Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVEMENTS RELATllVG TO SIGNALLING IN ELECTRICITY
DISTRIBUTION SYSTEMS
FIELD OF THE INVENTION
This invention relates to signalling in electrical power systems, and
particularly to
systems and methods in which information is broadcast to electricity consumers
through
variations in the fundamental supply frequency. The invention also relates to
decoding
devices for use in such systems.
BACKGROUND TO THE INVENTION
Ripple control is a well known method of communicating with consumer loads
over an
electricity transmission and distribution network. Signals having frequencies
between
I S about 170 and 350 Hz are injected to the network at subtransmission level
for direct
control of local loads including water and space heating, night storage
heating, and special
municipal services such as street lights and pumps. For example, small
capacity water
heaters are typically turned off for between 4 and 8 hours per day and
domestic consumers
obtain corresponding cost benefits. Around 1500 MW of load is ripple
controlled
throughout New Zealand. Information such as day/night tariff changeovers and
other
pricing signals are also transmitted one-way to consumers.
Another more sophisticated electrical power signalling system is TWACS, or two-
way
automatic communication system, which provides various remote metering, load
management and other service functions. Participating consumers install
intelligent
transducers which are able to both receive and transmit signals over the power
network
and communicate comprehensively with their utility organisations. Once again
however,
signals are injected at subtransmission levels and are received locally only,
rather than
throughout an entire network. Further, both ripple control and TWACS methods
require
extra generation equipment to produce coded signals at relatively high
frequencies
compared to the network fundamental frequency. Both work acceptably well
however,
and are in use around the world in a range of network systems.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an alternative signalling
system for an
electricity network in which the fundamental frequency of the supply is varied
throughout
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the network. It is also advantageous to provide a system which avoids the need
for
additional and usually costly generation or injection equipment for control
signals.
In a first aspect the invention may broadly be said to consist in a method of
signalling by
an electricity power organisation in which variations of fundamental mains
frequency are
used for control purposes such as the shedding, adding or adjusting of loads
by electricity
consumers. Other control purposes are also envisaged.
In a second aspect the invention may broadly be said to consist in a method of
signalling
1 o in an electricity supply system comprising: generating alternating current
at a fundamental
frequency subject to noise fluctuations, and generating coded variations in
one or more
characteristics of the fundamental frequency which are distinguishable from
the noise.
Preferably the method further comprises monitoring the fundamental frequency
to provide
1 S feedback for creation of the coded variations. Preferably the variations
are created in
magnitude and/or rate of change characteristics of the fundamental frequency.
In a third aspect the invention broadly consists in a method of controlling
reconnection
of interrupted loads in an electrical power network following load shedding,
comprising:
20 determining present supply capability for the network at a central site,
determining a load
category for reconnection according to the supply capability, transmitting a
frequency
control signal representing the load category from the central site to an
electricity
generation system, varying a fundamental frequency characteristic of the
generation
system in response to the control signal, and supplying electricity having the
varied
25 frequency characteristic within the network to provide a reconnection
signal for the load
category.
Preferably the fundamental frequency characteristic which is varied by the
generation
system is either frequency magnitude or rate of change of magnitude.
Preferably the
30 reconnection signal is created to be detectable over predetermined noise
levels in the
fundamental frequency. Preferably two or more load categories are determined
for
reconnection. Preferably the load categories include predetermined heating,
lighting,
motor drive and Like systems by arrangement with electricity consumers.
35 In a fourth aspect the invention broadly consists in an electrical supply
system in which
a control organisation creates variations of fimdamental mains power frequency
to signal
adding or shedding of loads by consumers.
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In a fifth aspect the invention broadly consists in an electricity power
system comprising:
generation systems which produce the electricity as alternating current at a
fundamental
frequency, transmission systems which convey the electricity from the
generation systems
to consumers, and a control system which signals to consumer detection devices
over the
transmission systems by creating coded variations in the fundamental
frequency.
