The present invention relates to signalling over power lines, and is mainly concerned with signalling over overhead power lines of low or intermediate vol¬ tage.

Mains distribution - general

In most major countries, electricity is supplied on a wide scale by electricity generating and distribution companies (electricity utilities). The distribution network normally consists of a large number of low voltage networks (often termed the mains) to which domestic and small business consumers are connected, with the low voltage networks being supplied through a higher voltage distribu¬ tion network or system (often termed the grid). The low voltage (consumer) networks may for example operate at 230 V (or 440 V 3-phase).

The distribution network will normally operate at more than one voltage, with long-distance distribution at voltages of say 132 kV or 275 kV which are stepped down (possibly through 2 or more stages) to a voltage of say 1 1 k V or 33 kV. We will term the former voltages (ie the voltages used for long-distance distribution) high voltages and the latter voltages (ie the voltages relatively close to the final mains voltages) intermediate voltages.

Mains signalling - general

The use of the mains for signalling has often been proposed. Systems are available for intercommunication between rooms in domestic premises (typically for "baby alarms"), for coupling to the telephone system, and for transmission of data between computer units. Many proposals have also been made for the use of mains signalling for remote meter reading (primarily for electricity meters, though gas and other meters can be coupled to the mains for this purpose, pref¬ erably through electricity meters).

There is in fact an international standard now for such signalling, using frequencies in the general region of 3 to 150 kHz. The standard is CENELEC EN50065.1, which specifies that frequencies in the band 3 kHz - 148.5 kHz are

available for signalling on low voltage electrical installations. This bandwidth is divided into several smaller bands with various uses and permissions associated with them; for example, the 9 kHz - 95 kHz band is reserved for electricity sup¬ pliers and their licencees.

The signalling which is performed by the electricity suppliers is likely to be largely concerned with metering and more generally with load and system con¬ trol. This will therefore largely operate over the low voltage portions of the mains. However, as noted above, the distribution network will normally include intermediate and high voltage levels, all coupled through power transformers. It will often be desirable for metering information collected over the low voltage portions of the network to be passed on over the intermediate and/or high vol¬ tage portions, and for control information to be passed similarly in the opposite direction. This control information may include information to be passed to the consumers connected to the low voltage level, and also signals for controlling the electricity distribution system itself.

Coupling to intermediate voltage networks

Techniques are therefore required for coupling signals to intermediate voltage networks. These signals may be generated or used at the coupling points, ie the substations where the intermediate voltage networks are coupled with either the high or the low voltage networks, or may be passed between the intermediate voltage network and a low voltage network coupled to it.

It may be noted that signalling frequency signals do not pass through power (distribution) transformers effectively. Some means of coupling PLC signals round such transformers is therefore necessary if signalling between low and intermediate voltage portions of a network is to be achieved. This will normally involve signal reception and retransmission. The signals are thus coupled separately with the two sides of a transformer and passed around the transformer between its two sides, with the signals being processed to remove noise. It may also be desirable to use different frequency bands on the two sides of the transformer. (This has the advantage that signal feedthrough at power trans¬ formers will be irrelevant.)

Mains signalling - relevance of mains voltage level

Signal transmission and reception techniques are relatively straightforward for low voltage (mains) networks. The signal transmission and reception equip¬ ment can be connected directly to the network wiring.

An intermediate voltage network, however, presents more difficulty, for both electrical and mechanical reasons. Intermediate voltage networks require physi¬ cally robust insulation which is largely incompatible with direct connections to the intermediate voltage. Also, fairly delicate and sensitive electronic equipment is largely incompatible with direct connection to intermediate voltages (we are using the term "intermediate" voltage, of course, in connection with distribution networks; 1 1 kV, for example, is exceedingly high relative to most electronic equipment).

Overhead and underground networks

Distribution networks may be overhead, underground, or both. The high voltage portions are normally overhead, since they generally cross long distances of fairly open country, and the cost of burying them underground would be pro¬ hibitive. In many countries the low voltage portions are normally underground, since they are in densely populated areas where overhead wires would be unduly intrusive and potentially dangerous. The intermediate voltage portions may be overhead or underground; as with the low voltage portions, they are generally underground in urban and suburban areas. We are here concerned primarily with overhead intermediate-voltage networks.

For mains signalling over intermediate-voltage overhead networks, it is obviously necessary to couple the signal to the network at one point and to be able to pick up the signal from the network at another point. Various ways of coupling signals onto overhead networks have been proposed, including inductive coupling. For this, a transducer comprising a magnetic core is placed around one of the conductors, forming a transformer. The core has a signal winding wound round it as a primary winding and the conductor itself effectively forms a single-turn secondary winding (for transmission; for reception, the conductor forms a single-turn primary and the signal winding forms a multi-turn secon¬ dary). We are here concerned with such inductive coupling.

