Note: Descriptions are shown in the official language in which they were submitted.
CA 02223666 1997-12-04
WH-9944CA
TITLE: POWER DISTRIBUTION GRID COMMUNICATION SYSTEM
FIELD OF THE INVENTION
The present invention relates to transmission of
communication signals over a power transmission grid from
at least a power distribution station to individual
households and to a system which uses the received
communication for control of electrical appliances and user
viewing of other information. The system optionally
includes an outbound communication channel from the
household to a predetermined computer over a different
channel such as a telephone channel, cable channel or RF
link. This outbound channel allows a report signal to be
sent and the report is carried out by a device in the
household using this different channel.
BACKGROUND OF THE INVENTION
Electrical power systems have long been recognized
as having the potential to be used as an effective
communication channel but in practice, this potential is
severely restricted due to the strong power signal being
transmitted and the harmonics of the power signal, as well
as the frequency characteristics of local power
distribution equipment. To overcome this problem, some
powerline carrier line systems have used a high frequency
communication signal which is not affected by the power
signal. Unfortunately, such high frequency signals
encounter problems where the power distribution system
includes power factor correction capacitor banks. In order
for the high frequency signal to be transmitted, these
capacitor banks have to be trapped. Unfortunately, it is
difficult and expensive to carry this out and it is even
more difficult to ensure that this always occurs. Failure
CA 02223666 1997-12-04
WH-9944CA
to take this corrective step results in the loss of the
communication signal. The high cost and the difficulty of
controlling this arrangement renders a high frequency
transmitter system ineffective. The potential of powerline
transmission systems being used as a communication channel
continues to be attractive, however, the effective use of
this potential communication path has proven difficult to
realize.
Powerline carrier systems (PLC) can be divided into
two segments, a powerline transmission segment and a
downstream powerline distribution network segment.
PLC systems applied to distribution networks have a
group of special technical obstacles that are not
experienced in transmission level PLC. Distribution line
carrier (DLC) signals must propagate through networks that
are extremely hard to model as the networks branch and mesh
with each other while experiencing highly variable levels
of loading. DLC signals must traverse a network that was
designed to carry power signals at 50/60 Hz and are
optimized for this task. Power transformers at PLC
frequencies are modelled primarily, at PLC frequencies, by
their leakage reactance and tend to block DLC signals.
Capacitor banks used for power factor correction present a
low-impedance path to ground and sink DLC signals unless
they are trapped out with reactors.
Finally, standing-wave phenomena cause many nodal
points to occur throughout the distribution network when
carrier frequencies become greater than 5 kHz. To obviate
this problem, many carrier frequencies can be employed
simultaneously but at the cost of increased receiver
complexity.
These obstacles lead to the development of so-
called ripple control system in the 1960s for use with
residential and commercial load shedding systems that
helped utilities offset peak demand and maintain service
CA 02223666 1997-12-04
WH-9944CA
during periods of generation shortfall. Traditional
residential load-shedding systems controlled appliances by
transmitting signals having very low information content
and low bandwidths. Ripple control systems relied on
binary on/off signalling, or amplitude shift keying (ASK),
at typical signalling rates of between 0.5 and 5 baud.
Messages were sent as broadcast "telegrams" to residential
loads to either turn on or off consumption, typically with
a safety time-out mechanism in case the load failed to
receive a turn on signal after a load shed request. This
was necessary as signals were commonly not received or not
recognized. Carrier frequencies of ripple systems were
kept very low, between 30-1000 Hz, in order to avoid the
cost of distribution network changes but placed the signals
in the most noisy area of the powerline spectrum. The
magnitude of power frequency harmonics can be very large,
with respect to the fundamental, below 2.5 kHz and jam any
communication systems that use this frequency range to
transmit.
Ripple systems with their low signal rates require
only about 10 Hz of bandwidth to communicate and could
easily fit between two 50/60 Hz harmonics. The problem
during the 1960s and 1970s was that the narrow band filters
used to isolate the ripple signal from the noise usually
let in more power frequency harmonics than they did signal.
