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Claims  |
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What is claimed is:
1. A power line carrier communication system for communicating between a
first subscriber site and a second subscriber site of a power distribution
system which conveys power at an alternating current power line frequency,
where the first subscriber site is coupled to a common primary
distribution line through a first distribution transformer and the second
subscriber is coupled to the common primary distribution line through a
second distribution transformer, comprising:
a transmitter at the first subscriber site which outputs a signal at a
transmitter output, said signal being referenced to an earth ground and
being a modulated carrier signal characterized by a carrier frequency,
said carrier frequency being different from the power line frequency;
first secondary distribution means coupled to a secondary side of the first
distribution transformer and to a first power feeder at the first
subscriber site for distributing power to said first power feeder;
second secondary distribution means coupled to a secondary side of the
second distribution transformer and to a second power feeder at the second
subscriber site for distributing power to said second power feeder;
a first plurality of coupling capacitors, wherein each of said coupling
capacitors is coupled at a first terminal to said transmitter output and
is coupled at a second terminal to a line of said first secondary
distribution means; and
a second plurality of coupling capacitors, wherein each of said coupling
capacitors is coupled at a first lead to a line of said second secondary
distribution means; and
a receiver at the second subscriber site, commonly coupled at an input to a
second lead of each one of said second plurality of coupling capacitors,
said input receiving said signal from said transmitter as a voltage
relative to said earth ground, said signal received after propagating
through said first plurality of coupling capacitors, said first
distribution transformer, the common primary distribution line, said
second distribution transformer, and said second plurality of coupling
capacitors.
2. The apparatus of claim 1, wherein said receiver and said transmitter are
an element of separate transceivers, respectively, whereby communication
between the first subscriber site and the second subscriber site is
bi-directional communication.
3. The apparatus of claim 1, further comprising a plurality of receivers at
a plurality of subscriber sites wherein each of said plurality of
subscriber sites comprising a second secondary distribution means, wherein
each of said plurality of subscriber sites receives electrical power from
one of said second secondary distribution means of said plurality of
subscriber sites and a plurality of similar distribution means.
4. The apparatus of claim 1, wherein said first secondary distribution
means and said second secondary distribution means are three-phase
distribution lines, respectively.
5. The apparatus of claim 1, wherein said first secondary distribution
means and said second secondary distribution means are single phase
distribution lines, respectively, and a center tap of a secondary winding
of each of said first and second distribution transformers is connected to
said earth ground.
6. A circuit for coupling a transmitter and a receiver through an
electrical power distribution system, where power in the electrical power
distribution system originates at a power station and is transmitted
through high-voltage lines at a power line frequency to at least a first
and a second distribution transformer, where at least one primary winding
of the first distribution transformer is electrically coupled to the
high-voltage lines and magnetically coupled to at least one secondary
winding of the first distribution transformer which is electrically
coupled to at least a first secondary distribution line, the first
secondary distribution line for providing electrical power to at least a
first electrical utility subscriber site, where at least one primary
winding of the second distribution transformer is electrically coupled to
the high-voltage lines and magnetically coupled to at least one secondary
winding of the second distribution transformer which is electrically
coupled to at least a second secondary distribution line, the second
secondary distribution line for providing electrical power to at least a
second electrical utility subscriber site, and where the first and second
distribution transformers are optimized to transfer power at the power
line frequency, comprising:
an output of the transmitter which imposes a common-mode signal relative to
an earth ground onto a transmitter output wire, where said signal is a
modulated carrier with a carrier frequency other than the power line
frequency;
a first coupling capacitor coupled at a first terminal to said transmitter
output wire and coupled at a second terminal to an end tap of one of the
at least one secondary winding of the first distribution transformer; and
a second coupling capacitor coupled at a first lead to a signal node and
coupled at a second lead to an end tap of one of the at least one
secondary winding of the second distribution transformer, wherein said
signal node is coupled to an input of the receiver.
7. A method for communicating between a first subscriber site and a second
subscriber site of a power distribution system, where a communication path
crosses at least a first distribution transformer and a second
distribution transformer which are both coupled to a common primary
distribution line, comprising the steps of:
common-mode coupling a transmitter to at least two lines of a first
secondary distribution line, said first secondary distribution line also
coupling electrical power from the first distribution transformer to the
first subscriber site;
common-mode coupling a receiver to at least two lines of a second secondary
distribution line, said second secondary distribution line also coupling
electrical power from the second distribution transformer to the second
subscriber site;
generating a packet comprising data to be communicated and an address
corresponding to said second subscriber site;
outputting a signal representative of said packet from said transmitter,
said signal conveyed as a varying voltage relative to earth ground on both
of said at least two lines of said first secondary distribution line, said
signal being of equal phase and amplitude on each of said at least two
lines of said first secondary distribution line;
receiving said signal at least at said receiver, said signal conveyed as a
varying voltage relative to earth ground on both of said at least two
lines of said second secondary distribution line, said signal being of
equal phase and amplitude on each of said at least two lines of said
second secondary distribution line;
converting said signal into a received packet, said address being readable
in said received packet;
comparing said address to a receiver address; and
accepting said received packet when said address and said receiver address
match, and discarding said received packet when said address and said
receiver address do not match.
