WO1996023368A1

WO1996023368A1 - Method and apparatus for communicating by means of an electrical power cable

Info

Publication number: WO1996023368A1
Application number: PCT/GB1996/000177
Authority: WO
Inventors: Michael Andrew Yuratich; Andrew Jewell
Original assignee: Tsl Technology Ltd.; Expro North Sea Limited
Priority date: 1995-01-27
Filing date: 1996-01-26
Publication date: 1996-08-01
Also published as: GB9501615D0; NO973448D0; NO973448L; AU4493396A

Abstract

A system which allows transmission of data on the same electrical cable which is used to transmit power to an electrical load, such as an Electrical Submersible Pump (ESP), is described. This is achieved, in one embodiment, by using the cable insulation as the dielectric of capacitors whose electrodes are the cable current-carrying conductors and external conducting sheaths. This obviates the need for direct contact or special high voltage connections with the current-carrying conductors which makes it especially advantageous in systems like oil well ESPs where high reliability of the system is essential. The system can be single-phase or three-phase. In a preferred arrangement two such capacitors are used to connect to two separate power cables lines to complete a circuit to allow bidirectional data transmission and the data, when transmitted across two such capacitors, avoids the need for an earth connection and this further improves the reliability and permits routine insulation testing. Various embodiments of the invention are described.

Description

Method and apparatus for communicating by means of an electrical power cable

The present invention relates to a method and apparatus for communicating over an electrical cable. In particular, the invention relates to the bidirectional transmission of data and instrumentation equipment power over an electrical cable particularly, but not exclusively, where the primary purpose of the cable is the transmission of large amounts of electrical power.

In the electrical power distribution industry it has long been desirable to be able to transmit information on the same electrical cables which are used to carry the electrical power. There are many other cases where data transmission is required between points following essentially the same route as cables installed to provide electrical power. In such cases, it would be economically advantageous if the data could also be transmitted in the power cable rather than in separate and costly data cables. Some examples of apparatus where this is, or could be, implemented are domestic wiring where data transmission is provided on the mains loop to provide control of "smart" household appliances, in large factories and in road and in rail signalling equipmen . In the oil industry there is a need for a system where information is transmitted on the same electrical cables as electrical power. This is particularly true for downhole environments where electronic equipment is situated near and/or in large electrical motors installed in deep oil wells.

In deep oil producing wells an electrical submersible pump 10 (ESP) is located in a well seen in Fig. 1. A motor 12 coupled to the pump drives the pump 10 which lifts reservoir fluids like crude oil 14 and water to the surface via metal tubing 16. Electrical power for the motor 12 is carried by an armoured high voltage cable 18 which runs from the surface to the motor 12. The armoured cable 18 is secured (not shown) to the tubing 16 and normally passes through a gas and fluid impermeable bulkhead known as a packer 20, inside a packer penetrator 22 which is a sleeve or conduit passing through the packer 20. The packer penetrator contains a cable joint (not shown in the interest of clarity) as part of its construction and the part of the cable 18 beneath the cable joint is known as the motor lead extension 24. This extension is usually of high grade construction in order to withstand the hostile environment of the well fluids and the downhole temperature: individual cable lines are often sheathed in lead or copper to prevent or minimise gas impregnation and subsequent damage.

In such installations the overriding consideration is reliability of the ESP and the cable because these must work as expected over a period of time in order to provide the production targets which are set for the well.

However, in order to optimise production management and to extend the ESP operating life and also to build up a database of pre-failure information, it is desirable to monitor many parameters in the vicinity of the ESP. These parameters may include pump flowrate, well fluid pressure, oil/water ratio, fluid density, motor vibration and motor temperature. If the measurements of such parameters are transmitted to the surface on a separate cable, ESP reliability is not effected. However, there is a very large capital cost associated with a second cable to carry such measurements and also with the installation of such a cable. Moreover, with a separate cable of this nature it is possible to damage the cable which can add to the cost and this will also leave the well operator without information until the ESP itself is pulled from the well.

Existing ESPs are universally driven by three-phase electrical power and the motors are of a squirrel-cage induction type with a typical somewhat simplified electrical circuit being shown in Fig. 2. The point where all the motor windings 26 are connected together is known as the star point 28. As part of a routine inspection it is desirable to be able to test the cable 18 and the motor windings insulation. This is possible if the star point is not connected to the surrounding metal work and the ESP surface power supply is transformer isolated from the distribution supply, which is normal. In such a case, at the surface, an insulation test voltage can be applied between the cable conductors and earth to test the cable and motor windings insulation.