The solution for many utilities, even today, is to
drastically increase the DLC injected power to the point
where the DLC signal becomes many times larger than the
nearest harmonics. The amount of injected power is
measured in kilowatts so most ripple systems use motor-
generator pairs to inject the signal; a very costly
solution.
Higher frequency DLC systems, above 5 kHz, were
also available for utilities that required higher data
throughput or two-way communications. Westinghouse,
General Electric, and Rockwell all offered such systems
CA 02223666 1997-12-04
- WH-9944CA
during the 1980s. As mentioned before, all these higher
frequency systems required distribution network changes to
accommodate the DLC signals and most used multiple carrier
frequencies to overcome standing-wave phenomena.
Today, only a few companies produce DLC systems, as
the cost has become prohibitive for the typical
applications of automatic meter reading (AMR) and load
shedding. The plummeting cost of radio communications, the
need for more bandwidth, and the proliferation of estimated
consumption billing, has caused many utilities to abandon
DLC systems. The original advantages of DLC systems still
exist, if only the cost can be dramatically reduced, and
preferrably the data-rate brought up to a level that would
enable other revenue generating/customer attention services
such as real-time pricing, residential information
(weather, news etc.) and remote service disconnection.
For the above reasons, power carrier systems have
not proven popular. The present application overcomes
these difficulties and combines this form of communication
path with electrical equipment to be placed in the home.
The powerline carrier system is used to broadcast signals
into the homes. These broadcast signals can be saved
temporarily for review by the occupants of the home and the
broadcast signals can include instruction signals for
controlling certain devices in the home. The present
invention also combines this powerline carrier broadcast
system with a different communication channel out of the
home or premise for reporting to a central source or
predetermined computer.
SUMMARY OF THE INVENTION
A data communication network for transmitting an
outbound communication signal over an AC power transmission
grid having a low frequency power carrier signal, according
CA 02223666 1997-12-04
WH-9944CA
to the present invention comprises a signal input device
located at an upstream point on the power grid, a signal, a
signal receiving device located on the grid at a point
downstream of the input device. The signal input device
includes a signal input connection, a spread spectrum
arrangement for coding the input signal, and an arrangement
for injecting the spread spectrum coded input signal onto
the low frequency power carrier signal of the power grid.
The signal receiving device is connected to the power grid
and receives the powerline signal receiving device,
processes the coded powerline signal to substantially
remove the effects of the power frequency signal,
digitizing the remaining signal and despreading the
r~m~;n;ng signal to reconstruct the input signal. The
coded input signal is placed in a low frequency band to
allow passage thereof through the power grid between said
input and the receiving device and past any capacitor banks
and transfer meter.
According to an aspect of the invention, the signal
input device is located at a transmission or distribution
substation of the power grid.
According to a further aspect of the invention, the
signal receiving device is located at a distribution
transformer.
According to a further aspect of the invention, the
spread spectrum powerline carrier signal has a frequency
band with an upper limit below about 2.5 kHz and
preferrably, below 2.0 kHz.
A system for managing user-determined discretionary
electrical loads in a household according to the present
invention, comprises a plurality of discretionary
electrical devices connected to a control arrangement for
controlling or switching on and off electrical devices in
accordance with user input and information from an outside
source, a user interface for entering user information with
CA 02223666 1997-12-04
- WH-9944CA
respect to a scale indicating the level of power
conservation aggression that is to be used with respect to
management of the discretionary devices, a receiver for
receiving electrical information from a source outside the
household, and a microprocessor connected to the user
interface, the receiver, the control arrangement and the
discretionary electrical devices for varying the operation
of said electrical devices in accordance with the user
information and information received from said outside
source.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in
the drawings, wherein:
Figure 1 is an overview of the system involving the
power distribution grid and the eventual termination
thereof in various households;
Figure 2 is a schematic showing further details of
the communication arrangement for transmitting of signals
from a distribution station to the household;
Figure 3 is a depiction showing various devices of
the system located in a household.
Figure 4 is a circuit diagram of a signal injecting
arrangement; and
Figure 5 is a schematic of the receiver processing
arrangement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 shows a power distribution system 2
including a power generation source 4, which supplies power
CA 02223666 1997-12-04
- WH-9944CA
to a transformer station 6, and the distribution station 8.
