors of the power networkO Another feature of

3 this lnvention is to lmpress and detect carrier current - 44,042 . , signals by current trans~ormers coupled to the grounding wire conductors which are connected to a neutral line conductor of a distribution network and to real earth ground. ;
BRIEF DESCRIPTION OF TH~ DRAWING ~ -Further advantages and uses of this invention will be more apparent when considered in view of the .~ .
following detailed description and drawing, in which, :~
Figure 1 is a combined block and schematic 10 diagram o~ a ground wire distribution network power line ..
carrier communication system constructed in accordance with `
the present invention;
Figure 2 is a o-ne-line schematic diagram of carrier current signal paths existing in the system `
illustrated in Figure lj Figure 3 is a fragmentary portion of the system illustrated in Figure 1 including a signal repeater;
Figure 4 is a view of a coupler used for coupling carrier current signals to the system illustrated in Figure 1;
Figure 5 is a view of an alternative coupler which may be used according to the teachings o~ this ~ -invention; : -Figure 6 is a view o~ a further alternative coupler which may be used according to the teachings of this invention; and -.
Figure 7 is a diagram of an alternative coupler arrangement for coupling carrier current signals to a fragmentary portion of the system illustrated in 30 Figure 1. -6- -~, . . ~, . . . . .

~)5~301~3 DESCRIPTION OF THE PREFERRED EMBODIMENT
Throughout the following description, similar reference characters refer to similar elements or members in all of the ~igures of the drawing.
Referring now to -the drawing, and to Figure 1 in par-ticular, there is shown a ground wire power line carrier communication sys-tem 10 constructed according to this invention. The communication system 10 includes a power distribu-tion network 11 and communica-tion links for carrier current signals which are provided by the distri-bution network 11 and associated earth ground return paths described in further detail hereinafter. Initially, the general arrangement of the distrlbution network 11 is described for a better understanding of the invention.
By way of example and not by limitation, -the distribution network 11 is provided by an electric utility company to distributealternating electrical power signals, -~
typically in the order of 60 Hz, from a substation 15.
The substation high-vol-tage transformer 16 has a primary grounded conductor 21 which is connected to the secondary winding of the transformer 16 and conduct the 60 Hz power signals in the network 11 at moderate voltage levels.
The primary conduc-tor 20 is connected to each of -the distribution transformers in -the network 11, such as the distribution transformers 23 and 2~. The primary grounded neutral conductor 21 is connected to on]y -the closest distribution transformer -to -the substation 15, such as the transformer 23. Locations 25, 26 and 27 provide connection points for serving -the various branches of the 0 secondary portion 12 of the distribution network 11.
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Locations 25 and 27 are adjacent to the transformers 23 and 24. The location 26 is adjacent -to a branch line not located adjacent -to a distribution transformer. The secondary distribu-tion portion 12 conducts the power signals at low-voltage levels derived from the secondary of the transformers 23 and 24. A large number of customer locations or premises may be associated with the various branch lines, such as represented by -the customer premises 29, 30, 31 and 32.
Ungrounded secondary wire conductors 34 and 35 and a grounded neutral secondary conductor 36 are connec-ted to the secondary winding of the distribution transformer 23. This conventional three-wire signal-phase connection delivers low-voltage distribution power which is utilized by the loads at the customer premises. The service conductors 34A, 35A and 36A connect the customer wiring at the customer premise 30 to the secondary conductors 34, 35 and 36, respectively. The service conduGtors 34B, 35B and 36B connect the secondary conductors 34, 35 and 36, respectively, to the customer wiring at -the customer premise 29.
The secondary conductors 37, 38 and 39 are connected to the junctions of the secondary conductors `
34, 35 and 36 at the secondary winding of the transformer 23. These conductors extend between the pole locations 25 and 26, which would normally be at poles or other supporting structures, to serve secondary conductors 40, 41 and 42 which are connected to the conductors 37, 38 and 39 at pole location 26. The service conductors 40A, 41A and - 8 ~