The use of built-in sensors connected between the star point 28 and earthed metal work 30 as part of a data transmission system is disclosed in U.S. Patent No. 1,589,546 to TRW and U.S. Patent No. 3,340,500 to Borg- Warner. In these documents, a specially operated relay disconnects the star point and hence the transmission system when insulation testing is required. This is undesirable because it causes a failure if the relay fails to open when commanded.

In addition, the large motor winding inductance results in a large reactance at all but very low frequencies. This means that data transmission via the star point and windings is limited to essentially slowly- varying analog or digital signals. Besides limiting the updating rate of data, it also limits the number of distinct sensors that can be used.

U.S. Patent No. 4,620,189 to Oil Dynamics discloses how to connect a data transmission signal above the motor which attempts to overcome the large reactance problem. In this disclosure the circuit is connected between earth and a joint which is made onto one of the cable conductors itself. This allows the use of relatively high frequency data transmission as the motor inductance is not part of the connection to surface. However, it is affected by the variable characteristics of the earthed metal works which are usually of constructional grade steel. This is not a high grade electrical material and can be affected by corrosion. In addition, however, the system requires a separate high voltage instrument cable to be connected directly to the power cable. This instrument cable runs to a separate instrument housing containing a high-voltage isolating capacitor and low-voltage data measurement and transmission circuitry. With this arrangement there are therefore four potential points of failure which could cause the ESP system to fail and these are: the capacitor short circuits through to the instrument case; the high voltage cable feed through into the instrument housing (similar short circuit) ; the instrument cable (insulation breakdown) ; and the joint to the power cable (insulation leakage to well fluids) .

An object of the present invention is to obviate or mitigate at least one of the aforementioned disadvantages of existing data transmission systems.

A further object of the present invention is to be able to transmit data over the same cable which carries electrical power, at a relatively high frequency.

A further object of the present invention is to provide a single cable for the transmission of electrical power and transmission of data which does not require a high voltage instrument wire or an earthed connection.

A further object of the present invention is to provide a single cable which allows the transmission of electrical power and measurement data and which permits two-way communication of data and control over downhole equipment operation.

A further object of the present invention is to provide a single cable which allows the transmission of data and electrical power and relatively low electrical power from the surface to operate downhole instrumentation.

The present invention solves the problems of the art and allows transmission of data and instrumentation on the same electrical cable which is used to transmit power to an electrical load, such as an ESP. This is achieved by using the cable insulation as the dielectric of capacitors whose electrodes are the current-carrying conductors and external conducting sheaths. This obviates the need for direct contact or special high voltage connections with the current-carrying conductors which makes it especially advantageous in systems like oil well ESP pumps where high reliability of the system is essential.

In a preferred arrangement two such capacitors are used to connect to two separate cable lines to complete a single-phase circuit to allow bidirectional data transmission and the data when transmitted across two such capacitors avoids the need for an earth connection and this further improves the reliability and permits routine insulation testing.

Electrically conducting electrodes are preferably placed over the insulation of individual ESP power cable lines thereby forming capacitors between the electrodes and the power conductors themselves. The electrodes are made to be substantially close and full circumferential fit to the cable line insulation in order to maximise the capacitance per unit length and to reduce any electrical stress on the insulation.

Data transmission equipment is then connected to these electrodes and transmits through the capacitors. This provides the considerable advantage that there is no high-voltage connection to the cable, no separate high- voltage coupling component is required because the ESP cable insulation itself provides the required isolation. This is particularly advantageous in oil well systems using electrical submersible pumps because the power cable being an existing and integral of such an ESP system does not have its reliability affected by the connection.

According to a first aspect of the present invention there is provided apparatus for transmitting data signals over the same cable carrying electrical power to a load, said apparatus comprising providing capacitors by using the insulation of a current-carrying conductor as the dielectric of a capacitor and the conductor and an external conductive sheath as capacitor electrodes.

Preferably, two capacitors are used for two or three power lines in a single-phase system. Alternatively, three capacitors may be used, one for each line in a three-phase system.

Conveniently, electrically conducting electrodes are disposed over at least one line to form a capacitor between the line conductor and the electrodes. Advantageously the electrodes are a close circumferential fit to the cable line insulation.

Preferably also, the electrodes are applied over power cable lines in downhole wells. Conveniently transmitting data on a three-phase power cable to a downhole motor, electrodes are disposed over each power cable line and are located inside an oil-filled unit, with each electrode being coupled to an instrumentation package for communicating data signals to be transmitted over said power cable lines.

According to another aspect of the present invention there is provided apparatus for transmitting data signals over the same conductor as that carrying electrical power to a load, said apparatus providing capacitors coupled to the power conductor whereby data signals can be transmitted to and from said power conductor via said capacitors.

