Patent Publication Number: US-9903373-B2

Title: Dual motor drive for electric submersible pump systems

Description:
BACKGROUND 
     The subject matter disclosed herein relates to variable frequency drives (VFDs), and more specifically to VFDs for driving electric machines used with electric submersible pumps (ESPs) in oil and gas applications. 
     In typical oil and gas drilling applications a well bore is drilled to reach a reservoir. The well bore may include multiple changes in direction and may have sections that are vertical, slanted, or horizontal. A well bore casing is inserted into the well bore to provide structure and support for the well bore. The oil, gas, or other fluid deposit is then pumped out of the reservoir, through the well bore casing, and to the surface, where it is collected. One way to pump the fluid from the reservoir to the surface is with an electrical submersible pump (ESP), which is driven by an electric motor (e.g., an induction motor or a permanent magnet motor) in the well bore casing. 
     A variety of components may be used to receive power from a power source, filter, convert and/or transform the power, and then drive the electric motor. For example, a variable frequency drive (VFD) may receive power from a power source (e.g., utility grid, batteries, a generator, etc.). The power may then pass through a filter and a step up transformer before being provided to the electric motor via a cable that passes through the well bore. 
     In some conditions (e.g., startup of a synchronous motor, seizure of the pump, transient load conditions, etc.), the motor may not operate as intended because magnetic saturation of the transformer prevents adequate voltage from reaching the motor. Accordingly, it may be desirable to improve the system to be capable of providing the motor with sufficient voltage to reduce or eliminate motor stalling. 
     BRIEF DESCRIPTION 
     Certain embodiments commensurate in scope with the original claims are summarized below. These embodiments are not intended to limit the scope of the claims, but rather these embodiments are intended only to provide a brief summary of possible forms of the claimed subject matter. Indeed, the claims may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
     In one embodiment, an electric submersible pump (ESP) control system includes a primary variable frequency drive (VFD), a transformer, and a secondary VFD. The primary variable frequency drive (VFD) is configured to receive power from a power source and output a variable voltage and variable amplitude AC voltage. The transformer has a low voltage side and a high voltage side of the transformer. The primary VFD is coupled to the low voltage side. The transformer is configured to receive the AC voltage from the primary VFD and output a stepped up AC voltage. The secondary VFD is coupled to the high voltage side of the transformer, wherein the secondary VFD is configured to provide a supplemental voltage in addition to the stepped up AC voltage when the operational values of an electric motor exceed a threshold value. 
     In a second embodiment, an ESP system includes a pump, an electric motor, and an ESP control system. The pump is configured to extract deposits from a reservoir. The electric motor is coupled to the pump, and is configured to receive an output voltage via a cable and drive the pump. The ESP control system includes a primary VFD, a transformer, and a secondary VFD. The VFD is configured to receive power from a power source and output a variable voltage and variable amplitude AC voltage. The transformer has a low voltage side and a high voltage side. The primary VFD is coupled to the low voltage side of the transformer. The transformer is configured to receive the AC voltage from the primary VFD and output a stepped up AC voltage. The secondary VFD coupled to the high voltage side of the transformer and is configured to provide a supplemental voltage in addition to supplement the stepped up AC voltage when the operational values of an electric motor exceed a threshold value. The stepped up AC voltage and the supplemental voltage combine to form the output voltage. 
     In a third embodiment, a method of controlling an ESP system includes monitoring one or more operational values of an electric motor in an ESP system, determining whether the one or more operational values of the electric motor are below a threshold value, and utilizing a secondary VFD when the one or more operational values of the motor exceed the threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE 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 a schematic of a hydrocarbon extraction system extracting fluid from an underground reservoir in accordance with aspects of the present disclosure; 
         FIG. 2  is a wiring schematic of the electric submersible pump (ESP) control system in accordance with aspects of the present disclosure; 
         FIG. 3  is a wiring schematic showing an alternative embodiment of coupling the secondary variable frequency drive (VFD) to a high voltage side of a transformer in accordance with aspects of the present disclosure; 
         FIG. 4  is a wiring schematic showing an alternative embodiment of coupling the secondary variable frequency drive (VFD) to the transformer using switches in accordance with aspects of the present disclosure; 
         FIG. 5  is a plot of transformer voltage capability versus system required voltage for two synchronous motor torques; 
         FIG. 6  is a plot of the minimum allowable operating frequency of the system as a function of motor output torque; and 
         FIG. 7  is a flow chart for a process of operating a system with two variable frequency drives (VFDs) in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Furthermore, any numerical examples in the following discussion are intended to be non-limiting, and thus additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments. 
