WO2003012968A1

WO2003012968A1 - Improvements in or relating to a power distribution system

Info

Publication number: WO2003012968A1
Application number: PCT/GB2002/003283
Authority: WO
Inventors: Philip John Rimmer
Original assignee: Tunewell Technology Ltd.
Priority date: 2001-07-30
Filing date: 2002-07-18
Publication date: 2003-02-13
Also published as: GB2380871A; GB0118554D0; GB2380871B

Abstract

An electric power distribution system comprising: a source of high frequency alternating current having a resonant output circuit with a predetermined operating frequency; a transmission line spur connected to the source, the spur comprising at least one transmission line element having two conductors, the or each transmission line element having a high capacitance and low inductance; and a spur inductor connected in parallel across the conductors of the transmission line spur, between the transmission line spur and the source, wherein the combination of the inductance of the spur inductor and the capacitance of the transmission line spur is resonant substantially at the operating frequency.

Description

"IMPROVEMENTS IN OR RELATING TO A POWER DISTRIBUTION

SYSTEM"

The present invention relates to a power distribution system, and more particularly relates to a power distribution system adapted to distribute electric power from a power source over a relatively large area to at least one load using a transmission line preferably in the form of a track.

Various power distribution systems have been used to distribute power from a power source to a load.

A problem exists in connection with a high frequency power distribution systems in that substantial H field "interference" can be generated by the cable or cables of the power distribution system. There is a desire to minimise power loss and radiated interference from the cable or cables.

In a known configuration, power is distributed on a track system, consisting of a cable comprising two flat copper conductors separated by a thin film of insulator. This cable is carried in a plastic extrusion that facilitates mounting to a variety of ceiling surfaces and permits various two-terminal connectors to be electrically connected and mechanically retained. The power supply therefor has an output resonant circuit having a predetermined resonant frequency.

The cable structure in the track possesses high capacitance and low inductance (i.e. it is a transmission line of very low impedance). Long lengths of track will apply a significant capacitive loading to the output resonant circuit of the power supply. Despite the lower impedance of the output resonant circuit the track capacitance will undesirably lower its resonant frequency. It is desirable, however, for the power supply, to maintain a constant operating frequency. Power is to be distributed over a large area and loads attached to the system. The number, location and type of loads to be attached will vary. Accordingly, the demands on the power source and the configuration of the track will change. Presently available systems are only able to cope with the varying system requirements by over-specifying the power supply, its tank circuit, and cabling requirements for a worst case scenario. Such over-specification results in an unnecessarily expensive and over-engineered system.

The present invention seeks to provide an improved power distribution system which does not suffer from the above-mentioned disadvantages.

According to this invention there is provided an electric power distribution system comprising: a source of high frequency alternating current having a resonant output circuit with a predetermined operating frequency; a transmission line spur connected to the source, the spur comprising at least one transmission line element having two conductors, the or each transmission line element having a high capacitance and low inductance; and a spur inductor connected in parallel across the conductors of the transmission line spur, between the transmission line spur and the source, wherein the combination of the inductance of the spur inductor and the capacitance of the transmission line spur is resonant substantially at the operating frequency.

Preferably, the transmission line spur comprises a plurality of transmission line elements connected in series to one another and an element inductor located between each pair of adjacent transmission line elements and connected in parallel across the conductors thereof, wherein the combination of the inductance of each element inductor and the capacitance of each transmission line element subsequent to its pair is resonant substantially at the operating frequency.

Advantageously, the element inductor is adjustable such that the operative inductance of the element inductor is selectable. Preferably, the spur inductor is adjustable such that the operative inductance of the spur inductor is selectable.

Conveniently, at least one load is electrically connected to the transmission line spur.

Advantageously, the or each transmission line element is in the form of a track having two conductors separated by an insulator.

Preferably, the conductors are of substantially flat or rectangular cross-section and the insulator is a sheet of insulating material.

Conveniently, at least one transmission line element is mechanically connected to another by a connector, the connector including the element inductor for the subsequent one of the transmission line elements.

Advantageously, the transmission line spur is mechanically connected to an interface box provided between the source and the transmission line spur, the interface box including the spur inductor for that transmission line spur.

Preferably, the transmission line elements each have a predetermined length which defines the capacitance thereof.

Conveniently, transmission line elements are available in lengths which are integral multiples of a base length of transmission line element.

