Abstract:
A data communications system and associated method of data transmission for transmitting data over a three phase power system between a surface and a sub-surface location for an arrangement such as an oil field electrical submersible pump, on each of the three conductors of a three phase cabled connection with an isolation mechanism operable to isolate any given conductor when a fault associated with that conductor is detected. With the system providing a separate AC signal and data transfer on each conductor isolation can be achieved while enabling continued operation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to data transmission to and from down hole equipment and in particular, though not exclusively, to an improved data communication system and a method of data transmission through a three phase power system between the sub-surface and a surface location. 
     “Down hole equipment” is understood to refer to any tool, equipment or instrument that is used in a wellbore. 
     Data needs to be transmitted between down-hole equipment and the surface for various reasons. For example, monitoring performance of motors/pumps; transmission of control signals for control of valves; measuring device orientation and position, and making physical measurements. 
     For motorised down hole equipment, such as an Electric Submersible Pump (ESP) motor system, data needs to be sent from below the equipment in a circuit that includes motor windings and the equipment&#39;s power cable which can be considered as a three phase power system. In such arrangements, as power cables are already present, there is the rationale that the cost of the solution of using these should be proportionately less than a solution where an appropriate length of communication cable is also supplied. It is also generally accepted that being able to maintain power on the down hole monitoring instrumentation when the main three phase power system is not powered up is needed, as this provides essential information in the event of pump shut downs or other major events in the well. 
     Thus these systems are challenging to design and operate to ensure data is successfully transmitted and an independent power supply is maintained at all times. 
     Due to the motor and power cable properties of a three phase power system, DC current based devices which are coupled to the power system using inductive couplings have been developed and are extensively used. Power is provided from a low current DC power supply at surface and data is transmitted to surface by modulating the current drawn from this supply. 
     Examples of digital and processor based devices are disclosed in U.S. Pat. No. 5,515,038; GB2283889 and U.S. Pat. No. 6,396,415. These systems utilise DC current injected onto the power signal and extracted through inductive Y-point couplings. These systems are all susceptible to failure when insulation on the power cable is lost or damaged, as any fault is in parallel with the independent power source, and the fault becomes another current modulation source thus causing signal integrity to be lost. These prior art systems are also typically either analogue in nature, thus introducing noise into the measurements or, where digital data is transmitted, it is at a very slow data rate. 
     AC based systems which make use of AC power and/or signal transmission have been developed to overcome these problems. However, these AC based systems introduce disadvantages of their own. A typical prior art AC based system is disclosed in U.S. Pat. No. 7,982,633 being a data communication system for use in down hole applications wherein electrical energy is supplied over a multiple-conductor power cable to an ESP motor assembly. A down hole unit is AC-coupled to the conductors of the power cable through the wye point of the ESP motor assembly. A surface unit is AC-coupled to the conductors of the power cable. Uplink communication of telemetry data occurs over an AC communication scheme supported by the down hole unit and the surface unit. Downlink communication of remote control command data occurs over a different AC communication scheme supported by the surface unit and the down hole unit. These AC communication schemes provide an independent supply of power to the down hole environment. All communication between the surface and down hole environment is accomplished through the power cable without the use of additional communication lines. Data communication is maintained in the event of a ground fault on the power cable. 
     The expressed intention of such prior art AC based systems is to operate when the insulation on the power cable is damaged or at least imperfect. However, a disadvantage of these systems is that when a fault in the ground insulation exists the load presented by the ESP power system may be excessively high. When such an excessive load fault exists it is necessary to completely disable the system in order to protect the instrument power system from the excessive power supply load and reduce loading on signal transmission. In U.S. Pat. No. 7,982,633 there is disclosed an arrangement where high pass filtering is used to remove the low frequency ESP motor power, typically around 25-60 Hz, applied across the down hole unit. In practice, however, the insulation fault will also pull the surface and down hole star points down toward the shorted phase. This in turn loads the downhole signal driver and consequently attenuates the recovered data signal. The level of attenuation may be sufficient to render the signal as irrecoverable and thus the system cannot function. It also loads the surface power supply and if the current loading is too high this may collapse also rendering the system inoperable. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a system and method of data transmission for transmitting data over a three phase power system where the system can remain operable in the event of a heavily loaded fault. 
     According to a first aspect of the invention there is provided a data communications system for transmitting data over a three phase power system on a multi-conductor power cable between a surface and a sub-surface location, said data communications system comprising: a surface system module provided with a power supply providing an AC power signal coupled to each conductor of the power cable; a sub-surface system module providing a first powered module and a second powered module wherein the second powered module provides a data communications signal coupled to each conductor; the multi-conductor power cable being provided between the surface system module and the sub-surface system module to transmit the power and data communication signals; and wherein the surface system module includes an isolation unit configured to selectively isolate each conductor of the power cable. 
