Abstract:
A method of operating a pumping system includes an electric motor and a variable speed drive. The method includes the steps of setting a first preference for the operation of the motor, loading a first optimized motor model into the variable speed drive and controlling the motor in accordance with the first preference. The method continues with the steps of monitoring the performance of the motor and setting a second preference for the operation of the motor, wherein the second preference is different from the first preference. The method concludes with the steps of loading a second optimized motor model into the variable speed drive and controlling the motor in accordance with the second preference.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of electric submersible pumping systems, and more particularly, but not by way of limitation, to an improved control system for electric submersible pumping systems that include a permanent magnet motor. 
       BACKGROUND 
       [0002]    Pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, the submersible pumping system includes a number of components, including one or more electric motors coupled to one or more high performance pumps. In the past, large induction motors have been used to drive the pump. The induction, or “squirrel cage,” motors tend to be long. These long motors present deployment problems in certain applications, including deviated wellbores and surface applications with limited space. 
         [0003]    The electric motor is often driven by a variable speed drive located on the surface. The variable speed drive produces an alternating current that is transferred to the electric motor through a power cable. In many modern pumping systems, the variable speed drive produces a low voltage, pulse width modulated (PWM) current at a selected frequency. The waveform produced by the variable speed drive can be adjusted manually or automatically to adjust the operating parameters of the pumping system. Step-up transformers can be used to modify the output of the variable speed drive to the design voltage range of the motor. 
         [0004]    Recently, motor drives have been provided with have control features called “Field Oriented Control,” or “Vector Control”, that attempt to use the motor voltage and current information to identify motor rotor position. With this position information the drive can commutate the applied voltage in a way that yields better performance and a higher level of control than other open-loop drive techniques. In vector control schemes, the stator currents of the three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor (d), the other the torque (q). The control system of the drive calculates from the flux and torque references given by the drive&#39;s speed control the corresponding current component references. Vector control can be used to control AC synchronous and induction motors and can be used to operate a motor smoothly over the full speed range, generate full torque at zero speed, and have high dynamic performance including fast acceleration and deceleration. 
         [0005]    Although effective, the vector control algorithms are based on calculations using a motor model that is established during manufacture, prior to operation. In applications where the load varies during the service life of the motor, the static vector control algorithm does not always obtain optimal performance from the motor. Additionally, the existing control algorithms are set during manufacture so that the motor operates at a relatively constant efficiency or power output. Once the efficiency of the motor has been established, the power output from the motor is increased by making the motor longer. The inability to adjust efficiency and power output in the field presents a significant drawback in existing systems. There is, therefore, a need for an improved motor control system that is well-suited for use with permanent magnet motors and that provides a greater range of operational characteristics in the field. 
       SUMMARY OF THE INVENTION 
       [0006]    In an embodiment, the present invention includes a method of operating a pumping system that includes an electric motor and a variable speed drive. The method includes the steps of setting a first preference for the operation of the motor, loading a first optimized motor model into the variable speed drive and controlling the motor in accordance with the first preference. The method continues with the steps of monitoring the performance of the motor and setting a second preference for the operation of the motor, wherein the second preference is different from the first preference. The method concludes with the steps of loading a second optimized motor model into the variable speed drive and controlling the motor in accordance with the second preference. 
         [0007]    In another embodiment, the present invention includes a method of operating a pumping system that includes an electric motor and a variable speed drive. The method include the steps of setting a first preference for the operation of the motor, loading a first optimized motor model into the variable speed drive, controlling the motor in accordance with the first preference and monitoring the performance of the motor by comprises monitoring the theta ratio of the motor under a torque load. 
         [0008]    In another embodiment, the present invention includes an electric submersible pumping system that has a pump, a motor configured to drive the pump, wherein the motor is a permanent magnet motor, and a variable speed drive configure to control the operation of the motor, wherein the variable speed drive includes a field oriented control scheme. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a pumping system constructed in accordance with an exemplary embodiment. 
           [0010]      FIG. 2  is a functional block diagram of the motor control system of the pumping system of  FIG. 1 . 
           [0011]      FIG. 3  is a functional depiction of the motor of  FIG. 1  showing the direct (d) and quadrature (q) components of the current vector. 
           [0012]      FIG. 4  is a functional block diagram of an exemplar vector control scheme employed by the motor control system of  FIG. 2 . 
           [0013]      FIG. 5  is a process flow diagram for a motor control process. 
           [0014]      FIG. 6  is a process flow diagram for a method of optimizing the theta ratio of the quadrature (q) and direct (d) components of the current vector. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In accordance with exemplary embodiments of the present invention,  FIG. 1  shows a perspective view of a pumping system  100  attached to production tubing  102 . The pumping system  100  and production tubing  102  are disposed in a wellbore  104 , which is drilled for the production of a fluid such as water or petroleum. As used herein, the term “petroleum” refers broadly to all mineral hydrocarbons, such as crude oil, gas and combinations of oil and gas. The production tubing  102  connects the pumping system  100  to a wellhead  106  located on the surface. Although the pumping system  100  is primarily designed to pump petroleum products, it will be understood that the present invention can also be used to move other fluids. It will also be understood that, although the pumping system  100  of  FIG. 1  is depicted in a deviated or non-vertical wellbore  104 , the pumping system  100  and methods disclosed herein will find also utility in traditional vertical wellbores. 