Advantageously, the source has a tank circuit and parts of that tank circuit comprise the inductance and capacitance associated with the transmission line spur, the spur inductor and/or the or each element inductor. Preferably, the impedance of the tank circuit at the operating frequency is less than the load impedance by a factor of at least two.

Advantageously, the factor is at least five.

Conveniently, a plurality of isolatable spurs are provided for connection to the source.

Advantageously, the load comprises a low voltage incandescent lamp, a discharge lamp, or a heater.

Another aspect of the present invention provides an electric power distribution system comprising a source of high frequency alternating current having a resonant output tank circuit with a predetermined operating frequency operable to supply a plurality of individually operable loads.

Preferably, one or more of the loads are coupled to the output tank circuit.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIGURE 1 is a block diagram illustrating a power distribution system embodying the invention;

FIGURE 2 is a perspective view of a power transmission line and a connecting clip structure suitable for use in a power distribution system embodying the invention.

Referring to Figure 1, a large central power supply 1 have a rating in the region of 100W to 10,000 W is used to provide a relatively high frequency sinusoid i.e. (10kHz to 200kHz) via a cable or cables to a relatively large area (e.g. 10 to 1000 sq. metres). The cable or cables are in the form of a low impedance transmission line 2, i.e. possessing low series inductance and high parallel capacitance. This form is chosen because of its lower tendency to radiate power, hence minimising power loss and radiated interference. Preferably, the transmission line 2 is in the form of a track 2 to be described below.

The transmission line 2 has two conductors 3,4 which may be connected, as illustrated in Figure 2, directly to a load 5. The load 5 may comprise an appropriate incandescent lamp, or could comprise a heater or some other equivalent device. The load may also be connected indirectly to the transmission line 2 by, for example, a transformer or by means of an inductor.

Preferably, there are a plurality of loads each comprising, for example, a discharge lamp. The relatively high frequency permits the use of small inductive, capacitive and transformer components to provide power at the appropriate voltages and currents for the lamps. Typically an inductor will be used to limit current flow into a discharge lamp and a transformer used to accommodate differing voltage requirements.

The number and variety of lamps which may be utilised and the substantial load variation during the brief period of lamp ignition results in a substantial variation in load power factor. These variations will tend to produce substantial waveform distortions that will contribute to radiated interference. These variations are ameliorated by the use of a power supply 1 with a very low output impedance.

The transmission line 2 is conveniently in the form of track which is provided in various standard lengths which can be connected together using inter-track connectors 6 to create the total lengths required. The inter-track connectors 6 provide both mechanical and electrical connection of the tracks 2 to one another. A single power supply may power several such lengths of track 2; each referred to as a spur 7. Where lengths of track are exposed to view an extruded plastic cover may be provided to conceal the track within. Figure 1 shows two spurs 7 of track 2. The power is provided from the power supply 1 to each spur 7 via an interface box 8. The interface box 8 connects at the end of a spur 7. Connection to the spur 7 is made using an inter-track connector 6, a short stub of extrusion-like profile containing a short stub of track 2 being offered from the end of the interface box 8 for this purpose. Power from the power supply 1 is brought to the interface box 8 via conventional twisted pair (possibly screened) cable 9. The connection of the cable 9 to the interface box 8 uses simple screw terminals, a paralleled pair of which will allow daisy chaining out of the interface box 8.

In the interface box 8, power is applied to the track 2 via isolating relay contacts 10. These are used to provide a sequenced turn-on from spur to spur, thereby reducing power supply stress, providing isolation on detection of an overload or short to ground and also a means of servicing loads 5 on the track 2 whilst the power supply is still working. These relays 10 can provide area switching.

Each spur 7 is terminated by an end cap 7a which electrically connects the two conductors 3,4 at the end of the last of track 2 of the spur so as to complete the circuit in that spur 7.

Figure 2 illustrates part of a length of a transmission line 2 in the form of a track 2. The track 2 comprises two principal conductors 3,4. Each of the conductors is of substantially flat or rectangular cross-section, with two opposed parallel flat faces. The conductors may be formed of copper or copper alloy and may have a thickness of between 0.025 and 0.25 mm, and a width of up to 30 mm, depending upon the current to be carried. The two conductors 3,4 are separated by a sheet of insulating material 11. The insulating material may be an appropriate plastic such as a polyester, polypropylene or polyphenylene sulphide. The thickness of the insulating sheet 11 depends upon the voltage to be carried by the conductors 3,4 and may typically be of the order of 0.1 millimetres. It has been found that distribution lines of the type described above provide very low inductance, low resistance and high capacitance. The capacitance provided by the distribution line can be incorporated into the circuit of the power supply so as to provide power storage through the transmission line, for example as part of a resonating circuit.

