GB2198605A

GB2198605A - Filtering electrical signals

Info

Publication number: GB2198605A
Application number: GB08723750A
Authority: GB
Inventors: Edward Christopher Th Crampton; Barry Stanley Ford
Original assignee: EMLUX Ltd
Current assignee: EMLUX Ltd
Priority date: 1986-11-22
Filing date: 1987-10-09
Publication date: 1988-06-15
Anticipated expiration: 2007-10-09
Also published as: GB2198605B; GB8723750D0; GB8628004D0

Abstract

An electrical filter for filtering out all or some signal(s) from AC currents in a plurality of cables comprises: at least one line cable (11), at least one neutral cable (12), and optionally at least one earth cable (13), the filter comprising a first loop (21) for surrounding and being magnetically coupled to at least one line cable, or to at least one neutral cable, or to at least one earth cable, the remaining said cable(s) not being so surrounded; and a second loop (22) for being surrounded by and magnetically coupled to the first loop, the second loop comprising a capacitor (23), the inductance of the first loop being adapted for coupling the capacitor to said at least one surrounded cable so as to form a parallel LC circuit in said at least one surrounded cable. The plurality of cables and said filter may be adapted for e.g. line/neutral signalling, or line/earth signalling, or neutral/earth signalling. A bandwidth of signals may be in the range e.g. 20 to 500 kilohertz. The first loop may comprise ferrite materials(s) or a laminate construction. <IMAGE>

Description

FILTERING EISCTXICAL SIGNALS Electrical cables (e.g. power cables for supplying AC mains currents to buildings) may carry AC currents of different frequencies. It is often desired to filter out currents of one band of frequencies while leaving substantially unaffected currents in another band of frequencies. Power cables within a building carry electrical currents at mains frequencies, e.g. normal AC currents at frequencies of 50 to 60 hertz and voltages of 100 to 500 volts. Higher or lower frequency currents (which may be termed signals,) may be superimposed on normal or other AC currents. For example, the same power cables within a given building may be used for carrying communication signals.Those signals are only wanted within the building, and should not travel to other buildings or to the central power station that supplies normal AC currents to the power cables, where the unwanted signals may cause interference. Power cables carrying a mains supply to a building carry high currents. It is dangerous to make connections and/or disconnections with the power cables while those high currents are flowing. But, it is not convenient to cut off the mains supply while such connections and/or disconnections are made, It has now been found in accordance with the present invention that a novel filter can be used to filter out all or some signal(s) from AC currents (e,g.

normal AC currents in power cables) in a plurality of cables comprising at least one line cable (alternatively termed live cable or phase cable), at least one neutral cable (alternatively termed return cable), and at least one earth cable (alternatively termed ground cable), The filter does not require ceasing of normal current flow.

h first aspect of the invention provides an electrical filter for filtering out all or some signal(s) from AC currents (e,g. normal AC currents in power cables) in a plurality of cables comprising: at least one line cable, at least one neutral cable and optionally at least one earth cable, the filter comprising: a first loop for surrounding and being magnetically coupled to at least one line cable, or to at least one neutral cable, or to at least one earth cable, the remaining said cable(s) not being so surrounded; and a second loop for being surrounded by and magnetically coupled to the first loop, the second loop comprising a capacitor, the inductance of the first loop being adapted for coupling the capacitor to said at least one surrounded cable so as to form a parallel LC circuit in said at least one surrounded cable, said inductance and the capacitance of the capacitor being chosen so that the parallel circuit has maximum impedance at the centre of a bandwidth of signals to be filtered out of said plurality of cables.

A second aspect of the invention provides an electrical filter system for filtering out all or some signal(s) from AC currents (e.g. normal AC currents in power cables) in a plurality of cables comprising: at least one line cable, at least one neutral cable, and optionally at least one earth cable, the filter system comprising at least one filter according to the first aspect of the invention.

A third aspect of the invention provides an electrical distribution system (e.g. power cable distribution system for supplying AC currents to buildings), comprising at least one filter according to the first aspect of the invention, and/or at least one filter system according to the second aspect of the invention.