As can be appreciated, there will be many distribution
stations and each distribution station 8 in turn,
distributes power to a series of distribution transformers
10. These distribution transformers are sometimes referred
to as feeder transformers. Typically, in North America,
each distribution transformer 10 feeds anywhere from four
to ten households. The segment between transformer station
6 and the distribution station 8 is normally referred to as
a power transmission network and the signal between
distribution station 8 and the distribution transformers is
the distribution network.
The schematic of Figure 2 shows further details of
the power distribution network between the distribution
station 8 and individual households shown as 12. The power
distribution network has been modified by including at the
distribution station 8, a device 14 for injecting a coded
communication signal on to the powerline signal. The
injected signal is a broadcast signal, which is received
and decoded on the low voltage side of the distribution
transformer 10. The injected signal is a coded signal and
is not affected by the corrective capacitor banks 16 which
often are present between the originating station and the
distribution transformers 10. The frequencies of the
communication broadcast signal are relatively low and are
unaffected by the corrective capacitor banks 16. This is
in contrast to high frequency communication signals which
could be introduced at the distribution station 8, however,
with such high frequency signals, the corrective capacitor
bank 16 would have to be trapped to allow the signal to
pass therethrough. Unfortunately, this is a relatively
expensive process and it also requires constant monitoring
and quality control to ensure that all capacitor banks that
are added, are properly trapped. By using a lower
frequency signal, the signal passes through the capacitor
banks, however, as will be further discussed, it has the
CA 02223666 1997-12-04
WH-9944CA
disadvantage that the lower frequency signal is adversely
affected by harmonics of the power signal.
Outbound communication receiver and transmitter 18
are located on the low voltage side of the distribution
transformers 10. The precise location is not important as
the households are interconnected by the transformer 10. It
may be desirable to locate this receiver at the meter
interface unit of a household for more convenient access
while all households on the low voltage side of the
transformer, continue to receive the retransmitted signal,
as will be explained. The receiver, as generally shown by
arrow 11, receives the incoming signal from the
distribution transformer station. This signal is a
combination of the power signal and the communication
signal. The receiver/transmitter 18 processes the signal
to strip out the communication signal and decode the
signal. It then encodes the signal according to a
different protocol and injects it on the secondary side of
the distribution transformer 10 as indicated by line 13.
This signal is preferrably injected, using CEBus
transmission protocols and will be received by any of the
plurality of households located downstream of the
distribution transformer 10 as a broadcast signal.
Each household 12 is connected to the distribution
transformer 10 for receiving power and for receiving the
CEBus translated communication signal. Each household has
a meter interface unit 20 which is an electronic device
associated with the meter of the house for electronic
tracking of power consumption.
A communication signal processor 22 receives the
now CEBus communication broadcast signal and processes the
signal to extract and/or accumulate the information
contained in the broadcast signal. It can be seen that the
meter interface unit 20 and the various devices of the
household are all interconnected by the household wiring,
generally shown as 24. Hot water heater 26 is directly
CA 02223666 1997-12-04
- WH-9944CA
connected to the household wiring 24, as is Air Conditioner
28 and the Pool Pump 30. These have been labelled as smart
devices in that they are CEBus devices which can be turned
on or off. A load control box can be provided for
controlling conventional air conditioners and other
devices. The load control box is associated with the
distribution panel and the communication signal processor
22. The load control box interrupts the power to these
devices when instruction to do so by the signal processor
22.
Also associated with the system is a user interface
unit 36 which communicates with the communication processor
22, and the meter interface unit, via the household wiring
24, as the user interface unit plugs into an electrical
receptacle of the house wiring. In addition, the house may
have a smoke detector 32, and a thermostat 34 which
communicate with signal processor 22, either via the house
wiring or their own dedicated wiring or by RF as is well
known, particularly in the security field.