It is preferred to use an appropriate insulating material for the insulating sheet 11 between the conductors 3,4 so that the capacitance is of a high quality, that is to say with a low power loss. The polyester material mentioned above provides good characteristics and is suitable for use at a low to medium temperature. Polypropylene provides better characteristics, but is only suitable for use at low temperatures. Polyphenylene sulphide provides good properties, and is suitable for use at high to medium temperatures.

A clip 12 is provided which has two spaced apart blades 13,14 which engages the opposed faces of the transmission line 2. Since the blades 13,14 of the clip 12 engage a substantial area of the conductors 3, 4 and the arms of the clip 12 provide an appropriate power-take-off. The arms of the clip may be connected to, for example, a housing 15 which contains an appropriate transformer to a load 5 which is to be supplied with electrical power. Thus, a clip as shown establishes electrical contact between the load 5 and the transmission line 2.

If the voltage present on the two conductors of the transmission line 2 is always equal and opposite, about earth, - that is to say if the two conductors of the distribution line are provided with an alternating voltage in anti-phase about earth - then only a very small electric field, E, is generated.

Importantly, each inter-track connector 6 includes a small inductor 6A, the inductance thereof having a value which is associated with the capacitance of the length of track 2 which is connected to the inter-track connector and "downstream" from the power supply 1. As previously mentioned, each length of track 2 has a high capacitance and long lengths of track 2 connected to the power supply will apply a significant capacitive load on the output resonant circuit of the power supply. Such a capacitive load would undesirably lower the resonant frequency of the output resonant circuit. The provision of the inductance, in parallel with the conductors 3,4 of each length of track 2, neutralises the capacitance of the track 2 immediately downstream of the inter- track connector 6. This balances the output resonant circuit which does not, therefore, go out of equilibrium so that the desired predetermined operating frequency of the power supply 1 is maintained.

As well as the inter-track connectors 6 being provided with an inductance 6A, each output of the interface box 8 to which track 2 is intended to be connected, is also provided with an inductance 8 A so as to accommodate the capacitance of the track connected thereto and to make the interface box resonant at the operating frequency of the power supply unit. The inductor 8 A may be a tapped inductor.

The loads 5 which are connected by the clips 14 to the track 2 may also include components of a virtual tank circuit. Such components in the load should, however, be neutral, i.e. balanced for capacitance and inductance. In the case of the loads 5 being gas discharge lamps, the end connectors of these loads would carry appropriate igniting and ballasting components. This effectively includes a resonant tank circuit within the components associated with the load 5. The tank circuit would typically have an impedance of one fifth of the apparent impedance of the load 5 in operation. The components would be split so as to be applied symmetrically (with regard to their capacitive and inductive contributions) at both ends 17 of the tube 5 allowing a single type of lamp connector to supply inverse phase power by simply rotating the phase through 180°.

Track 2 is preferably supplied in integral multiples of a base length such as, for example, 1, 2, 3 or 4 metre lengths. Each length has an associated capacitance which needs to be balanced by the appropriate inductance. In order to -Si- accommodate different lengths of track, each inter-track connector 6 can include an adjustable inductor, the inductance of which can be readily selected by turning a dial on the outside of the inter-track connector 6. Thus, if track is sold in the four different lengths described above, then the inter-track connector would have four possible inductance settings, each inductance setting matching one of the track lengths so that the circuit is balanced and the power supply unit is maintained at its constant operating frequency.

The provision of discrete inductors along the transmission line also confers other advantages to the power distribution system. Each discrete inductance along the transmission line 2 provides an effective or virtual distribution of the power supply tank circuit along the transmission line thereby reducing the required size of the power supply unit tank circuit and reducing the cost thereof. In fact, the provision of the virtual tank circuit distributed around the transmission line means that the actual tank circuit of the power supply unit can have a very low output impedance at its operating (resonant) frequency which is at the very least half of but preferably in the order of one fifth of the maximum load impedance. This provides substantial engineering benefits and cost savings.