Preferably the control system monitors the fundamental frequency at a point in
the.
transmission system to provide feedback in creating the coded variations.
Preferably the
variations are created in magnitude and/or rate of change characteristics of
the
l0 fundamental frequency.
In a still further aspect the invention broadly consists in an electricity
network having
controlled reconnection for interrupted loads, comprising: generation systems
which
provide supply capability for alternating current at a fundamental frequency,
transmission
systems by which the current is delivered from the generation systems to the
loads, and
a control system which determines the supply capability following loss of a
generation
system, determines intemrpted load categories for reconnection as generation
is restored,
and transmits frequency control signals representing the load categories to
the generation
systems; wherein the generation systems vary the fundamental frequency in
response to
the frequency control signal to broadcast a reconnection signal to the
interrupted loads
over the transmission systems.
Preferably the fundamental frequency is varied in magnitude or rate of change,
or both,
to signal the loads. Preferably the frequency control signal is calculated by
the control
system to provide a reconnection signal which is detectable despite noise in
the
fundamental frequency. Preferably the control system contains a database of
load
categories including industrial, commercial and residential power systems.
In a further aspect, the invention also consists in a load control device
which monitors
alternating voltage in an electricity distribution system and acts to cause
shedding or
adding or other control of load according to signals transmitted as variations
in the
fundamental frequency of the system.
In a further aspect, the invention consists in apparatus for enabling an
electricity consumer
to receive load control or other data signals from a supply organisation,
comprising a
monitor of fundamental frequency in the electricity supply which determines
variations
in the fundamental frequency and decodes the var7ations to produce a
representation of
the signals as an output.
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Preferably, the variations in fundamental frequency comprise departures in
magnitude
and/or rate of change of magnitude from predetermined ranges of values.
Preferably the
variations are decoded to provide an output which is able to actuate a relay
and shed or
add loads operated by the consumer as signalled by the supply authority. It
would also
be possible for organisations other than the supply authority to use a system
of this kind.
In a still further aspect, the invention consists in apparatus for controlling
reconnection
of loads operated by a consumer to an ac electricity supply network,
comprising:
measuring means which determines fundamental frequency of the ac supply,
decoding
means which determines variations in the fundamental frequency and translates
the
variations into predetermined control signals, and actuating means which
reconnects the
loads to the supply network according to the control signals.
Preferably the measuring means comprises a filter and hysteresis detector
circuit which
produces pulses at a rate proportional to the fundamental frequency.
Preferably the
decoding means comprises a processor which counts the pulses to determine
variations
in the magnitude or rate of change of the fundamental frequency and produces
the control
signals from a translation table. Preferably the actuating means comprises one
or more
relays which control power to respective loads.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention will be described with respect to the
drawings,
of which:
Fig. 1 is a schematic diagram showing an electricity power system in which
signals
may be broadcast according to the invention,
Fig. 2 is a schematic diagram of a frequency decoder for load control in the
system
of Fig. 1,
Figs. 3a and 3b are example frequency detectors for the decoder of Fig. 2,
Figs. 4a, 4b and 4c are graphs showing variations of fundamental frequency in
the
power system,
Fig. 5 is a flowchart showing part of the operation of the decoder in Fig. 2,
and
Fig. 6 is a flowchart indicating operation of the preferred decoder in more
detail..
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to these drawings it will be appreciated that the invention may be
implemented
by various means in an electricity power system, and that a preferred
embodiment has
S been described in general terms as one possibility. The concept of
broadcasting
information over a power distribution system using variations in the
fundamental
frequency of the electricity supply is defined in the claims which then
follow.
Fig. 1 indicates very schematically the elements of an electricity system
which supplies
electric power from a system of generator stations 10, typically fuel and
hydro stations,
to a large number of consumers 11 who operate residential, municipal,
commercial or
industrial loads. Electricity is supplied through a network transmission
system 12 and
distributed to the various consumers such as C I, C2 and Cn. The generation,
transmission
and distribution elements of an electricity system are assumed to be well
known to the
reader and will not be described in detail. Each consumer is required to
record their
consumption of power, generally on their premises such as through a meter
device 13.