Preferably, the capacitors are provided by using the insulation of the power-carrying cable lines as a dielectric and external sheaths as the other electrodes of the capacitors. Alternatively, the capacitors are separate capacitors.

According to a further aspect of the invention there is provided a method of transmitting a data signal on a power conductor carrying electrical power to a load, said method comprising the step of creating a capacitive coupling from said power conductor by using the insulation of the power cable as the dielectric and providing an external conductive sheath on said insulation, said external conductive sheath acting as an electrode and the other electrode being the conductor of the power cable, and applying a data signal to said power cable via said capacitor.

According to a further aspect of the invention there is provided a tuned coupler arrangement for facilitating the transfer of data signals to and from a power carrying conductor whereby signals transmitted at high frequencies onto a cable line have minimal attenuation, said apparatus comprising a capacitor formed by using the conductive core of a power cable line and an external electrode, the external electrode being disposed over the insulation of a power cable line whereby the insulation of the power cable line acts as a dielectric, an inductor coupled to the external electrode forming a tuned coupler arrangement whereby at resonance the reactance of said inductance and capacitance substantially cancel or offer minimal impedance to data signals coupled from instrumentation to and from said power cable via said coupling unit.

Conveniently, two separate coupling units may be coupled together to provide single-phase coupling. Alternatively, in a three-phase communication system a coupling unit may be associated with each power cable line and the coupling nits connected to form a common isolated neutral point via a downhole load to facilitate the transfer of data transmission signals to said three- phase power cables.

Conveniently, the three-phase connection may be a star or delta configuration.

Alternatively, a data signal may be connected to the power conductor by making a connection between a tuned coupler and the earthed metal work. Conveniently, the capacitance per unit length of cable may be increased by joining two or three electrodes together and placing them in series with a single coil.

According to a further aspect of the present invention there is provided apparatus for implementing bidirectional data transmission on a power-carrying cable, said apparatus comprising: a first coupling means coupled to the power-carrying cable at a first location along its length for receiving and transmitting data signals onto said cable, a second coupling means coupled to said cable and spaced from said first coupling means, said second coupling means being arranged to transmit and receive data along said power cable, each of said first and said second coupling means including respective first and second capacitive couplers each of which comprises a dielectric provided by the insulation of the power cable, and electrodes provided by the power cable conductors and external electrodes, the external electrodes being adapted to be coupled to a source of data whereby data can be transmitted or received by said first and second coupling means to travel along said conductors of the power cable.

Conveniently, each coupling means includes an inductive coil to create a tuned coupler. Preferably, a single carrier frequency is used on a single-phase coupling or a three-phase coupling.

Alternatively, a single-phase coupling can be used to transmit up/down on one pair of lines and a second single-phase coupling can be used to transmit down/up on a second pair of lines. Conveniently, these transmission are the same carrier frequency and on different pairs of lines. Alternatively, these transmissions are at different frequencies.

The first and second coupling means are the two ends of a link for one power cable and one, two or three of each capacitive coupler means can be provided depending on how the system operates. Conveniently, cable capacitors are used downhole and ordinary high voltage capacitors are used at surface.

Further coupling means coupled to the power-carrying cable spaced from the first and second coupling means may be used to connect to separate pieces of instrumentation.

Two single-phase couplings between the same two lines can transmit signals at different frequencies.

More than one coupler may be provided per line, with signals sharing the same two lines using different frequencies and/or operating at different times.

Conveniently also, each coupler is coupled to instrumentation via rectifying regulating means, said instrumentation also being coupled to switch means which actuates the switch means in response to instrumentation signals whereby data transmission signals are transmitted via said couplers along said cable lines.

Preferably, said couplers are located in a downhole power cable whereby a data signal is transmitted along said power cable lines between the surface and downhole instrumentation, for example to measurement instrumentation associated with an electric submersible pump.

Conveniently, the data signal carried up the hole is achieved by varying power consumed downhole from the carrier signal transmitted from the surface whereby variation in electrical power which is consumed is detectable at the surface.

Alternatively, data transmission along said power cable down the hole can be readily achieved by varying the carrier frequency using FSK modulation or other suitable encoding schemes.

In a system- for supplying instrumentation electrical power to downhole equipment which uses single-phase or three-phase power, the improvement comprising each cable line carrying data having a series tuned coupler at the surface and downhole and each tuned coupler at surface is substantially the same as the downhole couplers but instead use conventional capacitors.

Conveniently, at the surface the series tuned couplers are connected via three-phase carrier power generators to a common isolated neutral point and between the three-phase isolated neutral point and each carrier power generator is located a power sensor from which downhole data measurements are communicated to surface.

