|Publication number||CA1127710 A|
|Application number||CA 329040|
|Publication date||13 Jul 1982|
|Filing date||4 Jun 1979|
|Priority date||9 Jun 1978|
|Also published as||CA1127710A1, DE2923471A1, DE2923471C2, US4359644|
|Publication number||CA 1127710 A, CA 1127710A, CA 329040, CA-A-1127710, CA1127710 A, CA1127710A|
|Inventors||Peter M. Foord|
|Applicant||Peter M. Foord, Electricity Trust Of South Australia (The)|
|Export Citation||BiBTeX, EndNote, RefMan|
|Classifications (4), Legal Events (1)|
|External Links: CIPO, Espacenet|
~1277~0 This invention relates to a means of and method for transmitting information, for example for control of consumer loads in an alternating current electrical power distribution network, and is particularly applicable where there is a 5. need to shed a load from the network under peak conditions for example. However, the invention is not limited to such uses and can be used to control relays, for example, for the control of street lighting, shop lighting, traffic lights and other control functions for which ripple control has 10. been, or could have been, used previously.
BACKGROUND OF THE INVENTION
Because of the variable nature of electrical loads, it is desirable to be able to control selectively the loads to which power is supplied. Thus much use is made of time 15. clocks for switching in various loads, for example pumping facilities, hot water storage devices and the like, but time clocks are not in themselves sufficiently versatile to meet extraordinary conditions and are expensive. This problem has been recognised previously, and attempts to improve 20. control over load shedding facilities and other functions have included the provision of the so-called ripple control.
Reference is made to the Proceedings of the Institute of Electrical and Electronic Engineers of America, Volume 67, No. 2, February, 1979 on page 241, wherein there is an 25. article entitled "Electric Power Load Management: Some Technical, Econom c, Regulatory and Social Issues". In the article there is a general review of load shedding techniques utilising ripple control.
`-` 1127~10 In a conventional ripple control system, an audio-frequency signal is produced by a separate generator and then coupled by means of a high voltage capacitor or transformer onto a high voltage bus. The capital plant required is costly, and 5. is physically bulky. Furthermore ripple control is capable ; of causing inconvenience to consumers under certain conditions because of the relatively large signal magnitude required.
Ripple control receivers as presently used can respond to a relatively wide band width of the ripple frequency, and lO. a range of plus or minus 5% is not uncommon, but such a wide frequency band width of response does not give the security against operation by spurious signals which is sometimes required, and reduces the number of channels which can be used in a relatively small range of signal frequencies. It is 15. found that the frequency at the transmitter of a ripple control unit can vary slightly with both time and temperature and this i5 also inconvenient and further contributes to a lower standard of security against spurious signals~actuating a receive relay than the standard which can be achieved by use 20, of this invention.
An alternative proposal which has been devised by the inventor herein is a voltage distortion caused by switching in a heavy inductive load. This has been proved to be satisfactory but the equipment was found to be expensive and 25, switched such heavy currents that the apparatus operated near its maximum limit.
Further to this, it is known that a system exists whereby the power ~ystem voltage wave-form is distorted on the zero crossing, but this again means a very large energy ~ .
1127~10 pulse, and each distortion must be separately recognised by the receive circuit.
The main object of this invention is to provide a relatively inexpensive consumer load control means and method.
5. A second object is to provide a means and method which will result in a very hlgh degree of security against actuation of a receive circuit by a spurious signal. A still further object of this invention is to provide a system which is simple to use, wherein the constituent elements of both the transmitter 10. and receiver are of rugged and simple construction and not likely to have maintenance problems, and yet wherein a large number of codes can be transmitted for a comparatively low cost to thereby control a large number of functions.