8. The method of claim 7, further comprising the steps of:
designating at least one transceiver as a store-and-forward transceiver;
accepting said packet at said store-and-forward transceiver from said
transmitter; and
retransmitting said packet from said store-and-forward transceiver to said
receiver. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to the field of power-line carrier
communications, more specifically to an apparatus and method for
communicating among transceivers separated by power distribution
transformers.
Power-line carrier (PLC) systems provide a means for electronically
communicating between two points using the existing wiring of a power
distribution system. PLC systems are common for communicating within a
building, whether the building is a house, apartment building, business,
or industrial building. Each point on the power lines within the building
is directly connected to each other point in the building, except in the
rare case where a distribution or isolation transformer is installed
inside a building. More typically, however, power is supplied from a
distribution transformer installed on a power pole to the building.
In distributing power from a utility generating station to a utility
subscriber's building, the power passes through successively smaller
branching networks, until it is finally split off to several distribution
transformers, each of which have secondary windings directly connected to
one or more subscribers. Because each of the subscribers connected to a
single distribution transformer have power lines electrically connected to
the secondary side of the same distribution transformer, PLC communication
is easily possible between subscriber sites which share a common
distribution transformer, but communication beyond the typically small
number of subscribers on one distribution transformer requires passing a
PLC signal through or around one or more distribution transformers.
One application of a PLC system is for remote meter reading where meters at
each subscriber site record usage and transmit a signal indicating the
amounts used to a local receiver, which collects reading from several
transmitters using PLC communications, and then relays the data to a
central utility computer via radio or telephone lines. The transmitters
can also be configured as receivers to receive data such as load shedding
and power blocking commands from a local transceiver. With one local
transceiver or receiver connected at the secondary side of each
distribution transformer, no PLC communications through a distribution
transformer is necessary. However, such a system is uneconomical, and in a
practical system, a local transceiver must be able to collect data from
and distribute data to subscribers on more than one distribution
transformer, thus requiring an ability to communicate through one or more
distribution transformers.
Since the distribution transformers are optimized to pass power at the
power line frequency, typically 60 Hz, a PLC carrier frequency at a higher
frequency is greatly attenuated by the transformer. A higher frequency PLC
carrier is necessary to achieve adequate data rates and to allow the PLC
signal to be separated from the power being delivered to the subscriber's
power lines. Loss of signal through distribution transformers is
especially troublesome when communicating between two subscribers coupled
to separate distribution transformers, since signals must pass through
both distribution transformers and therefore the loss is squared.
One method of transmitting higher frequency PLC carriers through a
distribution transformer is to bypass the transformer. For example, U.S.
Pat. No. 4,473,817, issued to Perkins, illustrates a communication system
where a signal is passed from the primary side of a distribution
transformer to the secondary side using a separate communications
transformer with a capacitor in a circuit optimized for the carrier
frequency. However, such a system requires additional hardware and the
second transformer absorbs some of the power destined for subscribers.
From the above, it is seen that an improved means for communicating through
a distribution transformer is needed.
SUMMARY OF THE INVENTION
The invention provides an improved method and apparatus for communicating
among transceivers in an electrical utility distribution system where a
signal path between the transceivers includes one or more distribution
transformers.
In accordance with the present invention, a first transceiver is provided
at one subscriber site which is to communicate with a transceiver provided
at a second subscriber site, where the two sites are coupled to separate
distribution transformers. The first transceiver at the first site
transmits a modulated carrier signal referenced to earth ground. The
signal passes through the secondary distribution lines which run between
the first site and a first distribution transformer.
The signal then passes through the first transformer to a second
distribution transformer which is coupled to the first transformer by a
primary power line in common with both transformers, and then passes
through secondary distribution lines running from the second distribution
transformer to a transceiver at the second site. Coupling is provided
according to the invention by capacitors coupled to the secondary
distribution lines, where the same polarity signal is applied to each
coupled line of the secondary distribution lines, thereby effecting an
alternating current (AC) common-mode coupled connection to the secondary
distribution lines. Because the signal is common-mode at the transformers,
as opposed to differential mode, the transmission from one site to another
suffers negligible loss, the loss being due to parasitic resistances and
reactances, since according to the invention each distribution transformer
operates as a capacitor to convey the AC common-mode coupled signals
through the transformer. Furthermore, whereas differential signal transfer
through a transformer requires tight magnetic coupling between the primary
and secondary windings, common-mode coupling, which is, in this case,
capacitive as opposed to magnetic, is less affected by the degree of
magnetic coupling between the windings.