It will also be appreciated that each of the foregoing aspects of the invention associated with the apparatus will have corresponding associated methods involved in the transmission of data signals on the same conductor as carrying electrical power signals for both unidirectional and bidirectional data transmission.

Conveniently, it will also be understood that the aspects of the invention are not limited to use with power cables for use in wells or similar environments.

These and other aspects of the present invention will be become apparent from the following description when taken in combination with the accompanying drawings in which:

Fig. 1 is a diagrammatic view of part of an oil well with an ESP and motor located in the well downhole of a packer;

Fig. 2 is a schematic circuit diagram showing the three-phase electrical connections from the surface to an ESP;

Fig. 3 and 3a depict an embodiment of the present invention where an electrically conducting electrode is placed over the insulation of an individual ESP power cable line forming a capacitor between the electrodes and the power conductor itself;

Fig. 4 depicts a preferred embodiment of apparatus in accordance with the present invention comprising an oil-filled coupling unit in which each of three power cable lines are converted into capacitors; Fig. 5 depicts a second embodiment of the invention in which a tuning coil is coupled in series with the capacitor shown in Fig. 3 to form a tuned coupler to minimise impedance to the data transmission signal and effect efficient line coupling;

Fig. 6a depicts a block diagram of a circuit arrangement in which two tuned couplers, as shown in Fig. 5, are connected to provide single-phase data transmission coupling from downhole;

Fig. 6b depicts a block diagram which shows a "star connection" for generating a data signal in a three-phase form;

Fig. 7 depicts a schematic circuit diagram of a data transmission system in accordance with the present invention using a twin coupler configuration for making bidirectional connections;

Fig. 8 is a circuit diagram of a surface coupler for use with the downhole coupling equipment, and

Fig. 9 is a detailed circuit diagram of the power sensor and carrier generator shown in Fig. 8.

Reference is first made to Fig. 3 of the drawings which illustrates the principle of a central element of the present invention. A cable conductor 40 has a sheath of cable insulation 42 coaxially located thereover and on top of the cable insulation 42 is located a conducting electrode 44 thereby forming a capacitor between the electrode 44 and the power conductor 40. Using typical power cables, the individual capacitors so formed are about 300 picofarads per metre of length as is confirmed by calculation or experiment. It is desirable, but is not essential, to make the electrodes substantially close and full circumferential fit to the cable insulation because this maximises the capacitance per unit length and reduces any electrical stress on the insulation. In the embodiment shown the electrode 44 is a close and full circumferential fit to the cable insulation 42. Because cable insulation materials are of a high electrical quality they make excellent capacitor dielectrics.

As will be later described data transmission equipment is connected to such electrodes 44 and transmits through the capacitors. The electrodes 44 are made of copper tape wound over the cable insulation but, as will be later described, there are various other ways of implementing the electrodes over the cable insulation.

Reference is now made to Fig. 4 of the drawings which depicts a practical arrangement for making a clean insulating oil-filled capacitor coupling unit, generally indicated by reference numeral 46, defined by a housing 47, in which each of three power cable lines 48,50,52 is made into a capacitor by respective coaxial electrodes 54,56,58 which are metallic almost-closed cylinders and are connected by suitable cables 60 to a feedthrough connector 62 in the housing 47 and then (for illustration purposes only) to the instrumentation package 64 via an instrument lead 66. A folded internal buffer/expansion tube 68 of considerable length is open to the well fluid 70, thereby allowing the internal pressure of the clean insulating oil in unit 46 to equalise with respect to the well fluid pressure. Dissimilar fluids have their interface trapped in the tube 68 , thereby preventing migration of conductive well fluids into the chamber 69 for a period of some years. In order to reduce the oil volume and the volume of the expansion tube 68 the unit 46 is largely filled with insulating solids, such as sand or glass beads.

While the oil-filled coupling unit is shown as being separate, it will be appreciated that the packer penetrator shown in Fig. 1 may have an extended housing to form such an oil-filled coupling unit, thereby eliminating a separate unit. While the capacitor electrodes ideally should be perfectly isolated from the surrounding fluid, in practice, some electrical leakage is permissible adding to the fault tolerance of the system.

In practice, the cable capacitors are limited in capacitance by considerations of length. The reactance of each capacitor is very high at low frequencies. At frequencies above several hundred kilohertz the reactance becomes quite small and direct coupling of data signals to the line conductor through the capacitors becomes feasible. However, at these high frequencies the cable itself considerably attenuates the signals on the way to the surface and this is unacceptable in long cables which can be 20,000 ft. (6,000m) in length.

