WO1996017444A1

WO1996017444A1 - Power line signalling system

Info

Publication number: WO1996017444A1
Application number: PCT/GB1995/002813
Authority: WO
Inventors: Donald Stuart Armstrong; Alan Dennison Craig; Paul Martin Moore; Joseph Anthony Wells
Original assignee: Remote Metering Systems Ltd.
Priority date: 1994-12-01
Filing date: 1995-12-01
Publication date: 1996-06-06
Also published as: SK68097A3; IL116202A0; NO972478L; GB9424389D0; CA2206300A1; ZA9510203B; EP0806094A1; PL320753A1; JPH10510115A; HUT77613A; NO972478D0; KR987000737A; AU3988895A

Abstract

A 3-phase overhead distribution network operates at intermediate voltage (e.g. 11 kV to 33 kV), between a high voltage supply network (at e.g. 110 kV or 275 kV) and a consumer mains system (at e.g. 110 V or 230 V). A signalling system uses signals at frequencies preferably in the region of 10 kHz to 100 kHz for signalling over the network, the signals being coupled inductively to and from the network. The signals are coupled to different phases at different points in the network cross-coupling between the phases ensures that signals coupled onto the system on one phase give adequate signal strength for detection at other points in the system regardless of which phase the detector is coupled to.

Description

Power Line Signalling System

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

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 distribution network or system (often termed the grid). The low voltage (consumer) networks may for example operate at 1 10 V or 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 kV 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, preferably 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 suppliers 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 control. 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 voltage 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 generally 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 any signal feedthrough which does occur at power transformers 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 equipment 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 physically 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 prohibitive. 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 a 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 secondary). We are here concerned with such inductive coupling. 3-phase systems

Distribution systems are generally 3-phase at intermediate (and high) voltages, and often at low voltages as well. The distribution system therefore consists generally of 3 live supply lines, and usually a neutral (earth) line as well. The supply lines are conventionally termed R, Y, and B (red, yellow, and blue), forming a star connection with the neutral line.

Large consumers are often supplied with a 3-phase supply. However, small consumers (such as domestic customers) are normally only provided with a single-phase supply. The supply companies endeavour to arrange the connections of the (single-phase) consumers so that the loads on the three phases are broadly matched or balanced. In particular, an overhead intermediate voltage 3-phase network can have various single-phase branches or spurs, because the cost of installing such a spur is significantly smaller than that of installing a 3-phase spur. (Single-phase spurs are also possible in principle with underground networks, but for various reasons are rare in practice.)

A true single-phase spur would use a single one of the 3 phases (R, Y, and B) together with earth or neutral, but for various reasons this is generally undesirable. So-called single-phase spurs therefore normally use 2 of the 3 phases at the intermediate voltage, with the transformer at the low voltage end reducing the voltage between those 2 phases to the normal mains voltage (eg 1 10 V or 230 V).

With a 3-phase system and inductive coupling, the signals are carried on whichever phase the injecting transducer is coupled to, and the detecting transducer detects signals on whichever phase it is coupled to. This has dictated that a single phase be used for signalling, with all transducers coupled to that phase. (It is convenient to describe the phase to which the transducers are coupled as the primary phase, and the other two phases as the secondary phases.) This has required that the primary phase be identified at all points in the system where transducers are attached. It has also meant that signalling could only be carried out over single-phase spurs if they included the primary phase. The present invention

According to the present invention, there is provided a signalling system for signalling over a 3-phase distribution network, characterized in that the signals are coupled inductively to and from the network, and are coupled to different phases at different points in the network. The signal frequency is preferably in the region of 10 kHz to 100 kHz.

The present invention rests on the discovery or realization that the primary phase is coupled to the secondary phases (at the signal frequency) sufficiently well for the signal injected onto the primary phase to be satisfactorily detectable on the secondary phases as well as the primary phase. Of course, the "primary phase" is now defined by reference to a particular signal injecting transducer; if another injecting transducer is considered, its primary phase may be different.