         [0016]    The pumping system  100  includes a pump  108 , a motor  110  and a seal section  112 . The motor  110  is an electric motor that receives power from surface facilities  114  through a power cable  116 . When energized, the motor  110  drives a shaft (not shown) that causes the pump  108  to operate. The seal section  112  shields the motor  110  from mechanical thrust produced by the pump  108  and provides for the expansion of motor lubricants during operation. The seal section  112  also isolates the motor  110  from the wellbore fluids passing through the pump  108 . 
         [0017]    In exemplary embodiments, the motor  110  is a permanent magnet motor in which the rotor includes one or more permanent magnets. The permanent magnets within the rotor may be constructed of ferrite or rare earth magnetic materials. Suitable materials include neodymium and alloys of neodymium, iron and boron. The use of a permanent magnet motor creates a power dense motor  110  that can be made shorter than conventional induction motors. In other embodiments, however, the motor  110  is a squirrel cage, induction motor. 
         [0018]    The surface facilities  114  provide power and control to the motor  110 . The surface facilities  114  include a power source  118 , a variable speed drive (VSD)  120  and a transformer  122 . The power source  118  includes one or both of a public electric utility  124  and an independent electrical generator  126 . Electricity is fed by the power source  118  to the variable speed drive  120 . 
         [0019]    Turning to  FIG. 2 , the variable speed drive  120  includes a user interface  128 , system software  130  and drive software  132  that cooperatively control the operation of the drive power module  134 . The drive power module  134  receives input power from the power source  118 . During normal operation, the drive power module  134  produces a low voltage, pulse width modulated (PWM) voltage at a selected frequency. The output of the drive power module  134  is provided to the motor  110 . The waveform produced by the drive power module  134  can be adjusted manually or automatically through the user interface  128 , system software  130  and drive software  132  to adjust the operating parameters of the motor  110 . The combination of the user interface  128 , system software  130 , drive software  132  and drive power module  134  are collectively referred to as the motor control system  136 . 
         [0020]    Turning to  FIG. 3 , shown therein is a graphical depiction of the rotation of the rotor at a speed “ω” of the motor  110  in a reference frame, showing the direct (d) and quadrature (q) components of the current vector. The motor  110  can be approximated using a complex stator current space vector defined in a (d,q) coordinate system with orthogonal components along the d and q axes. The field flux linkage component of current is aligned along the d axis and the torque component of current is aligned along the q axis. A significant benefit of the field oriented control (FOC), vector control (VC) functionality is the ability to largely decouple the attributes of speed and torque within the motor  110 . The permanent magnet synchronous motor  110  with FOC/VC control is capable of full load torque at nearly any rotational speed. 
         [0021]    Turning to  FIG. 4 , shown therein is a functional block diagram of an exemplar vector control scheme employed within the motor control system  136 . Generally, the motor control system  136  incorporates an iterative control scheme in which reference currents I d  and I q  are initially generated based on a “Speed Reference” input. The reference currents are provided to a current controller module, which outputs representative voltages, V d  and V q . The voltages are presented to the PWM generator, which produces a three-phase current to an inverter. Power is fed to the inverter from the power source  118 . In response to the three-phase signals from the PWM generator, the inverter produces controlled, three-phase power to the motor  110 . 
         [0022]    The motor control system  136  monitors the power provided to the motor  110  and estimates rotor speed and position. The estimated rotational speed of the motor  110  is then provided back through the motor control system  136  and adjustments can be made to the output of the variable speed drive  120  to more closely approximate the Speed Reference input. It will be appreciated that the functional depiction of the motor control system  136  in  FIG. 4  is merely exemplary and additional and alternative Field Oriented Control schemes are contemplated as within the scope of the present invention. It will be appreciated that the motor control system  136  can be embodied as control software within the system software  130 , drive software  132  or drive power module  134 . 
         [0023]    Turning to  FIG. 5 , shown therein is a flow chart for a motor control process  138 . The motor control process  138  generally permits the motor  110  to be controlled at various efficiencies and power ratings after the motor  110  has been placed into operation. This represents a significant improvement and departure from prior art designs in which the efficiency of the motor was fixed during the manufacturing process and adjustments to power output could only be made by changing the effective length of the motor. 
         [0024]    The process  138  begins at step  140  when the operator inputs into the motor control system  136  a preference for increased efficiency, increased power output or a balanced performance from the motor  110 . Based on this input, the motor control system  136  automatically loads a reference motor model at step  142  that is optimized for the operational criteria selected at step  136 . Once the motor model has been loaded, the motor  110  can be operated at step  144  in accordance with the parameters associated with the loaded motor model. 
