AU620058B2

AU620058B2 - Power line carrier coupling method and circuit

Info

Publication number: AU620058B2
Application number: AU27018/88A
Authority: AU
Inventors: Ronald James Coomer; Ah Loy Hoi
Original assignee: SOUTH EAST QUEENSLAND ELECT; South East Queensland Electricity Board
Current assignee: South East Queensland Electricity Corp
Priority date: 1987-12-17
Filing date: 1988-12-16
Publication date: 1992-02-13
Anticipated expiration: 2008-12-16
Also published as: AU2701888A

Description

HELMUT EICHBERGER.

TO: The Commissioner of Patents, Commonwealth of Australia.

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S20058 COMMONWEALTH OF AUSTRALIA The Patents Act 1952-1969 Name of Applicant: Address of Applicant: Actual Inventors: THE SOUTH EAST QUEENSLAND ELECTRICITY BOARD 150 CHARLOTTE STREET BRISBANE, Q. 4000 RONALD JAMES COOMER and AH LOY HOI

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Address for Service: G.R. CULLEN COMPANY, Patent Trade Mark Attorneys, Dalgety House, 79 Eagle Street, Brisbane, Qld. 4000, Australia.

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COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: C t POWER LINE CARRIER COUPLING METHOD CIRCUIT The following statement is a full description of the invention including the best method of performing it known to us.

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*5 a I *q t ct CS St t tL C 1 C C *I V 2 This invention relates to a power line carrier coupling method and a circuit for applying signals to a high voltage power distribution feeder.

There are many instances or situations in an electrical distribution system where it is desirable to provide for two-way communication between remote locations employing power line carriers. For example, it is sometimes necessary or desirable to communicate between an electrical substation and a low voltage transformer with a view to communicating directly with one or more electrical consumers associated with the low voltage transformer.

A number of techniques have been employed for applying communication signals onto the distribution or 15 feeder lines. In one earlier proposal, signals were directly applied by arranging a transmitter to extend between a selected distribution line of a three phase distribution network and the neutral line or conductor of that network.

In this way, a signal could be applied to a winding of the 20 low voltage secondary of a low voltage transformer. The signal was made available on the high voltage primary of the transformer because of the transformer action. Such an arrangement would only function correctly where the signal applied to the distribution line was at a sufficiently high power level and could only be at frequencies up to about kHz because of the low frequency response of low voltage transformers. These schemes cause noticeable power waveform distortions and can interfere with sensitive equipment of the C C cc i'i 3 power system. Generating equipment is usually expensive and it is difficult to limit propagation of the signals on the power system.

In another earlier proposal communication signals were applied to the high voltage feeder lines employing either coupling capacitors or a coupling transformer. Where a transformer was employed, that transformer would typically be another low voltage transformer identical to the one with which the plurality of consumers were associated. Thus, where the feeder lines are typically at 11 kV a low voltage transformer capable of providing a voltage step down to 415 volts between distribution line phases was necessary. This transformer was necessary. In addition to this, the s [t r, transformer required isolation of at least 11 kV with a tr r breakdown insulation level of about 95 kV to cater for over voltage surges and lightning strikes. This added to the Sc expense. In addition, the high voltage feeder needed to be i. r taken off line to enable the coupling transformer to be installed and thus the supply of power was interrupted.

Where the prior method involved the use of a oupiling capacitor, it was once again necessary to provide.

isolation of 11 kV with a breakdown insulation level of about 95 kV. In addition, to install such a coupling capacitor it was once again necessary to take the feeder off line and this was not desirable. The need for high level isolation and the Slarge level of breakdown insulation made the use of coupling capacitors to inject or apply signals to the feeder lines is expensive to achieve.

Typically, capacitive coupling was employed where high signalling frequencies of above 10 kHz were used and low voltage transformers were employed for signals of about kHz or lower. At these frequencies, the distribution or feeder lines of the network functioned as radiators and particularly high power levels were necessary because of this. In addition, the high power levels caused interference in radio and other equipment.

It is an object of the present invention to provide a method of and a circuit for applying signals to a distribution or feeder line which at least minimise the disadvantages referred to above.

