Abstract:
This invention relates to a data communication system/method for use in a downhole application wherein electrical energy is supplied over a multiple-conductor power cable to a motor assembly of a downhole tool such as an electric submersible pump. A power leg coupling interfaces a surface controller of a downhole instrument to the conductors of the tool&#39;s power cable. Uplink communication of telemetry data occurs via current modulation generated by the downhole instrument and interpreted by a surface controller. Downlink communication of downhole instrument data occurs over a different communication scheme supported by the downhole and surface controllers. Downlink communication scheme provides a supply of power to the downhole instrument. Protection of downhole electronics and continuity of communication is ensured in the event of a ground fault on the power cable. Both downlink and uplink communication frequencies are adaptive based on frequencies and voltages present on the power cable.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This United States application is the National Phase of PCT Application No. PCT/US2014/064982 filed Nov. 11, 2014, which claims priority to United States Provisional patent application Ser. No. 61/903,266 filed Nov. 12, 2013, each of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally relates to power supply and data communication systems for downhole tools or instruments. More particularly, this invention relates to a data communication system for a downhole instrument over a power cable. 
     Various communication systems exist for downhole instruments such as, but not limited to, electric submersible pump (“ESP”) gauges and surface controllers. 
     An ESP system includes a downhole motor and pump assembly, a surface- located control unit, and one or more downhole gauges or instruments. A three-phase AC power supply located at the surface provides an AC power signal over a three-conductor power cable to the downhole motor and pump assembly. Depending on the motor size and length of the power cable, the operating voltage of the motor can be very large. The three-phase AC power signal is coupled to the motor by a balanced inductor network having a neutral, ungrounded node. This node is referred to as the wye point of the motor or the downhole wye point. 
     The downhole instrument associated with the ESP measures physical parameters of the wellbore such as temperature and pressure. The telemetry data that represents those physical parameters must be communicated to the control unit and various schemes for doing so have been implemented. Because the instrument and its control circuitry include sensitive electronic components, they must be protected from high voltage events such as those that occur during a ground fault. Most of these ESP systems use large inductive isolation chokes—which have the disadvantage to limit the data transfer rate —but also make use of direct current power supplies, which can cause operations to stop in case of a ground fault on the power cable. For example, where DC power is tapped from the wye point of the motor, a ground fault can lead to higher than desired power levels at the wye point, thereby jeopardizing the instrument&#39;s sensitive electronic components. 
     Some systems couple the downhole instrument to the motor wye point and provide a surface-located AC power supply to generate power at a higher frequency than the motor power supply frequency. These systems require high voltage capacitors located between the downhole instrument and the motor wye point (see e.g. U.S. Pat. No. 7,982,633 B2 to Booker et al. and U.S. Pat. No. 8,138,622 B2 to Layton et al.). The capacitors are large in size, expensive, and have uncertain reliability. 
     Some systems protect the downhole electronics from high AC voltage using semiconductor devices by adding circuitry below the wye point which makes use of a diode (and associated voltage clamp) that conducts during positive polarity voltage and a silicon-controlled rectifier (and associated resistor) that conducts during application of a negative polarity voltage to the instrument (see e.g. U.S. Pat. No. 8,149,552 B1 to Cordill). However, those systems still require the use of a large inductive choke (e.g., in a range of about 80 H or greater), and the semiconductor devices only function to keep the choke current balanced during ground fault conditions (see also e.g. U.S. Pat. No. 6,176,308 B1 to Pearson). 
     A need exists for a system which eliminates the need for large inductive isolation chokes and high voltage capacitors while still protecting the downhole instrument and allowing the downhole instrument to operate and communicate during ground fault conditions. 
     SUMMARY OF THE INVENTION 
     A power and bi-directional data communication system for a downhole instrument made according to this invention makes use of a megger test diode located below the wye point of a downhole motor assembly and a high voltage protection circuit located after the megger diode. The megger diode blocks current in case a negative voltage is applied to it which happens during a megger test. A downhole wye point sensor analyzes the voltage and frequency seen after the diode and listens for downlink communication between the downhole instrument and its surface controller. The downhole instrument&#39;s electronics are protected against any high voltage event by the high voltage protection circuit. The circuit allows the use of low voltage components at the output of the circuit, thereby limiting reliability issues and component cost. 