Electricity metering is also assumed to be well known to the reader and will
not be further
described.
The generator stations rely on rotating machinery to produce electricity. Each
generates
a sinusoidal alternating voltage and current at a fundamental frequency which
in New
Zealand is 50 Hz, for example. Although the nominal voltage and frequency are
defined
at a11 parts of the overall system, in practice they tend to vary and must be
constantly
monitored and adjusted by small amounts. One frequency keeping station on the
network
performs fine tuning and generation control 14. Frequency is measured at one
or more
points in the network by standard means such as a zero crossing detector, and
information
is continually sent to the other stations from the frequency keeping station
over the
telephone system to ensure synchronizarion. Under normal operating conditions
the
frequency variations are small and slow, having amplitudes generally less than
0.1 Hz and
varying at up to perhaps O.OlSHz/s, as seen in Figs. 4a and 4b for example.
Each country has an electricity supply system tailored to meet particular
geographical and
regulatory requirements. Continental and island countries generally have
different
arrangements for example. The New Zealand system consists of two ac networks,
in
which the South Island generates excess power for transmission to the North
Island
through a do link. However, the demand for electricity varies daily and
seasonally, so that
from time to time generators in either island must be connected or
disconnected to ensure
that supply matches demand. The fundamental supply frequency of mains power
changes
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at a rate which is proportional to the mismatch at any instant. Before a
generator can be
added to the system it must be run up to speed and synchronized with those
which are
already supplying power. This takes several minutes so the system is generally
run with
a number of generators already operating as "spinning reserve" ready for
connection to
ensure that ordinary variations in demand can be met immediately.
Nevertheless, small
random imbalances from moment to moment create a noiselike variation in the
fundamental frequency as seen in Figs. 4a and 4b.
Failure of a generator or transmission line creates a substantial mismatch
between supply
and demand in the electricity system. The frequency then falls quickly such as
shown in
Fig. 4c for example, and in practice is used as an indicator that a fault has
occurred.'
Voltage levels may also fall. Failure of the do link in New Zealand for
example, would
remove about I000 MW of supply from the South Island and frequency may fall in
the
North at around 1 Hz/s. Should the frequency fall below 45 Hz the transmission
system
1 S as a whole is likely to fail with serious consequences nationwide if the
spinning reserve
is not sufficient to boost the supply. Selective load disconnection or "load
shedding" has
been used to reduce demand under these circumstances, and the supply of whole
streets
or suburbs may be terminated without warning. In New Zealand the generation
companies contract the distribution companies to provide nominated 2x20% load
blocks
for shedding to meet such emergency fault conditions. The loads must then be
reconnected once the fault has been remedied, which is a significant problem
in itself.
Most electricity consumers operate a number of kinds of loads L 1, L2, L3, L4
and so on,
as indicated in Fig. 1, which may be more or less dispensable. Some loads such
as general
water heating, space heating and pool heating would not cause great
inconvenience if
disconnected for a period of minutes or perhaps up to an hour. Loss of supply
to others
such as household lighting, could indeed be inconvenient but not of great
concern. Non
essential categories of load can account for hundreds of megawatts in New
Zealand.
Other loads such as some industrial facilities can be disconnected provided
the consumer
is given a few minutes warning. In some cases such as water heating and street
lighting
the load may be under control of the transmission or distribution supply
organizations
already, by way of ripple signals for example. Local connection and
disconnection of
loads can thereby be planned and cazried out selectively under price discount
agreements
between suppliers and consumers. An interruptible demand resource of a more
general
3 5 nature is currently being developed in New Zealand with the aim of
creating arrangements
for controlled shedding and reconnection of loads totalling around 500 MW when
required by suppliers during emergencies.