S oAccording to one aspect of the invention, there is t tt SE provided a method of applying signals onto a distribution or feeder line in a distribution network having a low voltage c transformer with a multiple earth neutral and a high voltage t: 20 earth typically provided by separate conductors, the transformer providing stray capacitance between windings thereof and the high voltage earth, said method including coupling a voltage signal either directly or indirectly to the high voltage earth such that the stray capacitance of the transformer couples the signal onto the feeder lines of the distribution network extending between the transformer and a substation.

According to another aspect of the invention, there is provided a circuit for applying signals to a distribution or feeder line in a distribution network and for receiving signals from the line, the network having a low voltage transformer with a multiple earth neutral and a high voltage 3earth typically provided by separate conductors, the circuit including a coupling element for applying a voltage signal either directly or indirectly to the high voltage earth of the low voltage transformer whereby stray capacitance between the transformer and the high voltage earth is employed to couple the signal onto the distribution or feeder lines, the circuit including a modulator connected to the coupling t C element for applying the voltage signal to the element and a demodulator associated with the coupling element for e receiving via the high voltage earth and the coupling element t t t4' voltage signals applied to the feeder lines at a remote location.

t 4In a preferred aspect of the method of the C 20 invention, the voltage signal is either inductively or capacitively coupled directly to the high voltage earth.

Where the voltage signal is inductively coupled it is preferred that this be done by arranging a coil to extend around the conductor used to achieve the high voltage earth between the transformer case or housing and ground.

Preferably, a toroidal core is employed and the earth conductor is arranged to extend through that toroidal core.

dI i i i i 6 The toroidal core may be provided with two core pieces which may be brought together around the earth conductor and in this way, the coil may be arranged relative to the earth conductor without interrupting the supply of power to the consumers.

In another aspect of the invention, the voltage signal may be induced onto the high voltage earth by coupling the signal between the high voltage earth and the multiple earth neutral conductor.

Regardless of which of the abovementioned methods are employed, the stray capacitance between the primary windings of the low voltage transformer and the transformer case or housing is used to advantage in ensuring that the voltage signal applied or induced onto the high voltage earth 04*4 is coupled to the feeder lines. Typically, where the voltage induced or applied to the high voltage earth is between about 1 30 kHz to 100 kHz the stray capacitance is of the order of nF.

0* 4 20 The coupling element of the invention may consist of a transformer. The transformer may be arranged to have I C two primary windings and a respective one of these two windings is associated with each of the modulator and demodulator referred to above. Where the voltage signal is inductively coupled to the high voltage earth, the secondary winding of the coupling transformer may consist purely of the high voltage earth conductor and that conductor may then have a common magnetic circuit with the windings associated with the modulator and demodulator.

i i I- In an alternative arrangement, where the voltage signal is induced or applied to the high voltage earth by connecting the signal between the high voltage earth and the multiple earth neutral, the coupling element may consist of a transformer having two primary windings (one for each of the modulator and demodulator) and a secondary winding for connection between the high voltage earth and multiple earth neutral.

It is preferred that protection elements be associated with each of the modulator and demodulator to protect them against induced voltage surges or lightning strikes or the like. Where the voltage signal is applied e Oacross the high voltage earth and multiple earth neutral by a 4 000 *@Goo: secondary winding associated with a coupling transformer, it ~oe.

is preferred that a protection element be coupled across the 9 secondary winding. A suitable protection element may comprise a surge suppressor.

20Any suitable type of modulation and demodulation o may be employed. For example, the modulation and demodulation may consist of frequency shift keying, frequency Smodulation, spread spectrum modulation, phase shift keying or rrsnc 0 any other suitable modulation/demodulation. Since the voltage signal is coupled onto the feeder lines via the stray S 25 capacitance of the transformer, the voltage signal does not have the same frequency constraints which would apply if the signal was induced onto the feeder lines via the transformer windings themselves. Thus, the frequency of the voltage signal may be selected such that the impedance represented by the feeder line and the stray capacitance is at a predetermined or desired minimum level. For example, the voltage signal employed in the invention may have a frequency within the range of 30 kHz to 100 kHz. Typically, a frequency of 50 kHz is employed.

