Patent Publication Number: US-9835160-B2

Title: Systems and methods for energy optimization for converterless motor-driven pumps

Description:
BACKGROUND 
     This description relates to converterless motor-driven pumps, and more particularly, to systems and methods for energy optimization for converterless motor driven pumps. 
     Electric submersible pumps (ESPs) are sometimes used in the oil and gas industry for pumping operations in off-grid applications. Typically, one or more prime movers are directly coupled to generators to produce an AC voltage having a fixed frequency and amplitude to supply electrical loads. The generated AC power is fed to a variable speed drive (VSD). The VSD uses a power converter to adjust the frequency and amplitude of the AC power to control operation of the ESPs. The output of the VSD is provided to the motors of the ESPs via a suitable transformer. 
     Most known systems for operating ESPs are large, complex, expensive systems. Such known systems use a large amount of energy. To the extent possible, this energy consumption should be minimized, thereby reducing the operating temperatures of the components of the systems, thereby potentially increasing the service life of one or more of the components of the systems. 
     BRIEF DESCRIPTION 
     In one aspect, a converterless motor-driven pump system includes an off-grid prime mover, an electric power generator driven by the off-grid prime mover to generate a power output, an electric submersible pump (ESP) system, and a system controller. The ESP system includes a motor coupled to the electric power generator to receive the power output, and a pump driven by the motor to pump a fluid. The system controller includes a processor and a memory. The memory includes instructions that, when executed by the processor, cause the system controller to control the off-grid prime mover as a function of an operational parameter of the ESP system to maintain a desired operating point of the pump, and control the electric power generator to reduce the power output generated by the electric power generator while the desired operating point of the pump is maintained. 
     In another aspect, a method of operating an electric submersible pump (ESP) system including an off-grid prime mover driving an electric power generator to produce a power output for a motor driving a submersible pump is provided. The method includes controlling a rotational speed of the off-grid prime mover as a function of an operational parameter of the ESP system to maintain a desired operating point of the submersible pump, and controlling a voltage output of the electric power generator to reduce the power output generated by the electric power generator while maintaining the desired operating point of the submersible pump. 
     In a further aspect, a system controller for a converterless motor-driven pump system includes a prime mover, an electric power generator driven by the prime mover to generate a power output, and an electric submersible pump (ESP) system including a motor powered by the power output and a pump driven by the motor. The system controller includes a processor and a memory. The memory includes instructions that, when executed by the processor, cause the system controller to control a rotational speed of the prime mover as a function of an operational parameter of the ESP system to maintain a desired operating point of the pump, and control the electric power generator to reduce the power output generated by the electric power generator while the desired operating point of the pump is maintained. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is an exemplary converterless electric submersible pump (ESP) system; 
         FIG. 2  is a block diagram of an exemplary embodiment of the system shown in  FIG. 1  including the flow of power and information through the system; 
         FIG. 3  is a block diagram of a portion of a control scheme for the ESP system shown in  FIG. 1 ; and 
         FIG. 4  is a flow diagram of an exemplary perturb and observe algorithm that may be used by the ESP system shown in  FIG. 1 . 
     
    
    
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     DETAILED DESCRIPTION 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     As used herein, the terms “processor” and “computer” and related terms, e.g., “processing device”, “computing device”, and “controller” are not limited to just those integrated circuits referred to in the art as a computer, but broadly refers to a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. In the embodiments described herein, memory may include, but is not limited to, a computer-readable medium, such as a random access memory (RAM), and a computer-readable non-volatile medium, such as flash memory. Alternatively, a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the embodiments described herein, additional input channels may be, but are not limited to, computer peripherals associated with an operator interface such as a mouse and a keyboard. Alternatively, other computer peripherals may also be used that may include, for example, but not be limited to, a scanner. Furthermore, in the exemplary embodiment, additional output channels may include, but not be limited to, an operator interface monitor. 
     Further, as used herein, the terms “software” and “firmware” are interchangeable, and include any computer program stored in memory for execution by personal computers, workstations, clients and servers. 