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The present invention enables broadcast signalling to electricity consumers as
required
to implement arrangements of this kind. Predetermined variations in the
fundamental
mains power frequency can be created and used to control the connection of a
range of
load categories to the power supply system, particularly their reconnection
following
shedding caused by a fault. Signals are broadcast to the network as a whole
from a
control system and the loads are reconnected at timed intervals as the
frequency is
stabilized and full supply capacity is restored. The signalling is inherently
one-way from
the control system to the consumers, and is also slow, due to the nature of
the generation
and transmission systems, but su~cient for the purpose of load control.
Consumers who
wish to become part of the interruptible demand resource install a mains
frequency
decoder at the switchboard in their premises to enable the load categories
which they
operate to be automatically shed, and powered up or down when required and
permitted
under the supply agreement. In New Zealand the North and South Islands would
have
coordinated but separately controlled resources, with the arrangements being
more
complicated in continental countries.
Fig. 1 shows a highly generalized electrical power system in which load
control
arrangements have been set up. The fundamental frequency of this normally
complex
system is varied by a frequency change controller FCC 15 which is typically a
PC system
or part of a larger existing system. This monitors and stores frequency data
relating to the
network and provides a control output signal to the generation system 10
through the.
existing generation control equipment 14. In New Zealand the FCC would be
located in
the Transpower control centre to automatically manipulate the network
frequency when
required. A frequency change master decoder FCMD 16 is preferably set up at
detection
point in the transmission system to provide feedback for the FCC. The FCMD is
connected to the FCC by via telephone line or other telemetry system for real
time
communication of information on the frequency changes. The FCC may be required
for
example, to speed up or slow down variations in order to effect a detectable
pattern under
the particular circumstances. Each participating consumer 11 installs a
frequency change
decoder FCD 17 as either an integral metering and decoding unit or perhaps a
standalone
device in conjunction with an existing meter 13, as shown for consumers C 1
and Cn
respectively.
The FCD devices 17 sample the mains voltage or possibly current and decode
changes in
the fundamental frequency which may be directed at a particular consumer. Each
is able
to actuate one or more relays 18 or similar devices through which power
reaches the
respective consumer loads 19. The loads are schematically indicated as various
categories
L 1 to L4 in Fig. 1 and may effectively be turned on or off by their
respective relays.
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Each FCD is generally programmed to react to sudden decays in the frequency
during a
fault emergency as well as to predetermined patterns of variation introduced
by the FCC.
The patterns are programmed into both the FCD and FCMD devices on connection
to the
system and may also be programmed over the power system or perhaps a telephone
line
by the FCC later if required. These patterns are preferably based on frequency
rates of
change as indicated below, although combinations of rate of change and
absolute
frequency values or other characteristics of the supply may also be used. A
one-way
signalling capability as mentioned above can thereby be incorporated in an
existing
electrical power system.
Fig. 2 is a block diagram for a microprocessor based FCD with integral
metering and
decoding functions. The microprocessor 20 is connected to a memory 21, display
22 and
clock 23 by a bus. A frequency detector 24 is electrically connected to a
single phase
mains supply having phase and neutral lines P and N. A meter circuit 25 is
inductively
connected to the supply in the usual fashion but plays no part in the
signalling capability.
Electrical energy for the FCD is derived from the mains by a power circuit 26
which is
also of standard construction. The microprocessor receives output from the
frequency
detector, typically as pulses to be counted, and thereby monitors the mains
frequency for
comparison with patterns which are stored in the memory. On detecting an
emergency
decay or a recognizable pattern the microprocessor takes appropriate action
through load
control ports 27 which are connected to actuate the relays 18 in Fig. 1 for
example. A
suitable communications port 28 may also be present if required.
Figs. 3a and 3b are example frequency detector circuits which may be used in
an FCD
such as shown in Fig. 2. These produce accurately timed pulses from the mains
voltage
by which the microprocessor is able to measure the mains frequency despite
noise and
other distortions which are always present in the voltage waveform. In Fig. 3a
mains
phase is connected to the circuit through terniinal 30 and a low pass filter
formed by
resistor Rl and capacitor C 1. This produces a sine wave having a peak to peak
amplitude
of several volts which is clipped about a positive reference value by diodes D
1 and D2.