It is preferred that both the modulator and demodulator be controlled by a microprocessor to enable the reception and transmission of the voltage signal to be selectively controlled. It is preferred that a filter be interposed between the demodulator and the signal obtained from the high voltage earth. A band pass filter may be employed.

SrrE 4 IceParticular preferred embodiments of the invention 15 will now be described by way of example with reference to the r accompanying drawings in which: Figure 1 is a schematic circuit diagram of a circuit according to an embodiment of the invention; Figures 2a and 2b are block diagrams of an embodiment of a circuit according to the invention; and, Figure 3 is a graph illustrating the typical impedance of a low voltage transformer.

With reference to figure 1, the high voltage feeder lines 10, 11 and 12 are shown coupled to the delta configured S 25 primary windings of a low voltage transformer 13. The transformer 13 has a housing or casing identified by d 1 tted outline 14. Surge suppressors 15, 16 and 17 are arranged to extend between the feeder lines 10, 11 and 12 and the housing 14.

9 The stray capacitance existing between each of the windings of the primary of the transformer 13 are identified by the code Cs The stray capacitance Cs exists between each primary winding of the transformer and the housing 14. The transformer has a star configured set of secondary windinags from which are obtained the low voltage distribution lines.

The star point of the secondary of the transformer 13 provides a neutral connection which forms a multiple earth neutral (MEN). The housing 14 is earthed as shown by the numeral 18 and provides a high voltage earth for the housing.

In some installations the MEN and the high voltage earth are provided by the same conductor.

r Block 19 is representative of the modulator and 2:4 15 demodulator and associated circuitry. Block 19 is shown as I, deriving its source of power via connection 20 which extends ,4 between it and one of the low voltage distribution lines.

The signal impressed onto the high voltage earth is, shown as 1t being inductively applied to the conductor 21 which t 20 establishes the high voltage earth. As the stray capacitance Cs extends between the housing 14 and the primary windings of the transformer (which in turn are coupled to feeder lines 11 and 12) the alternating voltage signal impressed on conductor 21 is capacitively coupled onto the feeder lines 10, 11 and 12.

With reference to figures 2a and 2b, common numerals are used for like components illustrated in figure 1. In figures 2a and 2b, an alternative way in which the 4l voltage signal may be induced or applied to the high voltage earth is shown. In figure 1, that voltage was applied or induced on conductor 21. In figures 2a and 2b, a voltage signal Vs is applied across the high voltage earth and the multiple earth neutral and in this way, a signal is impressed on the high voltage earth and coupled to the feeder lines 11 and 12 via stray capacitance Cs.

With particular reference to figure 2b, there is shown a controller or computer which functions to control the modulator 31 and demodulator 32. The control may consitt of .o a frequency select feature provided on lines 33 and 34 respectively as well as providing for alternate operation of the modulator and demodulator as may be required. The 15 circuit shown includes a coupling £rement 35 which in this case comprises a transformer. The transformer has two primary windings 36 and 37. Winding 37 is coupled to modulator 31 via a surge suppressor 38 and a load element 39 whilst winding 37 is coupled to a filter 40 via surge suppressor 41 and load element 42.

S.C. The secondary 43 of transformer 35 is tuned via 1 capacitor 44 so as to be broadly resonant within a desired I frequency range of about 30 kHz to 100 kHz. A surge suppressor 45 extends across the tuned secondary winding and the voltage Vs is available across this tuned circuit.

Figure 3 is a logarithmic plot of the individual impedance and combined impedance of the feeder lies 10, 11 and 12 and the frequency dependence of the combined

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11 impedance of these lines is indicated. When the voltage signal Vs has a frequency between 30 kHz to 100 kHz, the impedance represented by the feeder lines is relatively low.

The method and circuit of the invention may be used to first of all collect communication signals applied to the low voltage distribution lines by the consumers associated with the low voltage transformer 13. These communication signals, if at relatively high frequencies, will be blocked by the transformer and will not appear on the feeder lines.

These communication signals may be collected by the circuit of the invention and selectively applied or coupled to the i high voltage earth whereby the stray transformer capacitance Cs then in turn couples, the signals onto the feeder lines I 15 11 and 12. Conversely, any signals coupled onto the feeder lines at a substation remote from the transformer 13 will be available for detection by the demodulator since those 4: C signals will be coupled to the high voltage earth via the 8 4, transformer stray capacitance.

t 20 Typically, the voltage signal impressed or applied C C to the high voltage earth may be about 0.5 Vrms and of a low power level typically of the order of 1 milliwatt.