This is overcome using a tuned coupler arrangement shown in Fig. 5 of the drawings, and generally indicated by reference numeral 72. Each tuned coupler comprises a tuning coil, generally indicated by reference numeral L, in series with each capacitor 46 so that, at the frequency of transmission, the coils (L) and capacitors (C) combine in series resonance to offer substantially zero impedance to the signals. This allows a satisfactory choice of frequency for efficient coupling to the line conductor 40 and relatively low loss to the surface. Frequencies in a range of a few tens of kilohertz to a few hundred kilohertz are useable. It will be understood that the coils L are of high quality design so that the coupler is effective.

There is usually also unwanted capacitance between the cable electrodes 44 and the instrument connecting wires and nearby metal work as indicated by the capacitance CP in Fig. 5. This unwanted capacitance both shifts the resonant frequency and causes loss of some signal power. In order to overcome this a second coil LP may be placed between the electrode 44 and earth 76 or a neutral point 92, as will be later described. The inductance of the coil LP is chosen so as to be in parallel tuned resonance with the stray capacitance CP at the communications carrier frequency. The parallel tuned circuit has a high impedance at the carrier frequency and so effectively removes itself from the series circuit. Another advantage of the tuned coupler 72 coupling is that it also filters out some of the high power electrical noise present on the cable.

The various circuit arrangements located downhole for facilitating data transmission along the line conductors will now be described. In order to facilitate understanding, each coil/capacitor pair is referred to as a coupler. It will be understood that the list of methods for connecting data hereinbelow described is not intended to be exhaustive but merely to indicate that the couplers permit advantageous configurations with a final choice in any case to be determined by consideration of cost, fault tolerance and cable length.

Reference is now made to Fig. 6a which depicts a first single-phase coupling arrangement for connecting a signal between two couplers on separate cable lines, generally indicated by reference numerals 78,80. This provides a satisfactory method of connection because transmission is of a consistent quality based on known cable characteristics.

For clarity, the downhole equipment is depicted as a voltage generator 86, although it will be understood that the actual downhole equipment may both generate and receive power. The length of cable between the couplers leading to the motor acts as an unimportant capacitive load and at the relatively high carrier frequencies used, the motor reactance is so high as to in effect disconnect the motor from the circuit. Consequently, only the couplers and cable leading to the surface need be considered.

An alternative arrangement of connecting data to the power conductors is shown in Fig. 6b where a data transmission signal is generated in three-phase form using the circuit arrangement which has three couplers 86, 88 and 90 a star configuration, although it will be understood that a delta configuration would also be possible. The term 'star' and 'delta' are being well known in the art of electrical power distribution. The couplers 86,88,90 are connected to the neutral point 92 via downhole equipment represented as voltage generators 94,96 and 98. The arrangement shown in Fig. 6b has substantial advantages in that: the signal carrier frequency is spread over three phases, thereby reducing electrical stress on all components; if one of the couplers fails the other two couplers are sufficient to operate in the aforementioned twin coupler manner, but with an increased power per coupler to maintain the same signal:noise ratio. It will also be appreciated that this method provides fault tolerance.

A third method of connecting data to the power conductors is to make a connection between one coupler and the earthed metal work. In such a case it is possible to make a single larger value capacitor by joining two or more electrodes together and placing them in series with a single coil or joining two or more complete tuned couplers. This method, however, has the disadvantage mentioned previously of depending on the electrical quality of the metal work, although the capacitor provides sufficient isolation to permit insulation testing.

In the foregoing description of coupling circuits, it will be understood that there is no particular distinction between data signals sent through the coupler and up to the surface equipment, known as an "uplink", and signals sent from the surface equipment to be received downhole, known as a "downlink". The almost universal purpose of the telemetry system, when used for ESP applications, is to provide an uplink for sending measurements, such as mentioned previously. However, there are also good reasons for using a downlink. Firstly, the downhole system itself must have electrical power with which to operate. As the ESP is not always running (for example its power is sometimes removed) and batteries are unacceptable for permanent downhole use, the downlink is the only source of instrumentation power. Secondly, the instrumentation itself can be controlled with a downlink: if the instrumentation contains a programmable circuit, the parameters it measures and the various associated measurement conditions such as sampling rates, gain and filter settings can all be altered via a downlink. Indeed, an instrumentation microprocessor can be completely reprogrammed from the surface. Finally, down hole actuators can be controlled and, in the simplest case, the downlink signal, whether present or not present, may, in fact, form the entire system.