BRIEF SUMMARY OF THE INVENTION
15. Briefly in this invention, an electrical information transmitting means for consumer load control comprises a transmitter which transmits a coded signal through the conductors of an alternating current electrical distribution ~- network as a sequence of waveform distortions to a receiver20. which decodes the signal. The receiver comprises two correlators , connected in quadrature, and the transmitters and each correlator synthesise synchronous signals from the power frequency, all of which are locked thereto. The D.C. voltage outputs of the correlators are squared and summed so that 25. the receiver is independent of phase angle.
More specifically, the invention consists of means for transmitting information for example for control of a load in an alternating current electric power distribution system having a power frequency which comprises a transmitter coupled to the system to derive power from the system and generate a signal the frequency of which is synthesised from and locked to the power frequency, and to superimpose said signal upon the power frequency wave-form as a sequence of wave-form distortions, 5. a receiver also coupled to the system, said receiver having a detector circuit comprising a pair of correlators driven ninety degrees out of phase with each other by a control fre-quency which is synthesised from and locked to the power frequency and synchronous with said signal frequency, such that 10. a signal input to the correlators appears as a D.C. voltage output thereof, a squaring circuit coupled to the signal output of each correlator and a summing circuit coupled to the squaring circuits, so arranged as to square and sum the D.C. voltage outputs of said correlators.
15. The absence of phase sensitivity in the receiver causes ; the receiver to function as a very narrow band width filter (in some cases, about one twentieth of the effective receiver band-width of a ripple control receiver). This in turn enables relatively low amplitude signals to be used, and results in 20. equipment very much less expensive than ripple control equipment.
Since the receiver band width is small, in some instances the 5ignal can comprise several frequencies which are so close to-gether that a number of circuit components can be common to all signal frequencies, but the several frequencies may be 25. employed in a frequency shift binary code.
Locking of both the signal frequency and the receiver corxelator control frequency to the power frequency enables the receiver to always track the transmitted signal ~requency.
~127710 In another aspect, the invention relates to a method of transmitting electrical information, for example for controlling an electrical load in an alternating current electrical power distribution system, having a power frequency, comprising, 5. coupling a transmitter to the system to derive power from the system and generating in the transmitter a signal the frequency of which is synthesised from and locked to the power frequency, and superimposing said signal upon the power frequency waveform as a sequence of waveform distortions, coupling a receiver to 10. the system to derive power from the system, and driving two correlators in the receiver ninety degrees out of phase with each other with a control frequency synthesised from and locked to the power frequency and synchronous with said signal frequency, so as to covert a signal input to each said correlator into a 15. D.C. output thereof, and squaring and summing said D.C. outputs.
The invention has a number of advantages over known ripple control and wave distortion techniques.
Firstly the method of producing the signal by waveform distortion is simple and low in cost and lends itself to be 20. driven at a precise frequency or a range of frequencies.
Secondly by using a frequency synthesised from the supply frequency it is a simple matter to make the transmitted ; frequency identical with the filter characteristics in the receiver by using a synchronous correlator as a detector.
25, This produces a very selective and stable d~ector without using any critically tuned components.
Thirdly, as a result of the close match between trans-mitted frequency and receiver characteristics a very low level of signal magnitude is required for reliable and secure transmission. The level of signal is lower than llZ~7710 with any system which has been devised heretofore and which is known to the Applicant.
BRIEF SUMMARY OF THE DRAWINGS
Embodiments of the invention are described hereunder in 5, some detail with reference to and are illustrated in the accompanying drawings in which:
Fig. 1 is a diagrammatic representation of the system arrangement, Fig. 2 is a block diagram of a transmitter control 10. unit, Fig. 3 is a diagrammatic representation of a basic transmitter, Fig. 4 is a diagrammatic representation of a boosted transmitter, 15. Fig. 5 is a diagram which shows the relationship between the system voltage and the transmitter load curxent on a test which was conducted by the Applicant herein, Fig. 6 is a block diagram showing the receiver com-ponents according to the first embodiment, 20. Fig. 7 is a diagrammatic representation of the basic synchronous correlator utilised in the receiver, Fig. 8 is a block diagram of a receiver according to the second emhodiment and using two integrated circuits one of which is a microcomputer, and 25. Fig. 9 is a diagrammatic representation of the functions included in the correlator integrated circuit 60 shown in Fig. 8.
