CA2271913A1 - Improvements relating to signalling in electricity distribution systems - Google Patents
Improvements relating to signalling in electricity distribution systems Download PDFInfo
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- CA2271913A1 CA2271913A1 CA002271913A CA2271913A CA2271913A1 CA 2271913 A1 CA2271913 A1 CA 2271913A1 CA 002271913 A CA002271913 A CA 002271913A CA 2271913 A CA2271913 A CA 2271913A CA 2271913 A1 CA2271913 A1 CA 2271913A1
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Abstract
An electricity supply system in which a control organisation creates variations of the fundamental frequency of the supply in order to signal shedding or adding of loads (L1-L4), or to cause some other process or transmission of data. The loads (L1-L4) are generally located at consumer premises and are able to be actuated in this fashion by prior agreement with the control organisation. Each consumer installs frequency change decoder device (17) to detect and interpret the signals above noise which is always present on the supply system. Each device (17) may also be used to detect uncontrolled frequency decays caused by a failure in the supply system and to shed loads (L1-L4) appropriately.
Description
WO 98I21803 PCT/NZ97l00156 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 SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26) 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..
SUBSTITUTE SHEET (RULE 26) -S_ 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 SUBSTITUTE SHEET (RUSE 26) 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.
SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26) _g_ 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.
SUBSTITUTE SHEET (RULE 26) 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 SUBSTITUTE SHEET (RULE 26) 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 SUBSTITUTE SHEET (RULE 26) WO 98l21803 PCT/NZ97/00156 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 SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26)
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 SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26) 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..
SUBSTITUTE SHEET (RULE 26) -S_ 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 SUBSTITUTE SHEET (RUSE 26) 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.
SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26) _g_ 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.
SUBSTITUTE SHEET (RULE 26) 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 SUBSTITUTE SHEET (RULE 26) 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 SUBSTITUTE SHEET (RULE 26) WO 98l21803 PCT/NZ97/00156 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 SUBSTITUTE SHEET (RULE 26) 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.
SUBSTITUTE SHEET (RULE 26)
Claims (9)
1. 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.
2. A method of signalling 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.
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.
3. A method of controlling reconnection of interrupted loads in an electrical power network following load shedding, comprising:
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 frequency characteristic within the network to provide a reconnection signal for the load category.
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 frequency characteristic within the network to provide a reconnection signal for the load category.
4. An electrical supply system in which a control organisation creates variations of fundamental mains power frequency to signal adding or shedding of loads by consumers.
5. 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.
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.
6. 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 interrupted 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.
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 interrupted 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.
7. A load control device which monitors alternating voltage in an electricity distribution system and acts to cause shedding or adding of load according to signals transmitted as variations in the fundamental frequency of the system.
8. An 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 variations to produce a representation of the signals as an output.
a monitor of fundamental frequency in the electricity supply which determines variations in the fundamental frequency and decodes the variations to produce a representation of the signals as an output.
9. An 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.
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.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NZ299,756 | 1996-11-14 | ||
| NZ2997596 | 1996-11-14 | ||
| PCT/NZ1997/000156 WO1998021803A2 (en) | 1996-11-14 | 1997-11-14 | Improvements relating to signalling in electricity distribution systems |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2271913A1 true CA2271913A1 (en) | 1998-05-22 |
Family
ID=29707812
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002271913A Abandoned CA2271913A1 (en) | 1996-11-14 | 1997-11-14 | Improvements relating to signalling in electricity distribution systems |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2271913A1 (en) |
-
1997
- 1997-11-14 CA CA002271913A patent/CA2271913A1/en not_active Abandoned
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