The arrangement shown in Fig. 7 is apparatus for implementing a two-way transmission system which includes but is not limited to the use of separate couplers operating at different frequencies on different lines. In the arrangement shown in Fig. 7 a twin coupler configuration is selected and a single carrier frequency is used. The surface equipment (not shown in the interests of clarity) transmits sufficient carrier power to couplers 100,102 for some of the power to be used, passed through a full-wave bridge rectifier 104 and regulator, generally indicated by reference 106, to power the downhole instrumentation 108. In turn, the downhole instrumentation is connected to a modulator 110 which it controls and by which data is transmitted to the surface. This modulator is provided with a simple resistor 111 which is alternately switched into and out of the circuit in a well known way, for example by using a transistor 112. When the resistor 111 is switched into the circuit it consumes additional power from the carrier. The regulator 106 contains a regulating unit 107 of a type well known in the art and includes a smoothing capacitor 109 and an induction coil 113. The coil or choke 113 allows the switch 112 and modulator 110 to operate without short circuiting the capacitor 109. The variation in electrical power which is consumed is easily detected in the surface equipment, as will be later described. The means of encoding the digital data may be achieved using a well known frequency-shift-keyed (FSK) technique. By way of example only, if the carrier frequency is 50 kilohertz and meets the normal power requirement downhole, then the downhole link demands additional power by switching the modulator at 5 kilohertz alternately with switching at 6 kilohertz. The rate of alternation is a data bit transmission rate. If power demand at 5 kilohertz is taken as digital zero and demand at 6 kilohertz is taken at digital one and the alternation is five hundred times per second, then the up link is termed in the communications art as a 500 bits/second FSK link.

A data transmission downlink can also readily be achieved by varying the carrier frequency, such as in FSK modulation, for example between 49 kilohertz and 51 kilohertz. It will be understood that more sophisticated encoding schemes, like differential phase shift keying, are unnecessary because there is ample carrier power for reliable detection. Alternatively, the carrier power level itself can be switched at surface and the switching detected downhole in a manner which is exactly the reverse of that described above for the uplink. In all of these cases, the signal:noise ratio make the preferred scheme one in which the uplink and downlink data transmission occur alternately: this is known as "half duplex". To improve communications reliability the data may also be redundantly encoded and compressed before transmission and repeatedly transmitted by any of means well known in the communications art.

In order to complete the description of the invention, reference is now made to Fig. 8 of the drawings which depicts a three-phase surface coupler. This is also a high power three-phase electrical power system with high-voltage harmonics from 60 Hz. to 10 or more kilohertz and in each phase 114,116,118 there is located an optional parallel tuned circuit known as a "line trap" 120 which serves to filter unwanted power supply voltages at the carrier frequency. The series tuned couplers 122,124 and 126 act in the same way as the downhole couplers but use conventional capacitors C. Each power cable is coupled via a series tuned coupler 122,124,126 to a respective three-phase carrier power generator 128,130,132 which are, in turn, coupled to an isolated neutral point 134 via power sensors 136, one of which is shown in more detail in Fig. 9. In the circuit shown in Fig. 9 the single-phase carrier generator output is coupled via transformer 138 to switching circuit 140 which has two FETs 142,144 connected to a quadrature carrier oscillator 146. The FETs are thereby switched on and off alternately. The output of the circuit is conventionally taken across a load 148 by a data current sensor 150 and the output of this sensor provides the uplink data. The sensed current is synchronously rectified at carrier frequency by the action of the FET switching, which gives a high degree of noise rejection, single low-pass filtering in the circuit 150, then recovers the uplink FSK signal. In each series tuned coupler, the conventional capacitors are of sufficiently high capacitance that the coupling efficiency is high and the resonance is not particularly sharp. This means that the surface resonance frequency does not have to be kept exactly the same as the downhole resonance frequency and this provides an advantage as the main carrier frequency can be manually or automatically varied from time to time so as to maintain it at the optimum downhole frequency without altering fixed tuning components. This is used to account for ageing and temperature/pressure drift in the downhole couplers.

The transformer 138 or a separate transformer (not shown) also provides a fail-safe high voltage isolation function, with the primary of the transformer being connected to the electronic and data communications circuitry and the secondary to the series coupler.

Additional filtering components may be added to reject electrical noise including parallel-resonant circuits across the transformer secondary.

It will be appreciated that various modifications may be made to the apparatus hereinbefore described without departing from the scope of the invention. For example, the electrodes may be made of material other than copper tape. For example, they could be made of pairs of half cylinders of metal clamped over the cable insulation to make an almost closed cylindrical electrode or some cables may have their insulation sheathed in metal tubes like lead or copper: in such a case a section of the sheath can be isolated by removing rings of it and using the isolated section as an electrode in its own right. Furthermore, it will be appreciated that the electrode may, in turn, be insulated from the well fluids by means of many layers of electrical insulating tape. Alternatively, the electrodes may be insulated by plaiting in a suitable compound, such as epoxy resin or silicon rubber, and the electrodes can be insulated as part of the cable manufacturing process whereby additional cable insulation is moulded over the electrodes. The electrodes may be insulated by an enclosing chamber sealed from the well fluids and filled with insulating material, such as a compatible oil.