DF.TAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In all the embodiments described hereunder, the llZ''~710 sequential distortion of the power system voltage is used for controlling consumer loads in an alternating current electrical power distribution network. The distortions are transmitted via high voltage feeders and distribution 5. transformers and appear between the active and neutral lines of each phase at consumer premises.
The signalling device distorts the normal system voltage in a repetitive manner at a frequency which is derived from and hence locked to the power supply frequency 10. and which is a fractional multiple of the power frequency.
Desirably the frequency lies in the range of between 200 and 400Hz and an example of one frequency is 237.037Hz derived from the expr~ssion 2l78 X 50. A multiple bit code is used to obtain a number of separately identifiable channels, and 15. a plurality of transmission frequencies (in these embodi-ments, three) are used to enable frequency shift keying.
In the embodi~ents described hereunder, an ll bit code is used to obtain 256 separate channels (that is 128 ON-OFF
channels). Ea~h bit is formed by transmitting at a selected 20. frequency for 0.8 seconds and this results in a total transmission time of 8.8 seconds. By using the first two bits of the code only for channel 0, a fast signalling channel is formed. The normal time to send a command on this channel is one second and is provided or the purpose 25. of emergency load shedding.
Fig. 1 ilLustrates in diagrammatic form the entire system arrangement, wherein an incoming high ten~;ion three phase line l0 (66 kV) feeds a substation ~i-ansformer llZ'^~710 11 which reduces the voltage for distribution purposes (for eA~ample llkV) and the feeder line 12 (three phase) feeds into a distribution transformer 13, the output of whi~h feeds to a consumer network designated by the arrow 5. 14, and this includes a plurality of receivers 15. A
transmitter control unit is designated 18 (described in greater detail with reference to Fig. 2) and this feeds silicon controlled rectifiers (thyristors) 19. As shown in Figs. 3 and 4, each pair of thyristors 19 energise 10. a capacitor 20 and inductance 21 therebeing one LC load on each of the phases. This is transformed by a trans-mitter transformer 22 to the llkV feeder line 12, thereby providing an exceedingly simple coupling means to the llkV
bus of the transformer 11. In E'igs. 3 and 4, the alternators 15. designated 24 represent the power station alternators, and the inductance designated 25 designates the system source impedance of the lines (Ls).
The block diagram of Fig. 2 represents the transmitter control unit, wherein the power system reference frequency 20. enters the system on line 28, and the switch or other means used to initiate the signal enters on the control line 29. A timing control device 30 is thereby placed into operation upon a command from the control line 29.
The device utilises the frequency of the supply as a 25. base for all timing functions and controls the logic of frequency selection device 31, this being set up by a manually set switch 32 for channel selection ~y anll bit code.
_ g _ -- ` . . _ .. ~ .. ~ ._ .__~ .. _.~.. _~.. K_ :. _: _ _ . _ ___ '_ _ __ . ~.. ~ _ . ~_ _ ._ ._ ._ . _ . _ _ _ _ _.__ .. _ .__ __.. _ . ._ .. . ~ . .
o The output from -the logic of frequency selection, lines 33, 34 and 35 control the programmable divider 36 and thus determine which one of three frequencies is to be trans-mitted. The programmable frequency divider is fed 5. with a synthesised frequency, (in this embodiment 256 times the frequency of the supply, that is for a 50Hz power supply, 12800Hz), through a coupling line 37 from a phase locked loop frequency synthesiser 38, which receives its reference frequency from the power supply 10. line 28. The programmable frequency divider divides ~he 12800Hz either by 54, 57 or 60 to give the frequencies 237.04 224,56 and 213.33 Hz herein designated FC3, FC2 and FC1 in this embodiment. The selected frequency is fed to the thyristors 19 of the three phases of power through their 15. respective firing circuits generally designated 39.