     In this way, by providing such a data communications system, whereby any given conductor can be isolated should a system overload on a given phase occur, the system can continue to provide data and power transmission. 
     In an embodiment, the isolation unit comprises an isolation device on each conductor. In an embodiment, each isolation device is a relay. Alternatively, each isolation device is a solid state switch. In this way, power is prevented from operating on any selected conductor, independent of any other conductor. 
     In an embodiment, the isolation unit comprises a detection mechanism operable to detect a system fault. In an embodiment, the detection mechanism is operable to activate the isolation devices. In an embodiment, the detection mechanism comprises a current sensor on each conductor. By providing a detection mechanism to detect a system fault which can create a system overload, action can be taken to isolate the associated conductor prior to any system damage occurring. 
     In an embodiment, the isolation unit comprises a tuned circuit on each conductor. In this way, the frequency of the three phase power supply can be selected to be different from the frequency of the AC power signal. In an embodiment, the sub-surface module includes a tuned circuit on the motor star point to isolate the second powered module from the first powered module. 
     In an embodiment, the first powered module is a motor assembly. In an embodiment, the first powered module is an ESP motor system. 
     In an embodiment, the second powered module is a monitoring system. In an embodiment, the monitoring system comprises one or more gauges/sensors and the data communication signal comprises data from the one or more gauges/sensors. 
     In an embodiment, the three phase power signal is used to power the first powered module and the AC power signal is used to power the second powered module. In this way, isolating a conductor will still allow the AC power signal to reach the second powered module and transmit data to the surface. 
     In an embodiment, a frequency of the three phase power signal is in the range 20-60 Hz. In an embodiment, a frequency of the AC power signal is in the range 2-5 KHz. In this way, if the multi-conductor has three conductors, for example, isolating one or two conductors will still allow the AC power signal to reach the second powered module. 
     In an embodiment, the power and data signals on each conductor are identical. In this way, isolating one conductor still provides other conductors on which the data and power can be transmitted. Additionally, even if a majority of the conductors are isolated, an AC power signal can still reach the gauges down hole and send data signals to the surface. 
     According to a second aspect of the invention there is provided a method of data transmission for transmitting data over a three phase power system between a surface and a sub-surface location, the method comprising the steps of: (a) providing a three phase power multi-conductor cable connection from the surface to the sub-surface system; (b) coupling an AC power signal equally to each conductor of the three phase power cable connection; (c) coupling a data signal to each conductor of the cable connection; and (d) selectively isolating a conductor of the three phase power cable connection. 
     In this way, any given conductor can be isolated should a system overload on that given conductor occur, so that data and the AC power transmission can be maintained. 
     In an embodiment, the method includes the step of determining a fault on each conductor. In an embodiment, the conductor is isolated in response to the fault determination. In this way, damage is prevented to the sub-surface system when a fault occurs. The step of determining the fault may include the step of monitoring current on each conductor. 
     In an embodiment, the method includes the step of tuning the frequency of the three phase power supply to be different from the frequency of the AC power signal. In this way, an ESP motor system can be powered separately from a monitoring system at the sub-surface location. In an embodiment, the frequency of the three phase power signal is in the range 20-60 Hz. In an embodiment, the frequency of the AC power signal is in the range 2-5 KHz. 
     In an embodiment, the method includes the step of collecting data at the sub-surface location, the data being transmitted as the data signal. 
     In an embodiment, the AC power signal and data signals on each conductor are identical. In this way, for a three conductor cable, for example, isolating one conductor still provides two conductors on which the data and power can be transmitted. Additionally, even if two conductors are isolated, an AC power signal can still reach the gauges down hole and send data signals to the surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows the typical set up of a down hole equipment in a well, showing the positions of the equipment, the motor and the control interfaces at the surface; 
         FIG. 2  shows a schematic block diagram of a data communication system according to a first embodiment of the present invention; and 
         FIG. 3  shows a schematic block diagram of a data communications system surface module according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     One category of down hole equipment is artificial lift systems, for use in wells where there is insufficient pressure in the reservoir to lift the well&#39;s fluid (e.g. oil, water or gas) to the surface. Types of artificial lift systems include hydraulic pumps, Rod pumps, Electric Submersible Pumps (ESPs), Jet Pumps, Progressing-Cavity pumps (PCPs) and gas lift. 