Signals will normally be coupled to the system near a transformer, and this coupling to the secondary phases arises largely capacitively at the transformer. Assume that the transformer is a 3-phase delta transformer (if it is actually a star transformer, we can consider its delta equivalent circuit). The windings of the transformer present high impedance at the signal frequency, but there are effectively shunt capacitances across the windings from the primary phase to each of the two secondary phases, and these capacitances couple the signal on the primary phase to the two secondary phases. (If the actual configuration is a delta, there is no real neutral point. If the actual configuration is a star, there is a real neutral point, which may or may not have a neutral line connected to it; in either case, the neutral point is ideally at the same voltage as the earth but is not normally connected to earth (ground).)

There is also effectively a capacitance between each of the three delta points of the windings and earth. The two secondary-to-earth capacitances will act, in conjunction with the capacitances across the two primary-to-secondary windings, as signal droppers; also, the primary-to-earth capacitance will tend to shunt the signal on the primary phase to earth. But although these effects reduce the secondary signal strength, they do not reduce it to an unacceptable degree. There is also a capacitance across the winding between the two secondary phases. Under balanced load conditions, the two secondary phases receive equal signals, so this capacitance is irrelevant; if the conditions are unbalanced, this capacitance will help to equalize the signals on the two secondary phases.

At a 3-phase termination and receiving point, the signal coupling may be coupled to any of the three phases. If it is coupled to the primary phase, it will of course pick up the primary phase signal. If it is coupled to either of the two secondary phases, it will pick up a secondary phase signal, which will be smaller than the primary phase signal but still of acceptable strength. Similarly, at a single-phase spur and receiving point (ie one fed with 2 of the intermediate voltage phases), the signal coupling will pick up either a primary or a secondary phase signal, depending on which 2 phases are used for the spur and which of those 2 phases the signal coupling is coupled to. As with 3-phase terminations, the net signal current into the termination is zero, so there may also be an earth current.

For signals injected at a single-phase spur, one of the 2 intermediate voltage phases at the spur will necessarily be the primary phase for signals injected by the transducer there. There is likely to be an imbalance between the secondary phase for the spur and the remaining secondary phase, but at a 3- phase termination, signals on the primary phase will divide between the two secondary phases so that they can be received there on either secondary phase (as well as on the primary phase, of course), even though the two secondary phase signals may be of different strengths. Similar mechanisms will also normally ensure that signals injected on one single-phase spur will be received at other single-phase spurs.

Specific embodiment of the invention

A 3-phase intermediate voltage distribution network including a signalling system and embodying the invention will now be described, by way of example, with reference to the drawings, in which:

Fig. 1 is a general circuit diagram of the system; and

Fig. 2 is a more detailed circuit diagram of the supply transformer station. Referring to Fig. 1 , the system is fed from a transformer station 10 which is fed from a high voltage grid by means of a 3-phase transformer driving a 3-phase intermediate voltage power distribution system having 3 phases R, Y, and B. The 3 phases are fed to a 3-phase substation 1 1 at which the power is transformed down to low voltage by a 3-phase transformer. There are also two single-phase spurs from the system, a spur consisting of the R and Y phases feeding a substation 12 and a spur consisting of the Y and B phases feeding a substation 13. Obviously the system may have further 3-phase extensions and single-phase spurs.

For simplicity, only the intermediate voltage windings of the transformers are shown, with the high voltage windings (for transformer 10) and the low voltage windings (for transformers 1 1 to 13) omitted. The primary of the high-voltage transformer 10 will normally be a delta winding; the secondary of the low-voltage transformer 1 1 will normally be a star winding giving 3 separate low-voltage phases; and the secondaries of low-voltage transformers 12 and 13 will normally each be a single winding giving a single low-voltage phase.

The station 10 has a transducer 10T coupled to the R phase; this transducer comprises a magnetic core with the R phase power line passing through it (so forming a single-turn winding) and with (multi-turn) drive and sense windings coupled to it (indicated symbolically by a "U"). The 3-phase substation 1 1 has a transducer 12T coupled to its B phase power line; the single-phase substation 12 has a transducer 12T coupled to its Y phase power line; and the single-phase substation 13 has a transducer 13T coupled to its Y phase power line.