     During a ground fault, the circuit limits the voltage at its output to a lower value (preferably no greater than 80 V) and, therefore, protects the downhole electronics while still allowing communication with the surface controller during all positive cycles of the current waveform. 
     In a preferred embodiment, the high voltage protection circuit is a circuit having means such as a Zener diode or its equivalent to set or limit the voltage. At least one power semiconductor or an arrangement of power semiconductors (which can be several SiC FETS) see the voltage drop and dissipate significant power. Two or more stages of the protection circuit can be connected in series to distribute the voltage drop and power dissipation over the two or more stages. 
     An alternate embodiment of the high voltage protection circuit eliminates use of the Zener diode and instead uses a detection circuit that opens the connection between the downhole wye point and the downhole instrument when the downhole wye point voltage exceeds a predetermined value. 
     Uplink communication of telemetry data is generated by the downhole instrument by means of current modulation and is supported by the surface controller. By sensing voltage at the downhole wye point, the downhole electronics can perform frequency and voltage assessment. The current modulation passes through the high voltage protection circuit allowing for communication even during ground fault conditions. 
     The surface controller is AC-coupled to the multiple conductor power cable of the motor assembly and provides power to the downhole instrument by generating an AC power signal. Alternatively, during ground fault conditions the downhole instrument may be powered directly from the voltage generated at the downhole wye point. 
     Downlink communication occurs over a different communication scheme by modulating the frequency, the amplitude (or both frequency and amplitude) of the power supply generated by the surface controller. The surface power system is capable of analyzing the voltage signal at a surface wye point and adjusting the frequency of power transmission in order to avoid downstream communication interference caused by sources such as the downhole tool&#39;s (e.g., electric submersible pump (“ESP”)) variable speed drive (“VSD”). 
     The power and bi-directional data communication system eliminates the need for large inductive isolation chokes (e.g.  80  H or greater) or high voltage capacitors (e.g., 200 V or greater). 
     The objectives of this invention include: (1) limiting the downhole instrument&#39;s internal electronics&#39; input voltage by means of advanced semiconductor arrangements and without the use of expensive and large high voltage capacitors and (2) providing a communication system for use with a downhole instrument that (i) is reliable, cost competitive, and immune to ground faults; (ii) provides relatively high transfer rates (&gt;200 bps) for uplink communication; (iii) adapts upstream carrier frequency based on noise conditions; (iv) provides a downlink communication signal; and (v) adapts power signal frequency based on VSD conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the surface equipment in a preferred embodiment of the system. 
         FIG. 2  is a schematic of the power leg coupling between the surface controller and the power cable of a downhole tool motor assembly. 
         FIG. 3  is a block diagram and schematic of the AC power supply of the surface controller. 
         FIG. 4  is a block diagram of the downhole instrument located below the motor. 
         FIG. 5  is a schematic of an embodiment of a single stage high voltage protection circuit. 
         FIG. 6  is an example of the voltage seen at the output of the high voltage protection circuit. 
         FIG. 7  is a schematic of an embodiment in which two or more stages of high voltage protection circuits are used. 
         FIG. 8  is a schematic of an alternate embodiment of the high voltage protection circuit. A detection circuit opens the connection between the downhole wye point and the downhole instrument when the downhole wye point voltage exceeds a predetermined value. 
       Elements and Element Numbering Used in the Drawings 
       
           
             10  Gauge interface 
             11  Surface AC power supply or source 
             13  Surface receiver 
             15  Surface controller 
             20  Power leg coupling 
             30  Power cable 
             31  Conductor 
             33  Filter 
             40  Wye point 
             50  Megger test diode 
             60  Downhole instrument 
             61  Wye point sensor 
             63  High Voltage protection circuit 
             65  Transmitter 
             67  Zener diode 
             69  Power semiconductor 
             71  High voltage detection circuit 
             73  Commutation semiconductor 
         
      
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A bi-directional data communication system made according to this invention uses a high voltage protection circuit to pass a constant current to a downhole instrument during positive cycles of the power waveform even during a ground fault condition. The circuit lies between a megger test diode located below the downhole wye point of the motor and the downhole instrument. In one embodiment of the protection circuit, the voltage is limited by a Zener diode and a semiconductor arrangement dissipates power. In another embodiment of the protection circuit, the voltage is limited by a detection circuit which opens when a high voltage event occurs. The system limits voltage to the instrument but still allows current to pass to the instrument for communication during a ground fault. 