     Reference is initially made to  FIG. 1  of the drawings which illustrates a typical ESP completion in a wellbore. An ESP motor  10  is coupled through a seal  12  to a centrifugal pump  14  and used to lift the fluids through a tubing  16  to a surface  18  of the well  20  in a manner known to those skilled in the art. In order to monitor the operation, sensors or gauges  22  are located below the ESP  10 . Typically, the motor  10  is a three phase Y configuration. The motor is driven by a variable speed drive system  24  and is connected via a three phase power cable  26  having three connectors. The system can be considered to comprise two distinct parts, a surface system, generally indicated by reference numeral  28 , and a down hole system, generally indicated by reference numeral  30 . These two parts  28 , 30  communicate using the ESP power cable  26 . 
     Surface equipment relating to the gauge system is shown in  FIG. 1  where there is a HV unit  13  connected directly to the three phase power supply and to the down hole motor. There is a further LV or low voltage unit  8  which is safely isolated from the high voltage system. The LV system is primarily for data recovery and processing and data display etc. The HV unit is used to inject AC power and also make recovery of raw data from the three phase power system. 
     Referring now to  FIG. 2  of the drawings there is illustrated a functional block diagram of a data transmission system, generally indicated by reference numeral  40 , according to an embodiment of the present invention. In this arrangement data can be transmitted onto the three phase power cable  26  in either direction between the surface equipment  28  and subsurface or down hole equipment  30 . 
     At surface  28  the equipment is divided into a high voltage side  32  and a low voltage side  34 . The high voltage side  32  provides the power to the down hole system  30 . Tuned high-voltage AC coupling  36  is used to connect to each of the phases in the power cable  26 . Thus a tripling of circuitry is used in the high-voltage equipment  32 . A microprocessor  38  controls the power distribution on to the three-phase cable  26  and is linked to a corresponding microprocessor  41  on the low voltage side  34 . Additionally the high-voltage side  32  uses tuned high-voltage AC coupling  35   c , in parallel to pick off the data signals on the three-phase cable  26 . These signals are then filtered  42  and de-modulated  44  by known methods. Data signals then pass via the microprocessor  41  for display  46  or transport to a data logger or SCADA system. Additionally, the process can work in reverse where microprocessor  41  provides data on to the power lines  26  via the tuned high-voltage AC coupling  36  on the high-voltage side  32  as is known in the art. This can be achieved by modulation of the power frequency with a data pattern (FM), it could also be achieved with amplitude modulation of the power supply, and can be further enhanced by start and stop sequences of different amplitude and/or frequency to indicate start and end of messages. Frequency of surface power could be sequenced through a particular frequency pattern to differentiate the commands from normal power frequency adjustments. 
     Simple communication could be achieved by short interruptions to the power supply creating power pulses, which can be of differing pulse widths (PWM) or alternatively arranged in a particular pattern to signify particular commands. Power interruptions can be long enough to be detected at the down hole location but short enough so that power is not lost at the gauge. 
     Down hole an ESP system  48  is provided as described herein with reference to  FIG. 1 . Like parts have the same reference numerals to aid clarity. Below the motor  10  is a standard Y-point connector  50 . At the Y-point connector  50  is arranged a down hole system  52 . The down hole system  52  provides monitoring in the form of measurement devices sensors or gauges  54 , hooked up via a microprocessor  56 . Power to drive the gauges  54  is provided via tuned HV AC coupling circuits  37  to a power regulator  58 . Similarly, data from the measurement devices  54  is processed in the microprocessor  56 . Using a signal driver  60  and tuned HV AC coupling circuits  39 , the data is transmitted on to the power line  62  for transmission to the Y-point  50  and onward transmission up the three-phase power cable  26  to the surface units  28 . 
     In the present invention, a first AC power signal is generated at the drive system  24 . This is a three phase power signal which is typically large e.g. 2-3000 volts and 70-100 amps and at a low frequency, in the range 20 to 60 Hz. It is used to power the motor  10 . A second AC power signal is generated at the power driver  33  in the surface HV system  32 . This second AC power signal is modulated with any required data signal and passed onto each of the three conductors of the power cable  26 . The second AC power signal is at a single phase in contrast to the three phase first AC power signal. The second AC power signal is of a lower voltage and current with a higher frequency in the range 500 Hz to 5 kHz. The second AC power signal will pass through the wye point  50  and pass into the down hole system  52 . A tuned HV AC coupling circuit  37  at the input is tuned to prevent transmission of the first AC power signal which could damage the down hole instrumentation  54 . The power regulation circuit  58  will convert the second AC power signal into an appropriate form for powering the instrumentation  54 . Using this power, sensors and gauges  54  monitor conditions at and below the motor  10 . Data collected from the sensors and gauges  54  is modulated back onto each conductor of the cable  26  for return to the surface. 