The provision of the power supply unit tank circuit in isolation, or in combination with the virtual tank circuit distributed around the transmission line, is extremely important as the resultant tank circuit provides a significant energy storage function to compensate for any unusual loads or deviations from the resonant operation of the power distribution system. The tank circuit maintains the stability of the power distribution system by preserving the amplitude and waveform of the distributed voltage so that any deficiencies in the waveform, dropouts or the like will be made up for by the tank circuit. Further, any transient loads or spikes will be accommodated by the tank circuit within a few cycles. In short, the tank circuit pulls all the elements of the system, including the individually operable loads back to the power distribution resonant frequency - the constant predetermined operating frequency of the power supply unit. Further, by adding more transmission lines to the system, one is increasing the amount of stored energy available in the circuit, which energy can be used to power more loads. Effectively, the energy storage capacity of the circuit grows with the size of the circuit so that one does not need to over-specify or over- engineer the tank circuit for the power supply as the track 2 and inter-track connectors act as part of a virtual tank circuit. The increase in the size of the virtual tank circuit is also incremental and linked to the number and lengths of track 2 added to the circuit.

The overall system provides a balanced circuit which is important to maintain the constant predetermined operating frequency of the power supply unit.

The provision of two isolatable spurs 7 as shown in Figure 1 for each power supply unit 1 ensures that there is an element of redundancy in the circuit. Thus, a track failure on one spur 7 will not eliminate, for example, light production from the other spur 7 and, importantly, even if there is a track failure, the power supply unit 1 will still be maintained in equilibrium at its predetermined operating frequency.

In the present specification "comprise" means "includes or consists of and "comprising" means "including or consisting of.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLA S:

1. An electric power distribution system comprising: a source of high frequency alternating current having a resonant output circuit with a predetermined operating frequency; a transmission line spur connected to the source, the spur comprising at least one transmission line element having two conductors, the or each transmission line element having a high capacitance and low inductance; and a spur inductor connected in parallel across the conductors of the transmission line spur, between the transmission line spur and the source, wherein the combination of the inductance of the spur inductor and the capacitance of the transmission line spur is resonant substantially at the operating frequency.

2. A system according to Claim 1 wherein the transmission line spur comprises a plurality of transmission line elements connected in series to one another and an element inductor located between each pair of adjacent transmission line elements and connected in parallel across the conductors thereof, wherein the combination of the inductance of each element inductor and the capacitance of each transmission line element subsequent to its pair is resonant substantially at the operating frequency.

3. A system according to Claim 1 or 2, wherein the or each transmission line element is in the form of a track having two conductors separated by an insulator.

4. A system according to Claim 3 wherein the conductors are of substantially flat or rectangular cross-section and the insulator is a sheet of insulating material.

5. A system according to- any preceding claim, wherein at least one transmission line element is mechanically connected to another by a connector, the connector including the element inductor for the subsequent one of the transmission line elements.

6. A system according to any preceding claim, wherein the transmission line spur is mechanically connected to an interface box provided between the source and the transmission line spur, the interface box including the spur inductor for that transmission line spur.

7. A system according to Claim 6 wherein a plurality of isolatable spurs are provided for connection to the source.

8. A system according to any preceding claim, wherein the transmission line elements each have a predetermined length which defines the capacitance thereof.

9. A system according to Claim 8 wherein transmission line elements are available in lengths which are integral multiples of a base length of transmission line element.

10. A system according to any preceding claim, wherein the element inductor is adjustable such that the operative inductance of the element inductor is selectable.

11. A system according to any preceding claim, wherein the spur inductor is adjustable such that the operative inductance of the spur inductor is selectable.

12. A system according to any preceding claim, wherein the source has a tank circuit and parts of that tank circuit comprise the inductance and capacitance associated with the transmission line spur, the spur inductor and/or the or each element inductor.

13. A system according to Claim 12, including a load to which the power is supplied, wherein the impedance of the tank circuit at the operating frequency is less than the load impedance by a factor of at least two.

14. A system according to Claim 13, wherein the factor is at least five.

15. A system according to any preceding claim, wherein at least one load is electrically connected to the or each transmission line spur.

16. A system according to Claim 14 wherein the at least one load comprises a low voltage incandescent lamp, a discharge lamp, or a heater.

17. An electric power distribution system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

18. Any novel feature or combination of features disclosed herein.