         [0025]    At step  146 , the operation of the motor  110  is monitored  146  on a continuous or periodic basis. Due to the widely varying operational demands on the motor  110 , the assumptions initially used to produce the initial motor model may become inaccurate over time. In particular, the motor control system  136  monitors the torque load carried by the motor  110 . For every torque load, there is an optimized ratio of the q-axis current (I q ) to the d-axis current (I d ). The ratio of these currents is referred to as “theta.” As the torque load or other operational dynamics change, the theta ratio may become suboptimal. 
         [0026]    At decision block  148 , the motor control system  136  determines whether theta optimization is required. The decision to optimize the theta ratio arises and triggers an adjustment of the theta ratio at step  150  when the ratio falls outside a predetermined threshold variance from the optimal ratio for a given torque load. If the theta ratio remains within the threshold range, the motor control process  138  proceeds to step  152 . If, on the other hand, the theta ratio falls outside the prescribed range, a theta correction routine is undertaken and applied. 
         [0027]    A suitable theta correction routine  200  is provided in  FIG. 6 . The theta correction routine  200  presented in  FIG. 6  seeks to optimize the theta correlation between q-axis current (I q ) and the d-axis current (I d ), by increasing or decreasing one or both of the current vectors. It will be appreciated that additional and alternative theta correction routines  150  are contemplated as within the scope of the present invention. 
         [0028]    At step  202 , an initial theta value is determined and an initial performance variable (V 1 ) is measured at step  204 . The performance variable can be an actual or estimated value for an aspect of the operation of the motor  110 . Performance variables (V) include rotational speed, average current draw, efficiency, power output, and power density. At step  206 , the theta ratio is increased and at step  208  the performance variable is measured. The theta ratio is increased by manipulating one or both of the relative values of the q-axis current (I q ) and the d-axis current (I d ) vectors. At step  210 , the motor control system  136  determines if the second measurement (V 2 ) is better than the first measurement (V 1 ). The evaluation of whether the performance variable improved will be based on the type of performance variable evaluated and the desired improvement in that variable. 
         [0029]    If the second measurement reveals that the performance variable (V 2 ) is better, the process moves to step  212  and a determination is made whether the theta ratio has reached a predetermined maximum value (e.g., 1). If so, the process moves to step  214  and the optimized theta ratio is used to drive the motor  110 . If not, the process returns to step  204 , the performance variable is measured and the theta ratio is incrementally increased. 
         [0030]    If the iterative comparison of the performance variable indicates that the second measurement is not better than the first measurement at step  210 , the process moves to decision block  216  and a determination is made whether the theta ratio has been adjusted a predetermined threshold number of times (e.g., 10). If the theta ratio has been adjusted at least the threshold number of times, the process moves to step  218  and the optimized theta ratio is used to drive the motor  110 . If not, the process moves to step  220  and the theta ratio is decreased, again by manipulating one or both of the relative values of the q-axis current (I q ) and the d-axis current (I d ) vectors. At step  222 , the performance variable is reevaluated and compared against the previous value. The evaluation of whether the performance variable improved will be based on the type of performance variable evaluated and the desired improvement in that variable. 
         [0031]    If the comparison reveals that the performance variable is better, the process moves to step  226  and a determination is made whether the theta ratio has reached a predetermined minimum value (e.g., −1). If so, the process moves to step  228  and the optimized theta ratio is used to drive the motor  110 . If not, the process returns to step  220  and the theta ratio is incrementally decreased. 
         [0032]    If at step  224  the comparison determines that the performance variable has not improved, the process moves to decision block  230  and a determination is made whether the theta ratio has been adjusted a predetermined threshold number of times (e.g., 10). If the theta ratio has been adjusted the threshold number of times, the process moves to step  228  and the optimized theta ratio is used to drive the motor  110 . If the theta ratio has not been adjusted the threshold number of times, the process returns to step  204  and the theta ratio is incrementally increased. It is contemplated that the theta optimization routine  200  will be performed on a scheduled, periodic basis during the operation of the motor  110 . It will be appreciated that the theta optimization routine  200  is merely exemplary and additional or alternative optimization routines could also be employed. 
         [0033]    Turning back to  FIG. 5 , if no theta optimization is required, the motor control process  138  proceeds to decision block  152  and the operator is afforded the opportunity to change the baseline efficiency and power output of the motor  110 . If no change is required at step  152 , the motor control process  138  returns to step  146  and the operation of the motor  110  and motor control system  136  is monitored. If a change in the overall operational characteristics of the motor  110  is desired, however, the motor control process  138  permits the adjustment of the power output and efficiency of the motor  110 . The motor control process  138  returns to step  140  and the operator is prompted to enter a new preference for the efficiency and power output of the motor  110 . Thus, the motor control system  136  and motor control process  138  permit the modification of operational control scheme for the motor  110  that does not require an adjustment to the physical components of the pumping system  100 . 
         [0034]    It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functions of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.