     As used herein, the term “non-transitory computer-readable media” is intended to be representative of any tangible computer-based device implemented in any method or technology for short-term and long-term storage of information, such as, computer-readable instructions, data structures, program modules and sub-modules, or other data in any device. Therefore, the methods described herein may be encoded as executable instructions embodied in a tangible, non-transitory, computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. Moreover, as used herein, the term “non-transitory computer-readable media” includes all tangible, computer-readable media, including, without limitation, non-transitory computer storage devices, including, without limitation, volatile and nonvolatile media, and removable and non-removable media such as a firmware, physical and virtual storage, CD-ROMs, DVDs, and any other digital source such as a network or the Internet, as well as yet to be developed digital means, with the sole exception being a transitory, propagating signal. 
     Embodiments of the present disclosure relate to converterless systems for operating an electric submersible pump (ESP). The converterless ESP systems described herein eliminate the variable speed drive and, potentially, its associated transformer from typical motor driven submersible pump systems, resulting in a simpler system that reduces capital expense, weight and system footprint. Further, the exemplary ESP systems reduce the energy used by the system when operating, thereby increasing the efficiency of the system, reducing temperatures of components of the system, and decreasing the cost of operating the system. 
       FIG. 1  is an exemplary converterless electric submersible pump (ESP) system  100 . ESP system  100  includes power generation systems  102 , an electronics house (E-house)  104 , and an ESP  106 . Generally, power generation systems  102  produce electric power that is provided to E-house  104 . E-house  104  is connected to ESP  106  using an ESP cable  107  coupled to a junction box  108 . E-house  104  uses the electric power to operate ESP  106  to pump a fluid, typically from a well. 
     Each power generation system  102  includes a prime mover and a generator (neither shown in  FIG. 1 ). The prime movers are turbines, reciprocating engines fueled by natural gas or diesel fuel, or any other prime mover suitable for use as described herein. Each prime mover drives its associated generator to produce an alternating current (AC) output current. In some embodiments, each power generation system  102  produces a direct current (DC) output current to power a DC powered ESP  106 . In the exemplary embodiment, each prime mover is an off-grid prime mover that is not powered by an electric utility power grid. In the exemplary embodiment, the generators are synchronous generators. In other embodiments, the generators may be any other suitable type of generator. In some embodiments, each power generation system  102  is capable of producing about 250 kilowatt (kW) of output power, and the system may operate with a single power generation system  102 . In other embodiments, each power generation system  102  is capable of providing up to about a six MW output, as required by the pumping effort required from the ESP. In other embodiments, power generation systems  102  are configured to output an amount of power sufficient to power ESP  106 . Although two power generation systems  102  are shown in  FIG. 1 , system  100  may include more or fewer power generation systems. In some embodiments, a gearbox (not shown) may be coupled between the prime mover and the generator to match the shaft speeds of the prime mover and generator to facilitate the most productive use of the equipment. The gearbox may be a fixed ratio gearbox or any other suitable gearbox. 
     E-house  104  generally houses electronics components for controlling system  100 . In the exemplary embodiment, E-house includes a system controller, contactors, and sensors (none shown in  FIG. 1 ). The system controller controls overall operation of system  100 . The contactors facilitate providing and/or interrupting electric current flow from power generation system(s)  102  to ESP  106 . The sensors in E-house  104  are configured to detect characteristics of the electric power received from power generation systems  102  and/or provided to ESP  106 . In the exemplary embodiment, the sensors detect the electrical voltage, frequency, and current provided to ESP  106 . In other embodiments, the sensors detect any characteristics of the electric power that enable operation of ESP system  100  as described herein. 
     ESP  106  includes a motor  110 , a pump  112 , and sensors  114 . Power delivered to ESP  106  by E-house  104  is used to power motor  110 . Operation of motor  110  drives pump  112 , which may then pump a fluid. ESP  106  is typically located within a well for purposes of artificially lifting a fluid from the well. The fluid may be, without limitation, water, gas, oil, or a combination thereof. Some amount of solids, such as sand or proppant, will be entrained with the fluid. In the exemplary embodiment, motor  110  is an induction motor. In other embodiments, motor  110  may be any type of motor suitable for driving a pump. Sensors  114  detect characteristics associated with the ESP. In the exemplary embodiment, sensors  114  detect the inlet and outlet pressures of pump  112 , the temperature of the fluid being pumped, and the temperature of motor  110 . Other embodiments include any sensors configured to detect characteristics that enable operation of ESP system  100  as described herein, including, without limitation, vibration, fluid leakage, motor speed, and pump speed. 