       FIG. 1  is a schematic of a hydrocarbon extraction system (e.g., well  10 ) extracting fluid deposits (e.g., oil, gas, etc.) from an underground reservoir  14 . As shown in  FIG. 1 , a well bore  12  may be drilled in the ground toward a fluid reservoir  14 . Though the well bore  12  shown in  FIG. 1  is a vertical well bore  12 , well bores  12  may include several changes in direction and may include slanted or horizontal sections. A well bore casing  16  is typically inserted into the well bore  12  to provide support. Fluid deposits from the reservoir  14 , may then be pumped to the surface  18  for collection in tanks  20 , separation, and refining. Though there are many possible ways to pump fluids from an underground reservoir  14  to the surface  18 , one technique is to use an electrical submersible pump (ESP), as shown in  FIG. 1 . 
     When using an ESP, an ESP assembly or system  22  is fed through the well bore casing  16  toward the reservoir  14 . The ESP assembly  22  may include a pump  24 , an intake  26 , a sealing assembly  28 , an electric motor  30 , and a sensor  32 . Power may be drawn from a power source  34  and provided to the electric motor  30  by an ESP control system  36 . The power source  34  shown in  FIG. 1  is a utility grid, but power may be provided in other ways (e.g., generator, batteries, etc.). The ESP control system  36  may include a primary variable frequency drive (VFD)  38 , a filter  40 , a transformer  42 , a secondary VFD  44 , and a cable  46 . It should be understood, however, that  FIG. 1  shows one embodiment, and that other embodiments may omit some elements or have additional elements. The primary VFD  38  synthesizes the variable frequency, variable amplitude, AC voltage that drives the motor. In some embodiments, the power output by the VFD may be filtered by filter  40 . In the present embodiment, the filter  40  is a sine wave filter. However, in other embodiments, the filter may be a low pass filter, a band pass filter, or some other kind of filter. The power may then be stepped up or down by a transformer  42 . In the present embodiment, a step up transformer is used for efficient transmission down the well bore  12  to the ESP assembly  22 , however, other transformers or a plurality of transformers may be used. A secondary VFD  44  may be disposed on the high-voltage side of the transformer  42  and configured to deliver full-rated current for a short period of time (e.g., one minute or less) when the electric motor  30  requires more voltage than the transformer  42  can support. In embodiments with multiple transformers (e.g., a step up transformer  42  at the surface, and a step down transformer in the well bore  12 , at the end of the cable  46 , the secondary VFD  44  may be installed between the transformers or at the termination of the second transformer. Power output from the secondary VFD may be provided to the ESP assembly  22  via a cable  46  that is fed through the well bore casing  16  from the surface  18  to the ESP assembly  22 . The motor  30  then draws power from the cable  46  to drive the pump  24 . The motor  30  may be an induction motor, a permanent magnet motor, or any other type of electric motor. 
     The pump  24  may be a centrifugal pump with one or more stages. The intake  26  acts as a suction manifold, through which fluids  14  enter before proceeding to the pump  24 . In some embodiments, the intake  26  may include a gas separator. A sealing assembly  28  may be disposed between the intake  26  and the motor  30 . The sealing assembly protects the motor  30  from well fluids  14 , transmits torque from the motor  30  to the pump  24 , absorbs shaft thrust, and equalizes the pressure between the reservoir  14  and the motor  30 . Additionally, the sealing assembly  28  may provide a chamber for the expansion and contraction of the motor oil resulting from the heating and cooling of the motor  30  during operation. The sealing assembly  28  may include labyrinth chambers, bag chambers, mechanical seals, or some combination thereof. 
     The sensor  32  is typically disposed at the base of the ESP assembly  22  and collects real-time system and well bore parameters. Sensed parameters may include pressure, temperature, motor winding temperature, vibration, current leakage, discharge pressure, and so forth. The sensor  32  may provide feedback to the ESP control system  36  and alert users when one or sensed parameters fall outside of expected ranges. 
       FIG. 2  is a wiring schematic of the ESP control system  36  shown in  FIG. 1 , in accordance with aspects of the present disclosure. As previously discussed, the primary VFD  38  receives power from a power source  34  (e.g., utility grid, battery, generator, etc.), modifies the power, and outputs a power signal of the desired frequency and amplitude for driving the electric motor  30 . The primary VFD  38  may include power electronic switches, current measurement components, voltage measurements components, a process, or other components. The primary VFD  38  may be installed on the primary side of the transformer  42  and is programmed to operate the motor. 
     The output from the primary VFD  38  may then be filtered using the filter  40 . In the embodiment shown, the filter  40  is a sine wave filter, however in other embodiments, the filter may be any low pass filter, or any other kind of filter. As shown in  FIG. 2 , the filter  40  may include inductors  80 , capacitors  82 , or other electrical components. 