This requires the coupling of the power line to the transducer. While this is relatively easy to achieve if the transducer is being installed at the same time that the power line itself is being installed, it is considerably more difficult to achieve if the transducer is to be fitted to an existing line. There are two main techniques which can be used in this situation: breaking the line, inserting it through the transducer, and reconnecting the line; and using a transducer with a split core which can be opened up, placed around the line, and closed to restore the magnetic circuit.

The main object of the present invention is to provide an improved device and technique for coupling signals to a power line.

The present invention

According to one aspect the present invention provides a combined trans¬ ducer, insulator, and support rod for a power line, comprising insulating means supported by the support rod and through which the power line passes generally transversely to the support rod, and including a magnetic core. The core also has, of course, one or more drive and sense windings with connections thereto passing along the support rod.

In some forms, the insulator is generally collinear with the support rod and the power line is of generally U-shaped form, passing through the core and with both ends protruding from the end of the insulator. In another form, the insu¬ lator is essentially transverse to the support rod and the power conductor passes essentially straight through the insulator.

The insulator may include a strain (tension) mounting to act as the termina¬ tion of a power line under (mechanical) tension, either forming one of the ends of the power conductor or located between its ends. This form of insulator is intended for original installation as the termination of a power line under ten¬ sion, with the power line being connected to one end of the power conductor and the other end of that conductor being connected to utilization means such as an intermediate voltage to low (mains) voltage transformer.

Preferably however the insulator is located between the power line and the utilization means, with the power line being connected to one end of the power conductor and the other end of that conductor being connected to the utilization

means. With this arrangement, the current in the power line flows through the power conductor on its route between the power line and the utilization means (which we will take to be a transformer).

With this latter arrangement, the insulator can be installed while the system is live and without interrupting the power supply to the transformer. Initially, the power line is already connected directly to the transformer. To provide a signal coupling to the power line, the insulator is installed at a position inter¬ mediate between the power line and the transformer, connections are made between the power line and one end of the power conductor and between the other end of that conductor and the transformer, and the existing connection between the power line and the transformer is removed.

In conventional overhead power transmission systems, the connection between the power line and the transformer may require support at an intermediate point. To achieve this, a conventional insulator (which fixes the position of the connec¬ tion at the intermediate point) on a conventional support rod may be used. The present insulator may of course be used in place of the conventional insulator in such a case. The present insulator can be supported by a support rod which is broadly similar to the conventional support rod, but carries the signal connec¬ tions to the transducer (ie the connections to the drive and sense windings) through a hollow core.

The support rod may incorporate a fuse, connected in one or more of the signal connections, comprising a transparent tube having inside it a fuse element which produces a distinctive change of appearance when "blown".

Specific embodiments of the invention

A power line termination incorporating the present invention will now be described, by way of example, with reference to the drawings, in which:

Fig. 1 is a general view of the power line termination;

Fig. 2 is a longitudinal section through a coupling insulator;

Fig. 3 is a longitudinal section through a strain insulator; and

Fig. 4 is a longitudinal section through a further coupling insulator.

With reference to Fig. 1 , a power line 10 terminates at a termination con¬ sisting of a pole 1 1 mounted in the ground 12. (Only a single line is shown, though there will obviously be more than one line.) The line 10 is connected to a transformer 18, mounted on a platform 13 on the pole; the transformer pro¬ vides a low voltage (eg 230 V) supply, not shown, for local consumers.

The line 10 is connected to a strain insulator 15 mounted at the top of the pole. The line 10 is under a considerable tension, and the insulator 15 forms a secure mounting which will withstand that tension. The line is coupled to the transformer 18 by a connection 14 A, 14B which is attached to the line 10 adja¬ cent to the insulator 15 and to the intermediate voltage terminal of the trans¬ former 18.

The coupling from the line 10 to the transformer 18 is formed by a first section 14A between the line 10 and an insulator 16 and a second section 14B between the insulator 16 and the transformer 18. The insulator 16 is supported on a support rod 17, which is typically in the region of 500 mm long, and incor¬ porates signal coupling means.

Fig. 2 shows the insulator 16 is more detail. It comprises a body of insulating material 20, having conventional corrugations 2 1 around its length, with a mounting socket 23 for mechanically mounting it on the end of the moun¬ ting rod 17.

A magnetic core 26 is located towards the opposite end of the insulator, the core being in the form of a loop whose axis (vertical in the drawing) is trans¬ verse to the longitudinal axis of the insulator (horizontal in the drawing). A power conductor 25 of generally U-shaped form passes through the body of the insulator and through the magnetic core 26, with its ends curved somewhat back from the U shape and protruding from the end face 22 of the insulator as shown. A set of drive and sense windings 27 is formed around the core 26, with connecting wires forming a cable 28 which passes through the insulator and emerges at the mounting end of the insulator.