The resulting largely positive voltage signal is input to a comparator formed
by R2, R3,
R4 and an op-amp A such as LM 358. Hysteresis of about 0.5V is provided by R4.
In
use the comparator switches between high and low output states at positive and
negative
peak points on the mains waveform with considerable noise immunity. Fig. 3b is
an
alternative detector circuit using a CMOS gate such as MC 14093 to achieve the
same
result, both more effectively than a conventional zero crossing detector.
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Figs. 4a, 4b and 4c are graphical examples showing typical background
variations in the
fundamental frequency of an electrical power system, primarily due to
momentary
mismatches between supply and demand. Examples of the variations which might
be
added as described above to provide signalling according to the invention are
also shown.
Fig. 4a covers a period of about 47 minutes with averaging over approximately
1 second
. and 1 minute intervals. Two signal variations 40, 41 of about 3 minutes
duration can be
seen. Over minute intervals the background variations stay generally within
0.05 Hz of
the ideal frequency 50 Hz while the signal variations are easily distinguished
by an FCMD
or FCD with departures of about 0.1 Hz. A signal of this kind could be sent to
turn street
l0 lighting on or off or reconnect certain loads which were previously shed,
or transmit
simple items of data for example. Fig. 4b covers a period of about 2 minutes
and shows
in more detail an 0.1 Hz signal variation superimposed on the background.
Maximum
rates of frequency change for the signal and background are indicated by
dashed lines 42,
43. The signal rate of change is greater than that observed in the background
and is
readily distinguished.
Fig. 4c also covers a period of about 47 minutes with averaging over
approximately 1
minute intervals. A sudden frequency decay 44 of more than 2 Hz is shown which
drops
well below the usual operational limit of the system. This event exceeded the
spinning
reserve capacity and will have caused automatic shedding of various loads
which were
connected at the time. The decay is much greater in magnitude and rate than
the proposed
signal amplitudes 45, 46 and would be immediately recognised by FCDs
controlling
interruptible loads, which would then be disconnected to reduce demand. A
subsequent
reconnection process 47 is also shown in which the mains frequency is allowed
to briefly
stabilise at 48.5 and 49.8 Hz while three load categories are consecutively
added. In this
case the signals to reconnect are transmitted as predetermined high rates of
increase which
are not evident on the scale.
Fig. 5 is a flowchart outlining operation of the microprocessor 20 in the FCD
of Fig. 2
when monitoring frequency of the system supply. Operation for the purposes of
metering
and other functions of an integral device will not be described. In step 50
the
microprocessor determines the frequency and/or rate of change of frequency, or
other
characteristic, from the output of detector 24. This will usually involve an
average of
several instantaneous values recorded over a period of perhaps a minute as
indicated in
Fig. 4a. In step 51 if the measured characteristic falls within the expected
range of
background values then nothing is required of the FCD which continues to
monitor
without effect. In step 52 the characteristic has fallen outside the normal
range and the
microprocessor now checks a predetermined signal code range. In step 53 the
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characteristic has been found in the code range and the microprocessor checks
for a
recognised code. If the code is not recognised there is no effect once again.
Otherwise
in step 53 the microprocessor actuates a load control device such as a relay
18 through an
appropriate control port 27 and returns to monitor the frequency as before.
In step 55 the microprocessor has detected a frequency characteristic outside
the range of
both background and code values, and checks whether emergency action is
required. If
an emergency event is not recognised, such as a sudden frequency decay for
which the
particular consumer is not required to disconnect load, then the
microprocessor returns
to monitor the frequency. Otherwise in step 54 a predeterniined load
disconnection takes
place with one or more appropriate control devices being actuated as required.
Other
action may also be initiated through the communication port 28, such as
startup of a local
generator or an alarm. Data may also be received by the microprocessor, such
as supply
pricing information or new code values for example.