     Generally, the system controller controls operation of system  100 , at least in part, through control of power generation systems  102 . More specifically, the system controller controls the speed of the prime movers to set the frequency of the output of power generation systems  102 . The frequency of the output sets the speed of motor  110 . The speed of motor  110  determines (in combination with other factors such as the viscosity of the fluid and the presence or absence of obstructions) the pressure at the inlet of pump  112 . Accordingly, the system controller controls the speed of the prime movers to regulate an operational parameter of ESP  106  to an operating setpoint (also referred to as a desired operating state). In the exemplary embodiment, the operational parameter is the inlet pressure of pump  112 . In other embodiments, the operational parameter is the speed of the motor  110  or any other variable of motor  110  or pump  112  that permits operation as described herein. The system controller controls the excitation current provided to the generators of power generation systems  102  to control the voltage of the power generation systems&#39; output. In other embodiments, the system controller controls the voltage of the power generation system using any other suitable control method. The magnitude of the output voltage determines the amount of current delivered to motor  110 , and thereby affects the amount of power delivered to motor  110 . The system controller monitors the voltage current and frequency of the output and controls the excitation current to reduce the power used by ESP  106  while remaining at the operating setpoint. 
       FIG. 2  is a block diagram of an exemplary embodiment of system  100  showing the flow of power and information through system  100 . To maintain simplicity of illustration, a single power generation system  102  is shown in  FIG. 2 . It should be understood, however, that system  100  may include any suitable number of power generation systems  102 . In the exemplary embodiment, power generation system  102  includes a prime mover  200 , a throttle control  202  of prime mover  200 , a synchronous generator  204  driven by prime mover  200 , and a generator exciter  206  to provide excitation current to synchronous generator  204 . Generator exciter  206  may be a component of generator  204  (e.g., integrated within generator  204 ) or may be component separate from generator  204 . ESP cable  107  connects power generation system  102  to ESP  106 , and more particularly, to induction motor  110  driving pump  112 . Power flows from prime mover  200  through generator  204  and ESP cable  107  to motor  110  and subsequently the pump  112 . The power between prime mover  200  and generator  204  is mechanical driveshaft power, as is the power between the induction motor  110  and pump  112 . In some embodiments, a gearbox between prime mover  200  and generator  204  is employed for purposes of system optimization. Pump motor  110  may be any electric motor that can be line started, including, without limitation, an induction motor or a permanent magnet motor. 
     A system controller  208  is responsible for monitoring pump operating conditions, including without limitation input and output pressures, pump temperature(s), pump vibration levels, and pump rotational speed, and commanding throttle control  202  of prime mover  200  to a position that will drive pump  112  output to the desired pump operating point in response to one or more of the monitored operating conditions. System controller  208  also monitors, via a sensor  209 , the shaft speed of prime mover  200  and commands generator exciter  206  of the synchronous generator  204 . 
     In the exemplary embodiment, controller  208  is implemented in a computing device. Controller  208  includes a processor  210  and a memory  212 . Generally, memory  212  stores non-transitory instructions that, when executed by processor  210 , cause controller  208  to operate as described herein. It should be understood that the term “processor” refers generally to any programmable system including systems and microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), programmable logic circuits, programmable logic controller, and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term “processor.” Memory  212  may include, but is not limited to only include, non-volatile RAM (NVRAM), magnetic RAM (MRAM), ferroelectric RAM (FeRAM), read only memory (ROM), flash memory and/or Electrically Erasable Programmable Read Only Memory (EEPROM). Any other suitable magnetic, optical and/or semiconductor memory, by itself or in combination with other forms of memory, may be included in memory  212 . Memory  212  may also be, or include, a detachable or removable memory, including, but not limited to, a suitable cartridge, disk, CD ROM, DVD or USB memory. In other embodiments, controller  208  is implemented in analog circuitry, digital circuitry, or a combination of analog circuitry, digital circuitry, and/or computing devices. 
     System controller  208  monitors, using a suite of sensors  214 , the voltage, frequency and current being supplied to the motor  110 . As described above, system controller  208  also monitors characteristics (such as inlet pressure, outlet pressure, temperatures, etc.) of ESP  106  via sensors  114 . System controller  208  controls operation of system  100  based, at least in part, on the data received from sensors  214  and  114 . 
       FIG. 3  is a block diagram of a portion of a control scheme  300  for a converterless ESP system, such as ESP system  100 . Control scheme  300  is performed by system controller  208 . In other embodiments different and/or additional controllers may perform one or more portions of control scheme  300 . 