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are connected, respectively, to conductors 40, 41 and 42 for connecting the load a-t the customer premise 31 to the secondary conductors 40, 41 and 42. The secondary conductors 37 and 38 extend also to the pole location 27 where they are "dead-ended" near the secondary trans-former 24.
The distribution -transformer 24 provides a three-wire single-phase secondary distribution portion including -the ungrounded conductors 44 and 45 and the grounded conductor 46 which are connected to the secondary winding of the transformer 24. These conductors supply -~ ;
the service conductors 44A, 45A and 46A which are connected .
to the cus-tomer wiring and loads at the customer premise 32. Normally, a watthour meter 48 is connec-ted to -the service conductors at each of the customer premises 29, 30, 31 and 32.
A common grounded neutra] conductor is formed in the network 11 by the interconnection of the grounded conductors which include the customer wiring, -the grounded 20 service conductors, the grounded secondary conductors, and .
the grounded primary conductor 21. The interconnected grounded conductors form a solid metallic wire connection which is uninterrupted by the distribution trans~ormers between the customer premises and the substation 15. The conductors forming the common grounded neutral lines are connected to real earth ground at numerous points in the network 11. At the subs-ta-tion 15, a grounding conductor 50 is connected to one terminal o~ the primary and secondary windings and to real earth ground terminal 50G. A second grounding conductor 51 is connected to -the ground terminal _ g _ 44 ~ 042 51G and to the grounded primary conductor 21 for purposes which will be descrlbed in more detail hereinafter.
At each of the pole locations 25 ~ 26 and 27 grounding conductors 52 ~ 53 and 54 are provided in accordance with usual distribution network practices~
These conductors are connected to real earth ground at grounding ter~inals 52G~ 53G and 54G~ At the distribution :~ .
trans~ormers 23 and 24~ one terminal o~ the primary ls connected to ground by connections to the grounding 13 conductors 52 and 54 as shownO Further, the grounded primary terminal is connected to the center tap o~ the secondary winding terminals, as shown, so that the neutral secondary conductors 36 and 46 are connected to real earth `~
ground through grounding conductors 52 and 54 ~ respectivelyO
Each o~ the neutral serv:Lce conductors are . . . .
grounded at a customer premise, usually at a location between the connection o~ the watthour meter and the entrance to the customer building or dwelling. For -example~ the grounding conductor 60 is connected between 20 the watthour meter 48 and the customer wiring and load 61 at the customer premise 31~ The conductor 60 is connected to real earth ground at terminal 60G~ Further, the customers wiring and loads are typically grounded by additional conductors, such as by connection to "water pipe" grounds within the building or dwelling. There~ore, the grounding conductors 63~ 64~ 65 and 66 are connected in common with the neutral service conductors by the customers wiring at the customer premises 29 ~ 30 ~ 31 and .

32~ Grounding terminals ~or these conductors, such as 30 pro~ided by "water pipe connections~" are indicated at ,, ' ' ' ' ' '', ' ' :., ,, ~ , . . . . .
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63G, 6~G, 65G and 66G for the corresponding numbered grounding conductors. The ground wires described are all "directly" connected to ground. That is, there is subs-tantially a straight unlooped conductor extending to ground which does no-t have any significan-t lumped impedance element in series therewith. Such a description of "directly to ground" applies to such terminology used in the claims.
The ground wire carrier communication system 10 will now be described with respect to -the distribution network 11 previously described. A communication link is further described between the customer premise 31 and the substation 15. However, the communication system 10 is intended -to establish a communication link between the -substation and all of the customer premises.
A central or interrogation terminal 70, as shown `
in Figure 1, is loca-ted at or adjacent to the substation 15 and a remote or response communication terminal 71 is located at the customer premise 31. These terminals may be of the general -type disclosed in U.S. Patent No. 3,911,415 issued October 7, 1975, which is assigned to the assignee of this invention. In accordance wi-th the aforementioned U.S. Patent, the substation terminal includes a logic circuit 73, an interroga-tion transmitter 74, a response receiver 75, and a ground line coupler 76~ Correspondingly, the customer remote terminal 71 includes a logic circui-t 77, an interrogation receiver 78, a response transmitter 79, and a ground line coupler 80. The couplers 76 and 80, which will be described fur-ther hereinafter, link carrier current signals between the transmitters or receivers and ~s~

the grounding wires 51 and 60 at the subs-ta-tion 15 and at customer terminal 71, respec-tively.
The substation terminal 70 and the customer terminal 71 are preferably arranged to transmit and receive carrier current signals which are of the frequency-shift-key modula-ted type in which -two frequencies represent binary states of logic which correspond to information signals handled in the logic circuits 73 and 77. The reference carrier frequency of these signals may be in the order of about 25 to 400 -~
kilohertz for representing one of the binary states. A
small fractional bandwidth deviation, for example from 2 to 10 kilohertz, may represent the other binary state. -~
I-t is contemplated, as stated in the aboue referenced U.S. Patent No. 3,911,415, -that the substa-tion -terminal 70 may be connected to a central communication terminal by radio or commercial telephone line or other land line communication links. The customer terminal 71 may be associated with remote meter reading or load control apparatus, such as the watthour meter 48 at the customer premise 31.
The readings of the wat-thour me-ter 48 are encoded by the ]ogic circui-t 77 as described in the referenced application. A logic encoded word format includes an encoded address por-tion so that the carrier current signals originating from the in-terrogation transmitter 7~ would be sensed by -the interrogation -receiver 78 at the customer premise 31. Accordingly, -the logic encoded word format initiated from the response transmitter 79 would include an encoded identification .