The communication signal processor 22 has
associated therewith a communication transceiver 37
connected to a telephone 38 and thus to the publicly
switched telephone network. In this way, the communication
signal processor 22 can initiate an outbound communication
to a remote computer from time to time or when instructed
by the communication signal. This is of benefit in that
instructions for meter reading, for example, can be
transmitted over the powerline system and be received by
the communication system processor 22 which then interacts
with the meter interface 20 to determine power consumption
and reports the results using the communication transceiver
37 and the publicly switched telephone network. Other
electronic metering devices can be in communication with
the signal processor 22 and these can also be reported on a
regular basis or on a demand basis, if desired.
CA 02223666 1997-12-04
- WH-9944CA
The broadcast signal includes general information
of different types and the user can select and view any of
this information, using the user interface unit 36. The
user selects the particular information to be displayed
from a listing of all the information and then the
particular information is displayed on the user interface.
The signal processor tracks these requests to provide
market feedback information.
The schematic, as shown in Figure 2, has additional
benefits. The transmission of the CEBus communications
within a household from smart devices to the communication
transceiver or from the communication transceiver, to the
smart devices, is also effectively transmitted to other
households. This broadcast transmission between associated
households is not a problem as each of the devices can have
a unique address. This transmission has the added benefit
that a communication signal can be sent to other
households, for example, in the event of a security breach,
or disconnection of the publicly switched telephone
network. The transmitted signal is received by the
communication signal processor of other households on the
feed from the distribution transformer 10. These devices
can then place an outbound communication and report to a
security monitoring service, for example.
It is also possible that a communication device can
be provided at the distribution transformer 10, as a backup
for all households. In some cases, it may be desirable for
the household to have its own transmissions limited to that
particular household and appropriate blocking techniques
can be used. It is also possible to selectively remove the
block, if desired.
With the present system, it can be appreciated that
the user interface 36 can provide information regarding the
operation of his power system as well as other information.
It has been found that it is possible to use spread
spectrum coding techniques for injecting a communication
- 10 -
CA 02223666 1997-12-04
- WH-9944CA
signal on the powerline system, and achieve a reasonable
data transmission rate and much higher than existing ripple
control systems and without the high power requirements of
ripple control systems. This higher transmission rate
allows information, such as weather conditions, highway
conditions, stock market results and other general
information to be effectively transmitted over the
powerline system as well as control signals for households.
With this higher transmission rate, the DLC transmission
system can additionally be used as an effective information
communication channel for many different applications and
the above list is provided for example only.
The user interface 36 also allows the user to input
information to his communication signal processor 22
regarding the use of electrical power. The user interface
36 includes an eight line display capable of showing the
user many screens of information. One of these screens
allows inputting information to control the signal
processor according to a user-set desired level of power
conservation aggression that is to be carried out with
respect to certain non-essential or predetermined
electrical appliances.
For example, a water heater requires substantially
more energy if a high certainty is required that one will
never run out of hot water. On the other hand, if one,
from time to time, can tolerate potentially running out of
hot water, a substantial energy saving can be realized. If
the probability of running out of hot water is increased
further, further power savings can be realized. The user
determines a power consumption aggression level for the
discretionary loads. The communication signal processor 22,
includes algorithms for the various discretionary loads to
determine an operating procedure which takes local
information and the command information into account, to
satisfy the power supplier and the home owner. The
algorithm for a particular device can take into account,
CA 02223666 1997-12-04
- WH-9944CA
the time of day and the time of year, for example. The
strategy can also be varied for weekends.
It is anticipated that the power generation source
may provide the user with a further rate discount if they
are willing to allow for load shedding of these smart
appliances. As can be appreciated, peak power demand is
very costly and the ability to turn off or manage certain
devices, and thus reduce power, is of great benefit to the
power generation supply. With this arrangement, user
control and the power supplier control is carried out using
the powerline broadcast capability.
With the present arrangement, a broadcast signal
can be sent over the powerlines from the distribution
station, and predetermined devices can be efficiently
manger to reduce peak demand. The user interface also
allows the user to access various other devices, such as
security systems which can be combined in the communication
signal processor 22.
With the system as described, it is possible to
have a narrow band broadcast delivered to each household.