The power cable capacitor may be constructed by means of a metallic sheath solid or braided slid over the insulation of each power cable conductor, said sheath further insulated from the surrounding well fluid by additional insulations made up of any or all insulating compounds, tapes, rubbers, heat shrinkable tubing and dielectric oil.

The present invention allows two-way transmission to be achieved in various ways including the use of separate couplers operating at different frequencies.

It will also be understood that the invention disclosed herein has applications to fields other than ESP downhole links. Indeed, it can be applied to any link where isolation is desired. The main condition is that the insulation of the apparatus to be isolated forms substantially the capacitive part of a tuned coupler. For example, electrodes placed over the insulation of domestic power cable or overhead power lines and planar electrodes placed opposite each other across the walls of a plastic enclosure.

Advantages of the invention are that it allows bidirectional transmission of data and instrumentation equipment power over a single electrical cable, in either single-phase or three-phase mode, where the cable primary purpose is the transmission of large amounts of electrical power. One such capacitor and a separate direct connection such as an earth can be used to complete the circuit. A f rther advantage is that there is no direct contact with special high voltage connections required with the current carrying conductor which makes it especially useful in systems like oil well electric submersible pumps where a high reliability of the power system is essential. Furthermore, by transmitting across two or more such capacitors to make a circuit across two cable lines means that there is no need for an earth connection and this further improves the reliability and permits routine insulation testing. A further advantage of this arrangement in three-phase mode, that is providing three such capacitors and connecting across three lines, is that it provides a fault tolerant system by reducing electrical stress and providing redundancy. A further advantage of the invention is that the carrier frequency can be manually or automatically varied from time to time so as to maintain it at the optimum downhole frequency without altering fixed tuning components, thereby accounting for ageing and any drift due to temperature or pressure in the downhole couplers.

A further advantage is that the use of a single carrier to transmit data in each direction and instrumentation power allows single-tuned couplers and is generally economical with downhole components, which is important for the highest reliability. The advantage of using a tuning coil to cancel the reactance of a coupling capacitor is that it allows a very small coupling capacitor to be used, such as found on the said power cable capacitors.

Claims

1. Apparatus for transmitting data signals over the same conductor carrying electrical power to a load, said apparatus comprising providing a capacitor by using the insulation of a current-carrying conductor as the dielectric of the capacitor and the conductor and an external conductive sheath as capacitor electrodes.

2. Apparatus as claimed in claim 1 wherein two or three capacitors are used for two or three power lines in a three-phase system.

3. Apparatus as claimed in claim 2 wherein the two capacitors are used in a single-phase or a three-phase system.

4. Apparatus as claimed in claim 1 wherein three capacitors are used, one for each line in a three-phase system.

5. Apparatus as claimed in claim 1 wherein electrically conducting electrodes are disposed over at least one power cable line to form a capacitor between the power conductor and the electrodes.

6. Apparatus as claimed in claim 5 wherein the electrodes are a substantially close circumferential fit to the cable insulation.

7. Apparatus as claimed in claim 5 or claim 6 wherein the electrodes are applied over power cable lines in downhole wells.

8. Apparatus as claimed in any one of claims 5 to 7 wherein for transmitting data on a three-phase power supply to a downhole motor, electrodes are disposed over each power cable line and are located inside an oil- filled unit, with each electrode being coupled to an instrumentation package for communicating data signals to be transmitted over said power cables.

9. Apparatus for transmitting data signals over the same conductor as that carrying electrical power to a load, said apparatus providing a capacitor coupled to the power conductor whereby data signals can be transmitted to and from said power conductor via said capacitor.

10. Apparatus as claimed in claim 9 wherein the capacitor is provided by using the insulation of the power-carrying cable as a dielectric and an external sheath as the other electrode of the capacitor.

11. Apparatus as claimed in claim 10 wherein the capacitor is a separate capacitor.

12. A method of transmitting a data signal on a power conductor carrying electrical power to a load, said method comprising the step of creating a capacitive coupling from said power conductor by using the insulation of the power cable as the dielectric and providing an external conductive sheath on said insulation, said external conductive sheath acting as an electrode and the other electrode being the conductor of the power cable, and applying a data signal to said power cable via said capacitor.

13. A tuned coupler arrangement for facilitating the transfer of data signals to and from a power carrying conductor whereby signals transmitted at high frequencies onto the cable have minimal attenuation, said apparatus comprising a capacitor formed by using the conductive core of the power cable and an external electrode, the external electrode being disposed over the insulation of the power cable whereby the insulation of the power cable acts as a dielectric, an inductor coupled to the external electrode forming a tuned coupler arrangement whereby at resonance the reactance of said inductance and capacitance substantially cancel or offer minimal impedance to data signals coupled from instrumentation to and from said power cable via said coupling unit.