     An outer control loop  301  in control scheme  300  controls operation of system  100  to a desired operating point (e.g., a desired rate of pumping). In the exemplary embodiment, the desired operating point is associated with a shaft speed of induction motor  110 , which will drive pump  112  at a desired speed to achieve the desired rate of pumping. At  302 , a nominal speed command is summed, by system controller  208 , with a detected speed (negated) of motor  110  (from one of sensors  114 ). The resulting difference between the commanded and actual speeds is the speed error. The speed error is provided to a throttle control module  304  of controller  208 . Throttle control module  304  generates a throttle command for prime mover  200  that will drive generator  204  to produce an output with a frequency that will cause motor  110  to operate at the speed indicated by the nominal speed command. 
     A first inner control loop  306  controls the excitation current applied to generator  204 . Sensor  209  detects the rotational speed of the shaft (not shown) of prime mover  200 . An excitation controller module  308  determines the excitation current that should be applied to generator  204 . The excitation current is determined through use of a formula or a look-up table and is based at least in part on the detected speed of prime mover  200 . The excitation controller module  308  generates a nominal excitation command that will cause generator exciter  206  to provide the determined excitation current. 
     A second inner control loop  310  interacts with first inner control loop  306  to optimize the operation of system  100  to limit the power expended in the operation. In the exemplary embodiment, second inner control loop  310  is not activated until outer control loop  301  has brought system  100  to its desired operating point. In some embodiments, second inner control loop  310  is inactive until system  100  remains at about the desired operating point for a period of time. The period of time may be a fixed (e.g., a preset or predetermined) period of time or variable (e.g., a period of time determined/calculated as a function of another variable). In the exemplary embodiment, second inner control loop  310  is deactivated if system  100  deviates from the desired operating point by more than a threshold amount. The threshold amount may be a fixed threshold or a variable threshold. For example, the threshold may be, without limitation, an absolute speed difference of motor  110 , a fixed percentage speed difference, or a speed difference (whether an absolute speed or a percentage) that varies depending on another variable (such as temperature, inlet pressure, or a length of time). When second inner control loop  310  is inactive, the nominal excitation command generated by first inner control loop  306  is utilized by generator exciter  206  unmodified by second inner control loop  310 . 
     Second inner control loop  310  receives voltage, frequency, and current measurements from sensors  214 . An energy optimization module  312  determines the amount of power being output by generator  204  and used by motor  110 . Energy optimization module  312  determines whether the output power of generator  204  is at a minimum output power that will maintain the current operating conditions (e.g., current desired operating point, current speed of motor  110  and/or prime mover  200 , and/or current inlet pressure). Energy optimization module  312  determines an adjustment to be made to the nominal excitation command generated by excitation controller module  308 . If the output power is substantially at a minimum, the adjustment will generally be zero (i.e., no adjustment is needed if output power is already at a minimum). Otherwise, a positive or negative adjustment to the nominal excitation command is determined. At  314 , system controller  208  sums the nominal excitation command from excitation controller module  308  and the adjustment from energy optimization module  312  to produce the excitation command that is delivered to generator exciter  206 . 
     In the exemplary embodiment, energy optimization module  312  utilizes a perturb and observe algorithm. An exemplary perturb and observe algorithm suitable for use in system  100  will be described below with reference to  FIG. 4 . In other embodiments, any other suitable energy optimization algorithm may be utilized. In some embodiments, the minimum output power and/or the adjustments may be determined from a look-up table based on one or more current operating condition. 
     Second inner loop  310  repeats periodically to attempt to minimize (and maintain the minimized) the power output by generator  204  while maintaining the desired operating point. In the exemplary embodiment, second inner loop  310  acts to produce an adjustment to the excitation command at a frequency that is slower than the frequency at which outer loop  301  is performed. Thus, for example, outer loop  301  may make several adjustments to the throttle command before inner loop  314  determines whether or not to change the adjustment to the nominal excitation command. In other embodiments, the frequency of inner loop  314  may be the same as or greater than the frequency of outer loop  301 . 