The broadcast signal is transmitted over a powerline,
initially using a spread spectrum encoding technique which
is then decoded and retransmitted according to CEBus for
reception by the signal communication processors 22 of the
various households. Processors 22 of the communication
system provides outbound telephone signals and in
particular, allows for the household telephone system to
complete a communication with a predetermined computer
located, for example, at the power utility office to report
the various conditions within the house. The fact that the
household initiates such a telephone communication signal
is of benefit as the outbound communications system can
monitor the line and use the line when it is not in use.
As such, the telephone system provides a broadband
communication path for fast transmission of data between
- 12 -
CA 02223666 1997-12-04
WH-9944CA
the communication signal processor 22 and such
predetermined computer.
Figure 4 shows details of the structure necessary
for introducing the coded input signal, generally
represented as 14 in Figure 2.
A coded broadcast signal generally shown as 50 is
provided in Figure 4 to amplifier 52. The amplifier is
separated from the power transmission line 15 by various
components to effectively isolate the amplifier from the
power transmission signal and its related harmonics. The
circuit consists of six basic blocks: a power amplifier
52, a 60 Hz blocking filter 66 and autotransformer 64, a 60
Hz tuned shunt defined by capacitor 64 and coil 56, a 300
Hz tuned shunt defined by capacitor 58 and coil 60, and a
50 ohms resistive shunt generally shown as resistor 68.
The power amplifier 52 is capable of driving at least 20
volts and 5 amps into a 4 ohms inductive load.
The 60 Hz blocking filter 66 protects the amplifier
from the 60 Hz power that comes through the high voltage
capacitor. The autotransformer is used to optimize the
coupling for different values of the high voltage coupling
capacitor and station inductance. There is enough
variation of the values of these parameters that
flexibility is required. The non-coupled part of the
transformer winding essentially acts as an inductor, that
in series with the station transformer inductance, forms a
high pass filter, with a high voltage capacitor. Lower
values of station inductance or coupling capacitance
require a larger section of the winding that is not
coupled.
The 60 Hz tuned shunt carries the 60 Hz current
from the high voltage coupling capacitor and keeps the
voltage requirements for the auto transformer within the
range of available standard products. The 300 Hz tuned
shunt reduces the fifth harmonic voltage that is amplified
- 13 -
CA 02223666 1997-12-04
WH-9944CA
by the resonance of the high voltage coupling capacitor and
the inductor of the 60 Hz tuned shunt. The resistive shunt
limits the voltage of harmonics other than 300 Hz that
could damage the power amplifier. The coupling circuits
were installed in three ventilated metal boxes; one circuit
for each phase. 50 ohms resistive shunts for all three
phases were installed in a separate ventilated box so that
the main coupling circuit boxes could be removed for
repairs or design modifications without de-energizing the
high voltage capacitor. This resistor box was also fitted
with three switches that can be used to short out the S0
ohm resistors, thus solidly grounding the capacitor bank.
The protective spark gaps are installed in a plastic box on
the pole supporting the high voltage capacitor bank. At
this location, they protect all circuits from the 8000
volts that would result from accidental open circuiting of
the capacitor ground conductors. Typical values for the
various components are shown in the following tables:
- 14 -
CA 02223666 1997-12-04
WH-9944CA
TABLE A
Com~onent SYmbol - Value
Filter Capacitor CF 60 ~F
Filter Inductor LF 100 mH
Auto Transformer TA variable ratio
60 Hz Shunt Capacitor cs6o 120 ~F
60 Hz Shunt Inductor Ls60 60 mH
300 Hz Shunt Capacitor Cs300 8 ~F
300 Hz Shunt Inductor Cs300 30 mH
Resistive Shunt R 50 ohm
High Voltage Capaci-tor C~ 0.5 - 5.7 ~F
Station Transformer LST 4 - 16 mH
Inductance
CA 02223666 1997-12-04
- WH-9944CA
TABLE B
ComPonent Descri~tion Ratinq
60 ~F Capacitor ASC Capacitors 440 V
Type X386S
60 mH Inductor Hammond 159ZC 2 A
(0.7 ohm)
Autotransformer Superior Electric 1 kVA
Powerstat 126U
single phase 140 V
15 A
30 mH Inductor Hammond 195P20 20A
(0.0175 ohms) . 2500 V
8 ~F Capacitor 440 V
30 mH Inductor Hammond 195P10 10 A
(0.16 ohms) 2500 V
100 ohm Resistor ohmite L175J100 175W
Ventilated Box Hammond 1416T
Handles (Pair)
Terminal Strips
Spark Gap TII Industries Inc. 200 V
325-2M station
(fail short) 30 A
protector
CA 02223666 1997-12-04
WH-9944CA
It was found that this circuit is capable of
protecting the amplifiers and allows the input signal 50 to
be appropriately amplified and placed on the power carrier
line.