Experiments have shown that using voltage signals of this level, frequency and power a signal to noise ratio in excess of 10 dB is obtainable for feeder lines in industrial areas.

To receive signals at a substation it is a simple matter to arrange for inductive coupling between the high voltage earth at the substation and a circuit like that 12 illustrated in figure 2b. To achieve this, the high voltage earth needs to be arranged on a common magnetic circuit with coils 36 and 37. Where a plurality of low voltage transformers are associated with respective feeder lines terminating at a common substation, the circuit of the invention may be advantageously employed by having such a circuit associated with each low voltage transformer but arranging for each of those circuits to function at unique I 10 groups of frequencies.

The method and circuit of the invention can readily be installed without the need for interruption of supply to consumers since the voltage signal need merely be coupled or applied to the high voltage earth both at the low voltage 15 transformer and at the substation. The circuit of the S *invention will enable two way communication at relatively low power levels and because it is not associated directly with the feeder lines or distribution lines, there is no need for isolation or high levels of breakdown insulation.

I 20 The method and circuit of the invention is equally E..c applicable to ground or pole mounted low voltage transformers as they use the same earthing principle.

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Claims

1. A circuit for applying signals to a distribution or feeder line in a distribution network and for receiving signals from the line, the network having a low voltage transformer with a multiple earth neutral and a high voltage earth typically provided by separate conductors, the circuit including a coupling element for applying a voltage signal either directly or indirectly to the high voltage earth of the low voltage transformer whereby stray capacitance between the transformer and the high voltage earth is employed to couple the signal onto the distribution or feeder lines, the circuit including a modulator connected to the coupling o 4 element for applying the voltage signal to the element and a ree r' demodulator associated with the coupling element for S receiving via the high voltage earth and the coupling element voltage signals applied to the feeder lines at a remote location. S.

2. The circuit of claim 1 wherein the voltage signal applied or induced onto the high voltage earth has a frequency between 30 kHz to 100 kHz.

3. The circuit of claim 2 wherein the frequency is about 50 kHz. 4 9

4. The circuit of claim 1, 2 or 3 wherein the coupling element is a coil and the signal is inductively coupled onto the high voltage earth. The circuit of claim 1i, 2 or 3 wherein the coupling element is a coupling transformer.

IZ 0 B i 00 00C CO 0C *0 0 CO CC C) q CO a Co 00 C 0a 0 0 C

6. The circuit of claim 5 wherein the coupling transformer has a secondary winding connected between the multiple earth neutral and the high voltage earth.

7. The circuit of claim 6 wherein the coupling transformer has two primary windings and a respective one of which is associated with the demodulator and the modulator.

8. The circuit of claim 7 including a surge protection element coupled across each said primary and said secondary winding.

9. A circuit substantially as herein described with reference to figure 1 or figures 2a and 2b of the drawings.

10. A method of applying signals onto a distribution or feeder line in a distribution network having a low voltage transformer with a multiple earth neutral and a high voltage earth typically provided by separate conductors, the transformer providing stray capacitance between windings thereof and the high voltage earth, said method including coupling a voltage signal either directly or indirectly to the high voltage earth such that the stray capacitance of the transformer couples the signal onto the feeder lines of the distribution network extending between the transformer and a substation.

11. The method of claim 10 wherein the voltage signal is inductively or capacitively coupled to the high voltage earth. -r

12. The method of claim 10 wherein the voltage is inductively coupled to a conductor which provides the high voltage earth by a coil through which the high voltage earth conductor passes.

13. The method of claim 10 or 11 wherein the voltage signal is induced onto the high voltage earth by coupling the signal between the high voltage earth and the multiple earth neutral.

14. A method as claimed in claim 10 substantially as herein described. DATED this SIXTEENTH day of DECEMBER, 1988. THE SOUTH EAST QUEENSLAND ELECTRICITY BOARD By their Patent Attorneys, CULLEN CO. a t It *1 c 0* t. t t t e t t