A fourth aspect of the invention provides a method of filtering signal(s) from AC currents, comprising utilising any one or more of the first, second, and third aspects of the invention.

A fifth aspect of the invention provides a power line communication system for a plurality of cables comprising: at least one line cable, at least one neutral cable, and optionally at least one earth cable, the system comprising at least one filter according to the first aspect of the invention, and/or at least one filter system according to the second aspect of the invention, The present invention may be applied in any suitable environment, and embodied in any suitable manner.

The magnetic coupling of the first loop to said at least one surrounded cable can be provided in any suitable manner, e.g. by the first loop being assembled around said at least one surrounded cable, for instance the first loop may be an assembly for being around said at least one surrounded cable, e.g. without having to make any physical connection to said at least one surrounded cable, and/or e.g. without ceasing mains current flow in respect of power cables providing said plurality of cables. Preferably, the first loop is constituted by ferrite material(s), e.g. said assembly can be of pieces of ferrite material(s), for instance a split ferrite core securely clamped in place, Ferrite loops are known for use in electrical filters, but not for the present invention.In the present invention, ferrite material(s) may correspond to any suitable signal current frequencies, e.g, at least 20 kilohertz. As an alternative to ferrite material(s), the first loop may comprise a laminate construction (e.g. comprising iron) corresponding to signal current frequencies of at most 25 kilohertz, The magnetic coupling together of the first and second loops can be provided in any suitable manner.

The first and second loops may have any suitable dispositions, which may be adjustable or fixed, e.g.

at least one of: the first loop may interlink the second loop, and vice versa; the first and second loops may have respective first and second general planes substantially parallel to each other, or angularly disposed relative to each other; and the first loop may touch or not touch the second loop, It will be appreciated that the first and second loops may face each other.

A first embodiment of a filter according to the first aspect of the invention comprises: a said first loop for surrounding and being magnetically coupled to at least one line cable, the remaining said cable(s) not being so surrounded; and a said second loop. This embodiment may be suitable for line/ neutral signalling, or line/ earth signalling.

A second embodiment of a filter according to the first aspect of the invention comprises: a said first loop for surrounding and being magnetically coupled to at least one neutral cable, the remaining said cable(s) not being so surrounded; and a said second loop. This embodiment may be suitable for neutral/ line signalling, or neutral/earth signalling.

A third embodiment of a filter according to the first aspect of the invention comprises: a said first loop for surrounding and being magnetically coupled to at least one earth cable, the remaining said cable(s) not being so surrounded; and a said second loop, This embodiment may be suitable for earth/line signalling, or earth/neutral signalling.

In some embodiments of the invention, a filter of the first aspect of the invention may be coupled to a series resonance circuit tuned to the frequency of signal(s) to be excluded. The series resonance circuit may be coupled at its opposite ends respectively to the line cable and earth cable, or to the neutral cable and earth cable. The filter system of the second aspect of the invention may comprise first and second filters of the first aspect of the invention, those filters being disposed to form the cross arms of a T-filter, whose stem is formed by a series resonance circuit tuned to the frequency of signal(s) to be excluded. The series resonance circuit may be coupled at its opposite ends respectively to the linejneutral cables, the line/earth cables, or to the neutral/earth cables. Any suitable coupling manner may be used.Any said series resonance circuit may provide a sufficiently low impedance at the centre of said bandwidth so as to short circuit any signals which may have passed through parallel circuit(s) e,. from inside or outside a building.

The electrical distribution system of the third aspect of the invention may be disposed in any suitable manner. For example, the distribution system may be a 3-phase electrical distribution system. An advantage of connecting a communication system to the neutral and earth cables of a mains supply is that in a 3-phase electrical distribution system, the communication system may extend to all parts of a building to which any phase extends. In a communication system connected between the line and neutral cables, the coverage will only extend to the line cables of the one phase to which the system has been connected, The present invention may filter out communication signal(s) from hC mains currents, e.g. normal AC currents in power cables. The present invention may be applied to any frequencies outside those used by an AC mains supply.For example, a filter of the first aspect of the invention may be applied to a bandwidth of signals in the range 20 to 500 kilohertz for instance such signal(s) used in a communication system.