The performance of the circuit in coupling the
signal onto the high voltage conductors was measured by
injecting a steady signal of 990 Hz. It was found that it
was best to use three phase injection with all signals in
phase.
From a review of Figure 4, it can be appreciated
that the amplifier 52 will receive signal 50 which is then
amplified. This signal as will further be explained, is a
spread spectrum coded version of the broadcast signal. In
any event, Figure 4 shows how the signal can be injected
onto the power carrier transmission system.
Figure 5 is a block diagram of the receiver
provided on the low voltage side of the distribution
transformer 10 for .receiving the coded communication signal
in combination with the power carrier signal and processing
the signal to extract the coded communication signal and
translate the same for retransmissions. The signal is
first treated with a high pass filter 72 which is used to
remove the first third and fifth harmonics of the power
carrier signal and the signal below 500 Hz. The signal is
then processed by the automatic gain control 73 and the
anti aliasing filter 75. The signal is next converted from
analogue to digital by the analogue to digital converter
74. The digital signal is then processed using comb
filters 76. This filter is designed to remove further
harmonics, however, in actual practice, it is far from
ideal, as only signals at harmonic frequencies plus 30 Hz
are unattenuated and all other frequencies follow a sine
rolloff characteristic. The benefit of this type of filter
is its simplicity of implementation. A comb filter has
only two non zero taps and requires relatively few taps for
its response. Basically, the comb filter removes the
CA 02223666 1997-12-04
WH-9944CA
harmonics, however, it also removes portions of the signal
which are valuable. This trade-off is tolerated due to the
simplicity of implementation and the ability to process the
signal quickly. The signal is then passed through band
pass filters 78 and 79 which allow the signal between 500
Hz and 1500 Hz to pass. Band pass filters 78 and 79 are
part of the phase locking demonstration block 81. This
block adjusts for variations in phase and removes the 990
Hz reference frequency. The broadcast signal initially had
a target date rate of about 60 bits/second and a seven
digit pseudo random code is used to spread this signal.
With this arrangement, the transmitted signal is less than
420 bits/second. Passing the frequency less than about 400
Hz will allow the target rate of 60 bits/second to be
realized. Frequencies above this level are removed by the
low pass filter 83.
The signal is then sampled by the down sample block
85 having a sample rate of 1 ks/second. Equalizing filter
8 adds some of the signal back in which the comb filter
removed in removing the harmonics. Up sample block 89 then
resamples the revised signal prior to being filtered by the
low pass filter 91 (400 Hz). The signal is then passed to
the despreading arrangement 93.
The despreading arrangement includes inverse filter
95, bit tracking logic 97, bit clock 99, sampler 101, frame
tracking logic 103, frame clock 105, and frame decoder 107.
Raw output data blocks are outputted at 108 and are
provided to a forward correction decoding circuit 110. The
collected signal is then sent to the translator 92 for
conversion into CEBus and subsequent transmission on the
secondary side of the transformer to the households 12.
The use of spread spectrum effectively codes the
communication or broadcast signal over a larger frequency
band and much of the signal is located in the gaps between
the harmonic frequencies of the power carrier. In this
case, the spread signal is between 500 Hz and 1500 Hz and
- 18 -
CA 02223666 1997-12-04
WH-9944CA
.
is spread about the carrier frequency of 990 Hz. The 990
Hz midpoint was selected to be between two power harmonics
and it results in advantageous benefits.