With single phase power lines, two coupling capacitors at each site are
used, with one lead of each capacitor connected to a secondary winding of
a distribution transformer and the other lead coupled to a transceiver.
The center tap of the secondary winding is connected to earth ground. The
coupling capacitors are selected such that the series combination of a
first set of parallel coupling capacitors, the primary-to-secondary
capacitance of a first distribution transformer, the primary-to-secondary
capacitance of a second distribution transformer, and a second set of
parallel coupling capacitors form a low impedance capacitive coupling
between two transceivers at the carrier frequency, and a high impedance
path at the power line frequency. In one embodiment, the carrier signal
frequency is 230 kHz.
With three-phase power lines, a transceiver is coupled through three
coupling capacitors to each of three end taps of the respective
secondaries of distribution transformers.
In other various embodiments, two-way communication is effected between
secondaries of different power distribution transformers, or communication
is effected between one secondary of one transformer and secondaries of a
plurality of transformers, thus forming various communication network
architectures.
In yet another embodiment, some transceivers are designated as being
store-and-forward nodes, where such transceivers store packets addressed
to other transceivers and then retransmit such packets, thereby allowing a
packet to travel further than a single transceiver-transceiver hop would
allow.
A further understanding of the nature and advantages of the inventions
herein may be realized by reference to the remaining portions of the
specification and the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an embodiment of a power line
communication system according to the present invention, where subscribers
are provided with single phase power;
FIG. 2 is a schematic of a model of a communication path between two
transceivers.
FIG. 3 is a schematic diagram of an embodiment of a power line
communication system according to the present invention, where subscribers
are provided with three phase power through wye-configured distribution
transformers;
FIG. 4 is a schematic diagram of an embodiment of a power line
communication system according to the present invention, where subscribers
are provided with three phase power through closed-delta configured
distribution transformers; and
FIG. 5 is a block diagram of a power distribution system wherein various
subscribers coupled to a common primary line and transceivers at each
subscriber site are shown.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a power line carrier communications system 10 where electrical
power is distributed from an electrical utility generating plant (not
shown), along primary power lines 12, to utility subscribers A, B, C, and
D. Three distribution transformers 14, 16, 18 are shown. Each distribution
transformer has a primary winding, such as winding 20, 22, or 24, coupled
to primary power lines 12, and each primary winding is magnetically
coupled, and provides power, to a secondary winding, such as winding 26,
28, or 30. Subscriber power lines at subscriber sites are coupled to the
secondary winding of one of the distribution transformers. A single
distribution transformer may supply more than one subscriber, such as
distribution transformer 18, which supplies power to subscriber C and
subscriber D.
Electrical power is conveyed to exemplary subscriber site A, through
secondary power lines 32a. Power is also conveyed to other subscribers
through other secondary power lines 32b-d. In a typical power distribution
system, one distribution transformer provides power for more than one or
two subscribers. However for clarity, only a sample of subscribers is
illustrated in the figures.
At subscriber site A, transceiver 34a is coupled to secondary power lines
32a through coupling capacitors 36 and 38. Other transceivers 34b-d are
similarly coupled to respective secondary power lines at subscriber sites
through similar coupling capacitors. The coupling capacitors are selected
to block the power at the power line frequency of 60 Hz from reaching the
transceivers, but not to block a communication signal at a higher carrier
frequency. In one embodiment of the present invention, data is modulated
onto a 230 kHz carrier signal, and coupling capacitors 36, 38 each have a
capacitance sufficient to convey the carrier with an impedance
substantially less than power frequency signals, typically in the small
microfarad range. In other embodiments, the carrier frequency ranges from
10 kHz to 400 kHz. The coupling capacitors may be selected to establish a
high-pass filter circuit with the ambient inductance of the system.
Because of the particular arrangement of circuit elements, a communication
path at the carrier frequency between two subscribers, such as A and B, is
modelled by the circuit shown in FIG. 2. Significantly, because signals
are placed on the transformer secondary leads as common-mode signals, and
because of the impedance of the distribution transformer's
primary-to-secondary capacitance at the carrier frequency, the path from
subscriber A to subscriber B is a low attenuation path compared with
differential coupling.