     Referring to  FIGS. 1 to 8 , a bi-directional data communication system for a downhole instrument  60  associated with a downhole tool such as an electric submersible pump (“ESP”) modulates a current of the downhole instrument  60  through a high voltage protection circuit  63  for uplink communication and modulates a power waveform of the surface AC power source  11  for downlink communication to the instrument  60 . Means such as a transmitter  65  generates a current modulated signal that encodes data collected by the sensors of the downhole instrument  60 . The current modulation occurs on positive cycles of the power waveform. The output frequency of the surface AC power source  11  can be dependent on power cable spectrum components measured at a surface. Modulation carrier frequency of the downhole instrument  60  can be dependent on power cable spectrum components measured at the downhole three-phase wye point  40 . 
     The instrument  60  is coupled to the surface AC power source  11  through a multi- conductor power cable  30  of a downhole tool. A megger test diode  50  is located below a wye point  40  of the downhole tool. At least one high voltage protection circuit  63  is connected to the megger test diode and includes means such as a Zener diode  67  or its equivalent for setting the limiting voltage seen by the electronics of the downhole instrument  60 . The circuit  63  includes means such as one or more power semiconductors  69  for dissipating power. A wye point sensor  61  senses the voltage and frequency downstream of the megger test diode  50  and provides frequency assessment prior to uplink communication. 
     Preferably, the inductive isolation choke used in connection with this invention is in a range of 2.5 to 3 H to filter high frequency spikes. The system does not require the use of large inductive isolation chokes (e.g., 80 H or above) or high voltage capacitors (e.g. 200 V or above). Referring first to  FIGS. 1 &amp; 2 , a gauge interface  10  includes a surface AC power supply  11 , receiver  13 , and controller  15  in communication with the receiver  13 . The surface controller  15  powers the downhole instrument (e.g., an electric submersible pump (“ESP”) gauge) and provides an interface for communication with the downhole instrument. 
     A power leg coupling  20  interfaces the surface controller  15  to the conductors  31  of the power cable  30  connected to a motor assembly of a downhole tool. The power leg coupling  20  makes use of capacitors and inductors to create a high pass filter  33  that attenuates high voltage drive frequencies (&lt;100 Hz). 
     The surface power supply  11  provides power to the downhole instrument and an AC-DC converter/regulation stage (see  FIG. 3 ). Power cable impedance (which rises with frequency) attenuates leakage on the power supply  11  in case of a ground fault, thereby allowing some current to flow to the downhole instrument for its proper operation. 
     The surface receiver  13  analyzes the current drawn by the downhole instrument and looks for specific current patterns or frequencies to discriminate uplink communication signals from noise. Downlink communication is done by modulating the frequency, amplitude, or both frequency and amplitude of the power supply  11 . 
     The surface controller  15  manages the power supply  11  and controls the frequency and amplitude of the voltage being generated. The surface controller  15  also analyzes uplink telemetry data and provides data to a user through means well known in the art. Both power supply and uplink telemetry frequencies can be changed based on noise and operating conditions on the power cable  30 . The noise spectrum on the power cable  30  can be analyzed by embedded systems using well known methods (such as Fourier transform or digital filtering). 
     The downhole instrument  60  is coupled to the motor assembly&#39;s wye point  40  through a megger test diode  50 . This diode  50  blocks current when negative voltage is applied, which happens during a megger test (see  FIG. 4 ). Preferably, the diode  50  is a 10 kV diode (thereby accommodating a 5 kV megger test). A downhole wye point sensor  61  analyzes the voltage and frequency seen after the diode  50  and listens for downlink communication between the downhole instrument  60  and the surface controller  15 . The downhole instrument&#39;s electronics are protected against any high voltage event by a high voltage protection circuit  63 . The circuit  63  allows the use of low voltage components at the output of the circuit  63 , thereby limiting reliability issues. 