       FIG. 4  is a flow diagram of an exemplary perturb and observe algorithm  400  that may be used by system  100 , and more particularly by system controller  208 . At  402 , controller  208  sets an initial value for an old power variable (P_old) equal to zero, and sets a variable Sign equal to +1. At  404 , controller  208  reads the generator  204  output voltage and current values (v and i, respectively) detected by sensor  214 . Controller  208  computes the current output power (P_new) from the values of the output voltage and current at  406 . Controller determines, at  408 , if the current output power value P_new is greater than the old output power value P_old. If the current output power P_new is greater than the old output power value P_old, the variable Sign is set equal to −1 at  410 . If the current output power P_new is less than or equal to the old output power value P_old, the variable Sign is set equal to +1 at  412 . 
     At  414 , the excitation adjustment (also referred to as the excitation perturbation) is calculated by multiplying the variable Sign by an excitation increment (EXC_INC). The excitation increment determines how much the nominal excitation command will be adjusted each time second inner loop  310  perturbs the excitation current, and the value of the variable Sign determines in which direction (increasing or decreasing) the excitation current is perturbed. In the exemplary embodiment, the excitation increment is a predetermined, fixed value. In other embodiments, the excitation increment is a variable value and/or may be a calculated value. For example, the excitation increment may be increased or decreased as a function of how much the output power has changed after the last perturbation (i.e., based on the difference between P_new and P_old). The excitation increment may be increased to induce larger changes in the output power to move more quickly toward a minimum output power and decreased when near the minimum power point to limit overshooting the minimum. In some embodiments, the excitation increment value is periodically increased significantly for a single cycle. This will move the system off its previous operating point by a significant amount to combat the possibility that the perturb and observe algorithm has settled into an operating point that is a local minimum instead of the global minimum. 
     Controller  208  stores the current output power value P_new as the old output power variable P_old at  416 . At  418 , controller  208  implements the perturbation of the excitation current (e.g., by summing the nominal excitation current with the excitation perturbation calculated at  414 ). Algorithm  400  then returns to  404  to read the new voltage and current values. In some embodiments, algorithm  400  includes a delay, such as before returning from  418  to  404 , to permit the perturbation to affect the output power and to limit introduction of instability into system  100 . The time delay may be fixed or variable. A variable time delay may increase the time delay, for example, if controller  208  determines that the output power is relatively stable at the minimum power output. Thus, if the difference between P_new and P_old is small, or remains small for a certain number of cycles, controller  208  may increase the delay to avoid unnecessarily perturbing the excitation current for little or no efficiency gains. Conversely, if the difference between P_new and P_old is large, or remains large for a certain number of cycles, controller  208  may decrease the delay. 
     A converterless ESP system, such as system  100 , eliminates the variable speed drive and, potentially, its associated transformer from a motor driven submersible pump system, resulting in a simpler system that reduces capital expense, weight and system footprint. The use of power generated on-site advantageously reduces the time it takes to put a well into production resulting from delays in getting the utility to install requisite power lines. Further, the use of natural gas produced by the well itself advantageously reduces the operating expense. 
     Because the output of the system generator is substantially sinusoidal when compared with the output of a variable speed drive, a filter is not required between the generator and the pump motor. Moreover, the converterless systems do not generate harmonics that are not filtered and that may lead to accelerated aging of the insulation systems in the transformer, cable, and pump motor of submersible pump systems including converters. 
     Furthermore, the control systems described herein operate converterless ESP systems at their desired operating points and refine that control to attempt to minimize the power produced and expended, while still remaining at the desired operating point. Thus, the exemplary systems and controllers increase the efficiency of ESP systems and allow them to be operated at reduced costs and/or greater profitability. 
     An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) eliminating power converters in an ESP systems by controlling the speed of the prime mover and the excitation current of the synchronous generator driven by the prime mover to maintain a desired pump inlet pressure; (b) reducing the temperature of components of an ESP system; (c) reducing the size of ESP systems; (d) extending the useful life of the components of an ESP system; and (e) increasing the efficiency of an ESP system. 
     Exemplary embodiments of the systems and methods are described above in detail. The systems and methods are not limited to the specific embodiments described herein, but rather, components of the systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the system may also be used in combination with other apparatus, systems, and methods, and is not limited to practice with only the system as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other applications. Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     Some embodiments involve the use of one or more electronic or computing devices. Such devices typically include a processor or controller, such as a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic circuit (PLC), and/or any other circuit or processor capable of executing the functions described herein. The methods described herein may be encoded as executable instructions embodied in a computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term processor. 
     This written description uses examples to disclose the embodiments, including the best mode, and also to enable any person skilled in the art to practice the embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.