Fig. 5 illustrates the system voltage to load current relationship which is achieved by the thyristor arrangement.
The transmitter basic arrangement is shown ilL Fig. 3.
, 20. The transformer load consists of the air cored in-ductance 21 in series with the capacitor 20, and switched by the back to back thyristors 19 as describei:l above.
However, the load current does not flow continuously ~, and as illustrated in Fig. 5 has an off period be-tween 25. each pulse. This off period is achieved by making the resonant frequency of the LC combination 21/20 higher than 1.15 times the control frequency FC3, FC2 or FCl firing 11~7710 the thyristors. The actual resonant frequency is not criti~al and hence it is not necessary to use expensive critically tuned components. The off period is essential for the successful operation of the circuit and assists 5. in maintaining simplicity of design. The gap inbetween the pulses causes the LC circuit 21/20 to receive energy from the power system and hence oscillation is main-tained. The start of each current pulse is controlled by the firing of one or other of the thyristors by the 10. control frequency (~hich is the siynal frequency). The thyristors are commutated off as the load current pulse goes to zero.
The control frequency for firing the thyristors is always derived from the power frequency (in this embodiment 15. 50Hz), and for this reason it is always locked to it.
Suitable frequencies can be obtained by first multiplying the power frequency (50Hz) by an integer in a phase locked : loop circuit and then dividing by different integers. In this embodiment, three separate frequencies are used to 20, form a frequency shift system of coding. The supply frequency is first multiplied by 256 and then divided by i 54, 57 or 60 to produce the frequencies of 237.04, 224.56 an~ 213.33 ~z respectively. It is ad~antageous to use frequencies between 200 and 300 Hz because less signal is 25. then lost due to capacitor loads on the power system.
By using three frequencies close toyether, the same LC components can be used and only the control frequency alters.
11~7 710 As the source impedance of the power system (that is from ~:he alternators to the llkV bus) is almost entirely incluctive, the current desicJnated ~2 in Figs. 3 and 4, reflected from the oscillating load, will produce a 5. voltage distortion on the llkV bus and hence feeder line 12 (~ig. 1) which is proportional to the rate of change of current (i.e VD = s dt )- Hence a voltage of the same frequency as the current will be produced on the llkV
feeder line 12.
10. The level of oscillation obtained in the basic circuit shown in Fig. 3 is difficult to predict in practice as it is influenced by the amount of resistance in the circuit (within the inductance and transformer windings).
It is advantageous to boost this oscillation to control 15, its magnitude, to allow more resistance in the circuit (that is less copper in the windings) and to make more effective use of the capacitors, being the most costly transmitter components. Extra energy can be injected into the oscillator circuit by means of the small 1GW
20. voltage boost transformer 42 shown in Fig. 4. The four thyristors 19 nee~ to be selectively fired to ensure that the boost transformer voltage always assists the oscillation.
Control of oscillation magnitude is then obtclined by an adjustable damping resistor 43 across the reson.lllt 25. circuit.
The magnitude of signal voltage required for reliable and secure operation has been found to be about 0.4O of the 11~7710 system voltage with the receivers set at a threshold level of 0.1~.
The following description refers to the receiver which is used for signal detection and decoding, and 5. illustrated primarily in Figs. 6 and 7. The basic correla-tion circuit used for detection is shown in Fig. 7 and consists of a pair of solid state (CMOS) changeover switches 46 driven at the signal frequency, between an RC combination of resistor 47 and capacitor 48. This may be regarded as a 10. synchronous filter or correlator, and a pair of these driven in quadrature is generally designated in Fig. 6 by the designation 49. (Fig. 9 shows the details of circuit).