Besides wishing to keep communication signals in the power cables within a building from passing out of the building, it may be desirable to keep communication signals in the power cables outside the building from entering the building. Thus, a second filter of the first aspect of the invention may be provided outside the building. The resultant two filters (one inside the building, and one outside the building) may form the cross arms of a T-filter, whose stem is formed by a series resonance circuit tuned to the same communication frequency so as to provide a very low impedance at the centre Or the ban of communication frequencies which may have passed through parallel circuits either from inside or outside the building - cf. the description earlier above of the electrical filter system of the third aspect of the invention.

Some examples of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of a communication system within a building connected to an AC mains supply, and using one example of a filter of said first embodiment of the invention.

Fig. 2 is an equivalent circuit of the filter of Fig, 1.

Fig. 3 is a further equivalent circuit of the filter of Fig. 1.

Fig, 4 shows a T-filter arrangement using the filter of Fig. 1.

Fig. 5 is an equivalent circuit of the filter of Fig. 4.

Fig. 6 is a schematic diagram of a communication system within a building connected to an AC mains supply, and using one example of a filter of said second embodiment of the invention.

Fig. 7 is an equivalent circuit of the filter of Fig. 6, Fig. 8 is a further equivalent circuit of the filter of Fig. 6.

Fig. 9 shows a T-filter arrangement using the filter of Fig. 6.

Fig. 1 is an equivalent circuit of the filter of Fig. 9.

Fig, 11 is a schematic diagram of a communication system within a building connected to an AC mains supply, and using one example of a filter of said third embodiment of the invention.

Fig, 12 is an equivalent circuit of the filter of Fig. 11.

Fig. 13 is a further equivalent circuit of the filter of Fig. 11.

Fig. 14 shows a T-filter arrangement using the filter of Fig. 11.

Fig, 15 is an equivalent circuit of the filter of Fig. 14.

In Fig. 1, the AC mains supply of a building comprises a line cable 11, a neutral cable 12, and an earth cable 13. In the building, a communication system comprises a transmitter 14 connected between the line cable 11 and the neutral cable 12 A plurality of receivers (only one is shown as 15) is connected between the line cable 1 and the neutral cable 1;. The communication system operates at a very much higher frequency than the mains frequency, of the order of 100 kilohertz. A filter 20 is provided where the mains supply enters the building so as to stop communication frequency signals passing outside the building on the mains cables, and also to prevent spurious signals on the mains cables outside the building from interfering with the communication system in the building.Filter 2G corises a ferrite loop 21 encircling the line cable 11, Ferrite loop 21 also encircles loop 22 containing capacitor 23. Ferrite loop 21 may be made in two or more parts which are fitted around the line cable 11, without the need te break line cable 11 on assembly of filter 2. The neutral cable 12 and the earth cable 13 do not pass through ferrite loop 21, In Fig. 2, the ecuivalent circuit shows the interaction of the line cable 11 and ferrite loop 32.

The interaction is shown as a single turn 31 in the line cable 11 around a core 32 which transfers magnetic flux to a single turn 33 in the loop 22 containing the capacitor 23. The transformer ratio may be altered by providing more than one turn of loop 22 around ferrite loop 21, In Fig, 3, the equivalent circuit shows transformer coupling via ferrite loop 21 which brings capacitor 23 into a parallel resonance circuit L,C in line cable 11, the indutancu L being of the ferrite loop 21, and the capacitance C being of capacitor 23, The values of the inductance and capacitance are chosen so that the centre of the band of communication frequencies is equal to 1/2T , wherein L is measured in Henrys, C is measured in farads, and frequency is measured in hertz (i.e. cycles per second).Filter 20 provides a high impedance at the communication frequency where the cables leave the building, whereas the impedance between the line and neutral cables 11,12 without filter 20 is relatively low. Thus, communication signals are restricted within the building, In order to prevent communication signals outside the building from entering the building and interfering with the communication system in the building, a second filter may be provided where the cables enter the building so as to prevent communication frequency signals entering the building. Fig. 4 shows two filters 20a according to Fig. 1, and in a T-filter arrangement whose arms respectively contain those filters and whose stem is a series resonance circuit 20b having its opposite ends respectively connected to the line and neutral cables 11,12.The series resonance circuit is tuned to the centre of the communication frequency band, so as to absorb signals that may have passed through the filters 20a in either direction. The series resonance circuit may easily be connected without interfering with the mains supply, e.g. by piercing insulation of the line and neutral cables 11,12, and/or by external connection to them. Fig, 5 shows the equivalent circuit of the T-filter arrangement, In Fig. 5, the series resonance circuit 20b has a ferrite core in its coil.