4~,042 portion associated with the customer premise 31. It isnoted here that it is within the scope of this invention that each of the customer premises associated with the distribution network 11 could have a response communica-tion terminal, such as the terminal 71. Acc~rdingly, each of the customer premises would have a unique identi-fication code and/or have frequency response character-istics which are different from other response ~erminals, and the carrier current signals may have different frequency bandwidths for selectively signaling dif~erent communication terminals in the communication system lOo The multiple parallel circuit loops provided through the neutral line and earth ground paths are described by reference to Figure 2. Figure 2 is a one-line schematic diagram of the carrier current signal paths which are in parallel with the communication link between the substation terminal 73 and the customer terminal 71. The dashed lines 83 at the bottom of the schematic shown in Figure 2 represent the real earth ground path included in the return portion of the communica-tion link established for the carrier current signals.
The earth ground return path 83 includes resistances which represent small but measurable amounts of resistance between the ground terminals as the signals flow through the earth.

Assuming that a carrier communication signal is generated by the terminal 70 and applied, through the coupler 76, to the ground wire 51, several paths ~or the communication signal exist. The particular path of 30 propagation in this specific example of communication t :~ .

44,0112 signal transmission between substation and customer premise includes the ground conductor 51, the primary grounded conductor 21, the neutral secondary conductor 39, the ground conductors 42 and 63g and the real earth ;
ground return path 83. It is pointed out that other communication paths also exist in communicating between the terminals 70 and 71, and additional paths would be involved when the communication terminal 70 is communica-ting with a customer premise terminal located at another customer premise.
The signal coupled to the ground wire 51 by the -~
coupler 76 produces a voltage between the real earth ground terminal 51G and the conductor 21 due to the relativity low but finite impedance o~ the ground conductor 51. It is also pointed ~ut ~hat even though the impedance of the ground conductor is relatively low, the signal loss through the real earth ground path 83 is also relatively low since the path 83 also presents a low impedanceO Therefore, no high impedance power c~nsuming or restricting elements are present which o~er obstruction to propagation o~ the communication signal from one location to another. -;
Several parallel paths are connected across the real earth ground terminal 51G and the neutral ground wire 21. Each path conducts a porti.on of the communication signal current and has developed thereacross a portion o~
the communication signal voltage. However, due to the relatively low impedance of all o~ the elements involved, a somewhat equal distribution o~ signals is provided through each o~ the grounding conductors located at the varlous customer premises There~ore, suf~icient power ,, , . ,, ' ~' ' ",', ' ., . . .

~L~5~3~l39 is transmitted from the coupler 76 to the customer premise for communication purposes provided the number of parallel paths is not too excessive. Each parallel path consumes or diverts some of the communication signal back through the return path 83 to the coupler 76.
The coupler 76 is coupled to the grounding wire 51 which is located at the substation 15. However, the power current is conducted primarily through the substation ground wire 50 instead of through the ground wire 51. Using a separate ground wire for the communication system components assures that saturation of the coupler and the associated interference will not occur if unbalanced currents in the power distribution sys-tem flow through the substa-tion grounding wire 50.
Figure 3 shows a portion of the distribution network 11, for example, between the pole loca-tions 25 and 26. The coupler 85 is coupled to the neutral secondary conductor 39 and connects to a frequency translating and reconditioning repeater 86 as described in the referenced U.S. Patent No. 3,911,415. Signals transmitted between the substation -terminal 70 and the customer terminal 71 would then be reconditioned and amplified for transmission over longer distances.
Referring now in particu-lar to the arrangement of the current rransformer couplers 76 and 80, Figure 4 illustrates current transformers 87 and 88 which can be used to replace ei-ther or both of the couplers 76 and 80 shown in Figure 1. The transformers 87 and 88 consist of the two-piece, U-shaped core members 104, 105, 106 and 107 which are shown separated but which would normally be - 15 ~