     The output from the filter  40  is stepped up using the step up transformer  42 . The step up transformer steps up the voltage of the power signal for efficient transmission through the cable  46  to the electric motor  30 , which in some applications may as long as 1,000 to 10,000 feet. As will be discussed with regard to  FIG. 5 , because of magnetic saturation, the transformer  42  may be limited in the voltage it can supply to the electric motor  30  at low frequencies. 
     In order to deal with the limitations of the transformer, a secondary VFD  44  may be disposed in series or parallel with the line, on the high voltage secondary side of the transformer  42 , and configured to deliver full rated current for short periods of time (e.g., less than 1 minute). The secondary VFD  44  may interface with only one or all three phases of the system  36 . As shown in  FIG. 2 , the secondary VFD  44  may include transistors  84  (e.g., IGBT or MOSFET), diodes  86 , inductors  80 , capacitors  82 , and any number of other components. The secondary VFD  44  may also include power electronic switches, current measurement components, voltage measurement components, a processor, control circuitry, and the like. In addition to the single phase H-bridge topology shown in  FIG. 2 , the secondary VFD  44  may have a single phase half-bridge topology, or a polyphase half-bridge topology. In addition to the series topology, a parallel topology may be employed. 
     In some situations that require the electric motor  30  to operate at low frequency with high torque (e.g., startup of a motor, a temporarily seized pump, a transient load condition, etc.), magnetic saturation may prevent the primary VFD  38  and the transformer  42  from providing sufficient voltage or magnetic flux to keep the electric motor  30  from stalling. Because the secondary VFD  44  is on the high voltage side of the transformer, the secondary VFD  44  can provide full rated current for a short period of time (e.g., one minute or less), thus supplementing the voltage of the primary VFD  38  until the motor  30  reaches a high enough frequency for the primary VFD  38  to drive the motor  30  on its own. Motor  30  requirements (e.g., operational values, operational parameters, or parameters to drive the pump  24 ) and magnetic saturation of the transformer  42  will be discussed in more detail with regard to  FIG. 5 . As previously discussed, the power signal output by the ESP control system  36  is transmitted to the electric motor  30  via the cable  46 . 
       FIGS. 3 and 4  are wiring schematics of alternative embodiments of coupling the secondary VFD  44  to the transformer  42 . Specifically,  FIG. 3  is a wiring schematic showing an alternative embodiment of coupling the secondary VFD  44  to a high voltage side  90  of the transformer  42 . As shown, the transformer  42  has a low voltage side  88  and a high voltage side  90 . The transformer  42  receives a voltage at the low voltage side  88 , “steps up” the voltage, and outputs the stepped up voltage at the high voltage side  90 . In the embodiment shown in  FIG. 3 , the low voltage side  88  is shown in Y, but could also be in delta.  FIG. 4  is a wiring schematic showing an alternative embodiment of coupling the secondary VFD  44  to the transformer  42  using switches  92 . As shown in  FIG. 4 , the secondary VFD  44  is coupled between the transformer  42  and the electric motor by three lines, each corresponding to a phase of the voltage signal. Each of the three lines may include respective switches  92 . Though three phases are shown, it should be understood that a different number of phases may be possible. In such a configuration, the number of switches may or may not correspond to the number of phases. 
       FIG. 5  is a plot  120  of transformer  42  voltage capability versus system required voltage for two synchronous motor  30  torques. The x-axis  122  represents per-unit frequency (e.g., a percent of capability) and the y-axis  124  represents normalized voltage (e.g., a percent of capability). Line  126 , which has a slope of 1.0 and an intercept of 0.0, represents the maximum operating conditions of the transformer  42 . Lines  128  and  130  represent the voltage requirements of a prototypical synchronous motor  30  while supporting 25% and 75% rated torque, respectively. Due to magnetic saturation, the transformer  42  must operate below line  126 . At most frequencies, (e.g., higher than about 20% per unit frequency on the x-axis  122 ), the voltage requirements of the motor  30  are below the maximum operating conditions of the transformer  42 , meaning that powering the motor  30  is within the capabilities of the transformer  42  and the primary VFD  38 . However, at the low end of the frequency range (e.g., less than 10% or 20% per unit frequency on the x-axis), the voltages required to operate the motor  30  exceed the capabilities of the transformer  42  and the primary VFD  38 . Without the assistance of the secondary VFD  44 , situations that require high torque at low frequency (e.g., startup of a motor  30 , seizure of the pump  24 , transient load conditions, etc.) may result in the motor  30  stalling. When the capabilities of the transformer  42  are approached or exceeded by the requirements (e.g., operational values, operational parameters, or parameters to drive the pump  24 ) of the motor  30  (e.g., a threshold value is exceeded), the secondary VFD  44  may provide full rated power for a short period of time (e.g., less than one minute) to supplement the primary VFD  38  and the transformer  42 . 