In Fig. 6, the AC mains supply of a building comprises a line cable 11, a neutral cable 12, and an earth cable 13, In the building, a communication system comprises a transmitter 14 connected between the neutral cable 12 and the earth cable 13. A plurality of receivers (only one is shown as 15) is connected between the neutral cable 12 and the earth cable 13. The communication system operates at a very much higher frequency than the mains frequency, of the order of 100 kilohertz. A filter 20 is provided where the mains supply enters the building so as to stop communication frequency signals passing outside the building on the mains cables, and also to prevent spurious signals on the mains cables outside the building from interfering with the communication system in the building, Filter 20 comprises a ferrite loop 21 encircling the neutral cable 12.Ferrite loop 21 also encircles loop 22 containing capacitor 23. Ferrite loop 21 may be made in two or more parts which are fitted around the neutral cable 12, without the need to break neutral cable 12 on assembly of filter 20. The line cable 11 and earth cable 13 do not pass through ferrite loop 21.

In Fig, 7, the equivalent circuit shows the interaction of neutral cable 12 and ferrite loop 32.

The Interaction is shown as a single turn 31 in the neutral cable 12 around'a core 32 which transfers magnetic flux to a single turn 33 in the loop 22 containing the capacitor 23. The transformer ratio may be altered by providing more than one turn of loop 22 around ferrite loop 21.

In Fig. 8, the equivalent circuit shows transformer coupling via ferrite loop 21 which brings capacitor 23 into a parallel resonance circuit L,C in neutral cable 12, the inducatance L being of the ferrite loop 21, and the capacitance C being of capacitor 23, The values of the inductance and capacitance are chosen so that the centre of the band of communication frequencies is equal to 1/2t iLC, LC, wherein L is measured in Henrys, C is measured in farads, and frequency is measured in hertz.

Filter 20 provides a high impedance at the communication frequency where the cables leave the building, whereas the impedance between the neutral and earth cables 12,13 without filter 20 is relatively low, Thus, communicat communication signals are restricted within the building.

In order to prevent communication signals outside the building from entering the building and interfering with the communication system in the building, a second filter may be provided where the cables enter the building so as to prevent communication frequency signals entering the building. Fig. 9 shows two filters 20a according to Fig. 6, and in a T-filter arrangement whose arms respectively contain those filters and whose stem is a series resonance circuit 20b having its opposite ends respectively connected to the neutral and earth cables 12,13. The series resonance circuit is tuned to the centre of the communication frequency band, so as to absorb signals that may have passed through the filters 20a in either direction.The series resonance circuit may easily be connected without interfering with the mains supply, e.g. by piercing insulation of the line and neutral cables 11,12, and/or by external connection to them. Fig. 10 shows the equivalent circuit of the T-filter arrangement, In Fig. 10, the series resonance circuit 20b has a ferrite core in its coil.

In Fig. 11, the AC mains supply of a building comprises a line cable 11, a neutral cable 12, and an earth cable 13. In the building, a communication system comprises a transmitter 14 connected between the neutral cable 12 and the earth cable 13. A plurality of receivers receivers (only one is shown as 15) is connected between the neutral cable 12 and the earth cable 13. The communication system operates at a very much higher frequency than the mains frequency, of the order of 100 kilohertz. A filter 20 is provided where the mains supply enters the building so as to stop communication frequency signals passing outside the building on the mains cables, and also to prevent spurious signals on the mains cables outside the building from interfering with the conrunication system in the building, Filter 20 comprises a ferrite loop 21 encircling the earth cable 13.Ferrite loop 21 also encircles loop 22 containing capacitor 23. Ferrite loop 21 may be made in two or more parts which are fitted around the earth cable 13, without the need to break the earth cable 13 on assembly of filter 20. The line cable 11 and the neutral cable 12 do not pass through ferrite loop 21.