If desired, the power conductor 25 may be provided with one or more insu¬ lating coatings over that part of its length which passes through the insulator

body 20. Similarly, the windings 27 may be provided with a coating of insula¬ ting material. Also, the material of the magnetic core 26 will normally be of ferrite, which is insulating. The cable 28 may be brought out along the axis of the insulator 16, ie at the centre of the mounting socket 23, if desired.

The insulator is mounted on the end of the support rod 17, as described. The lower end of the connector section 14A is attached to the upper end of the power conductor 25, and the upper end of the connector section 14B is attached to the lower end of conductor 25. The current path from the line 10 to the transformer 18 therefore passes along the conductor 25 and hence through the core 26, so the drive and sense windings 27 are therefore coupled to the current path through the power line 10. The cable 28 passes along the hollow centre of the mounting rod 17, and emerges at the pole end of that rod.

The support rod 17 preferably has slots (not shown) towards its ends to allow the cable 28 to enter and emerge from it. The cable may include, in one or more of its wires, a transparent fuse containing a fuse wire or other material which changes its appearance when it "blows". This fuse may conveniently be contained within the rod 17 and visible though the slot.

Fig. 3 shows a strain (tension) insulator incorporating coupling means. Corresponding parts to the parts of Fig. 2 are shown by matching references, distinguished eg by primes.

The insulator is in two parts 20A and 20B, attached to each other by epoxy resin compound at 30. A metal strain post or peg 31 is anchored in the left- hand part 20B of the insulator, and passes through the right-hand part 20A. The power conductor 25' is offset from the strain post 31, and the magnetic core is divided into two parallel portions 26A and 26B to avoid interference with the strain peg 31. In use, the line 10 would be connected to the right-hand end of the strain peg 31 , a short connection would be attached between the line 10 and the upper end of the power conductor 25", and a connection would be attached between the lower end of the conductor 25' and the transformer.

Fig. 4 shows a second form of coupling insulator. In this form, the insu¬ lator is of generally linear shape, with the conductor passing straight through without significant deviation. The insulator is supported by a mechanical sup¬ port corresponding broadly to the support 17 of Fig. 1; the insulator and

support together form a T shape, the insulator forming the cross-bar of the T and the support forming the upright of the T. The ferrite core surrounds the power conductor, and has the drive and sense windings wound on it. A semi¬ conducting screen surrounds the insulated conductor along its length. A C- shaped element is attached to the insulator by two attachments, one at each inner end of the C shape, each attachment being formed by a metal wrapping having its inner turns encircling the insulator and its outer turns encircling the insulator and the inner end of the C shape. The support rod is attached to the centre of the C shape.

More specifically, referring to Fig. 4, the conductor 14 passes straight through the insulator, and is surrounded by an insulating layer 35 and a semi¬ conducting layer 36. Towards one end of the insulator there are 2 conventional insulating discs 39. Towards the other end of the insulator, there may be 2 similar discs (not shown), preferably with the same orientation as the discs 39 (ie with their flat faces facing to the left, like the discs 39).

The magnetic core 26 surrounds the conductor 14, outside the insulating layer 35 and the screening layer 36. Drive and sense windings 27 are wound on the magnetic core, and are connected via a cable 28 which passes along the mounting rod 17. The cable 28 may be clipped to the rod 17 by suitable clips (not shown).

The insulator is mounted on the mounting rod 17 by means of a C-shaped element 37, which can conveniently be of galvanized steel. The rod 17 is attached to the centre of the C shape. Each end of the C shape lies adjacent to the insulated conductor, and is attached thereto by means of a respective spring steel strap 38 which has a suitable number of turns around the insulated conduc¬ tor alone followed by a suitable number of further turns around the insulated conductor and the respective end of the C-shaped element 37. One of the straps 38 preferably contacts the semiconductor layer 36, to provide an earth for that layer; the other strap should be insulated from the layer 36, to avoid a closed loop and consequent inductive currents.

The magnetic core 26 with its windings 27, and the ends of the C-shaped member 37 with its mounting straps 38, are protected by means of protective tubing 40 of plastics material. This tubing may conveniently be heat shrink material, and may be in 2 (as shown) or more sections.

In summary, there is provided a transducer assembly ( 16) for an overhead power line operating at intermediate voltage (eg 1 1 k V). The power conductor ( 14A, 14B) passes through insulating means supported by a support rod ⁽ 17) and generally transversely to the conductor. A magnetic core (26) round the conductor has windings (27) with connections passing (28) along the support rod. The rod may be a transparent tube having inside it a fuse element which produ¬ ces a distinctive change of appearance when "blown". The insulator may be gen¬ erally transverse to the conductor (Figs. 1 , 2, &ι 3) or generally collinear there¬ with (Fig. 4), and may include a strain (tension) mounting (31 ). Some forms of the assembly may be installed on a live system.

The present invention can advantageously be employed in conjunction with the power line signalling system described in our copending application entitled "Power Line Signalling System", filed simultaneously herewith.