Spread spectrum was initially developed for
military applications and was in response to jamming
signals. Basically by spreading the coded signal over a
larger frequency band, it is difficult to jam, as it is
continually shifting frequencies and jamming at particular
frequencies only results in removal of a small portion of
the signal. If there are sufficiQnt jamming signals
present, then the holes in the transmitted signal can be
relatively high and higher error rates occur. In the
present application, jamming signals can be equated to the
harmonic signals that are present in a power transmission
system. These harmonic signals are relatively strong and
make the channel extremely noisy. Fortunately, the
harmonics are all at known frequencies and therefore, it is
possible to remove these frequencies from the signal
leaving the signal between harmonics for transmission of a
communication signal. In order to keep the sophistication
of the microprocessor at the distribution transformer
relatively inexpensive, the signal is processed using
simplified comb filters. It is possible to use more
accurate digital techniques for removing these frequencies,
however, it requires more processing speed and higher
costs. The spread spectrum coding uses the direct sequence
spread spectrum approach with a seven digit pseudo-random
code.
Spread spectrum systems have inherent jamming
protection due to spectrum diversity but they can still be
jammed if the jamming signal exceeds the jamming margin of
the spread spectrum system. In the case of spread spectrum
DLC power harmonics, they are so numerous within the
transmission band and are of such magnitude that special
steps have to be taken to remove them before passing the
signal on to the receiver. In military and certain
- 19 -
CA 02223666 1997-12-04
WH-9944CA
-
commercial systems, spread spectrum signals are first
preprocessed by an adaptive filter to remove unwanted or
jamming signals. The spread spectrum DLC arrangement can
be simplified as the jam signals in the transmission band
of known frequency and phase, and this allows for design of
static, harmonic rejection filters, and lowers hardware
requirements.
The design of a digital filter that will remove the
harmonics but will leave the desired signal intact is
dependent upsn certain hardware constraints. The DLC
receiver is implemented on a 16 bit, 40 MHz DSP having a 6
kHz sampling rate and 256 bytes of RAM.
In order to have the required characteristic of
linear phase across the band, an FIR filter is employed.
For a FIR filter running at 6 kHz to achieve the multiple
pass bands, between harmonics, that is from 500 Hz to 1500
Hz would require at least 500 taps. This would exceed the
RAM resources of the DSP processor which must store both
filter history values and tap co-efficients in RAM, in
order to use the fast multiplying and add instructions of
the DSP processor.
A simpler approach is to use a comb filter as
previously described. A comb filter has only two non zero
filter co-efficients, greatly simplifying the processing
requirement and reducing the amount of memory needed for
the DSP processor. Typically, the comb filter needs
approximately 100 history values but this is many times
less than would be required by a more tailored FIR filter
at this sampling rate. The phase response of this filter
has the desirable property of linear phase. However, there
are 180~ phase shifts at the locations of the zeros, so
this filter is only linear between harmonics. Such phase
distortion would be unacceptable for a wide band signal.
By selecting the modulating frequency of 990 Hz, exactly
between two harmonics, then all side bands that lie between
this frequency and the harmonics fall into the band of
- 20 -
CA 02223666 1997-12-04
WH-9944CA
-
linear phase. The side bands that are further than 30 Hz
from the carrier, experience a phase shift of 180~ with
respect to the carrier. The side bands, while phase
shifted, with respect to carrier are 3600 or Oo out of
phase with respect to each other's respective side band.
With no phase distortion of the double sided band signal,
no cancellation will occur when the modulated signal is
later digitally demodulated.
A major implementation issue exists when using a
comb filter to remove powerline harmonics. The comb
filter's rejection band for power harmonics is so small,
less than 10 mHz, that the only way such a filter can work,
is if the power frequency does not deviate for more than 16
~Hz. Such a requirement is not met in power transmission
systems which typically vary at least as much as 20 mHz
over a day. Therefore, the only way a comb filter can
work, is if it is phase locked to the powerline. The
spread spectrum DLC's 6 kHz nominal sampling rate, is
convenient. A phase lock loop to the powerline is used
and a frequency divider in the order of lOO's is placed in
the feedback path, allowing the sample clock rate to be
derived. This ensures that as the power system frequency
deviates from 60 Hz, the harmonics will also be rejected by
the front end of the DLC receiver. Therefore, this
arrangement provides phase lock.