In Fig. 12, the equivalent circuit shows the interaction of the earth cable 13 and ferrite loop 32.

The interaction is shown as a single turn 31 in the earth cable 13 around a core 32 which transfers magnetic flux to a single turn 33 in the loop 22 containing the capacitor 23. The transformer ratio may be altered by providing more than one turn of loop 22 around ferrite loop 21.

In Fig. 13, the equivalent circuit shows transformer coupling via ferrite loop 21 which brings capacitor 23 into a parallel resonance circuit L,C in earth cable 13, the inductance L being of the ferrite loop 21, and the capacitance C being of capacitor 23. The values of the inductance and capacitance are chosen so that the centre of the band of communication frequencies is equal to 1/2E Cs wherein L is measured in Henrys, C is measured in farads, and frequency is measured in hertz.

Filter 20 provides a high impedance at the communication frequency where the cables leave the building, whereas the impedance between the neutral and earth cables 12,13 without filter 20 is relatively low, Thus, communication signals are restricted within the building.

In order to prevent communication signals outside the building from entering the building and interfering with the communication system in the building, a second filter may be provided where the cables enter the building so as to prevent communication frequency signals entering the building. Fig. 14 shows two filters 20a according to Fig. 11, and in a T-filter arrangement whose arms respectively contain those filters and whose stem is a series resonance circuit 20b having its opposite ends respectively connected to the neutral and earth cables 12,13.The series resonance circuit is tuned to the centre of the communication frequency band, so as to absorb signals that may have passed through the filters 20a in either direction, The series resonance circuit may easily be connected without interfering with the mains supply, e.g. by piercing insulation of the line line and neutral cables 11,12, and/or by external connection to them. Fig. 15 shows the equivalent circuit circuit of the T-filter arrangement, In Fig. 15, the series resonance circuit 20b has a ferrite core in its coil, ln the drawings, single line, neutral, and earth cables 11,12,13 are shown. There may be a plurality of line cables, a plurality of neutral cables, a plurality of earth cables. For example, there can be a plurality of series resonance circuits 20b connected a single surrounded line cable 11 and one or more neutral cables 12 (cf. Fig. 1), or to a single surrounded neutral cable 12 and one or more earth cables 13 (cf. Fig. 6), or to a single surrounded earth cable 13 and one or more neutral cables 12 (cf, Fig. 11).

The disclosures with reference to the accompanying drawings can be modified according to the description given above before the first reference to the drawings.

In general, the present invention includes equivalents and modifications arising from all the disclosures within the present application.

Claims

1. An electrical filter for filtering out all or some signal(s) from AC currents in a plurality of cables comprising: at least one line cable, at least one neutral cable, and optionally at least one earth cable, the filter comprising: a first loop for surrounding and being magnetically coupled to at least one line cable, or to at least one neutral cable, or to at least one earth cable, the remaining said cable(s) not being so surrounded; and a second loop for being surrounded by and magnetically coupled to the first loop, the second loop comprising a capacitor, the inductance of the first loop being adapted for coupling the capacitor to said at least one surrounded cable so as to form a parallel LC circuit in said at least one surrounded cable, said inductance- and the capacitance of the capacitor being chosen so that the parallel circuit has maximum impedance at the centre of a bandwidth of signals to be filtered out of said plurality of cables.

2. A filter as claimed in claim 1, wherein said at least one earth cable is present.

3. A filter as claimed in claim 1 or 2, wherein said first loop is an assembly.

4. A filter as claimed in any one of claims 1 to 3, wherein said first loop comprises ferrite material(s).

5. A filter as claimed in claim 4, wherein said first loop comprises pieces of ferrite material(s).

6. A filter as claimed in claim 4 or 5, wherein said ferrite material(s) correspond to signal current frequencies of at least 20 kilohertz.