       FIG. 6  is a plot  150  of the minimum allowable operating frequency  154  of the system as a function of motor output torque  152 . The x-axis represents the per-unit torque (e.g., a percent of capability) and the y-axis represents the per-unit frequency (e.g., a percent of capability). A system with a single VFD  38  (e.g., a system without a secondary VFD  44 ) must operate above line  156 , which represents the minimum allowable operating frequency. Accordingly, an ESP control system  36  without a secondary VFD  44  will likely be unable to drive the motor  30  at low frequencies and high torques (e.g., 20% frequency and 80% torque). For example, starting a synchronous AC motor  30  requires high torque at low frequency. The addition of a secondary VFD  44  effectively increases the starting torque of the system  36  by providing full rated power for a short period of time. In operation, the secondary VFD  44  may start the motor  30  at full torque. Once the frequency increases and/or the voltage requirement of the motor decreases to within the capabilities of the primary VFD  38  and the transformer  42 , the primary VFD  38  and the transformer  42  takeover driving the motor  30 . 
       FIG. 7  is a flow chart for a process  200  of operating a system  10  with two VFDs ( 38 ,  44 ). In block  202 , the process  200  operates the electric motor  30  using the primary VFD  38  and the transformer  42 . In block  202 , the secondary VFD  44  may not provide any power to the motor  30 , or may provide a nominal amount of power to the motor  30  in comparison to the primary VFD  38  and the transformer  42 . In some embodiments, the motor  30  may be in a steady state or near steady state in block  202 . Referring back to plot  120  shown in  FIG. 5 , in block  202 , the motor  30  is likely operating at a voltage and frequency below line  126 . In block  204 , the process  200  monitors the requirements (e.g., operational values, operational parameters, or parameters to drive the pump  24 ) of the motor  30 . For example, the process  200  may monitor the frequency, voltage, and/or torque requirements of the motor. 
     At decision  206 , the process  200  determines whether the requirements of the motor  30  monitored in block  204  are within the capability of the primary VFD  38  and the transformer  42  (e.g., whether the requirements of the motor  30  monitored in block  204  are below a threshold value). For example, as discussed with regard to  FIG. 5 , the process  200  may monitor the voltage and frequency requirements of the motor  30  and determine whether the required combination of voltage and frequency fall below line  126 . Similarly, as discussed with regard to  FIG. 6 , the process  200  may monitor the frequency and torque requirements of the motor  30  and determine whether the required combination of voltage and frequency fall above line  156 . 
     In decision  206 , if the requirements of the motor fall well within the capability of the primary VFD  38  and the transformer  42  (e.g., the requirements of the motor  30  are below a threshold value), the process will continue to operate the motor  30  with the primary VFD  38  and return to block  204 , monitoring the requirements of the motor  30 . In block  208 , if the requirements of the motor  30  approach or exceed the capability of the primary VFD  38  and transformer  42 , the process  200  may utilize the secondary VFD  44  to provide additional power (e.g., voltage, magnetic flux, etc.) in order to reduce the likelihood of the motor  30  stalling. As previously discussed, conditions in which the process  200  may utilize the secondary VFD  44  may include startup of a synchronous motor  30 , seizure of the pump  24 , transient load conditions, and the like. The process  200  may continue to monitor the requirements of the motor. 
     In decision  210 , if the requirements of the motor approach or exceed the capability of the primary VFD  38  and the transformer  42  (e.g., the requirements of the motor  30  are above a threshold value), the process continues to utilize the secondary VFD  44  to drive the motor  30 . If the requirements of the motor  30  are within the capabilities of the primary VFD  38  and the transformer  42  (e.g., the requirements of the motor  30  are below a threshold value), the process  200  may return to block  204 , operating the motor  30  with the primary VFD  38  and monitoring the requirements of the motor  30 . 
     As the oil reservoir  14  is depleted, the torque, voltage, and frequency requirements of the motor  30  may be reduced. In such cases, it may be possible to remove the primary VFD  38 , relying only on the secondary VFD  44  to drive the motor  30 . 
     Technical effects of the disclosure include use of a secondary VFD  44  on the high voltage side of the transformer  42  that provides supplemental power (e.g., magnetic flux, voltage, etc.) when the requirements of the electric motor  30  approach or exceed the capabilities of the primary VFD  38  and the transformer  42 . The disclosed techniques may be used to provide short bursts of power to an electric motor  30  when the demands of the motor  30  exceed those of the primary VFD  38  and transformer  42  (e.g., startup of a synchronous motor, seizure of the pump, transient load conditions, and the like). 
     This written description uses examples to disclose the subject matter, including the best mode, and also to enable any person skilled in the art to practice the disclosure, 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 languages of the claims.