As can be appreciated from the above, it is
possible to use spread spectrum coding techniques on a
powerline carrier by appropriate selection of the frequency
of the communication signal for effective transmission
between the harmonics of the power carrier signal. It can
also be appreciated from the above, that the processor used
for despreading and effectively removing of the effects of
the power carrier signal, can also be relatively simple
while still achieving a satisfactory data rate, useful for
a broadcast type signal. By positioning the processing
arrangement at the distribution transformer, the cost for
CA 02223666 1997-12-04
WH-9944CA
the microprocessor used to process the signal at the
transformer, is effectively divided by the number of
households located downstream. It is possible to provide
signal processing arrangements in each household to
despread the signal, however, the cost for the system
increases. The cost for the equipment to receive a CEBus
transmission is relatively inexpensive, and thus, the cost
of the microprocessors within the home, is kept relatively
low.
It is possible to have enhancements to the system
by increasing the capabilities of the processor provided in
the de-spreading arrangement. This may be desirable where
high data rates are desired. Higher data rates are often
not necessary for broadcast signals as there is ample time
to receive the signals.
With the present arrangement, a broadcast signal is
sent over the distribution line carrier arrangement in a
spread spectrum coded format. It is received at a
downstream location within the dlstribution system and de-
spread and subsequently sent to households downstream in adifferent form, namely; translated into CEBus. Thus, the
communication signal from the distribution station is
transmitted in a first form received on the low voltage
side of the distribution transformer and converted to a
second form, and retransmitted to households located on the
low voltage side of the transformer. The equipment in the
household receives and processes the signal in accordance
with instructions which may be generally provided therein
and as may be provided by the household. A processor
within the house receives the general broadcast signal and
receives user determined characteristics and then takes
appropriate action by sending addressed CEBus signals to
control various electrical loads, or provides direct
control of the devices through a load control box
associated with the processor. This processing capability
within the house accommodates variations determined by the
CA 02223666 1997-12-04
WH-9944CA
user and adds a whole series of options not available with
conventional on/off controls.
The broadcast signal can include, as required, load
shed command signals. These command signals can include
the load shed command, the duration of the load shed, and
the outside temperature. This information is used by the
processor, together with the user input and parameters to
control the discretionary devices. The processor using
algorithms, which take into account, the time of day and
year, user parameters, and the broadcast load command
information, then determines how the devices will be
controlled. This does not necessarily mean the devices
will be off for the entire duration, but will be managed,
based on the combined needs and standards of the household
lS and the utility provider. The broadcast signal is used to
provide command signals when required and to provide other
information at other times.
As the number of homes effectively controlled by a
distribution station is not that large, it is possible to
send addressed signals for action of specific equipment
within a single household. This is valuable if you desire
to turn off the power to a certain user in the event of non
payment or for other reasons, or where a specific report is
desired. Individual addressed signals can be provided in
the broadcast signal, as the broadcast signal can be
continuous, and timing is not urgent.
For such desired events as reporting of meter
readings, a broadcast signal can be sent with the equipment
within an individual household recognizing the signal as a
request to report meter readings or other functions, and
these results can be reported to a predetermined computer
over the telephone lines as earlier described. Thus, the
meter reading can be caused by a broadcast initiation
signal, followed by a telephone reporting function. In
this way, the narrow band transmission channel of the
powerline carrier, is effectively used and the high speed
CA 02223666 1997-12-04
WH-9944CA
data communication capability of a telephone line is
effectively used for the more complicated reporting of
information. As can be appreciated, the signal to provide
instructions to report meter readings can be of a generic
nature, such that it is used by a host of households, and
is relatively short and suitable for the narrow band
transmission channel.
Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended
claims.
- 24 -