7. A filter as claimed in any one of claims 1 to 3, wherein the first loop comprises a laminate construction corresponding to signal current frequencies of at most 25 kilohertz.

8. A filter as claimed in any one of claims 1 to 7, wherein a said first loop surrounds and is magnetically coupled to at least one line cable.

9. A filter as claimed in claim 8, wherein said plurality of cables and said filter are adapted for line/neutral signalling.

10. A filter as claimed in claim 8, wherein said plurality of cables and said filter are adapted for line/earth signalling.

11. A filter as claimed in any one of claims 1 to 7, wherein a said first loop surrounds and is magnetically coupled to at least one neutral cable.

12. A filter as claimed in claim 11, wherein said plurality of cables and said filter are adapted for neutral/line signalling.

13. A filter as claimed in claim 11, wherein said plurality of cables and said filter are adapted for neutral/earth signalling.

14. A filter as claimed in any one of claims 1 to 7, wherein a said first loop surrounds and is magnetically coupled to at least one earth cable.

15. A filter as claimed in claim 14, wherein said plurality of cables and said filter are adapted for earth/line signalling.

16. A filter as claimed in claim 14, wherein said plurality of cables and said filter are adapted for earth/neutral signalling.

17. A filter as claimed in any one of claims 1 to 16, coupled to a series resonance circuit tuned to the frequency of signal(s) to be excluded.

18. A filter as claimed in claim 17, wherein the series resonance circuit t is coupled at its opposite ends respectively to a line cable and an earth cable.

19. A filter as claimed in claim 17, wherein the series resonance circuit is coupled at its opposite ends respectively to a neutral cable and an earth cable.

20. A filter as claimed in claim 1, substantially as hereinbefore described with reference to and as shown in Figs. 1 to 5 of the accompanying drawings.

21. A filter as claimed in claim 1, substantially as hereinbefore described with reference to and as shown in Figs. 6 to 10 of the accompanying drawings.

22. A filter as claimed in claim 1, substantially as hereinbefore described with reference to and as shown in Figs. 11 to 15 of the accompanying drawings.

23. A filter as claimed in any one of claims 1 to 22, adapted for a bandwidth of signals in the range substantially 20 to substantially 500 kilohertz.

24. An electrical filter system for filtering out all or some signal(s) from AC currents in a plurality of cables comprising: at least one line cable, at least one neutral cable, and optionally at least one earth cable, the filter system comprising at least one electrical filter as claimed in any one of claims 1 to 22.

25. A filter system as claimed in claim 24, comprising first and second electrical filters as claimed in any one or more of claims 1 to 22, those filters being disposed to form the cross arms of a T-filter, whose stem is formed by a series resonance circuit tuned to the frequency ofsignal(s) to be excluded.

26. A filter system as claimed in claim 25, wherein the series resonance circuit of said stem is coupled at its opposite ends respectively to line/neutral cables.

27. A filter system as claimed in claim 26, wherein the series resonance circuit of said stem is coupled at its opposite ends respectively to line/earth cables.

28. A filter system as claimed in claim 25, wherein the series resonance circuit of said stem is coupled at its opposite ends respectively to neutral/earth cables.

29. An electrical distribution system, comprising at least one filter as claimed in any one of claims 1 to 22, and/or at least one electrical filter system as claimed in any one of claims 25 to 28.

30. A method of filtering signal(s) from AC currents, comprising utilising at least one filter as claimed in any one of claims 1 to 23, and/cr at least one electrical filter system as claimed in any one of claims 24 to 28, and/or at least one electrical distribution system as claimed in claim 29.

31. A power line communication system for a plurality of cables comprising: at least one line cable, at least one neutral cable, and optionally at least one earth cable, the system comprising at least one electrical filter as claimed in any one of claims 1 to 23, and/or at least one electrical filter system as claimed in any one of claims 24 to 28, and/or at least one electrical distribution system as claimed in claim 29.

32. A communication system as claimed in claim 31, connected between line and neutral cables.