The t-requency driving the switches must be synthesised in the receiver in the same manner as was done in the 15. transmitter. This frequency is therefore also locked to the power frequency (50Hz). By means of the CMOS switches 46, the signal voltage is synchronously rectified and appears as a DC voltage on the capacitor 48. The actual value of DC
voltage will vary depending on the phdse relationship between 20. the correlator control frequency and the incoming signal.
If they are in phase maximum signal will be received and i they are 90 out of phase zero signal wlll be received.
Lf the signal to be detected = ~ sin ~t + ~), where A
is a constant representing the peak amplitude of waveform, 25. W is the angular frequency and0 is the phase anyle be-tween the signal and the correlator control frequency, then for maximum correlation (zero phase shift) the signal will be -` ~lZ7~10 full wave rectif:ied and smoothed to produce an average DC
voltage Vc~ = Al 57- When phase shift is present the relationship will result in Cl = - cos 0.
1.57 5. By using a second correlator in quadrature with the first, C2 1.57 0 Squaring and summing each result, V 2 + V 2 = A2 This gives a result which is independent of phase ~- 10. angle. Hence the signal detection section of the receiver con-sists of two synchronous filters 49, driven 90 apart by a synthesised frequency identical with the transmitted signal frequency. The output from the two correlators will normally be zero until our incoming signal is detected. When a 15. signal arrives from the transmitter, D.C. voltages will appear on the correlation capacitors, because the correlators synchronously rectify the incoming signal. The D.C. voltage on each correlation capacitor is then measured, squared and summed. This non phase-sensitive correlation method of 20. detection has several advantages for use in the receivers:
(1) it is made simply and is low in cost,
(2) it is very selective in frequency discrimination having a 3dB band width of approximately lHz, and hence gives high rejection to all other frequencies including 25. any random noise on the power line due to fluctuating - '1. 1 -11~7710 loads.
(3dB band width is the band width at half maximum signal power level.)
(3) It does not require any critically tuned components 5. and hence stability over a long period of time is achieved.
(4) Because of its very narrow band width response, a very low level of signal can be used without sacrificing reliability or security.
10. Reference is nGw made to the coding system.
A frequency shift keying system of coding is used to ensure security of transmission. An 11 bit code is used and a "1" or a "0" of the code is represented by one of three closely spaced transmission frequencies. If the 15. three transmission frequencies are FCl, FC2 and Fc3~ where FCl is the lowest frequency and Fc3 is the highest frequency, then the following general rule is used. The first bit of the code is always set at frequency Fc3 and is termed the start bit. Its detection by the receiver initiates 20. the receiver to search for the remainder of the code.
From then on a "l" in the code is represented by the higher of the two remaining frequencies and a "O" by the lower of the two remaining frequencies. By this means each successive bit of the code is always a different frequency.
25. This enables imp~ved bit separation and identification.
The second bit of the code i5 reserved to provide a FAST OFF flmction for emergency load shedding. If the ~lZ7710 second bit is sent as a "1" the FAST OFF function is immed-iately recognised and the receivers will respond to this if they are programmed to do so. For all other codes the second bit is a "O" and the receiver then continues to look
5. for the remainder of the code.
For these codes bit 3 is always sent as a "O" and is - termed a "continue" bit. It allows the receiver to set itself up correctly to search for the remainder of the code.
The receiver must set its correlator control frequency 10. correctly for each bit of the code according to the channel number code programmed into it. If a match is obtained between the signal transmitted and the code programmed into the receiver the receiver operates a bi-stable output ~ contactor controlling external load. A total of 128 ON--15. OFF channels are available and all of these can be made ~, responsive to the FAST OFF code if desired.
As shown in Fig. 6, the receiver may be divided into 6 separate functional blocks as follows:
Frequency synthesiser 50, synchronous filter 49, signal 20. measurement, computation and threshold detection 51, logic of code discrimination (decoder) and synthesiser control unit 52 output bi-stable contactor 53 and power supply 54.