     Reference is now made to  FIG. 3  of the drawings which illustrates an isolation unit  71  incorporated in the drive system  33  according to an embodiment of the present invention. Drive system  33  provides the first AC power signal  64  onto the three cable conductors  26   a ,  26   b ,  26   c  of the three phase power cable  26  via a star point  70 . This is a three phase supply as is known in the art. Each conductor  26   a ,  26   b  and  26   c  is provided with a current sensor  72   a ,  72   b ,  72   c , an isolator mechanism  74   a ,  74   b ,  74   c  which in this case are each a relay, and coupling components  76   a ,  76   b ,  76   c  respectively before being input to create high voltage cable connection  26 . In addition, to enable signal recovery, the conductors  26   a ,  26   b  and  26   c  each feed into a signal recovery system  34  via independent passive tuned circuits  35   a ,  35   b  and  35   c  respectively. The signal recovery system  35 , 42 , 44  may comprise components such as filters, amplifiers and demodulators (not shown) as is appropriate. 
     In use, a first AC power signal sufficient to power the motor  10 , is applied as a voltage at a selected frequency from the drive system  24 . 
     Also coupled to each conductor  26   a ,  26   b ,  26   c  is a second AC power signal, tuned to a second frequency and applied as a voltage from the power driver  33 . This is a single phase supply. The surface star point  70  enables the gauge system voltage  64  to be applied to each conductor  26   a ,  26   b  and  26   c  of the cable  26 . The current sensors  72   a ,  72   b ,  72   c  measure the current fed into each conductor  26   a ,  26   b ,  26   c  of the cable  26 . This second AC power signal is used to drive the gauges and sensors  54  down hole. The voltage applied will be identical on each conductor  26   a ,  26   b ,  26   c.    
     Further the surface low voltage system  34  is also connected to each conductor  26   a ,  26   b ,  26   c  via tuned HV coupling circuits  35   a ,  35   b ,  35   c . System  34  recovers the data from the gauges and sensors  54 . The data signal is modulated onto each conductor of the cable  26  downhole, via coupling circuits  39  and demodulated at surface as described herein before with reference to  FIG. 2 . 
     If a fault in the ESP power system, such as a fault in the ground insulation, exists, an excessive load can be created on one of the conductors  26   a ,  26   b  or  26   c . Upon detection of such an excessive load by current sensors  72   a ,  72   b  and  72   c  the associated isolator mechanism  74   a ,  74   b  or  74   c  is activated thus isolating the associated conductor  26   a ,  26   b  or  26   c  which the fault is affecting. In doing so, power is still provided to the sensors and gauges  54  and a data signal is still provided to signal recovery system  34  via the remaining two conductors from  26   a ,  26   b  or  26   c  and sufficient data is carried on the remaining two conductors to enable a data signal to be recovered whilst damage to the ESP system from the occurrence of an excessive load is minimised if not eliminated. Indeed, as the second AC power signal and the data signal is identical on each conductor  26   a ,  26   b ,  26   c  data can still be recovered if only a single conductor is operational. Such data could be important in determining the effect of the fault in the down hole environment. 
     As the signal recover circuit  34  and power driver  33  are provided with independent passive tuned circuits  76 ,  35 , the power and data signal coupling can be optimised for the frequency in use thus minimising interference between the power and data signal systems ensuring sufficient data signal is present to be recovered and converted into data. 
     The current sensors  72   a ,  72   b  and  72   c  may further be arranged to detect the occurrence of an insulation fault prior to the actual current levels of the system being affected. The current sensed  73  is also recorded at the microprocessor  38  so that the operation of an isolation mechanism  74   a ,  74   b  or  74   c  is recorded as an alert that a fault has occurred. 
     Such an isolation unit  71  is of particular use if an insulation fault is low resistance creating a ground short on one conductor effectively. When such a fault occurs, the load across the down-hole signal driver  60  increases thus attenuating the power and recovered data signal resulting in the gauge power failing and/or signal level dropping below a recoverable level. By detecting an effect of the shorting action occurring at the star point  70 , the appropriate conductor connection  26   a,b, c  can be isolated by isolator mechanism  74   a ,  74   b  or  74   c  thus reducing demand on the power supply and improving signal amplitudes and thus recoverable signal. 
     The principle advantage of the present invention is that it provides a system and method of data transmission over a three phase power system where isolating a conductor on which a system overload or ground fault has occurred can be implemented to protect the system whilst maintaining system operation. 
     A further advantage of the present invention is that it provides a system and method of data transmission over a three phase power system where system overload or ground fault occurrences are detected and isolation of the associated conductor is actioned to ensure ongoing operation of the system even in fault conditions. 
     Various modifications may be made to the invention herein described without departing from the scope thereof, for example whilst the isolation mechanism has been detailed as being a relay, it will be appreciated that a solid state switch or other similar component or components may be used. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.