Referring to FIG. 2, a PLC signal relative to earth ground is transmitted
from transceiver 34a through coupling capacitors 36, 38,
secondary-to-primary winding capacitances 70, 72 of distribution
transformer 14, primary distribution lines 12, primary-to-secondary
winding capacitances 74, 76 of distribution transformer 16, and coupling
capacitors 40, 42, to transceiver 34b, where the signal provided to
transceiver 34b on line 78 is referenced to earth ground. As should be
apparent from FIG. 2, communication is possible in the other direction,
from transceiver 34b to transceiver 34a, as well as communication in both
directions simultaneously. It should be further apparent that
communication is also possible using only one of the two parallel paths
shown in FIG. 2. However an embodiment using parallel paths is the
preferred embodiment.
FIG. 3 is a schematic diagram of a power line carrier communication system
80, which is similar to that shown in FIG. 1 except the secondary power
lines carry three-phase power on four lines, and the distribution
transformers are configured on in a wye configuration. In FIG. 3, only two
subscriber sites are shown. However, in a typical power distribution
system, several sites are powered from one transformer, and more than two
transformers receive power from a primary distribution line.
Three-phase electrical power is provided to subscriber A from primary
distribution line 82 through the primary windings of distribution
transformer 84, the secondary windings of distribution transformer 84, and
secondary distribution lines 88. Electrical power is provided to
subscriber B through distribution transformer 86 and secondary
distribution lines 90.
Transceiver 94 transmits a PLC signal to transceiver 100. Transceiver 100
also may transmit a signal to transceiver 94. However, only transmission
in one direction will be described here. A signal output from transceiver
94 is applied equally to three coupling capacitors 92a-c. These coupling
capacitors couple the high-frequency signal (10 kHz-400 kHz) onto each
line of secondary distribution lines 88 while preventing the high power at
the power line frequency (60 Hz or lower) from reaching transceiver 94.
Because the PLC signal is a common-mode signal relative to earth ground,
and the center node of the secondary windings of distribution transformer
84 is coupled to earth ground, the PLC signal is imposed on the three
secondary windings. Due to capacitive coupling, and some magnetic coupling
in this case, the signal is transmitted through to the primary side of
distribution transformer 84. In a similar manner, the signal is
transmitted through primary distribution lines 82, distribution
transformer 86 and a second set of coupling capacitors 98a-c, to reach
transceiver 100.
FIG. 4 is a schematic diagram of a power line carrier communication system
80'. It is similar to communication system 80, except that the
distribution transformers are configured as a closed delta. Enumeration of
elements is as in FIG. 3.
FIG. 5 is a block diagram of a communication system 110 in accordance with
the present invention, illustrating the transmission of data packets from
one site to another. Data originating at a subscriber site 120 is packaged
into a packet, such as packet 118, where packet 118 includes information
indicating a destination address. Data is transmitted through the power
system as shown in FIGS. 1, 3, or 4, and is transmitted as digital
modulation of a carrier signal, as is well known in the art.
The packet is transmitted to more than one transceiver by virtue of the
fact that many transceivers are coupled to power distribution lines which
can carry signals from the originating transceiver. To avoid unnecessary
duplication of packets, each transceiver contains hardware or software for
reading the address data in each received packet, and rejects those
packets which do not have an address matching the assigned address of the
transceiver. In the example shown in FIG. 5, packet 118 is transmitted to
all transceivers, but only the addressee, namely, transceiver 3A, accepts
and processes the packet.
In an alternate signal path, illustrated by the dashed lines in FIG. 5,
packet 118 is retrieved by transceiver 2B and retransmitted to its final
destination, transceiver 3A. Such an embodiment is useful where a packet
may need to be transferred further than line noise and attenuation would
otherwise allow. In some embodiments, each of the transceivers has such a
store-and-forward capability, and in other embodiments, only selected
transceivers may store and forward packets. Of course, the packets can be
easily encoded according to an error-correcting code, and error-detecting
and correcting protocols can be used as is known to conventional packet
communication systems.
The distance from a transceiver to a subscriber site is not limited in the
present invention, although in many cases a transceiver will operate in
conjunction with a power meter to record the amount of power used at the
subscriber site or used to disable the flow of power to the site. Not all
transceivers need be located near a subscriber site, however. For example,
some transceivers might be used to collect data from power meter
transceivers and to transmit the collected data via radio or other non-PLC
communication means, to a central utility computer. Such a transceiver
might be located high on a power pole to efficiently communicate using
radio signals, or near telephone lines to easily couple to telephone
lines.
The above description is illustrative and not restrictive. Many variations
of the invention will become apparent to those of skill in the art upon
review of this disclosure. Merely by way of example, a transceiver on a
power distribution transformer can broadcast data to a plurality of
transceivers on a plurality of other power distribution transformers;
two-way communication can be broadcast from transceivers on different
power distribution transformers; or communication can occur between sites
separated by more than one level of distribution, in which case signals
will travel up and down several levels of distribution transformers. The
scope of the invention should, therefore, be determined not with reference
to the above description, but instead should be determined with reference
to the appended claims along with their full scope of equivalents.
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