Figure 6 is flowchart indicating in more detail how a microprocessor based FCD
such as
shown in Figure 2 may monitor the mains frequency for significant variations.
In this
example the microprocessor 20 receives an output such as a pulse or transition
from the
frequency detector 24 during each cycle of the mains supply. Each pulse
triggers a new
count by the microprocessor according to an internal crystal oscillator. The
count may
be more or less than that expected for a perfect noise-free cycle at the
fundamental
fiequency of normal operation. Any difference between the expected and actual
counts
is termed a cycle error and by monitoring these errors over a period of time
the
microprocessor is able to determine whether a significant change in the
fundamental
frequency is underway. The cycle errors are expected to vary approximately
equally
between positive and negative values so that a running total of the errors
should remain
close to zero during normal operation of the network. Extracting a system
signal from the
noise requires a careful analysis of the cycle errors.
In step 60 the microprocessor first initialises a running total of the cycle
errors to zero,
and counts in step 61 to determine an error for the latest cycle. Abnormally
large bursts
of noise are eliminated in step 62 and are not added to the running total. In
most systems
cycle errors will rarely be more than a few percent of the expected count,
even when a
significant decay event is in progress. A threshold of perhaps 24-30% might
therefore be
3 5 set to provide a coarse threshold above which a cycle error will be
ignored. An
unreasonably large cycle error might set a flag, or trigger a special load
control process
as a precaution, although this has not been shown. Otherwise plausible errors
are added
to the running total in step 63 which may be positive or negative at any
instant as
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mentioned above. The total is then reduced by a standard error in step 64 or
is reset to
zero where the magnitude of the total is less than the standard error. This
serves to
. prevent slight drifts in either the mains or detector systems, such as a
drift in the
microprocessor crystal output for example, from accumulating and being
misinterpreted
as a significant frequency event. It also allows a range of acceptable
frequencies to be
defined within which the supply frequency may vary without effect.
In step 65 the microprocessor then checks whether the running total now falls
within a
range expected for normal or non-significant variations of mains frequency
over a length
of time. A frequency excursion must be sufficiently large and persist for
sufficiently long
to become significant. A normal range of non-significant variation is
predetermined and
programmed into the FCD according to characteristics of the particular supply
system.
If the rurming total is outside the normal range the FCD attempts to interpret
the variation
as either a coded signal or a failure in the power supply system, and may
actuate an
appropriate load control routine as indicated in Figure 5. If the signal
represents
transmission of data from the supply control organisation to the detector then
the data is
suitably processed and stored in memory 21. The data could include new values
for the
normal range of the running total for example.
Signals are best coded by changes in absolute frequency or rate of change of
fiequency
as mentioned above. In each case the variations or changes of state must be of
a nature
which is readily distinguished from the typical background variations inherent
in the
network. The magnitude of these variations can vary according to the network.
In small
networks such as that in New Zealand, the frequency can oscillate between
perhaps 51.2
Hz and 49.8 Hz at reasonably short intervals. Deliberate variations of
frequency must fall
outside these bounds. However, larger networks such as in Australia, will
generally be
much smoother in operation. Deliberate variations in frequency could then be
of a smaller
magnitude but the state changes may occur over a longer period. By
transmitting a
sequence of these state changes a binary message can be broadcast to the FCDs.
Each
FCD unit can receive the binary message but will not act upon the signal until
an entire
packet has been validated. Validation information such a parity bits or
checksum values
could conceivably be appended to the binary sequence which fom~s each packet
to reduce
the chance of a signal being misinterpreted.
3 5 The maximum speed of transmission will depend largely upon the background
frequency
noise and the size of the allowable mains frequency range within each
particular network.
Assuming one state can be transmitted in 20 seconds, and frequency variations
can be
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resolved into 16 states, Shannon's formula for channel capacity yields a rate
of 0.2 bits.
This is generally ample for load control purposes.
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