The driving transducer 10T is coupled to the R phase, so that phase is the primary phase and the Y and B phases are the secondary phases. In the present system, the receiving transducers 1 1T to 13T may each be coupled to any phase, and in particular may be coupled to the secondary phases as shown. Hitherto it has been regarded as mandatory for the receiving transducers to be coupled to the primary phase, so that the transducers at substations 1 1 and 12 would have to be located as indicated at 1 1T' and 12T'; it was not thought possible to couple a transducer to substation 13, as that substation is not fed by the primary phase.

It will of course be understood that while transducer 10T acts as the driving transducer and transducers 1 1T to 13T act as receiving transducers for signals being fed from the station 10, any of the transducers can act as a dri ving transducer for signals from its own substation, with the other transducers acting as receiving transducers. The R phase is by definition the primary phase for signals from transducer 10T, but other phases may be the primary phase for signals injected by other transducers.

Fig. 2 shows the effective circuit of the system at transformer 10 in more detail. The transformer has three intermediate voltage windings W1 to W3 in delta configuration. (If the transformer is actually a star configuration, it can be converted to the equivalent configuration shown by a standard transformation.) Each winding is, at the signal frequency, shunted by a shunt capacitance, shown as C 1 to C3. Each delta point is also coupled to earth by an earth capacitance, shown as C4 to C6.

Considering the system in voltage terms, the transducer 10T induces a voltage on the R power line. This voltage is coupled to earth through 3 parallel paths: capacitances C1 and C6 in series, capacitances C3 and C4 in series, and capacitance C5. The two series paths C 1-C6 and C3-C4 result in voltages being induced on the Y and B phase power lines. Hence all three power lines have voltages induced on them; a primary voltage on the primary (R) phase, and two equal and somewhat smaller voltages, of opposite phase, on the Y and B power lines.

In current terms, a primary current I _R is induced in the R phase power line, two equal and somewhat smaller secondary return currents 1 _Y and 1 _B, of opposite phase, are induced on the Y and B power lines, and an earth or ground return current I _C , also of opposite phase to the primary current, is induced in the earth at the transformer 10. Obviously I _R = I _Y + I _B + I _G . The primary current travels out along the primary phase power line to the various substations and passes to the secondary phases and earth at those substations. At the signal frequencies, the power lines act as transmission lines between the substations and switching points where the power distribution system forks (into 2phase or 3-phase branches).

It is evident that each of the two secondary phase return currents is effectively divided between the various substations, but that each of the substations will in general receive significant portions of the two total secondary phase return currents. In particular, substation 1 1 will receive a significant portion of the B phase return current, and substations 12 and 13 will each receive sig nificant portions of the Y phase return current. The receiving transducers

1 1T, 12T, and 13T will therefore all receive significant signals from the transducer 10T.

At signal frequencies, the windings of the transformers at the substations are each effectively shunted by capacitances, and are also effectively coupled to earth by earth capacitances. The currents in the power lines to which the transducers are coupled find their return routes through these capacitances. (The operation can of course also be explained in voltage terms.)

The present invention can advantageously employ the power line signalling device described in our copending application entitled "Power Line Signalling Device", filed simultaneously herewith.

Claims

1 A signalling system for signalling over a 3-phase distribution network (10-13), characterized in that the signals are coupled inductively to and from the network, and are coupled to different phases (R, Y, B) at different points in the network (10T to R, 1 1T to B, 12T to R, 13T to Y).

2 A signalling system according to claim 1 , characterized in that the signal frequency is in the region of 10 kHz to 100 kHz.

3 A signalling system according to either previous claim, characterized in that the system operates st a voltage between 1 1 kV and 33 k V.

4 A signalling system according to any previous claim, characterized in that the system includes at least one branch having only a single phase.

5 A signalling system according to any previous claim, characterized in that the system includes at least one branch (12, 13) having only two phases.

6 Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).