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arranged so that their facing sides are engaged to encircle a grounding wire, such as the wire 51. One-half of each transformer is made of a commercially available ferrite material and includes a few turns of wire -to provide a secondary winding having the desired number N of turns. The grounding wire 51 would form a single turn primary so as to provide a low-impedance transmission circuit. These low-impedance coupling circuits are a particularly important feature because they do not , interfere with the norrnal and proper use of -the grounding wire at the power frequency. In addi-tion, such ferrite cores saturate at large currents, thereby giving low ~
inductance and allowing only a small amount of voltage ;
to be developed across the magnetic cores during fault -current conditions.
In Figure 5, there is shown a transformer coupler which includes a substantially hollow cylindrical ferrite core 90 encircline the ground wire 51 in which the carrier current signals are to be coupled to or detected from. The winding 91 may be connected to the receiver of the communication terminal and the winding -92, which has a different l:N turns ratio than the winding 91, may be connected to the transmitter of the communica-tion terminal. The number of turns on either of the -windings 91 and 92 may vary depending upon the required turns ratio, the frequencies of a carrier current signal, and whether or not the windings are -to be tuned.
Figure 7 shows a coupler arrangement including current transformer couplers 96 c-ind 98 which include magnetic cores similar to those described herein. These .. . . . .
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Ll4,0~2 cores are applied around all three of the distribution secondary conductors or around all three service conductors, such as conductors ~OA, 41A and 42A. This can be done since the power current through the ungrounded or live conductors, such as 40A and 41A, will balance and neutrali~e each other so that the net current through the three wires is substantially equal to zero. It is also to be noted that the carrier current signals in the line conductors are conducted no further than the distribution trans~ormer neutral since the distribution transformer acts as a choke at carrier communication frequenciesO
A further modification of the coupler arrange-ment 76 and 80 is shown in Figure 6 wherein a magnetic core is used in which the wires lO:L and 102 are connected directly to the grounding conductor 51. A predetermined number of turns are provided on the ferrite core an~ a capacitor 100 is connected across the leads of the winding 103 formed by these turns. The wires 101 and 102 are connected either to the receiver or transmitter and the tuned wlnding leads are connected to other of the trans-mitter or receiver. The tuned circuit arrangement adds further flexlbility to the desired impedances under given conditions of the signal frequencies and power line impedances which would normally be encountered by the carrier current signal. Due to the impedance presented by the coupler between connection points of the wires 101 and 102 on the conductor 51, a true short circuit at carrier communication frequencies does not exist.
Since numerous changes may be made in the ;

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above described apparatus, and since different ernbodi-ments of the invention may be made without departing .
from the spirit thereof, it is intended that all the matter contained in the ~oregoing description, or shown in the accompanying drawing, shall be interpreted as illustrative rather than limiting.

~ ,. . .

Claims

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

1. A power line communication system for conveying carrier current signals between two locations in an electrical power system including a distribution transformer, comprising:
a first ground wire which connects an uninterrupted neutral conductor of the power system directly to real earth ground at a first of the two locations;
means including a first current transformer having a single turn primary formed by said first ground wire for inducing a firts current signal in said first ground wire in response to first communication signals;
a second ground wire which connects said neutral conductor of the power system directly to real earth ground at the second of the two locations; and means including a second current transformer having a single turn primary formed by said second ground wire for detecting a second current signal in the second ground wire which is responsive to the first current signal.

2. The communication system of claim 1 wherein the first location is at a power system substation.

3. The communication system of claim 2 wherein the first ground wire at the substation is substantially paralleled by another ground wire which conducts most of the power current between the neutral conductor and earth ground.

4. The communication system of claim 1 wherein the second location is at a power customer premise and wherein a repeater means is coupled to said neutral conductor for receiving said first current signal and transmitting said second current signal.

5. The communication system of claim 4 wherein the second ground wire at the customer premise is sub-stantially paralleled by another ground wire which conducts most of the power current between the neutral conductor and earth ground.

6. The communication system of claim 1 wherein the means for inducing a first signal in the first ground wire includes a closed magnetic core disposed around the first ground wire, and a winding disposed around said core and connected to means for generating the first communication signals.

7. The communication system of claim 1 wherein the means for detecting the second current signal in the second ground wire includes a closed magnetic core disposed around the second ground wire, and a winding disposed around said core and connected to means for processing second communication signals.

8. The communication system of claim 1 including means for inducing a third current signal in the second ground wire responsive to second communication signals, said second communication signals being transmitted in a direction opposite to the direction of transmission of the first communication signals, and means for detecting a fourth current signal in the first ground wire which is responsive to the second communication signals.

9. The communication system of claim 8 wherein the means for inducing the first current in the first ground wire includes a closed magnetic core disposed around the first ground wire, a winding disposed around said core and connected to means for generating the first communication signals, a capacitor connected in parallel relationship with said winding, and wherein the means for detecting the fourth current in the first ground wire includes a first lead connected directly to the first ground wire at one end of said magnetic core, and a second lead connected directly to the first ground wire at the other end of said magnetic core, said first and second leads being connected to means for processing the second communication signals.