The details of the synchronous filter section 49 which 25. extracts the signal from the power supply network have already been described above.
The frequency synthesiser section comprises a standard phase locked loop multiplier followed by a programmable divider to select the required three frequencies, this bei~g in accordance with known art.
5. Signal measurement and computation section can be implemented ln analogue form with analogue multipliers, summation amplifier and comparator for threshold detection.
Alternatively the signal can be converted t:~ digital form and all computation and threshold delection performe~
10. digitally. This arrangement is a second ~mbodiment which is a preferred embodiment and is illustrated in Fig. 8 It utilises a simple single chip microcomputer 56 to carry out the signal measurement, computation and threshold detection, corresponding to the portion 51 referred to in 15. Fig. 6. It also performs the logic functions of the decoder portic,n 52 of Fig. 6 as well as the frequency synthesis function of the synthesiser 50. By this means the hardware implementation of the receiver becomes very simple. It consists of one custorn integrated 20. circuit to perEorm the synchronous filter and analogue to digital conversion processes and one microcomputer integra-ted circuit to perform the signal computation, code discrimination locJic and frequency synthesiser functions. The output bi-s-table contactor 58 requires electronic drive circuitry so that it 25. can be actuated directly from the microcomputer. Both integrated circuits are fabricated in CMOS technology to keep power supply requirements to a minim-lm. 'Ihis enables a very ~imple power supply to bc used consistincJ o a capacitive divic~er, rectiEier and recJulator direct trom the -11'~7710 power supply. Hence the receiver is fabricated in a very simple low cost manner (when mass produced) witlnout requiring any critical, low tolerance or thermally stable components.
5. In Fig. 8, the power supply is designated 59, the correlator custom integrated circuit 60 and the power supply frequency reference is supplied to the microcomputer through resistor 61. Designations 71 are the inpu-t resistors to the synchronous filter in the correlator. The 10. correlated signal in the form of a pulse of variable width is transmitted to the microcomputer through line 62. 72 designates a code which is programmed into the micro-computer with manual switches. Fig. 9 is a diagrammatic representation of the functions performed by the correlator custom integrated 15. circuit 60 shown in Fig. 8. Voltage to pulse width conversion circuit 63 is fed from the filtered outputs of the two differential amplifiers 64 and 65 in turn fed from the CMOS
switches 67 and 68 respectively, the CMOS switches being controlled through the phase splitter 69 by the frequency 20, synthesised in the microcomputer through line 73. A voltage signal is supplied through the 240V. A.C. power leads 70.
From the above description it will be seen that the invention differs from conventional ripple control in two fundamental respects:
25. (1) The signal is produced as a wave-form distortion by applying a load to the system, and (2) by using a signal which is locked to the power system frequenc~ a non-phase sensitive correlator is used in the - - ~
receivers for detection. The fact lh;lt the transmitted signal and the receiver correlator are both locked to the supply frequency produces a very selective method o detection and results in a signal magnitude required for 5. reliable operation being considerably lower than for conventional ripple control.
The low level of ~ nal required combined wlth the simple method of signal generation and the fact that the receivers can be built using only two intégrated circuits 10. results in the overall cost of the equipment being lo~er by a significant amount.
As said above, the device can be used for remote control of any loads, for example shedding pea]c loads, switching storage wa*er heaters controlling the switching of ligh~s~
15. pumps or the like, th~ control of traffic lights~arld other functions. The number of functions can be increased to any desired number by increasing the nurnber of bits ~iansmitted in the code or ~he number of frequencies.
The embodiments described relate to transmitting 20. a signal from a sub-station to a COnSuJner. A similar means and method can be used for signalling from a consumer baclc to the sub-station. However~ the correlators are then arranged to monitor a current signal instead of a voltage signal.
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|International Classification||H04B3/54, H02J13/00|
|Cooperative Classification||Y10T307/477, H02J13/0027|