14. Apparatus as claimed in claim 13 wherein separate coupling units are coupled together to provide single- phase coupling.

15. Apparatus as claimed in claim 13 wherein in a three- phase communication system a coupling unit may be associated with each power cable line and the coupling units connected to form a common isolated neutral point via a downhole load and/or generator to facilitate the transfer of data transmission signals to said three-phase power cables.

16. Apparatus as claimed in claim 15 wherein the three- phase connection is a star or delta configuration.

17. Apparatus as claimed in claim 15 wherein a data signal is connected to the power conductor by making a connection between a tuned coupler and the earthed metal work.

18. Apparatus as claimed in claim 16 or claim 17 wherein the capacitance per unit length of cable is increased by joining two or three electrodes together and placing them in series with a single coil or coupling three tuned coils together.

19. Apparatus for implementing bidirectional data transmission on a power-carrying conductor, said apparatus comprising: a first coupling means coupled to the power-carrying conductor at a first location along its length for receiving and transmitting data signals onto said conductor, a second coupling means coupled to said conductor and spaced from said first coupling means, said second coupling means being arranged to transmit and receive data along said power cable, each of said first and said second coupling means including respective first and second capacitive couplers each of which comprises a dielectric provided by the insulation of the power cable, and electrodes provided by the power cable conductors and external electrodes, the external electrodes being adapted to be coupled to a source of data whereby data can be transmitted or received by said first and second coupling means to travel along said conductor.

20. A method as claimed in claim 19 wherein each coupling means includes an inductive coil to create a tuned coupler.

21. A method as claimed in claim 20 wherein a single carrier frequency is used on a single coupling or a three-phase coupling.

22. A method as claimed in claim 20 wherein a single- phase coupling can be used to transmit up/down on one pair of conductors and a second single-phase coupling can be used to transmit down/up on a second pair of conductors.

23. A method as claimed in any one of claims 19 to 22 wherein the transmissions are at the same or different carrier frequencies.

24. A method as claimed in any one of claims 19 to 23 wherein more than one coupler is provided per line, with signals sharing the same two lines using different frequencies and/or operating at different times.

25. A method as claimed in any one of claims 19 to 24 wherein each coupler is coupled to instrumentation via rectifying regulating means, said instrumentation also being coupled to switch means which actuates the switch means in response to instrumentation signals whereby data transmission signals are transmitted via said couplers along said conductors.

26. A method as claimed in claim 25 wherein said couplers are located in a downhole power cable whereby a data signal is transmitted along said power cables between the surface and downhole instrumentation to measurement instrumentation associated with power cables for an electric submersible pump.

27. A method as claimed in any one of claims 19 to 26 wherein the data signal carried up the hole is achieved by varying power consumed from the carrier signal transmitted from the surface whereby variation in electrical power which is consumed is detectable at the surface.

28. A method as claimed in any one of claims 19 to 26 wherein data transmission along said power cables, normally downlink, is readily achieved by varying the carrier frequency using FSK modulation or other suitable encoding schemes.

29. In a system for supplying instrumentation electrical power to downhole equipment which uses single-phase or three-phase power, the improvement comprising each cable line carrying data having a series tuned coupler at the surface and downhole and each tuned coupler at surface is substantially the same as the downhole couplers but instead use conventional capacitors.

30. A system as claimed in claim 29 wherein at the surface the series tuned couplers are connected via three-phase carrier power generators to a common isolated neutral point and between the three-phase isolated neutral point and each carrier power generator is located a power sensor from which downhole data measurements are communicated to surface.

31. A system as claimed in claim 29 wherein in a single- phase system the series tuned couplers are connected across two lines in said single-phase cable.

32. A system as claimed in claim 29 wherein in a single- phase system the series tuned couplers are coupled from any or all lines to earth.

33. A method of communicating over an electrical cable comprising the steps: modulating the data bit stream by known means; varying the instrumentation carrier power consumed by the transmitter in sympathy with said modulation, and synchronously detecting said carrier power at carrier frequency at the receiver to recover the modulated signed.

34. Apparatus for transmitting data signals over the same conductors carrying electrical power to a load in a downhole environment, said apparatus comprising providing a capacitor coupled to the cable, a coil coupled in series with the capacitor, and means for tuning the coil, capacitor or both so that the reactance of the coil and capacitor effectively carried at a predetermined frequency to provide efficient data signal communication through the insulator.