     In the preferred embodiment, the circuit  63  is comprised of multiple stages (see  FIG. 7 ). An alternative embodiment of the circuit  63  operates by opening the connection between the downhole wye point  40  and the downhole instrument  60  when the downhole wye point  40  voltage exceeds a predetermined value (see  FIG. 8 ). 
     The downhole instrument  60  utilizes sensors to acquire environmental parameters such as, but not limited to, pressure, temperature, and vibration and then converts the acquired sensor data into a data stream readable by the surface controller  15 . A downhole transmitter  65  modulates the current drawn by the instrument  60  from the power supply  11 . This modulated current represents the sensor data collected by the downhole instrument  60 . 
     Normal Mode of Operation 
     When no ground fault occurs along the power cable  30  and when imbalances are low, the voltage seen at the downhole wye point  40  is comprised of the power supply signal being generated by the surface controller  15  less any losses in the power cable  30  and motor windings. Due to the presence of the megger test diode  50 , the downhole instrument  60  is limited to draw the current required for its operation and telemetry only during the positive cycles of the current waveform from the surface power supply  11 . The negative cycles are not used. This current waveform is composed of the loading of the instrument&#39;s power supply, motor winding losses, cable losses, and the downhole instrument&#39;s telemetry current modulation. 
     Ground Fault Mode of Operation 
     When a ground fault occurs along the power cable  30 , the voltage seen at the wye point  40  is dominated by the motor supply voltage (which can be several thousand volts). The circuit  63  limits the voltage seen by the electronics at its output to a lower value (preferably no greater than 80 V, see  FIG. 6 ) and protects the downhole electronics while still allowing communication with the surface controller  15  during all positive cycles of the current waveform. The negative cycles are not used. For uplink communication, the downhole instrument  60  modulates the current being drawn through the high voltage protection circuit  63  during positive power cycles. 
     In a preferred embodiment of the high voltage protection circuit  63  (see  FIG. 5 ), a Zener diode  67  sets the limiting voltage. At least one power semiconductor  69  or an arrangement of power semiconductors  69  (which can be several SiC FETS) see the voltage drop (which can be several thousand volts) and must be able to dissipate significant power. Several stages of circuits  63  may be connected in series in order to distribute the voltage drop and power dissipation over the stages (see  FIG. 7 ). 
     In an alternate preferred embodiment of circuit  63  (see  FIG. 8 ), a high voltage detection circuit  71  and one or more additional commutation semiconductors  73  replace the Zener diode  67  arrangement. Upon detection of a high voltage event, the detection circuit  71  opens the high voltage protection circuit  63 . Similar to the other embodiment of circuit  63 , this embodiment limits the voltage seen by the electronics at its output to a lower value (preferably no greater than 80 V) and protects the downhole electronics while still allowing communication with the surface controller  15  when the circuit is closed and during positive cycles of the current waveform. Method of Use 
     Referring to  FIGS. 1 to 8 , a method of bi-directional data communication for a downhole instrument  60  includes the steps of modulating a current of the downhole instrument  60  through a high voltage protection circuit  63  for uplink communication and modulating a power waveform of the surface AC power source  11  for downlink communication. The telemetry carrier frequency of the downhole instrument  60  can be dependent on power cable spectrum components measured at the downhole three-phase wye point  40  with the modulating step occurring on a positive cycle of the power waveform. The output frequency of the surface AC power source  11  can be dependent on power cable spectrum components measured at a surface. 
     The method can also include the step of blocking a current to the downhole instrument  60  when in a negative voltage condition. The blocking step can be accomplished by a megger test diode  50  located between the wye point  40  of a motor assembly of a downhole tool and the high voltage protection circuit  63 . Voltage and frequency is sensed downstream of the megger test diode  50  and clear assessment of frequency prior to uplink communication is done. 
     The preferred embodiments described above are not all possible embodiments of the invention. The invention is defined by the following claims and the full range of equivalency to which each element of the claims is entitled.